I. MORPHOLOGY AND PHYSIOLOGY (part 2 of 2)

I. MORPHOLOGY AND PHYSIOLOGY (part 2 of 2)

—— Ueber die Palpen der Rhopaloceren, etc., 6 Taf. Helsingfors, 1896, pp. 1–578. (Acta Soc. Sc. Fennicæ, xxii, 1896.)

=Packard, A. S.= On the occurrence of organs, probably of taste, in the epipharynx of the Mecoptera (Panorpa and Boreus). (Psyche, 1889, v, pp. 159–164.)

—— Notes on the epipharynx, and the epipharyngeal organs of taste in mandibulate insects. (Psyche, 1889, v, pp. 193–199, 222–228.)

Also Lubbock’s Senses, etc., of animals, and the writings of Briant, Breithaupt (titles on p. 85).

d. The organs of hearing

Although it has been denied by Forel that insects have the sense of hearing, yet the majority of writers and experimenters agree that insects are not deaf. On general grounds if, as we know, many insects produce sounds, it must follow that they have ears to hear, for there is every reason to suppose that the sounds thus made are, as in other animals, either for attracting the sexes, for a means of communication, or to express the emotions. We will begin by briefly describing the structures now generally supposed to be auditory in function, and about which there can be no reasonable doubt, and then consider the more problematical organs, closing with an account of the extremely various means of producing sounds and cries.

=The ears or tympanal and chordotonal sense-organs of Orthoptera and other insects.=—The ears or tympana of locusts (Acrydiidæ) are situated one on each side, on the basal joint of the abdomen, just behind the first abdominal spiracle. That this is a true ear was first suggested by J. Müller, and his opinion was confirmed by Siebold, Leydig, Hensen, Graber, Schmidt, Lubbock, etc.[49]

The apparatus consists of a tense membrane, the tympanum, surrounded by a horny ring (Fig. 290). “On the internal surface of this membrane are two horn-like processes (o, u), to which is attached an extremely delicate vesicle (bi) filled with a transparent fluid, and representing a membranous labyrinth. This vesicle is in connection with an auditory nerve (n) which arises from the third thoracic ganglion, forms a ganglion (ga) upon the tympanum, and terminates in the immediate neighborhood of the labyrinth by a collection of cuneiform, staff-like bodies, with very finely pointed extremities (primitive nerve-fibres?), which are surrounded by loosely aggregated ganglionic globules” (Siebold’s Anatomy of the Invertebrates).

In the green grasshoppers, katydids, and their allies, the ears are situated on the fore tibiæ, where these organs can be found after a careful search (Figs. 291, 292).

The presence of the structure is indicated by the oval disc, the drum, which is a thin tense membrane covering the auditory apparatus of nerves, ganglion cells, and auditory rods beneath.

The tympana, or drums, are not present in all Locustidæ and Gryllidæ, and, as Lubbock states, it is an additional reason for regarding them as auditory organs, that in those species which possess no stridulating organs the tympana are also wanting. In many of the Locustidæ the tympana are covered or protected by a fold of the skin projecting over them. These covered ones are, Graber thinks, derived from the open ones.

On examining the apparatus within the leg under the drum, it is seen to consist of the trachea, the auditory vesicles and rods, ganglion cells, and acoustic nerve. The trachea is greatly modified (Fig. 292, Tr 1). On passing into the tibia the trachea enlarges and divides into two branches, which reunite lower down. The spiracles supplying the air to this enlarged trachea are considerably enlarged, while in the dumb species it is of the normal size. The enlarged trachea passes close to the tympanum, which thus has air on both sides of it: the open air on the outer, the air of the trachea on its inner surface. In fact, as Lubbock states, “the trachea acts like the Eustachian tube in our own ear; it maintains an equilibrium of pressure on each side of the tympanum, and enables it freely to transmit the atmospheric vibrations.”

These tracheæ, says Graber, though formed on a similar plan, present many variations, corresponding to those of the tympana, and showing that the tympana and the tracheæ stand in intimate connection with one another. For instance, in those species where the tympana are equal, the tracheæ are so likewise; in Gryllotalpa, where the front tympanum only is developed, though both tracheal branches are present, the front one is much larger than the other; and where there is no tympanum, the trachea remains comparatively small, and even in some cases undivided (Lubbock, ex Graber).

The acoustic nerve, which next to the optic is the thickest in the body, divides soon after entering the tibia into two branches, one almost immediately forming a ganglion, the supra-tympanal ganglion, the other passing down to the tympanum, where it expands into an elongated flat ganglion, the organ of Siebold (Fig. 293), and closely applied to the anterior tracheæ.

At the upper part of the ganglion is a group terminating below in a single row of vesicles, the first few of which are approximately equal, but which subsequently diminish regularly in size. Each of these vesicles is connected with the nerve by a fibril (Fig. 293, vN), and contains an auditory rod (Fig. 294). They are said by Graber to be brightly refractive, hollow (thus differing from the retinal rods, which are solid), and terminate in a separate end-piece (ko). The rods were first discovered by Siebold, and, as Lubbock remarks, may be regarded as specially characteristic of the acoustic organs of insects.

As will be seen in Fig. 293, at the upper part of the tibial organ of Ephippigera there is a group of cells, and below them a single row of cells gradually diminishing in size from above downwards. “One cannot but ask oneself,” says Lubbock, “whether the gradually diminishing size of the cells in the organ of Siebold may not have reference to the perception of different notes, as is the case with the series of diminishing arches in the organ of Corti of our own ears.”

These organs were supposed to be restricted to the Orthoptera, but in 1877 Lubbock discovered what seems to resemble the supra-tympanal auditory organ of Orthoptera in the tibia of the yellow ant (Lasius flavus). Graber confirmed Lubbock’s account, and also discovered these organs in the tibia of a Perlid (Isopteryx apicalis), and Fritz Müller has detected them in the fore tibiæ of the nymph of Calotermes rugosus (Fig. 295). To these structures Graber gave the name of chordotonal organs.

He has also detected these organs in all the legs of other insects (Trichoptera, Pediculidæ), and auditory rods have been discovered in the antennæ of Dyticus and of Telephorus by Hicks, Leydig, and Graber. Graber classifies the chordotonal organs into truncal and membral. In Coleoptera and Trichoptera they may occur on several joints of the leg; others are more localized,—thus he distinguishes femoral (Pediculidæ), tibial (Orthoptera, Perlidæ, Formicidæ), and tarsal organs (Coleoptera).

A type of chordotonal organ, observed in the body-segments of the larvæ of several insects by Leydig, Weismann, Graber, Grobben, and Bolles Lee, is to be seen in the transparent larva of Corethra (Fig. 296), where the auditory organ extends to the skin. It contains at the point cs two or three auditory rods. In the opposite direction a fine ligament (cl) passes from cg to the skin; in this way the auditory organ is suspended in a certain state of tension, and is favorably situated to receive even very fine vibrations. A similar apparatus has been detected in the larva of Ptychoptera.

=Antennal auditory hairs.=—It is not at all improbable that the antennæ of different insects contain auditory as well as olfactory structures. Lubbock has suggested that the singular organs which have only been found in the antennæ of ants and certain bees, and to which he gives the name of “Hicks’ bottles” (Fig. 281), may act as microscopic stethoscopes, while Leydig also regards them as chordotonal organs.

That, however, some of the antennal hairs of the mosquito, as first suggested by Johnson and afterwards proved experimentally by Mayer, are auditory, seems well established. Fastening a male mosquito down on a glass slide, Mayer then sounded a series of tuning-forks. With an Ut_{4} fork of 512 vibrations per second, some of the hairs were seen to vibrate vigorously, while others remained comparatively at rest. The lower (Ut_{3}) and higher (Ut_{5}) harmonics of Ut_{4} also caused more vibration than any intermediate notes. These hairs, then, are specially tuned so as to respond to vibrations numbering 512 per second. Other hairs vibrated to other notes, extending through the middle and next higher octave of the piano.

Mayer then made large wooden models of these hairs, the one corresponding to the Ut_{3} hair being about a metre in length, and on counting the number of vibrations they made when they were clamped at one end and then drawn on one side, he found that it “coincided with the ratio existing between the numbers of vibrations of the forks to which covibrated the fibrils,” or hairs. It should be observed that the song of the female mosquito corresponds nearly to this note, and would consequently set the hairs in vibration. Mayer observed that the song of the female vibrates the hairs of one of the antennæ more forcibly than those of the other. Those auditory hairs are most affected which are at right angles to the direction from which the sound comes. Hence from the position of the antennæ and the hairs a sound will be loudest or most intense if it is directly in front of the head. If, then, the song of the female affects one antenna more than another, the male turns his head until the two antennæ are equally affected, and is thus able to fly straight towards the female. From his experiments Mayer found that the male can thus guide himself to within 5° of the direction of the female. Hence he concludes that “these insects must have the faculty of the perception of the direction of sound more highly developed than in any other class of animals.” (Also see Child’s work.)

=Special sense-organs in the wings and halteres.=—Organs of a special sense, which Hicks supposed to be those of smell, were found by him near or at the base of the wings of Diptera, Coleoptera, and less perfect ones in Lepidoptera, Neuroptera, and Orthoptera, with a trace of them in Hemiptera; but these were considered by Leydig to be auditory organs, since he found the nerves to end in club-shaped rods, like those of Orthoptera.

Hicks found, as to the halteres and their sense-organs, that the nerve in the halter is the largest in the insect, except the optic nerve; and that at the base of the halteres is a number of vesicles arranged in four groups, to each of which the nerve sends a branch. Afterwards Bolles Lee discovered that the vesicles, undoubtedly perforated, contain a minute hair, those of the upper groups being protected by hoods of chitin. He regarded them as olfactory organs, while Lubbock seems inclined to consider them as auditory structures. Graber also regards the vesicles of Hicks as chordotonal organs.

In his elaborate account of the balancers, Weinland concludes that the organs of sense of varying structure occurring at the base of these appendages allow the perception of movements which the halteres perform and which enable the fly to steer or direct its course. The halteres can thus cause differences in the direction of the flight of a fly in the vertical plane. If the balancers act unequally, there is a change in direction.

e. The sounds of insects

Insects have no true voice; but sounds of different intensity, shrill cries, and other noises are produced mechanically by insects, either being love-songs to attract the sexes, to give signals, to communicate intelligence, or perhaps to express the emotions. The loud, shrill cry of the Cicada, or chirp of the cricket, is evidently a love-call, and results in the mating of individuals of separate broods more or less widely scattered, thus preventing too close interbreeding.

The simplest means of making a noise is that of the death-watch (Anobium), which strikes or taps on the wall with its head or abdomen. Longicorn beetles make a sharp sound by the friction of the mesoscutellum against the edge of the prothoracic cavity, the head being alternately raised and lowered, Burying-beetles (Necrophorus) rub the abdomen against the hinder edges of the elytra. Weevils make a loud noise by rapidly rubbing the tips of the abdomen on the ends of the elytra.

Landois offers the following summary of the kinds of noises produced by beetles:

1. Tapping sounds (Bostrycinæ, Anobium).

2. Grating sounds (Elateridæ).

3. Friction without special rasping organs (_Euchirus longimanus_).

4. Rasping sounds produced by friction:

a. Rubbing of the pronotum on the mesonotum (Cerambycidæ except Spondyli and Prionus).

b. Friction of the prosternum on the mesosternum (Omaloplia brunnea).

c. Elytra with a rasp at the end (Curculionidæ, Dyticidæ, Pelobius).

d. With a coxal rasp (Geotrupes, Ceratophyus). The male of Ateuchus stridulates to encourage the female in her work, and from distress when she is removed. (Darwin.)

e. Friction of the edge of the elytra against the femur (Chiasognathus grantii).

f. Pygidium with two rasps in the middle (Crioceris, Lema, Copris, Oryctes, Necrophorus, Tenebrionidæ).

g. Abdomen with a grating ridge and four grating plates (Trox).

h. Abdomen with two toothed ridges rubbing on a rasp on edge of wing-cover (Elaphus, Blethisa, Cychrus).

i. Rubbing the elytra on a rasp on the hind wings (Pelobius hermanni).

j. Friction of the wing against the abdominal segments (Melolontha fullo).

Mutilla makes a rather sharp noise by rubbing one abdominal segment against another. Ants (Ponera) have a stridulating apparatus, and other genera numerous (20) ridges between the segments.

Even certain moths and butterflies emit a rasping or crackling noise. The death’s-head moth and other sphinges cause it by rubbing the palpi against the base of the proboscis. These and certain butterflies are provided with parallel ridges forming a rasp on the “basal spot” of the inner side of the basal joint of each palpus (Reuter). A South American butterfly (Ageronia feronia) can be heard for several yards as it flies with a crackling sound. Hampson finds that the cause of the clicking sound is due to a pair of strong chitinous hooks attached to the thorax, against which play the spatulate ends of a pair of hooks attached to the fore wings. An Australian moth (Hecatesia) flies with a whizzing sound; Vanessa is said to be sonorous.

The males of Orthoptera produce their shrill cries or chirping noises, 1, by rubbing the thighs against the sides of the body (Acrydiidæ); 2, by the friction of the base of the fore wings on each other (Locustidæ); 3, by rubbing the base of the upper on the base of the hinder or under pair (Gryllidæ), in the two last there being a shrilling apparatus consisting of a file on the hind wings, which rubs on a resonant surface on the fore wings. The females are not invariably dumb, both sexes of the European Ephippigera being able to faintly stridulate. Corixa also produces shrill chirping notes. (Carpenter.)

Certain insects also hum, and have what may perhaps be called a voice. The cockchafer, besides humming with the wings, produces a sound almost like a voice. In the large trachea, just behind each spiracle, is a chitinous process, which is thrown into vibrations by the air during respiration, and thus produces a humming noise. (Lubbock.) Such is also the case with flies, the mosquito, dragon-flies, and bees. In flies and dragon-flies the “voice” is caused by the air issuing from the thoracic spiracles; while in the humble-bee the abdominal spiracles are also musical. The sound made by the spiracles bears no relation to that caused by the wings. Landois tells us that the wing-tone of the honey-bee is A′; its voice, however, is an octave higher, and often goes to B″ and C″.

The sounds produced by the wings are constant in each species, except where, as in Bombus, there are individuals of different sizes; in these the larger ones generally give a higher note. Thus the comparatively small male of Bombus terrestris hums on A′, while the large female hums an entire octave higher.

From the note produced the rapidity of the vibrations can be calculated. For example, the house-fly, which produces the sound of F, vibrates its wings 21,120 times in a minute, or 335 times in a second; and the bee, which makes a sound of A′, as many as 26,400 times, or 440 times in a second. On the contrary, a tired bee hums on E′, and therefore, according to theory, vibrates its wings only 330 times in a second. Marey has confirmed these numbers graphically, and found by experiment that the fly actually makes 330 strokes in a second. (Lubbock.)

A different kind of musical apparatus is that of the cicada, which has been elaborately described by Graber. The shrill, piercing notes issue from a pair of organs on the under side of the base of the abdomen of the male, these acting somewhat as two kettle-drums, the membrane covering the depressions being rapidly vibrated.

LITERATURE ON THE ORGANS OF HEARING

a. The auditory organs

=Siebold, C. Th. E. von.= Ueber das Stimm- und Gehörorgan der Orthopteren. (Archiv f. Naturgesch., 1844, x, pp. 52–81.)

=Johnston, Christopher.= Auditory apparatus of the culex mosquito. (Quart. Journ. Micr. Soc., 1855, iii, pp. 97–102, 1 Fig.)

=Hicks, Braxton.= On a new organ in insects. (Journ. Linn. Soc. Zool., London, 1857, pp. 130–140, 1 Pl.)

—— Further remarks on the organ found on the bases of the halteres and wings of insects. (Trans. Linn. Soc., London, 1857, xxii, pp. 141–145, 2 Pls.)

=Hensen, V.= Ueber das Gehörorgan von Locusta. (Zeitschr. f. wissens. Zool., xvi, 1866, pp. 190–207.)

=Graber, V.= Bemerkungen über die Gehör- und Stimmorgane der Heuschrecken und Cicaden (Wiener Sitzungsber. Math.-natur-wiss. Cl., lxvi, 1 Abt., 1872, pp. 205–213, 2 Figs.)

—— Die tympanalen Sinnesapparate, der Orthopteren. (Denkschr. d. k. Akad. d. wissens. Wien, xxxvi, 1876, 2 Abt., pp. 1–140, 10 Taf.)

—— Die abdominalen Tympanalorgane der Cicaden und Gryllodeen. (Ibid., 1870, xxxvi, pp. 273–290, 2 Taf.)

—— Ueber neue, otocystenartige Sinnesorgane der Insekten. (Archiv f. mikroskop. Anat., 1878, pp. 35–57, 2 Taf.)

—— Die chordotonalen Sinnesorgane und das Gehör der Insekten. (Archiv f. mikroskop. Anat., 1882, xx, pp. 506–640; 1883, xxi, pp. 65–145, Taf.)

=Mayer, Alfred Marshall.= Researches in Acoustics No. 5. 3. Experiments on the supposed auditory apparatus of the culex mosquito. (Amer. Jour. Sc. and Arts, Ser. 3, viii, 1874, pp. 81–103; also Amer. Naturalist, viii, pp. 577–592.)

=Schmidt, Oscar.= Die Gehörorgane der Heuschrecken. (Archiv f. mikroskop. Anat., xi, 1875, pp. 195–215, 3 Taf.)

=Ranke, J.= Beiträge zu der Lehre von den Uebergangssinnesorganen, das Gehörorgan der Acridier und das Sehorgan der Hirudineen. (Zeitschr. f. wissens. Zool., xxv, 1875, pp. 143–164, 1 Taf.)

=Lee, A. Bolles.= Les balanciers des Diptères, leurs organes sensifères et leur histologie. (Recueil Zool. Suisse, ii, 1885, pp. 363–392, 1 Pl.)

—— Bemerkungen über der feineren Bau der Chordotonalorgane. (Archiv f. mikroskop. Anat., 1883, xxiii, pp. 133–140, 1 Taf.)

—— Les organes chordotonaux des Diptères et la méthode du chlorure d’or. (Observations critiques.) (Recueil Zool. Suisse, 1884, ii, pp. 685–689, 1 Pl.)

=Weinland, E.= Ueber die Schwinger (Halteren) der Dipteren (Zeitschr. f. wissens. Zool., 1890, li, pp. 55–166, 5 Taf.)

=Adelung, N. v.= Beiträge zur Kenntnis des tibialen Gehörapparates der Locustiden. Inaug. Diss., Leipzig, 1892, 2 Taf.

=Child, Ch. M.= Ein bisher wenig beachtetes antennales Sinnesorgan der Insekten, mit besonderer Berücksichtigung der Culiciden und Chironomiden. (Zeitschr. f. wissens. Zool., lviii, 1894, pp. 475–528, 2 Taf.; also, Zool. Anzeiger, xvii Jahrg., pp. 35–38, and in Annals and Mag. Nat. Hist., 1894 (6), xiii, pp. 372–374).

Also the writings of J. Müller, Kirby and Spence, Burmeister, Gilbert White, Westwood, Guilding, Meinert, Paasch, Leydig, Viallanes, Minot, Forel, Mayer, Darwin (Descent of Man, i, ch. x.), F. Müller, Lubbock (Senses of animals), Westring, Köppen, Bates, Vom Rath, Peckham, Jourdan, Nagel, etc.

b. The sounds made by insects

=Scudder, S. H.= Notes on the stridulation of grasshoppers. (Proc. Bost. Soc. Nat. Hist., xi, 1868, pp. 306–313 and 316.)

—— The songs of the grasshoppers. (Amer. Naturalist, ii, 1868, pp. 113–120, 5 Figs.)

=Riley, C. V.= The song notes of the periodical Cicada. (Proc. Amer. Assoc. Adv. Science, xxxiv, 1885, pp. 330–332; also in Kansas City Rev., October, 1885, pp. 173–175.)

=Swinton, A. H.= (Ent. Month. Mag., 1877.) Sound produced in Ageronia by a modification of the hook and bristle of the wings.

=Hampson, G. F.= On stridulation in certain Lepidoptera, etc. (Proc. Zool. Soc. London, 1892, ii, pp. 188–193, Fig; also Psyche, vi, p. 491, 1 Fig.)

With the writings of Landois, Lubbock, Graber, Kolbe, Carpenter (Nat. Science), Bruyant, and others.

THE DIGESTIVE CANAL AND ITS APPENDAGES

It is in the higher adult insects differentiated into the mouth and pharynx, the œsophagus or gullet, supplementary to which is the crop (ingluvies) or “sucking stomach” of Lepidoptera, Diptera, and Hymenoptera; the proventriculus or gizzard; the ventriculus, “chyle-stomach,” or, more properly, mid-intestine, and the hind-intestine, which is divided into the ileum, or short intestine, the long intestine, often slender and coiled, with the colon and the rectum. Morphologically, however, the digestive or enteric canal is divided into three primary divisions, which are indicated in the embryo insect; i.e., the fore-intestine (stomodæum of the embryo), mid-intestine or “chyle-stomach,” and hind-intestine or proctodæum (Fig. 300). The three primary regions, with their differentiations, may be tabulated thus:—

Fore-intestine (Stomodæum). Mouth and pharynx. Pumping apparatus of Hemiptera, Lepidoptera, and Diptera. Œsophagus. Crop or ingluvies, food reservoir, or “sucking stomach.” Proventriculus.

Mid-intestine (Mesenteron). Mid-intestine, “chylific stomach,” or ventriculus (with cœcal glands).

Hind-intestine (Proctodæum). Ileum, or short intestine (with the urinary tubes). Long intestine. Colon. Rectum (with rectal glands). Anus (with anal glands).

The appendages of the alimentary canal are: (1) the salivary and poison glands, which arise from the stomodæum in embryonic life; (2) while to the chylific stomach a single pair of cœcal appendages (Orthoptera and larval Diptera, e.g. Sciara), or many cœca may be appended; (3) the urinary tubes, also the rectal glands and the paired anal glands. In a Hemipter (Pyrrhocoris apterus) appendages arise from the intestine in front of the origin of the urinary tubes. In certain insects a single cœcal appendage (Nepa, Dyticus, Silpha, Necrophorus, and the Lepidoptera) arises from the proctodæum.

In certain larval insects, as those of the Proctotrypidæ (first larval stage), the higher Hymenoptera (ichneumons, ants, wasps, and bees, Fig. 301), in the Campodea-like larvæ of the Meloidæ and Stylopidæ, the larva of the ant-lion (Myrmecoleo), and those of Diptera pupipara (Melophagus), the embryonic condition of the separation of the proctodæum and mid-gut (mesenteron) persists, the stomach ending in a blind sac; in such cases the intestine, together with the urinary tubes, is entirely secretory.

The anus is wanting in the larva of the ant-lion, as also in the wasps (in which there is a rudimentary colon) and in freshly hatched bees, though it becomes perfectly formed in the fully grown larvæ (Newport, art. Insecta, p. 967, and H. Müller).

In the larvæ of lamellicorn Coleoptera (Melolontha vulgaris) the digestive tube is nearly as simple as in bees, though there is a large colon, which at its beginning forms an immense cœcum, and has also one anal aperture (Newport).

The length and shape of the digestive canal is dependent on the nature of the food and also on the mode of life, especially the ease or difficulty with which the food is digested.

Newport, while stating that the length of the alimentary canal in larvæ is not in general indicatory of the habits of the species, makes this qualification after describing the digestive canal of Calandra as compared with that of Calosoma: “The length and complication of the intestines, therefore, appear to have some reference to the quality of the food to be digested, since it is well known that the food of these latter insects (weevils) is of difficult assimilation, being as it is chiefly the hard ligneous fibres of vegetable matter; but they cannot be received as always indicatory of a carnivorous [or] vegetable feeder, since, as above remarked, the length of the canal is considerable in one entirely carnivorous larva, while it is much shorter in some herbivorous, and particularly in pollenivorous larvæ, as in the Melolontha and the apodal Hymenoptera.”

Newport also contends that the length of the alimentary canal is not more indicative in the perfect insect of the carnivorous or phytophagous habits of the species than in the larva. It is nearly as long (being from two to three times the length of the whole body), and is more complicated, in the rapacious Carabidæ (Fig. 302) than in the honey-sipping Lepidoptera, whose food is entirely liquid. Referring to the digestive canal of Cicindelidæ, which is scarcely longer than the body, he claims that “we cannot admit that the length of the digestive organs, and the existence of a gizzard and gastric vessels, are indicatory of predacity of habits in the insect, because a similar conformation of parts exists often in strictly vegetable feeders. The existence and length of these parts seem rather to refer to the comparative digestibility of the food than to its animal or vegetable nature.” Newport then refers to the digestive canal of Forficulidæ (in which the gizzard is present, the canal, however, passing in an almost direct line through the body, making but one slight convolution), “a farther proof that the length of the canal must not be taken as a criterion whereby to judge of the habits of a species.” He adds this will apply equally well to the omnivorous Gryllidæ, in which there exists a short alimentary canal, but a gizzard of more complicated structure than that of the Dytiscidæ.

In larval insects and others (Synaptera, Orthoptera, etc.), in which the digestive canal is simplest, it is scarcely longer than the body, and passes through it as a straight tube.

In the caterpillar, which is a voracious and constant feeder, the digestive canal is a large straight tube, not clearly differentiated into fore-stomach, stomach, and intestine; but in the imago, which only takes a little liquid food, it is slender, delicate, and highly differentiated. In the larva the mid-gut forms the largest part of the canal; in the imago, the intestine becomes very long and coiled into numerous turns; at the same time the food-reservoir (the “sucking stomach”) develops, and the excretory tubes are longer.

a. The digestive canal

It will greatly simplify our conception of the anatomy of the digestive canal if we take into account its mode of origin in the embryo, bearing in mind the fact that during the gastrula condition the ectoderm is invaginated at each pole to form the primitive mouth and fore-gut (stomodæum) and hind-gut (proctodæum). The cells of the ectoderm secrete a chitinous lining (intima), which forms the continuation of the outer chitinous crust, and thus the lining of each end of the digestive canal is cast whenever the insect molts; while the mid-intestine (mesenteron), arising independently of the rest of the canal much later in embryonic life from the mesoderm, is not the result of any invagination, being directly derived from the mesoderm, and is not lined with chitin.

=The mouth, or oral cavity, and pharynx.=—This is the beginning of the alimentary bounded above by the clypeus, and labrum, with the epipharynx, and below by the hypopharynx, or tongue, as well as the labium. Into it pour the secretion of the salivary glands, which passes out through an opening at the base of the tongue or hypopharynx. On each side of the mouth are the mandibles and first maxillæ.

=The sucking or pharyngeal pump.=—This organ has been observed by Graber in flies and Hemiptera, but the fullest account is that by Burgess, who was the first to discover it in Lepidoptera. In the milk-weed butterfly (Danais archippus) the canal traversing the proboscis opens into a pharynx enclosed in a muscular sac (Figs. 303, 304, and 310).

The pharyngeal sac, says Burgess, serves as a pumping organ to suck the liquid food through the proboscis and to force it backwards into the digestive canal.

Meinert (“Trophi Dipterorum”) has made elaborate dissections of the mouth and its armature, including the pharynx of several types of Diptera, with its musculature. He describes the pharynx as the principal, and in most Diptera, as the only part of the pump (antlia), and says: “By the muscles of the pump (musculis antliæ) the superior lamina of the pharynx is varied that the space between the two laminæ may be increased, and the liquid is thus led through the siphon formed by the mouth-parts into the mouth” (Fig. 81).

=The œsophagus.=—This is a simple tube, largest in those insects feeding on solid, usually vegetable, food, and smallest in those living on liquid food. It usually curves upwards and backwards, passing directly under the brain, and merges into the crop or proventriculus either at the back part of the head or in the thorax, its length being very variable. Its inner walls longitudinally are folded and lined with chitin.

According to Newport, in the œsophagus of the Gryllidæ, of the two layers of the mucous lining the second is distinctly glandular and secretory, and in it there are many thousands of very minute granular glandular bodies, which probably secrete the “molasses” or repellent fluid often ejected by these and other insects when captured.

=The crop or ingluvies.=—This, when present, is an enlargement of the end of the œsophagus, and lined internally with a muscular coat. It is very large in locusts (Fig. 298), Anabrus (Fig. 299), and other Orthoptera (the Phasmidæ excepted), in the Dermaptera, and most adult Coleoptera. A crop-like dilatation in front of a spherical gizzard is also present in the Synaptera (Poduridæ and Lepismidæ), as well as in the Mallophaga (Nirmidæ).

In the larvæ of weevils (Calandra sommeri) there is a crop (Fig. 305), but not in the larva of Calosoma; also, according to Beauregard, in the pollen-eating beetles Zonitis, Sitaris, and Malabris it is wanting, while in Meloe it is highly developed (Kolbe).

The crop forms a lateral dilatation of the end of the œsophagus in the larvæ of weevils and of saw-flies (Athalia centifoliæ, Fig. 306).

=The “sucking stomach” or food-reservoir.=—This is a thin muscular pouch connected by a slender neck with the end of the œsophagus or the crop, when the latter is present. There is no such organ in Orthoptera, except in Gryllotalpa. It is wanting in the Odonata and in the Plectoptera (Ephemeridæ); in Platyptera (Perlidæ and Termitidæ), in Trichoptera, and in Mecoptera (Panorpidæ). In most adult Neuroptera (Myrmeleonidæ, Hemerobiidæ, and Sialidæ), but not in Rhaphidiidæ, the long œsophagus is dilated posteriorly into a kind of pouch or crop, and besides there is often a long “food-reservoir” arising on one of its sides, that of Myrmeleon (Fig. 307) and Hemerobius being on the right side.

A true food-reservoir is present in most Diptera (Fig. 308) as well as in the larvæ of the Muscidæ, but according to Dufour it is wanting in some Asilidæ and in Diptera pupipara, and according to Brauer in the Œstridæ. The food-reservoir in Diptera is always situated on the left side of the digestive canal; there is usually a long neck or canal, while the reservoir is either oval or more usually bilobed, and often each lobe is itself curiously lobed.

In Lepidoptera (Figs. 309, 310) the so-called “sucking stomach” is, as Graber has proved, simply a reservoir for the temporary reception of food; though generally found to contain nothing but air, Newport has observed that in flies it is filled with food after feeding. He has found this to be the case in the flesh fly, and in Eristalis he has found it “partially filled with yellow pollen from the flowers of the ragwort upon which the insect was captured,” the pollen grains also occurring in the canal leading to the bag, in the gullet, and in the stomach itself. Graber has further proved by feeding flies with a colored sweet fluid that this sac is only a food-receptacle. As he says: “It can be seen filling itself fuller and fuller with the colored fluid, the sac gradually distending until it occupies half the hind-body.”

The food-reservoir of the Hymenoptera is a lateral pouch at the end of the long, slender œsophagus, and has been seen in the bee to be filled with honey.

In the mole-cricket the hinder part of the crop is armed within with hook-like bristles directed backwards so as not to prevent the energetic pressure of the food backwards into the proventriculus, and to obviate the possibility of a regurgitation. (Eberli.)

=The fore-stomach or proventriculus.=—This is especially well developed in the Dermaptera, in the Orthopterous families Locustidæ, Gryllidæ, and Mantidæ, while in the Thysanura (Lepisma) there is a spherical gizzard provided with six teeth. It also occurs in many wood-boring insects, and in most carnivorous insects, notably the Carabidæ, Dyticidæ, Scolytidæ, in the Mecoptera (scorpion-flies), in the fleas, and in many kinds of ants, as well as Cynips, Leucospis, and Xyphidria. It is very muscular, lined within with chitin, which is usually provided with numerous teeth arising from the folds. These folds begin in the œsophagus or crop, and suddenly end where the mesenteron (“chylific stomach”) begins. It has been compared with the gizzard of birds, and is usually called by German authors the chewing or masticating stomach. (Kaumagen.)

The proventriculus is best developed in the Gryllidæ (Acrida viridissima), where the six folds at the end of the crop close together to form a valve between the crop and proventriculus. “They are each armed with five very minute hooked teeth; and, continued into the gizzard, develop many more in their course through that organ. These first teeth are arranged around the entrance to the gizzard, and seem designed to retain the insufficiently comminuted food and to pass it on to that organ.

“Next to these in succession on each of the longitudinal ridges are four flat, broad, somewhat quadrate teeth, each of which is very finely denticulated along its free margin. These extend about half-way through the gizzard. They appear to be alternately elevated and depressed during the action of the gizzard, and to serve to carry on the food to the twelve cutting teeth, with which each ridge is also armed, and which occupy the posterior part of the organ. These teeth are triangular, sharp-pointed, and directed posteriorly, and gradually decrease in size in succession from before backward. Each tooth is very strong, sharp-pointed, and of the color and consistence of tortoise shell, and is armed on each side by a smaller pointed tooth. These form the six longitudinal ridges of the gizzard, between each two of which there are two other rows of very minute teeth of a triangular form, somewhat resembling the larger one in structure, occupying the channels between the ridges. The muscular portion of the gizzard is equally interesting. It is not merely formed of transverse and longitudinal fibres, but sends from its inner surface into the cavity of each of the large teeth other minute but powerful muscles, a pair of which are inserted into each tooth. The number of teeth in the gizzard amounts to 270, which is the same number in these Gryllidæ as found formerly by Dr. Kidd in the mole-cricket. Of the different kinds of teeth there are as follows: 72 large treble teeth, 24 flat quadrate teeth, 30 small single-hooked teeth, and 12 rows of small triangular teeth, each row being formed of 12 teeth. This is the complicated gizzard of the higher Orthoptera.” (Newport.)

In the more generalized cockroach, there are six principal folds, the so-called teeth, which project so far inwards as to nearly meet (Fig. 312). The entire apparatus of muscles and teeth is, as Miall and Denny state, “an elaborate machine for squeezing and straining the food, and recalls the gastric mill and pyloric strainer of the crayfish. The powerful annular muscles approximate the teeth and folds, closing the passage, while small longitudinal muscles, which can be traced from the chitinous teeth to the cushions, appear to retract these last, and open a passage for the food.”

As in the fore-stomach or proventriculus of the lobster, the solid, rounded teeth do not appear to triturate the solid fragments found in the organ, but act rather as a pyloric strainer to keep such bodies out of the chylific stomach. We accept the view of Plateau that this section of the digestive canal in insects, which he compares to the psalterium of a ruminant, is a strainer rather than a masticatory stomach, and both Forel and Emery, as well as Cheshire, take this view.

The proventriculus of the honey-bee (Fig. 313, hs) is called by apiarians the “honey-sac” or “honey-stomach.” Cheshire states that if it be carefully removed from a freshly killed bee, its calyx-like “stomach-mouth” may be seen to gape open and shut with a rapid snapping movement. The entrance to the stomach is guarded by four valves, each of which is strongly chitinous within, and fringed along its edge with downward-pointing fine stiff bristles. By the contraction of the longitudinal muscles (lm), the valves open to allow the passage of food from the honey-sac to the “chyle-stomach.” It is closed at will by circular muscles (tm). Then the bee can carry food for a week’s necessities, either using it rapidly in the production of wax, or eking it out if the weather is unfavorable for the gathering of a new store.

Cheshire also shows that when bees suck up from composite and other flowers nectar together with much pollen, the outside wrinkled membrane (sm, A, Fig. 314) “is seen to continually run up in folds, and gather itself over the top of the stomach-mouth, bringing with it, by the aid of its setæ, the large pollen-grains the nectar contains.” The lips (l, l, B, Fig. 314), now opening, take in this pollen, which is driven forwards into the cavity made between the separating lips by an inflow of the fluid surrounding the granules. The lips in turn close, but the down-pointing bristles are thrown outwards from the face of the leaflet, in this way revealing their special function, as the pollen is prevented from receding while the nectar passes back into the honey-sac, strained through between the bristles aforesaid, the last parts escaping by the loop-like openings seen in the corners of C, Fig. 314. The whole process is immediately and very rapidly repeated, so that the pollen collects and the honey is cleared. “Three purposes, in addition to those previously enumerated, are thus subserved by this wondrous mechanism. First, the bee can either eat or drink from the mixed diet she carries, gulping down the pollen in pellets, or swallowing the nectar as her necessities demand. Second, when the collected pollen is driven forwards into the chyle-stomach, the tube extension, whose necessity now becomes apparent, prevents the pellets forming into plug-like masses just below p, Fig. 313, for, by the action of the tube, these pellets are delivered into the midst of the fluids of the stomach, to be at once broken up and subjected to the digestive process. And third, while the little gatherer is flying from flower to flower, her stomach-mouth is busy in separating pollen from nectar, so that the latter may be less liable to fermentation and better suited to winter consumption. She, in fact, carries with her, and at once puts into operation, the most ancient, and yet the most perfect and beautiful, of all ‘honey-strainers.’”

Forel’s experiments on the proventriculus of ants prove that through its valvular contrivance it closes the passage from the crop to the mid-intestine (“chylific stomach”), and allows the contents of the former to pass slowly and very gradually into the latter. Emery confirms this view, and concludes that the organ in the Camponotidæ and in the Dolichoderidæ provided with a calyx-bell, usually regarded as a triturating stomach (Kaumagen), but more correctly as a pumping stomach, consists of parts which perform two different functions. Under the operation of the muscles of the crop the entrance to the pumping stomach becomes closed, in order by such spasmodic contraction to prevent the flow of the contents of the crop into the proventriculus. By the pressure of the transverse muscles of the proventriculus its contents are emptied into the mid-intestine, while simultaneously a regurgitation into the crop is prevented. In the Dolichoderidæ and Plagiolepidinæ the closure in both cases is effected by the valves. In the true Camponotidæ there are two separate contrivances for closing; the calyx belonging to the crop-musculature, while the valves essentially belong to the proventricular pumping apparatus.

Opinions vary as to the use of this portion of the digestive canal. Graber compares it to the gizzard of birds, and likens the action of the rosette of teeth to the finer radiating teeth of the sea-urchin, and styles it a chopping machine, which works automatically, and allows no solid bits of food to pass in to injure the delicate walls of the stomach (mid-gut).

He also states that the food when taken from the proventriculus is very finely divided, while that found in the œsophagus contains large bits.

Kolbe says that this view has recently been completely abandoned, and that the teeth are used to pass the food backwards into the chylific stomach. “But Goldfuss had denied the triturating action of the proventriculus of the Orthoptera (Symbolæ ad Orthopterorum quorundam Œconomiam, 1843), stating that the contents of the same are already fluid in the gullet, so that the fore-stomach (Kaumagen) does not need to comminute the food” (Kolbe). In the Gryllidæ and Locustidæ, just before the posterior opening of the proventriculus into the stomach the chitinous lining swells into a ring and projects straight back as the inner wall of the cylindrical chylific stomach. The muscular layer forms two sac-like outgrowths or folds, which separate on the circular fold from the chitinous membrane. This apparatus only allows very finely comminuted food to pass into the stomach.

In the Acrydiidæ (Eremobia muricata) at the end of the proventriculus, where it passes into the stomach, is a small circular fold which hangs down like a curtain in the stomach.

=The œsophageal valve.=—Weismann[50] states that the origin of the proventriculus in the embryo of flies (Muscidæ) shows that it should be regarded as an intussusception of the œsophagus. While in the embryo the invaginated portion of the œsophagus is short, after the hatching of the larva it projects backwards into the mid-intestine. Kowalevsky also observed in a young muscid larva, 2.2 mm. in length, that the œsophagus, shaped like a tube, extends back into the expanded portion (proventriculus) and opens into the stomach (Fig. 315, A). In a larva 10 mm. long the funnel is shorter, the end being situated in the proventriculus (Fig. 315, B, pr). In the cavity between the outer (o) and inner wall (i) no food enters, and the use of this whole apparatus seems to be to prevent the larger bits of food from passing into the chylific stomach (Kowalevsky).

Beauregard has found a similar structure in the Meloidæ, and calls it the “cardiac valvule” (Fig. 318, Kl). It was observed by Mingazzini in the larvæ of phytophagic lamellicorn beetles, and Balbiani described it in a myriopod (Cryptops) under the name of the “œsophageal valvule.”

Gehuchten describes a homologous but more complicated structure in a tipulid larva (Ptychoptera contaminata), but differing in containing blood-cavities, as a tubular prolongation of the posterior end of the œsophagus which passes through the proventriculus and opens at various positions in the anterior part of the chylific stomach (Fig. 316).

The three layers composing this funnel are distant from each other and separated by blood-cavities, the whole forming “an immense blood-cavity extended between the epithelial proventricular lining and the muscular coat.”

According to Schneider the longitudinal muscular fibres of the fore and hind gut in insects pass into the stomach (mid-gut). The anterior part of the fore-gut has generally only circular fibres. When, however, the longitudinal fibres arise behind the middle, then they separate from the digestive canal and are inserted a little behind the beginning of the chylific stomach. Hence there is formed an invagination of the proventriculus, which projects into the cavity of the stomach.

Schneider describes this process, which he calls the “beak,” as an invagination of the fore-stomach which projects into the cavity of the stomach. The two layers of the invagination in growing together form a beak varying in shape, being either simple or lobed and armed with bristles or teeth. This beak is tolerably large in Lepisma, Dermaptera (Forficula), Orthoptera, and in the larvæ and adults of Diptera, but smaller in the Neuroptera and Coleoptera, while in other insects it is wanting.

=Proventricular valvule.=—Gehuchten also describes in Ptychoptera what he calls “the proventricular valvule,” stating that it is “a circular fold of the intestinal wall” (Fig. 310, vpr). He claims that it has not before been found, the “proventricular beak” of Schneider being regarded by him as the œsophageal valvule.

=The peritrophic membrane.=—This membrane appears first to have been noticed by Ramdohr in 1811 in Hemerobius perla. It has been found by Schneider, who calls it the “funnel.” On the hinder end of the fore-stomach, he says, the cuticula forms a fold enclosing the outlet of the fore-stomach, and extending back like a tube to the anus. This “funnel,” he adds, occurs in a great number of insects. It has been found in Thysanura, but is wanting in Hemiptera. In the Coleoptera it is absent in Carabidæ and Dyticidæ. It is generally present in Diptera and in the larvæ of Lepidoptera, but not in the adults. In Hymenoptera it has been found in ants and wasps, but is absent in Cynipidæ, Ichneumonidæ, and Tenthredinidæ. All those insects (including their larvæ) possessing this funnel eat solid, indigestible food, while those which do not possess it take fluid nourishment. It is elastic, and firmly encloses the contents of the digestive tract. Until Schneider’s discovery of its general occurrence, it had only been known to exist in the viviparous Cecidomyia larvæ (Miastor). Wagner, its discoverer, noticed in the stomach of this insect a second tube which contained food. Pagenstecher was inclined to regard the tube as a secretion of the salivary glands. Metschnikoff, however, more correctly stated that the tube consisted of chitin, but he regarded it as adapted for the removal of the secretions. (Schneider.) Plateau, however, as well as Balbiani, the latter calling it the “peritropic membrane,” considers this membrane as a secretion of the chylific stomach, and that it is formed at the surface of the epithelial cells. It surrounds the food along the entire digestive tract, forming an envelope around the fæcal masses. On the other hand, Gehuchten states that in the larva of Ptychoptera its mode of origin differs from that described by Plateau and by Schneider, and that it is a product of secretion of special cells in the proventriculus.

=The mid-intestine.=—This section of the digestive canal, often, though erroneously, called the “chylific stomach” or ventriculus, differs not only in its embryonic history, but also in its structure and physiology from the fore and hind intestine of arthropods, and also presents no analogy to the stomach of the vertebrate animals. In insects it is a simple tube, not usually lined with chitin, since it is not formed by the invagination of the ectoderm, as are the fore and hind intestine, the absence of the chitinous intima promoting the absorption of soluble food. Into the anterior end either open two or more large cœcal tubes (Fig. 299), or its whole outer surface is beset with very numerous fine glandular filaments like villi (Fig. 317 and Fig. 329).

The mid-intestine varies much in size and shape; it is very long in the lamellicorn beetles (Melolontha and Geotrupes), and while in Meloë it is very large, occupying the greatest part of the body-cavity, in the longicorn beetles and in Lepidoptera it is very small. The pyloric end consists of an internal circular fold projecting into the cavity. In the Psocidæ (Cæcilius) the pyloric end is prolonged into a slender tube nearly as long as the larger anterior portion.

The limits between the mid and hind intestine are in some insects difficult to define, the urinary tubes sometimes appearing to open into the end of the mid-intestine (“stomach”). The latter also is sometimes lined with an intima. The limits are also determined by a circular projection, directly behind which is an enlargement of the intestine in the shape of a trench (rigole), or circular cul-de-sac (the “pyloric valvule” of some authors, including Beauregard), while the walls of the small intestine contract so as to produce a considerable constriction of the cavity of the canal. This constriction exactly coincides with the beginning of the double layer of circular muscles in the wall of the small intestine. An internal layer, which is the continuation of the circular muscles of the chylific stomach, and an external layer much more developed probably belong to this part of the alimentary canal. Since the homologue of the circular fold occurs in the locust as well as in Diptera, it is probably common to insects in general.

Gehuchten adds that the limit set by the circular projection does not exactly coincide with the opening into the intestine of the urinary tubes and the two annexed glands. He shows by a section (his Fig. 133) that the tubular glands open into the alimentary canal in front of the circular fold. It is the same with the Malpighian tubes. They are not, therefore, he claims, dependences of the terminal intestine, but of the mid-intestine. Beauregard has observed the same thing in the vesicating insects (Meloidæ). The Malpighian tubes, he says, open into the “chylific stomach” before the valvular crown. This arrangement does not seem to be general, because, according to Balbiani, the Malpighian vessels open into the beginning of the intestine in Cryptops. Compare also Minot’s account of the valve in locusts separating the stomach from the intestine, and in front of which the urinary or Malpighian tubes open.

=Histology of the mid-intestine.=—The walls of the stomach are composed of an internal epithelium, a layer of connective tissue, with two muscular layers, the inner of which is formed of unstriated circular muscular fibres, and the outer of striated longitudinal muscular fibres.

In the cockroach short processes are given off from the free ends of the epithelial cells, as in the intestine of many mammals and other animals. “Between the cells a reticulum is often to be seen, especially where the cells have burst; it extends between and among all the elements of the mucous lining, and probably serves, like the very similar structure met with in mammalian intestines, to absorb and conduct some of the products of digestion.” (Miall and Denny.)

Gehuchten shows that the epithelial lining of the mesenteron (chylific stomach) of the dipterous larva Ptychoptera is composed of two kinds of cells, i.e. secreting or glandular cells and absorbent cells, the former situated at each end of the stomach, and the absorbent cells occupying the middle region. The part played by these cells in digestion will be treated of beyond in the section on digestion. (See p. 327.)

=The hind-intestine.=—In many insects this is divided into the ileum, or short intestine, and the long intestine. The limit between the intestine and stomach is externally determined by the origin of the urinary tubes, which are outgrowths of the anterior end of the proctodæum. Like the fore-intestine the hind-intestine is lined with a thick muscular layer, and, as Gehuchten states, the passage from the epithelial lining of the stomach (mid-intestine) to the muscular lining of the intestine is abrupt.

=Large intestine.=—In Ptychoptera, as described by Gehuchten, there are no precise limits between the small and large intestine; the epithelium of the large intestine has a special character, and its constituents present a close resemblance to the absorbed cells of the chylific stomach, being like them large and polygonal. The muscular layer is not continuous, and is formed of longitudinal and circular fibres, the latter being the larger.

=The ileum=—Though in most insects slender, and therefore called the small intestine, the ileum is in locusts (Fig. 298) and grasshoppers (Anabrus, Fig. 299) as thick as the stomach. In many carnivorous beetles (Dyticus, Fig. 320, il, and Necrophorus) it is very long, but rather slender and short in the Carabidæ and Cicindelidæ, as well as those insects whose food is liquid, such as Diptera. In the Lepidoptera it varies in length, being in Sphinx quite long and bent into seven folds (Fig. 309), while it is very short in the Psocidæ, Chrysomelidæ, and Tenthredinidæ.

In the locust the ileum is traversed by six longitudinal folds with intervening furrows; outside of each furrow is a longitudinal muscular band. Seen from the inner surface the epithelium has an unusual character, the cells in the middle of each of the flat folds being quite large, polygonal in outline, while towards the furrows the cells become very much smaller. The walls are double when seen in transverse section, the inner layer consisting of epithelial cells resting on connective tissue, the outer layer formed of circular muscles. The cuticula is thin, but probably chitinous; it resembles that on the gastro-ileal folds, except that there are no spinules, but unlike the cuticula of the stomach it extends equally over the folds and the furrows. (Minot.) In the cockroach the junction of the small intestine with the colon is abrupt, a well-developed annular fold assuming the nature of a circular valve. (Miall and Denny.)

=The gastro-ileal folds.=—In the locust the intestine is separated from the chylific stomach by what Minot calls “the gastro-ileal folds,” which form a peculiar valve. The urinary vessels open just underneath and in front of this valve. In Melanoplus, and probably in the entire family of Acrydiidæ, they are indicated as “dark spots, round in front and lying at the anterior end of the ileum so as to form a ring around the interior of the intestine.” They are 12 in number, and all alike. They are pigmented and round in front where they are broadest and stand up highest; they narrow down backwards, the pigment disappears, and they gradually fade out into the ileal folds. Directly beneath them, and just at the posterior end of the stomach, there is a strong band of circular striated muscular fibres. The epithelium of these folds is covered with minute conical spines, which are generally, but not always, wanting between the folds. (Minot.)

=The colon.=—This section of the intestine (Fig. 319) is sometimes regarded as a part of the rectum. In the locust the six longitudinal folds of the ileum are continued into the colon, but their surface, instead of being smooth as in the ileum, is thrown up into numerous irregular curved and zigzag secondary folds. The cells of the epithelium are of uniform size, and the layer is covered by a highly refringent cuticula without spines; and, like that in the ileum, it rests on a layer of connective tissue, beyond which follows (1) an internal coat of longitudinal, and (2) an external coat of circular striated muscular fibres. (Minot.)

In butterflies (Pontia brassicæ), in Sphinx ligustri, and probably in most Lepidoptera the colon is distinct from the rectum, and is anteriorly developed into a very large more or less pyriform or bladder-like cæcum (Figs. 309, 310), which in certain Coleoptera (Dyticus, Fig. 320, d; Silpha, Necrophorus, etc.) is of remarkable length and shape; it also occurs in Nepidæ (Fig. 327). In the cockroach a lateral cæcum “is occasionally, but not constantly, present towards its rectal end,” and a constriction divides the colon from the rectum. (Miall and Denny.)

=The rectum.=—The terminal section of the hind-gut varies in length and size, but is usually larger than the colon, and with thick, muscular walls. In Lepidoptera it is narrow and short.

The rectum is remarkable for containing structures called rectal glands (Fig. 298). Chun describes those of Locusta viridissima as six flat folds, formed by a high columnar epithelium and a distinct cuticula; there is a coat of circular bands corresponding to the furrows between the glands. Minot states that this description is applicable to the locusts (Acrydiidæ) he has investigated, the only difference being in the structural details of the single layers. He claims that the rectal folds “do not offer the least appearance of glandular structure,” neither is their function an absorbent one, as Chun supposed. From their structure and position Fernald regards the rectal glands of Passalus as acting like a valve, serving to retain the food in the absorptive portions of the digestive tract till all nutriment is extracted.

The epithelial folds of the larvæ of dragon-flies serve as organs of respiration, the water being admitted into this cavity, and when forcibly expelled serving to propel the creature forward. Paired and single anal glands (repugnatorial) enter the rectum of certain Coleoptera (Figs. 302, l; 317, s; 320, e).

=The vent (anus).=—The external opening of the rectum is situated in the end of the body, in the vestigial 10th or 11th abdominal segment, and is more or less eversible. It is protected above in caterpillars, and other insects with 10 free abdominal segments, by the suranal plate. It is bounded on the sides by the paranal lobes, while beneath is the infra-anal lobe.

The anus is wanting in certain insects, and where this is the case the hind-gut, owing to a retention of the embryonic condition, is usually separated from the mid-intestine. (See p. 300.)

Some remarkable features of the digestive canal in hemipterous insects are noteworthy. In the Coccidæ, according to Mark, the anterior end of the long mid-intestine forms, with the hinder end of the œsophagus, a small loop, whose posterior end is firmly grown to the wall of the rectum, and forms a cup-like invagination of the latter. Then the rest of the tube-like stomach turns sidewise and forms a large loop, which turns back on itself and occupies a large part of the body-cavity. This loop receives on the anterior end, near the œsophagus, the two urinary vessels, and forms just below the opening into the rectum a short cæcum.

In other homopterous genera (Psyllidæ and some Cicadidæ) Witlaczil describes nearly the same peculiarity, the mid-gut and part of the intestine forming a loop growing together for a certain distance and winding round each other (Fig. 321).

=Histology of the digestive canal.=—In all the divisions of the digestive canal of insects the succession of the cellular layers composing it is the same: 1st, a cuticula; 2d, an epithelium; 3d, connective tissue; 4th, muscular tissue. In the locust, the first division of the canal (fore-gut), there are two muscular coats, an internal longitudinal and an external circular coat; the fibres are all striated. The lining epithelium is not much developed, but forms a thick, hard, and refringent cuticula, which is thrown up into spiny ridges. In the second division (mid-gut, “stomach”) the epithelium is composed of very high columnar cells, which make up the greater part of the thickness of the walls, while the cuticula is very delicate, slightly refringent, with no ridges, and is probably not chitinous; the fibres of the muscular coats are not striated, while this division is also distinguished by the presence of glandular follicles and folds. The stomach and the cæcal appendages have all these peculiarities in common, while no other part of the canal is thus characterized.

The third division (intestine and rectum) is composed of an epithelium, the cells of which are intermediate in size between those of the fore and mid gut. The cells are often pigmented, and they are covered by a much thicker cuticula than that of the stomach, but which is not so thick and hard as that of the œsophagus and proventriculus. The very refringent cuticula is not thrown up into ridges, though in some parts it is covered with delicate conical spines, which are very short. “The epithelium and underlying connective tissue (tunica propria) are thrown up into six folds, which run longitudinally, being regular in the ileum and rectum (as the rectal glands), but very irregular in the colon. Outside the depression between each two neighboring folds there is a longitudinal muscular band, these making six bands. This peculiar disposition of the longitudinal muscles does not occur in any other part of the canal; it is, therefore, especially characteristic of the third division.” (Minot.)

LITERATURE ON THE ORGANS OF DIGESTION

=Treviranus, G. R.= Resultate einiger Untersuchungen über den inneren Bau der Insekten. (Verdauungsorgane von Cimex rufipes.) (Annal. d. Wetterau. Gesells., 1809, i, pp. 169–177, 1 Taf.)

=Ramdohr, C. A.= Abhandlungen über die Verdauungswerkzeuge der Insekten. 1811, vii, pp. 221, 30 Taf.

=Dutrochet, R. J. H.= Mémoire sur les métamorphoses du canal alimentaire dans les insectes. (Journal de Physique, 1818, lxxxvi, pp. 130–135, 189–204; Meckel’s Archiv, 1818, iv, pp. 285–293.)

=Suckow, F. W. L.= Verdauungsorgane der Insekten. (Heusinger’s Zeitschr. f. organ. Physik., 1828, iii, pp. 1–89.)

=Doyère, L.= Note sur le tube digestif des Cigales. (Ann. Sc. nat. Zool., 1839, 2 Sér., xi, pp. 81–85.)

=Grube, A. E.= Fehlt den Wespen- oder Hornissenlarven ein After oder nicht? 1 Taf. (Müller’s Archiv für Physiol., 1849, pp. 47–74.)

=Sirodot.= Recherches sur les sécrétions chez les insectes. (Ann. Sc. nat. Zool., 1858, 4 Sér., x, pp. 141–189, 251–328, 12 Pls.)

=Milne-Edwards, H.= Leçons sur la physiologie et l’anatomie comparée, v, 1859, pp. 498–536, 581–638.

=Dufour, L.= Recherches anatomiques sur les Carabiques et sur plusieurs autres insectes Coléoptères. Appareil digestif. (Ann. Sc. nat., ii, 1824, pp. 462–482, 2 Pls.; iii, 1824, pp. 215–242, 5 Pls., pp. 476–491, 3 Pls.; iv, 1824, pp. 103–125, 4 Pls.; iv, 1825, pp. 265–283.)

—— Recherches anatomiques sur l’Hippobosque des chevaux. (Ann. Sc. nat., 1825, vi, pp. 299–322, 1 Pl.)

—— Description et figure de l’appareil digestif de l’Anobium striatum. (Ibid., xiv, 1828, pp. 219–222, 1 Pl.)

—— Recherches anatomiques sur les Labidoures. Appareil de la digestion. (Ibid., xiii, 1828, pp. 348–354, 2 Pls.)

—— Recherches anatomiques et considerations entomologiques sur quelques insectes Coléoptères, compris dans les familles des Dermestins, des Byrrhiens, des Acanthopodes et des Leptodactyles. Appareil digestif. (Ibid., Sér. 2, Zool., i, 1834, pp. 67–76, 2 Pls.)

—— Résumé des recherches anatomiques et physiologiques sur les Hémiptères. (Ibid., pp. 232–239.)

—— Mémoire sur les métamorphoses et l’anatomie de la Pyrochroa coccinea. Appareil digestif. (Ibid., Sér. 2, Zool., xiii, 1840, pp. 328–330, 334–337, 2 Pls.)

—— Histoire comparative des metamorphoses et de l’anatomie des Cetonia aurata et Dorcus parallelepipedus. Appareil digestif. (Ibid., Sér. 2, Zool., 1824, xviii, pp. 174–176, 2 Pls.)

—— Anatomie générale des Diptères. Appareil digestif. (Ibid., Sér. 3, Zool., i, 1814, pp. 248, 249.)

—— Histoire des métamorphoses et de l’anatomie du Piophila petasionis. Appareil digestif. (Ibid., Sér. 3, Zool., i, 1844, pp. 372–377, 1 Pl.)

—— Études anatomiques et physiologiques sur les insectes Diptères de la famille des Pupipares. Appareil digestif. (Ibid., Sér. 3, Zool., iii, 1845, pp. 67–73, 1 Pl.)

—— Recherches sur l’anatomie et l’histoire naturelle de l’Osmylus maculatus. Appareil digestif. (Ibid., Sér. 3, Zool., ix, 1848, pp. 346–349, 1 Pl.)

—— Études anatomiques et physiologiques, et observations sur les larves des Libellules. Appareil digestif. (Ibid., Sér. 3, Zool., xvii, 1852, pp. 101–108, 1 Pl.)

—— Recherches anatomiques sur les Hyménoptères de la famille des Urocerates. Appareil digestif. (Ibid., Sér. 4, Zool., i, 1854, pp. 212–216, 1 Pl.)

—— Fragments d’anatomie entomologique. Sur l’appareil digestif du Nemoptera lusitanica. (Ibid., Sér. 4, viii, 1857, pp. 6–9, 1 Pl.)

—— Recherches anatomique et considerations entomologiques sur les Hémiptères du genre Leptopus. Appareil digestif. (Ibid., Sér. 4, Zool., 1858, x, pp. 352–356, 1 Pl.)

—— Recherches anatomiques sur l’Ascalaphus meridionalis. Appareil digestif. (Ibid., Sér. 4, xiii, 1860, pp. 200–202, 1 Pl.)

=Leydig, F.= Zur Anatomie von Coccus hesperidum. (Zeitschr. f. wissens. Zool., v, 1853, pp. 1–12, 1 Taf.)

=Lubbock, J.= On the digestive and nervous System of Coccus hesperidum. (Proc. Roy. Soc., ix, 1886, pp. 480–486; also Ann. Mag. Nat. Hist., 1859, Ser. 3, iii, pp. 306–311.)

=Scheiber, S. H.= Vergleichende Anatomie und Physiologie der Œstriden-Larven. V. Das chylo- und uropœtische System. (Sitzber. d. k. Akad. d. Wissens. Wien. Math.-naturwiss. Cl., 1862, xlv, pp. 39–64, 1 Taf.)

=Gerstaecker, A.= Bronn’s Klassen und Ordnungen des Tierreichs. V. Gliederfüssler. (Ernährungsorgane, pp. 87–105.)

=Graber, V.= Zur naheren Kenntnis des Proventriculus und der Appendices ventriculares bei den Grillen und Laubheuschrecken. (Sitzber. d. k. Akad. d. Wissensch. Wien. Mathem.-naturwiss. Cl., lix, 1869, pp. 29–46, 3 Taf.)

—— Ueber die Ernährungsorgane der Insekten und nächstverwandten Gliederfüssler. (Mitteil. d. naturwiss. Vereins für Steiermark. Graz, 1871, ii, pp. 181, 182.)

—— Verdauungssystem des Prachtkäfers. (Ibid., Graz, 1875.)

—— Die Insekten., i, 1877. (Verdauungsapparat, pp. 308–328.)

=Wilde, K. F.= Untersuchungen über den Kaumagen der Orthopteren. (Archiv f. Naturgesch., xliii Jahrg., 1877, pp. 135–172, 3 Taf.)

=Simroth, H.= Ueber den Darmkanal der Larven von Osmoderma eremita mit seinen Anhängen. (Giebel’s Zeitschr. f. d. ges. Naturwiss., 1878, li, pp. 493–518, 3 Taf.)

=Müller, H.= Ueber die angebliche Afterlösigkeit der Bienenlarven. (Zool. Anzeiger, 1881, pp. 530, 531.)

=Schiemenz, Paulus.= Ueber das Herkommen des Futtersaftes und die Speicheldrüsen der Bienen, nebst einem Anhänge über das Riechorgan. (Zeitschr. f. wissens. Zool., xxxviii, 1883, pp. 71–135, 3 Taf.)

=Rovelli, G.= Alcune ricerche sul tubo digerente degli Atteri, Ortotteri e Pseudo-Neurotteri. (Como, 1884, p. 15.)

=Beauregard, H.= Structure de l’appareil digestif des Insectes de la tribu des Vésicants. (Compt. rend. Acad. Paris, 1884, xcix, pp. 1083–1086.)

—— Recherches sur les Insectes vésicants., 1 Part, Anatomie. (Journ. Anat. Phys. Paris, 1885, xxi Année, pp. 483–524, 4 Pls.; 1886, xxii Année, pp. 85–108, 242–284, 5 Pls.)

—— Les Insectes vésicants, Paris, 1890, Chap. III, Appareil digestif, pp. 63–99; (Phénomènes digestifs, pp. 161–170; Pls. 6–9.)

=Wertheimer, L.= Sur la structure du tube digestif de l’Oryctes nasicornis. (Compt. rend. Soc. Biol. Paris, 1887, Sér. 8, iv, pp. 531, 532.)

=Kowalevsky, A.= Beitrage zur Kenntniss der nachembryonal Entwicklung der Musciden. (Zeitschr. f. wissens. Zool., xlv, 1887, pp. 542–594, 5 Taf.)

=Schneider, A.= Ueber den Darm der Arthropoden, besonders der Insekten. (Zool. Anzeiger, 1887, x Jahrg., pp. 139, 140.)

—— Ueber den Darmkanal der Arthropoden. (Zool. Beiträge von A. Schneider, ii, 1887, pp. 82–96, 3 Taf.)

=Fritze, A.= Ueber den Darmkanal der Ephemeriden. (Berichte der Naturforsch.-Gesellsch. zu Freiburg i. Br., 1888, iv, pp. 59–82, 2 Taf.)

=Emery, C.= Ueber den sogenannten Kaumagen einiger Ameisen. (Zeitschr. f. wissens. Zool., 1888, xlvi, pp. 378–412, 3 Taf.)

=Meinert, F.= Contribution à l’anatomie des Fourmilions. (Overs. Danske Vidensk. Selsk. Forhandl. Kjöbenhavn, 1889, pp. 43–66, 2 Pls.)

=Mingazzini, P.= Richerche sul canale digerente dei Lamellicorni fitofage (Larve e Insetti perfetti). (Mitteil. Zool. Station zu Neapel, ix, 1889–1891, pp. 1–112, 266–304, 7 Pls.)

=Fernald, Henry T.= Rectal glands in Coleoptera. (Amer. Naturalist, xxiv, pp. 100, 101, Jan., 1890.)

=Visart, O.= Digestive canal of Orthoptera. (Atti Soc. Toscana Scient. Natur., vii, 1891, pp. 277–285.)

=Eberli, J.= Untersuchungen an Verdauungstrakten von Gryllotalpa vulgaris. (Vierteljahresschr. d. Naturforsch. Gesells. Zurich, 1892, Sep., p. 46, Fig.)

=Holmgren, Emil.= Histologiska studier öfver några lepidopterlarvers digestionskanal och en del af deras Körtelartade bildningar. (Ent. Tidskr. Årg. xiii, pp. 129–170, 1892, 6 Pls.)

=Ris, F.= Untersuchung über die Gestalt des Kaumagens bei den Libellen und ihren Larven. (Zool. Jahrb. Abth. Syst., ix, 1896, pp. 596–624, 13 Figs.)

See also the works of Straus-Dürckheim, Newport, Mark, Witlaczil, Vayssière, Landois, Jordan, Oudemans, Berlese, List, Grassi, Verson, Miall and Denny, Leidy, Cheshire, Kowalevsky, Gehuchten, Locy, etc.

b. Digestion in insects

For the most complete and reliable investigation of the process of digestion, we are indebted to Plateau, whose results we give, besides the conclusions of later authors:

In mandibulate or biting insects, the food is conducted through the œsophagus by means of the muscular coating of this part of the digestive canal. Suctorial insects draw in their liquid food by the contractions followed by the dilatations of the mid-intestine (chylific stomach). Dragon-flies, Orthoptera, and Lepidoptera swallow some air with their food.

Where the salivary glands are present, the neutral alkaline fluid secreted by them has the same property as the salivary fluid of vertebrates of rapidly transforming starchy foods into soluble and assimilable glucose. In such forms as have no salivary glands, their place is almost always supplied by an epithelial lining of the œsophagus, or, as in the Hydrophilidæ, a fluid is secreted which has the same function as the true salivary fluid.

Nagel states that the saliva of the larva of Dyticus is powerfully digestive, and has a marked poisonous action, killing other insects, and even tadpoles of twice the size of the attacking larva, very rapidly. The larvæ not only suck the blood of their victims, but absorb the proteid substances. Drops of salivary juice seem to paralyze the victim, and to ferment the proteids. The secretion is neutral, the digestion tryptic. Similar extra-oral digestion seems to occur in larvæ of ant-lions, etc. (Biol. Centralbl., xvi, 1896, pp. 51–57, 103–112; Journ. Roy. Micr. Soc., 1896, p. 184.)

In carnivorous insects and in Orthoptera, the œsophagus dilates into a crop (ingluvies) ended by a narrow, valvular apparatus (or gizzard of authors). The food, more or less divided by the jaws, accumulates in the crop, which is very distensible; and, when the food is penetrated by the neutral or alkaline liquid, there undergoes an evident digestive action resulting, in carnivorous insects, in the transformation of albuminoid substances into soluble and assimilable matter analogous to peptones, and, in herbivorous insects, an abundant production of sugar from starch. This digestion in the crop, a food-reservoir, is very slow, and, until it is ended, the rest of the digestive canal remains empty.

“Any decided acidity found in the crop is due to the injection of acid food; but a very faint acidity may occur, which results from the presence in the crop of a fluid secreted by the cæcal diverticula of the mesenteron.” (Miall and Denny.)

When digestion in the crop is accomplished, the matters are subjected to an energetic pressure of the walls through peristaltic contractions, and then, guided by the furrows and chitinous teeth, pass along or gradually filter through the valvular apparatus or proventriculus, whose function is that of a strainer.

At the beginning of the “chyle-stomach” (mesenteron) of Orthoptera are glandular cæca which secrete a feebly acid fluid. This fluid emulsifies fats, and converts albuminoids into peptones. It passes forwards into the crop, and there acts upon the food.

In the mesenteron (mid-intestine) the food is acted upon by an alkaline or neutral fluid, never acid, either secreted, as in Orthoptera, by local special glands, or by a multitude of minute glandular cæca, as in many Coleoptera, or by a simple epithelial layer. It has no analogy with the gastric juices of vertebrates; its function differs in insects of different groups; in carnivorous Coleoptera it actively emulsionizes greasy matters; in the Hydrophilidæ it continues the process of transformation of starch into glucose, begun in the œsophagus. In the Scarabæidæ, it also produces glucose, but this action is local, not occurring elsewhere; in caterpillars, it causes a production of glucose, and transforms the albuminoids into soluble and assimilable bodies analogous to peptones, and also emulsionizes greasy matters. Finally, in the herbivorous Orthoptera there does not seem to be any formation of sugar in the stomach itself, the production of glucose being confined to the crop (jabot).

When digestion in the crop is finished, the proventriculus relaxes, and the contents of the crop, now in a semi-fluid condition, guided by the furrows and teeth, passes into the mesenteron, which is without a chitinous lining, and is thus fitted for absorption.

The contents of the mid-intestine (chylific stomach) then slowly and gradually pass into the intestine, the first anterior portion of which, usually long and slender, is the seat of an active absorption. The epithelial lining observed in certain insects seems, however, to indicate that secondary digestion takes place in this section. The reaction of the contents is neutral or alkaline.

The second and larger division of the intestine only acts as a stercoral reservoir. (The voluminous cæcum occurring in Dyticidæ, Nepa, and Ranatra, whether full or empty, never contains gas, and it is not, as some have supposed, a swimming-bladder.) The liquid product secreted by the Malpighian tubes accumulates in this division, and, under certain circumstances, very large calculi are often formed. In his subsequent paper on the digestion of the cockroach, Plateau states that in the intestine are united the residue of the work of digestion and the secretion of the urinary or Malpighian tubes, this secretion being purely urinary.

These organs are exclusively depuratory and urinary, freeing the body from waste products of the organic elements. The liquid they secrete contains urea (?), uric acid and abundant urates, hippuric acid (?), chloride of sodium, phosphates, carbonate of lime, oxalate of lime in quantity, leucine, and coloring-matters.

The products of the rectal or anal glands vary much in different groups, but they take no part in digestion, nor are they depuratory in their nature.

Insects have nothing resembling chylific substances.[51] The products of digestion, dissolved salts, peptones, sugar in solution, emulsionized greasy matters, pass through the relatively delicate walls of the digestive canal by osmose, and mingle outside of the canal with the blood.

Whatever substances remain undigested are expelled with the excrements; such are the chitin of the integuments of insects, vegetable cellulose, and chlorophyll, which is detected by the microspectroscope all along the digestive canal of phytophagous insects.

In his experiments in feeding the larvæ of Musca with lacmus, Kowalevsky found that the œsophagus, food-reservoir, and proventriculus, with its cæcal appendages, always remained blue, and had an alkaline reaction; the mid-intestine, also, in its anterior portion, remained blue, but a portion of its posterior half became deep red, and also exhibited a strong reaction. The hind-intestine, however, always remained blue, and also had an alkaline reaction. (Biol. Centralbl., ix, 1889, p. 46.)

=The mechanism of secretion.=—Gehuchten describes the process of secretion in insects, the following extract being taken from his researches on the digestive apparatus of the larva of Ptychoptera. The products of secretion poured into the alimentary canal are more or less fluid; for this reason, it is impossible to say when an epithelial cell at rest contains these products. For the secreting nature of these cells is only apparent at the moment when they are ready for excretion; then the cellular membrane swells out, and a part of the protoplasmic body projects into the intestinal cavity.

Before going farther, the terms secretion and excretion should, he says, be defined. With Ranvier, he believes that the elaboration in the protoplasm of a definite fluid substance is, par excellence, the secretory act, while the removal of this substance is the act of excretion.

A glandular cell of the chylific stomach, when at rest, is always furnished with a striated “platform,” or flat surface, or face, on the side facing the cavity of the stomach, and the free edge of the platform, or plateau, is provided with filaments projecting into the digestive cavity (Fig. 322, f). These glandular cells, when active, differ much in appearance. In a great number, the platform (plateau) has disappeared, and is replaced by a simple, regular membrane. During the process of secretion, a finely granular mass, in direct continuity with the protoplasm, swells, and raises the membrane over the entire breadth of the cell, causing it to project into the intestinal cavity (Fig. 322, A, B). These vesicles, or drops of the secretion, whether free or still attached by a web to the cells, are clear and transparent in the living insect, but granular in the portions of the digestive canal fixed for cutting into sections. Gehuchten then asks: “How does a cell gorged with the products of secretion empty itself?” Both Ranvier and also Heidenhain believe that one and the same glandular cell may secrete and excrete several times without undergoing destruction, but their researches made on salivary glands have not answered the question. Gehuchten explains the process thus: when the epithelial cell begins to secrete, the clear fluid elaborated in the protoplasm of the cell increases the intracellular tension, until, finally, the fluid breaks through certain weak places in the swollen basal membrane of the platform, and then easily passes through the closely crowded filaments, and projects out into the intestinal cavity as a pear-shaped vesicle of a liquid rich in albumens at first attached to the free face of the cell, but finally becoming free, as at Fig. 322, A, B.

When the elaboration of the substance to be secreted is more active, the mechanism of the secretion is modified. The basal membrane of the platform may then be raised at several places at once; instead of a single vesicle projecting into the intestinal cavity, each cell may present a great number more or less voluminous. If all remain small and rapidly detach themselves from the glandular cell, the filaments of the platform are simply separated from each other at different points of the free face, as in Fig. 322, C. On the other hand, when the different vesicles of a single cell become larger, the filaments of the platform are compressed and crowded against each other in the spaces between the vesicles remaining free, and the undisturbed portions of the platform appear homogeneous (Fig. 322, D). After the excretion of the secretory products by this process of strangulation, the cell then assumes the aspect of a glandular cell at rest, and may begin again to form a new secretion.

To sum up: The process of excretion may occur in two ways:

1. Where the membrane ruptures and the substances secreted are sent directly out into the digestive cavity. 2. Where the vesicles become free by strangulation, floating in the glandular or intestinal cavity, and ending by rupturing and coming into contact with the neighboring vesicles or with the food.

=Absorbent cells.=—Besides the glandular or secreting cells in Ptychoptera, there is between the two regions of the chyle-stomach lined with these cells a region about a centimetre long composed of absorbent cells. The absorbent cells are very large, polygonal, and contain a large nucleus, in which is a striated convoluted chromatic cord.

The food on entering the chyle-stomach is brought into contact with the products secreted in the proventriculus, in the first part of the chyle-stomach, and in the tubular glands. These products of secretion act on the food, extracting from them useful substances which they render soluble. These substances, after having been absorbed by the absorbent cells in the middle region of the stomach, undergo special modifications, and are transformed into solid products, which are situated at the bottom of these cells. Afterwards the alimentary substances freed from a portion of their useful substances are again placed in contact with the products of secretion in the distal part of the chylific ventricle, and reach the terminal part of the intestine.

“The products of secretion,” adds Gehuchten, “diverted into the intestinal canal do not come into immediate contact with the alimentary substances; they are separated from it by a continuous, structureless, quite thick membrane (the peritrophic membrane), which directly envelops the cylinder of food matters, extending from the orifice of the œsophageal valvule to the end of the intestine. Between this membrane and the free face of the epithelial lining there exists a circular space, into which are thrown and accumulate the excreted substances. The latter then cannot directly mingle with the aliments; but when they are liquid they undoubtedly pass through this membrane by osmose, and thus come into contact with the nutritive substances. It is the same with the products of absorption. The absorption of soluble products of the intestinal cavity is not then so simple a phenomenon as it was at first thought to be, since these products are nowhere brought into immediate contact with the absorbent cells” (pp. 90, 91).

The most recent authority, Cuénot, states that absorption of the products of digestion takes place entirely in the mid-intestine, and in its cæca when these are present. The mid-intestine exercises a selective action on the constituents of the food comparable to the action of the vertebrate liver.

LITERATURE ON THE PHYSIOLOGY OF DIGESTION

=Davy, J.= Note on the excrements of certain insects, and on the urinary excrement of insects. (Edinburgh New Phil. Journ., 1846, xl, pp. 231–234, 335–340; 1848, xlv, pp. 17–29.)

—— Some observations on the excrements of insects, in a letter addressed to W. Spence. (Trans. Ent. Soc. London, Ser. 2, iii, 1854, pp. 18–32.)

=Bouchardat, A.= De la digestion chez le ver à soie. (Revue et Mag. de Zool., Sér. 2, 1851, iii, pp. 34–40.)

=Lacaze-Duthiers, H., et A. Riche.= Mémoire sur l’alimentation de quelques insects gallicoles et sur la production de la graisse. (Ann. Scienc. natur., 1854, Sér. 4, ii, pp. 81–105.)

=Basch, S.= Untersuchungen über das chylopoetische und uropoetische System von Blatta orientalis. (Sitzungsber. d. math.-naturwiss. Classe d. Akad. d. Wissensch. Wien., 1858, xxxiii, pp. 234–260, 5 Taf.)

=Lambrecht, A.= Der Verdauungsprozess der stickstoffreichen Nährmittel, welche unsere Bienen geniessen, in den dazu geschaffenen Organen derselben. (Bienenwirtschaftl. Centralbl., viii Jahrg., 1872, pp. 73–78, 83–89.)

=Plateau, F.= Recherches sur les phénomènes de la digestion chez les insectes, (Mém. Acad. roy. de Belgique, Sér. 2, xli, 1 Part, 1873, pp. 124, 3 Pls.)

—— Note additionelle au mémoire sur les phénomènes de la digestion chez les insectes. (Bull. Acad. roy. de Belgique, Sér. 2, xliv, 1877, pp. 710–733.)

=Tursini, G. Fr.= Un primo passo nella ricerca dell’ assorbimento intestinale degli artropodi. (Rend. d. R. Accadem. di Sc. fis. e matemat. di Napoli, 1877, xvi, pp. 95–99, 1 Pl.)

=Jousset de Bellesme=, Physiologie comparée. Recherches expérimentales sur la digestion des insectes et en particulier de la blatte. Paris, 1876, vii, and 96 pp., 3 Pls.

—— Recherches sur les fonctions des glandes de l’appareil digestif des Insectes. (Compt. rend., lxxxii, Paris, 1876, pp. 97–99.)

—— Travaux originaux de physiologie comparée. (i, Insectes, Digestion, Métamorphoses.) Paris, 1878, 5 Pls.

=Simroth, H.= Einige Bemerkungen über die Verdauung der Kerfe. (Zeitschr. f. d. gesammten Naturwiss., xli, 1878, pp. 826–831.)

=Krukenberg, C. Fr. W.= Versuche zur vergleichenden Physiologie der Verdauung und vergleichende physiologische Beiträge zur Kenntnis der Verdauungsvorgange. (Untersuch, a. d. physiolog. Institut d. Universität Heidelberg, 1880, i, 4, pp 327, Figs.; ii, 1, p. 1, Figs.)

=Metschnikoff, E.= Untersuchungen über die intrazelluläre Verdauung bei wirbellosen Tieren. (Arb. d. zool. Instit. Wien., 1883, v, pp. 141–168, 2 Taf.)

=Locy, William A.= Anatomy and physiology of the family Nepidæ. (Amer. Naturalist, xviii, 1884, pp. 250–255, 353–367, 4 Pls.)

=Vangel, E.= Beiträge zur Anatomie, Histiologie, und Physiologie des Verdauungsapparates des Wasserkäfers, Hydrophilus piceus. (Termész. Füzet., x, 1886, pp. 111–126 (in Hungarian); pp. 190–208 (in German), 1 Pl.)

=Schönfeld.= Die physiologische Bedeutung des Magenmundes der Hönigbiene. (Archiv f. Anat. u. Physiol., Physiol. Abt., 1886, pp. 451–458.)

=Faussek, V.= Beiträge zur Histiologie des Darmkanals der Insekten. (Zeitschr. f. wiss. Zool., 1887, xl, pp. 694–712, 1 Taf.; Abstract in Zool. Anz., Jahrg. x, pp. 322, 323, 1 Taf.)

=Frenzel, J.= Ueber Bau und Thätigkeit des Verdauungskanals der Larve des Tenebrio molitor, mit Berücksichtigung anderer Arthropoden (Berlin. Ent. Zeitschr., 1882, pp. 267–316, 1 Taf.); Inaug.-Diss. Göttingen, 1882.

—— Einiges über den Mitteldarm der Insekten, sowie über Epithel-regeneration. (Archiv f. Mikrosk. Anat., 1885, xxvi, pp. 229–306, 3 Taf.)

—— Zum feineren Bau des Wimperapparates. (Ibid., 1886, xxviii, pp. 53–80, 1 Taf.)

—— Die Verdauung lebenden Gewebes und die Darmparasiten. (Archiv f. Anat., 1891.)

=Gehuchten, A. van.= Recherches histologiques sur l’appareil digestif de la Ptychoptera contaminata, I Part. Étude du revêtment épithélial et recherches sur la sécrétion. (La Cellule, 1890, vi, pp. 183–291, 6 Pls.)

=Cuénot, L.= Études physiologiques sur les Orthoptères. (Arch. Biol., xiv, 1895, pp. 293–341, 2 Pls.)

=Needham, James G.= The digestive epithelium of dragon-fly nymphs. (Zool. Bull., i, 1897, Chicago, pp. 104–113, 10 Figs.)

With the writings of Mingazzini (see p. 323), Kowalevsky, Ranvier, Haidenhain, Beauregard (p. 323), Sadones.

THE GLANDULAR AND EXCRETORY APPENDAGES OF THE DIGESTIVE CANAL

Into each primary division of the digestive canal open important glands. The salivary and silk-glands are offshoots of the œsophagus (stomodæum); the cœcal appendages open into the stomach (mesenteron), while the urinary tubes grow out in embryonic life from the primitive intestine (proctodæum), and there are other small glands which are connected with the end of the hind-intestine.

a. The salivary glands

We will begin our account of these glands with those of the Orthoptera, where they are well developed. In the cockroach a large salivary gland and accompanying reservoir lie on each side of the œsophagus and crop. The gland is a thin, leaf-like, lobulated mass, divided into two principal lobes. These open into a common trunk, which after receiving a branch from a small accessory lobe, and from the salivary reservoir, unites with its fellow to form the unpaired salivary duct which opens into the under side of the lingua. Each salivary reservoir is a large oval sac with transparent walls. (Miall and Denny, also Figs. 299, sr, and 327.) The ducts and reservoirs have a chitinous lining, and the ducts are, like the tracheæ, surrounded by a so-called spiral thread, or by separate, incomplete, hooplike bands, which serve to keep the duct permanently distended. In the locust (Fig. 298) the lobules are more scattered, forming small separate groups of acinose glands. In the embryo of Forficula Heymons has observed a pair of salivary glands opening on the inner angle of the mandibles, a second pair opening in the second maxillæ, while a third pair of glands, whose function is doubtful, is situated in the hinder part of the head, opening to the right and left on the chitinous plate (postgula) behind the submentum. In Perla, there are two pairs segmentally arranged (Fig. 343).

Here we might refer to a pair of glands regarded by Blanc as the true salivary glands. They do not appear to be the homologues of the salivary glands of other insects, though probably functioning as such. The functional salivary glands of lepidopterous larvæ have been overlooked by most entomotomists, and the spinning glands have been, it seems to us, correctly supposed to be modified salivary glands. Lucas also regards those of case-worms (Trichoptera) as morphologically salivary glands. Those of the silkworm were figured by Réaumur (Tom. i, Pl. v, Fig. 1), but not described; while those of Cossus, which are voluminous, were regarded by Lyonet as “vaisseaux dissolvans.” Dr. Auzoux (1849), in his celebrated model of the silkworm, represented them accurately, while Cornalia briefly described them as opening into the mouth. The first satisfactory description is that of Blanc (1891), who states that in the silkworm “the two salivary glands” are small, flexuous, yellow tubes, which occupy a variable position on the sides of the œsophagus (Fig. 323). The glandular portion passes into the head, ending at the level of the adductor plate of the mandibles (Fig. 324, o), and entering the buccal cavity at the base of the mandible, as seen in Fig. 323. It is plain, when we recognize the direct homology of the silk-glands of the caterpillars with the salivary glands of other insects, and of the spinneret with the hypopharynx, that these so-called “salivary glands” in lepidopterous larvæ are different structures. They are probably modified coxal glands, belonging to the mandibular segment.

The polygonal epithelial cells of these glands contain branched nuclei, recalling those of the spinning-glands. In those caterpillars which feed on leaves, the salivary glands are slightly developed, but in such as bore into and eat wood, as the Cossidæ, the glands are, as figured by Lyonet, very large, forming two sausage-shaped bodies passing back to the beginning of the mid-intestine, each ending in a long convoluted filament. The salivary glands of the imago are very long and convoluted (Fig. 310, sd).

In the Panorpidæ these glands differ in the sexes, the males having three pairs of very long tortuous tubes, while, in the females, they are reduced to two indistinct vesicles. (Siebold.)

In the Diptera in general there are two pairs, one situated in the beak, the other in the thorax. In the larvæ there is a single pair (Fig. 341). Kraepelin describes a third pair in the Muscidæ at the point of transition from the fulcrum to the œsophagus, but Knüppel has apparently found only what may be fat cells at this point, so that the supposed presence of a third pair in Diptera needs confirmation. In the Psocidæ there are two salivary glands, of simple tubular shape (Fig. 325).

In the Nepidæ the salivary glands are four in number, and of conglomerate structure, two being long and extending back into the beginning of the abdomen, while the other two are about one-fourth as long. (Figs. 327, 328.) In Cicada, besides a pair of simple tortuous tubes, there is in the head another pair of glands, each composed of two tufts of short lobes, situated one behind the other. (Dufour.) In many Hemiptera (Pyrrhocoris, Capsus, etc.) there is but a single pair, each gland consisting of four lobes; in the Coccidæ each gland is divided into two lobes (Fig. 326); in the Aphidæ, according to Witlaczil, they consist of two lobes grown together. In the Psyllidæ they are said to be absent.

In Phylloxera vastatrix the saliva is forced through a salivary passage out of the duct and into the mouth by a pumping apparatus furnished with special muscles. (Krassilstschik.)

In the Odonata acinose glands are present in the imago, but not in the nymph until in its last stage, Poletaiew accounting for their absence in the earlier stages by the fact that the larva swallows more or less water while taking its food.

In the Coleoptera, as we have observed in Anopthalmus, there are three pairs of salivary glands (Fig. 74). In the Blapsidæ these glands consist of many ramifying tubes united on each side of the œsophagus into a single duct; in others they are but slightly developed, while in still others they are wanting.

The salivary glands are most highly differentiated in the Hymenoptera, and especially in the bees (Bombus and Apis), where Schiemenz found not less than five systems of glands (Fig. 329; also 87), of which four systems are paired. One pair of these glands lies in the tongue, three in the head, and one in the thorax.

System I is situated in the head, and consists of unicellular glands; the duct from each cell leads into a common, strongly chitinized duct, opening into the gullet.

System II, composed of acinose glands, lies also in the head; its duct is united with that of System III, situated in the thorax. (Fig. 329, 2, 3.)

System IV is situated at the base of the upper surface of the mandibles, and forms a delicate sac lined within with glandular cells; its duct opens at the insertion of the mandibles.

System V lies in the beak, and is a single gland consisting of unicellular glands; it opens into the common opening of Systems II and III. This system is wanting in the honey-bee, but occurs in Bombus and other genera.

In all the five systems there constantly occur three cellular layers: the intima, epithelial, and propria. As regards their origin Schiemenz states that Systems I and IV are new structures, that System III arises in part, and Systems II and V wholly, from the silk-glands of the larva. As the glands differ much in the sexes, and in different species and genera, Schiemenz believes that their function is very manifold.

In addition to those previously discovered by Schiementz, Bordas has detected two additional pairs of salivary glands in the worker and male honey-bee, i.e. the internal mandibular and sublingual glands, so that in Apis there are in all six pairs, and apparently one unpaired.

The delicate chitinous external layer of the gland is perforated by many very fine pores through which the salivary fluid secreted by the epithelial cells passes into the salivary duct. The glands are externally bathed by the blood.

In many insects, including lepidopterous larvæ, the single median opening of the salivary duct is converted into a spraying apparatus.

In the adult Lepidoptera, according to Kirbach:—

“Its lower half forms a thick chitinous gutter, with a concave cover above, in which the similarly shaped upper half lies encased, so that between the two only a small semicircular opening remains. Powerful muscles extend from the cover to the lower side and to the two ridges of the bottom plate; through their contraction the upper channel is elevated, and presses out of the hinder part of the ducts into the space thus formed a great quantity of the saliva, which by allowing the contraction of the cover-muscle through the crevice-like opening, which is situated in the lower edge of the mouth-opening, becomes squeezed out in order either to mix with the fluid where the 2d maxillæ fuse, passing up into the canal in the proboscis, or to penetrate into and thus dilute the semi-fluid or solid substances taken, into the proboscis.”

The morphology and general relations of the salivary glands have been sketched out by Hatschek, Patten, and by Lucas, from observations on those of the case-worms or larval Trichoptera.

Patten states that the spinning-glands in Neophylax are formed by a pair of ectodermal invaginations on the ventral side of the embryo, between the base of the 2d maxillæ and the nervous cord. They increase rapidly in length, and “they also unite to form a common duct, which opens at the end of the upper lip.”

The salivary glands in the same insect are “formed by invagination of the ectoderm on the inner sides of the mandibles, in the same manner as are the spinning glands.”

Lucas has shown that in trichopterous larvæ (Anabolia) there are three pairs of salivary glands in the head, which are serially arranged. The first pair belong to the mandibular, the second pair to the 1st maxillary, and the third pair, or spinning glands, to the 2d maxillary segment. The first or mandibular glands open into the mouth at the base of the mandibles directly behind the dorsal condyle. The second pair open between the 1st and 2d maxillæ; at the base of the latter, near the ventral condyle of the mandibles. The third pair open into the hypopharynx, which is modified to form the spinneret. Lucas agrees with Korschelt in regarding them as modified coxal glands, Schiemenz having previously regarded the headglands of the imago of the bee as belonging to the segments bearing the three pairs of buccal appendages, so that each segment originally contained a pair of glands. It is thus proven that the silk-glands are modified salivary glands adapted to the needs of spinning larvæ, and indeed in the imago the sericteries revert to their primitive shape and use as salivary glands.

The serial arrangement of the salivary glands in the Hymenoptera, where the number varies from five to ten pairs, is clearly proved by Bordas. He has detected five more pairs than were previously known, and names the whole series as follows:—1, the thoracic salivary glands, which are larger than the others, and nine other pairs, which are all contained in the head as follows: 2, postcerebral; 3, supracerebral; 4, lateropharyngeal; 5, mandibular; 6, internomandibular, situated on the inner side of the base of mandible; 7, sublingual; 8, lingual (these and 1 to 7 common to all Hymenoptera); 9, paraglossal (in Vespidæ); 10, maxillary (very distinct in most wasps). These glands do not all occur in the same species, being more or less atrophied.

Bordas further shows the segmental arrangement of the cephalic glands by stating that the supracerebral glands correspond to the antennal segment, the sublingual glands to the labial, the mandibular glands (external and internal) to the mandibular segment, the maxillary glands to the 1st maxillary segment, the lingual glands to the 2d maxillary segment, while the thoracic and postcerebral salivary glands, he thinks, correspond to the ocular segment, a view with which we are indisposed to agree, although conceding that each of the six segments of the head has in it at least one pair of salivary glands.

=Functions of the different salivary glands in Hymenoptera.=—The secretion of the thoracic glands is feebly alkaline. The postcerebral salivary glands, considered by Ramdohr to be organs of smell, secrete, like the preceding, a distinctively alkaline fluid, which mingles with the products of the thoracic glands. The supracerebral glands, also equally well developed in all Hymenoptera, though much atrophied in the females and especially the males of Apis mellifica, also in the Vespinæ and Polistinæ, secrete an abundant, feebly acid liquid, which is actively concerned in digestion.

As to the mandibular glands, which Wolf supposed to be olfactory organs, their acid secretion, though smelling strongly, acts energetically on the food as soon as introduced into the mouth.

The sublingual glands, atrophied in most Apidæ, but relatively voluminous in Sphegidæ, Vespinæ, Polistinæ, Crabronidæ, etc., empty their secretion into a small prebuccal excavation, where accumulate vegetable and earthy matters collected by the tongue, and the saliva secreted by these glands, acts upon them before they pass into the pharynx. The lingual glands secrete a thick, sticky liquid, which causes foreign bodies to adhere to the tongue, and also agglutinates alimentary substances. The uses of the other glands, maxillary and paraglossal, are from their minuteness undetermined. (Bordas.)

LITERATURE ON THE SALIVARY GLANDS

=Leydig, F.= Zur Anatomie der Insekten. (Archiv Anat. und Phys. 1859.)

—— Untersuchungen zur Anatomie und Histiologie der Tiere. Bonn, 1883, pp. 174, 8 Taf.

—— Intra- und interzellulare Gänge. (Biolog. Centralblatt, x, 1890, pp. 392–396.)

=Dohrn, A.= Zur Anatomie der Hemipteren. (Stettin. Entom. Zeit., 1866, salivary glands, pp. 328–332.)

=Kupffer, C.= Die Speicheldrüsen von Periplaneta orientalis und ihr Nervenapparat. (Beiträge zur Anatomie und Physiol., 1875.)

=Schiemenz, P.= Ueber das Herkommen des Futtersaftes und die Speicheldrüsen der Biene. (Zeitschr. f. wissens. Zool., xxxviii, 1883, pp. 71–135, 3 Taf.)

=Korschelt, E.= Ueber die eigentümlichen Bildungen in den Zellkernen der Speicheldrüsen von Chironomus plumosus. (Zool. Anzeiger, 1884, pp. 189–194, 221–225, 241–246.)

=Hofer, B.= Untersuchungen über den Bau der Speicheldrüsen und des dazu gehörenden Nervenapparates von Blatta. (Nova Acta d. Kais. Leopold.-Carol. Deutsch. Akad. d. Naturforscher, li, 1887, pp. 345–395, 3 Taf.)

=Knüppel, A.= Ueber Speicheldrüsen von Insekten. (Archiv für Naturg., 1887, Jahrg. 52, pp. 269–303, 2 Taf.)

=Blanc, Louis.= La tête du Bombyx mori à l’état larvaire, anatomie et physiologie. (Extrait des Travaux du Laboratoire d’Études de la Soie, 1889–1890; Lyon, 1891, p. 180, many figs.)

=Bordas, L.= Anatomie des glandes salivaires des Hyménoptères de la famille des Ichneumonidæ. (Zool. Anzeiger, 1894, pp. 131–133.)

—— Glandes salivaires des Apides, Apis mellifica. ♂ and ♀. (Comptes rendus Acad. Sc., Paris, cxix, pp. 363, 483, 693–695, 1894; also two articles in Bull. Soc. Philomath. Paris, 1894, pp. 5, 12, 66.)

—— Appareil glandulaire des Hyménoptères. (Ann. Sc. Nat. Zool., xix, Paris, 1894, pp. 1–362, 11 Pls.) (See also p. 366.)

=Berlese, Antonio.= Le cocciniglie Italiane viventi sugli agrumi. Firenze, 1896, 12 Pls. and 200 Figs.

With the writings of Mark, Minot, Locy, List, Krassilstschik, Nagel (1896).

b. The silk or spinning glands, and the spinning apparatus

The larvæ of certain insects, chiefly those of the Lepidoptera, possess a pair of silk or spinning glands (sericteries) which unite to form a single duct opening in the upper lip at the end of the lingua, which is modified to form the spinneret. (See pp. 71, 75.) All caterpillars possess them, and they are best developed in the silkworms, which spin the most complete cocoon. Silk-glands also occur in the larvæ of the Tenthredinidæ, in the case-worms or larval Trichoptera, also in certain chrysomelid beetles (Donacia, Hæmonia), and in a weevil (Hypera). In a common caddis-worm (Limnophilus) the glands are of a beautiful pale violet-blue tint, and two and a half times as long as the larva itself; viz. the body is 20 mm. and the glands 55 mm. in length.

In caterpillars the glands are of tubular shape, shining white, and much like the ordinary simple tubular salivary glands of the imago. When only slightly longer than the body they are twice folded, the folds parallel and situated partly beneath and partly on the side of the digestive canal; not usually, when folded in their natural position, extending much behind the end of the stomach; but in the silkworms they are so long and folded as to envelop the hinder part of the canal. In geometrid caterpillars the glands when stretched out only reach slightly beyond the end of the body; in Datana they are half again as long as the body. Helm thus gives their relative length in certain Eurasian caterpillars, and we add that of Telea polyphemus:—

Vanessa io length of body 32 mm.; of the silk glands 26 mm. Smerinthus tiliæ length of body 63 mm.; of the silk glands 205 mm. Bombyx mori length of body 56 mm.; of the silk glands 262 mm. Antheræa yamamaya length of body 100 mm.; of the silk glands 625 mm. Telea polyphemus length of body 60 mm.; of the silk glands 450 mm.

Thus in Telea the silk-glands are about 18.50 inches in length, being about seven times as long as the body.

For the most complete accounts of the spinning glands of Lepidoptera and their mechanism we are indebted to Helm and to Blanc, and for that of the Trichoptera to Gilson.

The unpaired portion, or spinning apparatus (filière of Lyonet), is divided into two portions; the hinder half being the “thread-press,” the anterior division the “directing tubes.” The silk material, stored up in the thickest portion of the glands, passes into the thread-press (Fig. 334, A), which is provided with muscles which force the two double ribbon-like threads through the directing tube, as wire is made by molten iron being driven through an iron plate perforated with fine holes. The entire spinning apparatus, or filator, as we may call it, is situated in the tubular spinneret. The opening of the spinneret is directed anteriorly, and the anterior end of the directing tube passes directly into this opening so that the directing tube may be regarded as an invagination of the lingua.

The silk thread which issues from the mouth of the spinneret is, as Leeuwenhoek discovered, a double ribbon-like band, as may be seen in examining the silk of any cocoon.

=The process of spinning.=—Since the appearance of Helm’s account, Gilson, and also Blanc, have added to our knowledge of the way in which the silk is spun and of the mechanism of the process. Gilson has arrived, in regard to the function of the press or filator, at the following conclusions: 1, the press regulates the thread, it compresses it, gives it its flattened shape; 2, it regulates the layer of gum[52] (grès) which surrounds the thread; 3, it may render the thread immovable by compressing it as if held by pincers.

The process of spinning in the silkworm, says Blanc, comprises all the phenomena by which the mass of silk contained in the reservoir is transformed into the silk fluid of which the cocoon is spun. The excretory canals each contain a cylindrical thread of silk having a mean diameter of 0.2 mm. and surrounded by a layer of gum (grès) which in the fresh living organ exactly fills the annular space situated between the fibroin cylinder and the wall. Arrived within the common duct, the two threads receive the secretion of Filippi’s gland, where the silken fluid is formed, but has not yet assumed its definite external characters. The two threads press through the common canal and arrive at the infundibulum (Fig. 334, c) of the press, at the bottom of which is situated the orifice of the spinning canal, almost completely divided into two by the sharp edge of the rachis (Figs. 334, a, 335, l). The threads each pass into one of the two grooves, and the layer of gum (grès) fills the rest of the canal of the press or filator.

The silken substance is then pressed by the more or less powerful contractions of the muscles of the filator, so that the passage of the threads is facilitated. If the muscles totally contract, the spinning canal is opened wide, the threads pass easily upwards and assume the form of a triangular prism (Fig. 336).

If this contraction diminishes, the chitinous wall of the spinneret comes together, owing to its elasticity; the ceiling of the canal approaches the floor; the cavity tends to take the form of a semicircular slit, and the threads are compressed, flattened. As each mass or thread of silk is much more voluminous than the canal, except when the latter is extremely dilated, it follows that the two threads are always compressed, or squeezed together, and that each of them is compelled to mould itself in the groove it occupies and to take its shape. Hence the variations in the appearance of the two masses or divided portions of silk, which as stated present all grades between the form of an isosceles-triangular prism and that of a nearly flat ribbon; but this last case is quite rare. The use of the spinneret, then, is to compress the thread and to change its form more or less considerably, at the same time as it diminishes its diameter.

Moreover, this constant compression of the thread as it passes through the press keeps it in a certain state of tension so as to allow the caterpillar while spinning to firmly hold its thread.

Finally, when the worm suspends the contraction of its spinning muscles, the press flattens, vigorously compresses the thread, and arrests its motion, in such a way that if there was a strain on the silken fluid (bave), it would break rather than oblige the caterpillar to let go any more of it.

The press does not act directly on the silken thread, but through the gummy layer (grès) which transmits over the whole surface of the silken fluid (brin) the pressure exerted on it. After having overcome this difficult passage, the silk thread has acquired its definite form; it rapidly passes out of the spinneret.

=How the thread is drawn out.=—Having seen, says Blanc, how the two masses of silk (brins), in passing through the spinning apparatus (or press), join each other, constituting the frothy silken fluid, thus becoming modified in form, it remains to examine the way in which the thread is drawn out of the spinneret. If we examine a caterpillar while spinning, it will be seen that in moving its head it draws on the frothy mass of silk fixed to the web of the cocoon. This traction certainly aids very much the exit of the thread, but it is not the only cause.

The silk, Blanc affirms, is pushed out by a force a tergo, developed by different agents, such as the pressure of the distended cuticle or the silky mass contained in the reservoir, as seen in the section of a worm which has spun its cocoon. But if we consider a caterpillar before it has begun to spin, it is difficult to explain the mechanism of spinning. As Blanc has often observed, in making sections of the heads of silkworms, two cases arise. Sometimes the worm has already spun a little, and a certain length of the frothy silk (bave) issues from the orifice of the spinneret, where it forms a small twisted bundle. At other times the worm has not spun since its last moult or the frothy mass of silk has broken within the head, and we find the end in the common tube. In the first of these two cases, the worm, dilating its press, is able by a general contraction to discharge a little of the gritty material (grès) which lines the ball of silk hanging at the end of the spinneret. It can also reject a certain quantity of the secretion of Filippi’s glands and thus soften the gritty substance. The little plug of silk can then adhere to the body with which it comes in contact.

In the same case it is necessary that the two bits or portions of silk traverse the press, and this normally has a calibre less than their diameter. The worm should then distend the spinning tube as much as is practicable, so as to make the openings as large as possible. It has been stated that the press is, in this condition, at least as large as the mass of frothy silk. This Blanc believes (although Gilson thinks otherwise) is pushed by a force a tergo, and reaches the funnel of the spinning canal; its two bits of silk (brins) unite there, penetrate into the canal itself, and, owing to successive impulses produced by the general contractions of the worm, press through and pass out of the spinneret.

While the silkworm is engaged in spinning its cocoon, the spinneret and press execute very varied movements, determined by the elevator, depressor, retractor, and protractor muscles of the labium, as well as those of the press. These movements, originally very numerous, may combine among themselves, so that the spinneret is susceptible of assuming during the process of spinning still more diverse positions.

Histologically the silk-glands are composed of three layers,—the outer, or tunica propria (Fig 337); the inner, the tunica intima; the middle layer being composed of extraordinarily large epithelial cells which can be seen with the naked eye, and are also remarkable for the branched shape of the nuclei (a, b, c, 337), the branches being more or less lobed, and the larger the cells the more numerous are the branches of the nucleus. Gilson[53] finds that those of Trichoptera, Lepidoptera, Diptera, and Hymenoptera ordinarily consist of a small number of cells; and it is quite common, he says, to find only two cells in a transverse section (Fig. 338, A). In the Tenthredinidæ, however, “the organ still consists of a tube, the wall of which is composed of flat cells, but in addition to that, two series of spheroidal cells are attached to the sides. Each of these cells contains a system of tiny canals running through their cytoplasm (B, i. d). These cells are the secreting elements; they continually cast the silk substance into the tube.” A peculiarity of the tunica intima is its distinct transverse striation.

The lining of the glands and of their common duct is moulted when the caterpillar casts its skin, and this, as well as the mode of development, shows that the glands are invaginations of the ectoderm. Gilson finds that the silk-glands and silk-apparatus of Trichoptera are very similar to those of caterpillars, and that the silk is formed in the same way.

=Appendages of the silk-gland (Filippi’s glands).=—In most larvæ there is either a single or a pair of secondary glands which open into the spinning glands near their anterior end. They are outgrowths of the gland provided with peculiarly modified excretory cells or evaginations of the entire glandular epithelium. Those of Bombyx mori (Fig. 340) are very well developed, and, according to Blanc, form two whitish, lobulated masses in the labium on each side of the common duct of the spinning gland. Externally they appear to be acinose; but their structure, as described by Blanc and by Gilson, is very peculiar. Helm thinks, with Cornalia, that the function of these glands is to secrete the adhesive fluid which unites the silk threads, and also to make the silk more adhesive in the process of spinning, but Blanc states that this is done before the thread passes into the common excretory tubes, and he is inclined to think that the secretion serves to lubricate the spinneret, and thus to facilitate the passage of the thread. On the other hand, in certain caterpillars these glands are situated quite far from the spinning apparatus.

The silk-glands in the pupa state undergo a process of degeneration, and finally completely disappear. They are specific larval organs evolved in adaptation to the necessity of the insect’s being protected during its pupal life by a cocoon. (Helm.)

Morphologically the silk-glands are by Lang regarded as modified coxal glands, and homologues of the setiparous parapodial glands of chætopod worms, the coxal glands of Peripatus, and the spinning glands of spiders.

In Scolopendrella, spinning glands are situated in the two last segments of the body, opening out at the end of the cercopods (Fig. 15, s.gl), and the larvæ of the true Neuroptera (Chrysopa, Myrmeleon, etc.) which spin cocoons, have spinning glands opening into the rectum. The silk forming the cocoon of the ant-lion, as Siebold and the older observers have stated, is secreted by the walls of the rectal or anal sac. Siebold (Anatomy of the Invertebrates, p. 445) states that in the larva of Myrmeleon, the silk-apparatus is very remarkable, “for the rectum itself is changed into a large sac and secretes this substance which escapes through an articulated spinneret projecting from the opening of the anus”[54] (Fig. 307, e). The larvæ of the Mycetophilidæ have spinning glands at the hinder end of the body, as also the imago of the female of the tineid moth Euplocamus. (Kennel.) The larvæ of ichneumons, wasps, bees, of Cecidomyia, and other Diptera, spin silken cocoons, but their glands have not yet been examined.

It should also be observed that during the process of pupation the larvæ of butterflies, of certain flies (Syrphus), and beetles (Coccinellidæ and some Chrysomelidæ) attach themselves by silk spun from the anus, so that the pupa is suspended by its tail; such glands are probably homogenetic with the coxal glands.

The silk in its fluid or soft state is mucilaginous, and according to Mulder, in the silkworm consists of the following substances, varying somewhat in their relative proportions by weight:

Silk-fibre material 53.67 Glue (Leim) 20.66 Protoplasm 24.43 Wax 1.39 Coloring matter 0.05 Fat and resin 0.10

LITERATURE ON THE SPINNING GLANDS

=Helm, E.= Anatomische und histiologische Darstellung der Spinndrüsen der Schmetterlingsraupen. (Zeitschr. f. wissens. Zool., xxvi, 1876, pp. 434–469, 2 Taf.)

=Lidth de Jeude, Th. W. van.= Zur Anatomie und Physiologie der Spinndrüsen der Seidenraupe. (Zool. Anzeiger, 1878, pp. 100–102.)

=Engelmann, W.= Zur Anatomie und Physiologie der Spinndrüsen der Seidenraupe. (Onderz. Phys. Lab. Utrecht, iii, 1880, pp. 115–119.)

=Joseph, G.= Vorläufige Mitteilung über Innervation und Entwickelung der Spinnorgane bei Insekten. (Zool. Anzeiger, 1880, pp. 326–328.)

=Poletajew, N.= Ueber die Spinndrüsen der Blattwespen. (Zool. Anzeiger, 1885, pp. 22–23.)

=Meinert, Fr.= Contribution à l’anatomie des fourmilions. (Overs. Danske Vidensk. Selsk. Forh. Kjöbenhavn, 1889, pp. 43–66, 2 Pls.)

=Blanc, Louis.= Étude sur la sécrétion de la soie et la structure du brin et de la bave dans le Bombyx mori. Lyon, 1889, pp. 48, 4 Pls.

—— La tête du Bombyx mori à l’état larvaire. Anatomie et physiologie. (Extrait du volume des Travaux du Laboratoire d’Études de la Soie. Années 1889–1890, Lyon, 1891, pp. 180, 95 figs.)

=Gilson, G.= Recherches sur les cellules sécrétantes. La soie et les appareils séricigènes: I. Lépidoptères. (La Cellule, 1890, vi, pp. 115–182, 3 Pls. I, Lépidoptères (suite); II, Trichoptères. Ibid., x, pp. 71–93, 1893, 1 Pl.)

=Garman, H.= Silk-spinning dipterous larvæ (Science, xx, 1893, p. 215).

Also the writings of Meckel, Pictet, Duméril, Klapálek, Wistinghausen, Loew, Hagen, Fritz Müller, Kolbe, McLachlan, de Selys-Longchamps.

c. The cæcal appendages.

These diverticula of the mid-intestine (“stomach”) are appended to the anterior end, and in the living, transparent larva of Sciara, which has two large, long, slender cœca (Fig. 341), the partly digested food may be seen oscillating back and forth from the anterior end of the stomach into and out of the base of each cæcum. In the Locustidæ (Anabrus, Fig. 299) and Gryllidæ (Fig. 344, e) there are two large, short cæca, and in the locusts (Caloptenus) there are six cæca, while cockroaches have eight. In the Coleoptera (Carabidæ and Dyticidæ) these large cæca appear to be replaced by very numerous slender, minute villi or tubules, which arise from the anterior part of the stomach (Figs. 317, r, also 342).

These cæca differ in structure from the stomach, as shown by Graber, as well as by Plateau and by Minot. The latter states that a single transverse section of one of the diverticula of the locust demonstrates at once that its structure is entirely different from that of the stomach.

Its inner surface is thrown up into longitudinal folds, generally twelve in number. These folds shine through the outer walls, and are accordingly indicated in the drawings of Dufour, Graber, and others. The entire cæcum has an external muscular envelope, outside of which are a few isolated longitudinal muscular bands. The folds within are formed mainly by the high cylindrical epithelium which lines the whole interior of the cavity. Tracheæ ramify throughout all the layers outside the epithelium. There are appearances of glandular follicles in the bottom of the spaces between the folds. (Minot.)

Burmeister supposed that these cæca were analogous to the pancreas, and this view has been confirmed by Hoppe Seyler, Krukenberg, Plateau, and others, who claim that the digestive properties of the fluid secreted in them agrees with the pancreatic fluid of vertebrates.

d. The excretory system (urinary or Malpighian tubes)

The excretory matters or waste products of the blood tissue of worms are carried out of the body by segmentally arranged tubes called nephridia. As a rule they arise in the blood sinuses of the body and open externally through minute openings in the skin. As there is a pair to each segment (in certain oligochete worms two or three pairs to a segment), they are often called segmental organs. In the annulate worms each segment of the body, even the cephalic or oral segment, originally contains a pair of these excretory organs. These vessels may have survived in myriopods and perhaps do exist in insects as urinary tubes, and also occur in many of the Arachnida, and thus are characteristic of each important class of land arthropods, but are either wanting or are very rudimentary or much modified in the marine classes, notably the Crustacea and Merostomata (Limulus), where they are represented by the shell-glands of Copepoda, green glands of the lobster, and the brick-red glands of Limulus.

In the earliest tracheate arthropod, Peripatus, these tubes are well developed and are highly characteristic, each segment behind the head bearing a pair (Fig. 4, so{4}_-so{9}_). It has been suggested by some, but not yet proved, that the urinary tubes of insects are morphologically the same as the segmental organs of worms and of Peripatus; but there are no facts directly supporting this view, and, as Sograff states, it is a pure hypothesis and can only be confirmed or disproved by very detailed researches on the development of the urinary tubes of myriopods and of insects. Others regard them as probably homologous with the tracheæ, since they have a similar origin. As, however, they arise in the embryo as outgrowths of the proctodæum they may have arisen in myriopods and insects independently, and not be vermian heirlooms.

While in worms and in Peripatus a pair of these segmental organs occur in each segment, in insects this serial arrangement is not apparent; those with a purely excretory function are not segmentally arranged, with outlets opening externally, but arise as outgrowths of the hind-intestine or proctodæum of the embryo, not being segmentally arranged. The place of their origin is usually the dividing line between the mid and hind intestine (Fig. 343, mp); this applies to Scolopendrella (Fig. 15, urt) as well as to insects.

The urinary tubes are usually long, slender, blind, tubular glands varying in number from two to over a hundred, which generally arise at the constriction between the mid and hind intestine, and which lie loosely in the cavity of the body, often extending towards the head, and then ending near the rectum (Figs. 301, 310, vm). They were first discovered by the Italian anatomist Malpighi, after whom they were called the Malpighian tubes. While at first generally regarded as “biliary” tubes, they are now universally considered to be exclusively excretory organs, corresponding to the kidneys of the higher animals.

Usually arising from the anterior end of the hind-intestine where it passes into the mid-intestine, in certain forms they shift their position, in some Hemiptera (Lygæus, Cimex) opening into the rectum, while in the Psyllidæ they arise from the slender hinder part of the mid-intestine, being widely separated at their origin. (Fig. 321.)

The length varies in different groups; where they are few in number (two to four, six to eight), they are very long, but where very numerous they are often short, forming dense tufts, each tuft connecting with the intestine by a common duct (ureter), or, as in the mole-cricket, the numerous tubes empty into a single duct (Fig. 344); in the locusts (Acrydiidæ), however, they are arranged in 10 groups, each group consisting of about 15 tubes, making about 150 in all; and are much convoluted and wound irregularly around the digestive canal, and when stretched out being about as long as the entire body.

The urinary tubes occur in twos, or in multiples of two, though a remarkable exception is presented in the dipterous genera Culex and Psychodes, in which there are five tubes; the young and fully grown larvæ, as well as the pupa and imago of Culex, having this number (Fig. 433, mg.)

In many insects (Pentatoma, Cimex, Velia, Gerris, Haltica, Donacia, and often in caterpillars), the vessels open into a sort of urinary bladder connecting with the intestine on one side.

In the larvæ of some insects the blind ends of the tubes are often externally bound to the rectum, in the silkworms being attached by fine threads to the intestine, while in some flies (Tipula and Ctenophora), two vessels may unite to form a loop. In all larval Cecidomyiæ, the two tubes are united to form a loop which curves backward, opening near the vent, the proctodæum being very short. (Giard.)

While usually the urinary vessels form simple tubes, in many species of Lepidoptera and Diptera they are branched, thus resembling those of spiders and scorpions. Moreover, in many Lepidoptera and Diptera (Fig. 308), the tubes are not simple, but are lobulated, and in some Hemiptera (Pentatoma, Notonecta, and Tettigonia) are twisted or lace-like. In rare cases there are two kinds of urinary tubes; in Melolontha vulgaris, two of them are partly lobulated and yellow, while the other two are simple and white. Their color in beetles varies, some being whitish or yellowish; in Geotrupes, Dyticidæ, Hydrophilidæ, etc., reddish brown; in Gryllotalpa as well as Locusta viridissima, there are two different kinds of vessels, differing in contents and in color (white or yellow), as well as histologically. (Schindler.)

The exterior of the tubes is richly provided with tracheæ, which often form a web around them, and the fine branches often seem to attach them to the intestine. In Acheta they are enveloped by a very delicate, loose network of muscular fibres. (Schindler.)

The urinary tubes consist, according to Schindler, of at least three cellular layers (Fig. 345):—

1. An external, connective, nucleated membrane, the peritoneal membrane.

2. A very delicate homogeneous basal membrane, the tunica propria.

3. A single layer of large polygonal excretory cells.

4. Lining the internal canal a chitinous layer penetrated by pore-canals, the intima often wanting.

The secretory cells are usually of the same size, but in many cases are relatively small; sometimes four to six or more form the periphery of the canal, sometimes three or only two. In some insects the cells are so very large that a single cell forms the entire periphery. The nuclei in the Lepidoptera (Papilio, Pontia, Cossus) are large and irregularly branched.

The excretions of the Malpighian vessels, derived from the blood and from the fat-body, are more or less fluid and granular, sometimes pulpy. From the cells they pass into the canal, thence into the intestine, and thence out of the body. How, says Kolbe, the secretion passes into the intestine, whether by the contraction of the fine fibrillæ of the peritoneal membrane, or by the external pressure of the other organs, or by the pressure of the secretory matter behind, is not yet known. Grandis observed in living Hydrophilus that the urinary tubes moved, without the muscles seeming to show what caused the motion. Moreover, the cells incessantly changed their form. At a lower temperature such motions ceased. The tracheæ, ending freely in the cells, did not anastomose. (Kolbe.)

The different colors of the tubes (white, yellow, red, brown, or green) is due to the hue of the excretions, and is independent of the color of the blood and of the urinary substances held in the secreted matter.

Schindler found that insects of different stages, collected in winter, differed very much in their urinary secretions, the tubes in the adults being entirely empty, while in the larvæ they were filled full, so that he concluded that in the former the process of excretion during the winter hibernation is very slow, but in the latter very rapid.

As to the activity of the urinary vessels the following experiments will throw some light. Tursini fed a Pimelia with fuchsin; its urinary tubes were consequently colored red. Schindler fed insects with indigo-carmine, which was excreted by the urinary tubes; Kowalevsky arrived at the same results, which seems to prove that these vessels are analogous to the kidneys of vertebrates. Moreover, Schindler injected through the side of the first abdominal segment into the cavity of the body of a Gryllotalpa a concentrated solution of sodium salt of indigotin-disulphonic acid. After one or two hours the external portion of the epithelium of the urinary vessels was stained deep blue, while the inner portion remained of the normal transparency; the nuclei being for the most part deeply stained. Between one and two days after, the staining matter had not yet wholly passed through the central canal, the surface recently stained still appearing light blue.

The solid contents of the urinary tubes consist partly of crystals, which occur singly in the epithelial cells, or form scattered masses when situated in the central canal. Besides tabular rhombic crystals, there occur concretions which contain uric acid, and probably consist of urate of soda, also octahedral crystals of chloride of soda, and quadro-pyramidal crystals of oxalate of lime. Also acicular prisms occur; besides chloride of soda, phosphates, carbonate of lime, oxalate of lime in quantity, leucine, coloring matters, etc.; while the fluid secretion also contains urea (?), uric acid, and abundant urates; uric acid crystals were precipitated by the addition of acetic acid, and by adding hydrochloric acid crystals belonging to the dimetric system were formed. The often numerous spheroidal small granules are biurate of soda and biurate of ammonia. Pale, concentrically banded concretions are leucine pellets.

According to Kölliker the contents of the urinary vessels[55] in general are: (1) round granules of urate of soda and urate of ammonia; (2) oxalate of lime; and (3) pale transparent concretions of leucine. Crystals of taurin are also said to occur. (Claus’ Zoölogy, p. 531.)

Although uric acid is characteristic of the urinary tubes, yet sometimes it is wanting in them, while uric acid substances in quantity occur in the fat-body or in the mid-intestine.

In the living insect the urinary tubes remove urates from the blood; “the salts are condensed and crystallized in the epithelial cells, by whose dehiscence they pass into the central canals of the tubules and thence into the intestine.” (Miall and Denny.)

The process of excretion is carried on not only by the urinary tubes, but also, as Cuénot has recently shown (1896) in Orthoptera, by the pericardial cells and certain cells of the fat-bodies. In the last-named cells urates are stored throughout life; the pericardial cells apparently secrete but do not store waste products, which are finally eliminated by the urinary tubes, the latter constantly eliminating waste.

=Primitive number of tubes.=—Wheeler considers the primitive number of urinary tubules to be six, other authors regarding two pairs as the primary or typical number; and while Wheeler agrees that the more ancestral tracheate arthropods had but a single pair, Cholodkowsky supposes the primitive number in insects themselves to be a single pair. This view is strengthened by the fact that Scolopendrella has but a single pair (Fig. 15).

While Peripatus has no urinary tubes, in Myriopods a single pair arises, as in insects, from the hind-intestine.

When in insects the number of these tubes is few, they are, with rare exceptions, arranged in pairs, so that Gegenbaur and others have considered this paired arrangement as the primitive one. When the tubules are very numerous in the adult, as in Orthoptera, the embryos and larvæ have a much smaller number, Wheeler stating that “in no insect embryo have more than three pairs of these vessels been found.” We have observed 10 primary tubes in the embryo of Melanopus (Fig. 347), from each of which afterwards arise 15 secondary tubules. In the Termites, only, do the young forms have more urinary tubes than the adults.

In Campodea there are about 16 urinary tubes and in Machilis either 12 (Grassi) or 20 (Oudemans); but in other Thysanura the number is much less, Lepisma having either four, six, or eight, according to different authors, and both Nicoletia and Lepismina having six, opening separately into the hind-intestine. On the other hand, these organs have not yet been detected in Japyx. Whether they exist at all in the Collembola, which are degenerate forms, is doubtful. The weight of opinion denies their existence, though they may yet be found existing in a vestigial condition. They are said by Tullberg and by Sommer to exist in Podura, but are of peculiar shape.

Coming now to the winged insects, in what on the whole is perhaps the lowest or most generalized order, the Dermaptera, the number is over 30, and their insertions regularly encircle the intestine. (Schindler.) In the most ancient and generalized family of Orthoptera, the Blattidæ, Schindler detected from 60 to 70 tubes, but in a nymph of Periplaneta not quite 10 mm. in length he found from 16 to 18, and in nymphs 4 to 5 mm. long there were only eight vessels; while Wheeler has found in the embryo of Phyllodromia germanica but four tubes. In the adult Acrydiidæ there are as many as 150, in the Locustidæ between 40 and 50, and in the Gryllidæ about 100.

The Ephemeridæ with about 40, the Odonata with 50 to 60 tubules, the Perlidæ with from 50 to 60, are polynephrious; while the Termitidæ and Psocidæ are oligonephrious, the former having from six to eight and the Psocidæ only four tubes. So also all the other orders not mentioned, except the Hymenoptera, have few of these tubes. The Hemiptera, with none in Aphidæ, a single pair in the Coccidæ, and two in all the rest of the order, have the fewest number.

In the Neuroptera there are from six to eight, while in a larva, possibly that of Chauliodes, Wheeler finds the exceptional number of seven.

The closely allied order Mecoptera (Panorpidæ), and also the Trichoptera, agree with the Neuroptera (Sialis) in having six. According to Cholodkowsky all Lepidoptera have six of these vessels, except Galleria, which has but four. He finds that in Tinea biselliella (also T. pellionella and Blabophanes rusticella) the larva has six vessels, which, however, undergo histolysis during pupation, a single pair arising in their stead. On this account he regards the primitive number of urinary tubes as two, or a single pair, this return from six vessels in the larva to two in the imago being considered a case of atavism.

In the Coleoptera, the number of urinary tubes is from four to six; in what few embryo beetles have been examined (Doryphora, Melolontha), there are six vessels, but in the embryo of Dyticus fasciventris, Wheeler has detected only four, this number being retained in the adult. He thinks that in beetles in general, a pair of vessels must be “suppressed during post-embryonic development, presumably in early larval life.”

In Diptera and Siphonaptera, the number four is very constant, there being, however, a fifth one in Culex and Psychoda (Fig. 400.)

The number of these vessels is very inconstant in the Hymenoptera, varying from six (Tomognathus, an ant, worker) to 12 (Myrmica), and in Apis reaching the number of 150.

In the embryo of the honey-bee and wall-bee (Chalicodoma), there are only four; we still lack any knowledge of the number in embryo saw-flies.

The following is a tabular view of insects with few urinary tubes (Oligonephria) and many (Polynephria). It will be seen that the number has little relation to the classification or phylogeny, insects so distantly related as the Orthoptera and Hymenoptera being polynephrious:—

Oligonephria

Collembola, 2 (Podura), Tullberg and also Sommer. Thysanura, 4 (Lepisma); in Campodea, 16; in Machilis, 12 or 20; wanting in Japyx. Psocidæ, 4. Termitidæ, 6 (many in the young, Rathke). Mallophaga, 4. Physapoda, 4. Hemiptera, 2 (Coccidæ, none in Aphidæ). Neuroptera, 6–8. (In Sialidæ and Rhaphididæ 6; in Myrmeleonidæ and Hemerobiidæ, 8). Trichoptera, 6. Mecoptera, 6. Lepidoptera, 2–4–6 (2 in Tinea, Tineola, and Blabophanes; in Pterophorus and Yponomeuta, 4).

Coleoptera, 4–6; never more.

4 Carabidæ, Dyticidæ, Staphylinidæ, Gyrinidæ, Palpicornes, Lamellicornes, Cantharidæ, Buprestidæ (in larva, 6; in beetle, 4).

6 Byrrhidæ, Nitidulidæ, Dermestidæ, Cleridæ, Meloidæ, Pyrochroidæ, Bruchidæ, Bostricidæ, Cerambycidæ Chrysomelidæ, Coccinellidæ.

Diptera, branching into 4 (Gegenbaur); in Culicidæ, and Psychoda, 5. Siphonaptera, 4.

Polynephria

Orthoptera, 100–150. (In embryo Blattids, 4; in embryo locust, 10; in nymph of Gryllotalpa, 4.) Dermaptera, “over 30” (Schindler). Perlidae, 50–60. Plectoptera (Ephemeridæ), 40. Odonata, 50–60. Hymenoptera, 20–150. (In embryo bees only 4; Cynipidæ, Ichnenumonidæ, and Formicidæ have the smallest number, 6–12.)

Here should be mentioned the singular fact discovered by Koulaguine that in the larva of Microgaster, the urinary tubes have no connection with the intestine, but open dorsally on the outside of the body on each side of the anus. Ratzeburg had stated that the last segment of the body was in the form of a vesicle. Koulaguine now shows that this vesicle is in reality the end of the intestine opening upwards; as the result of this dorsal opening of the intestine the Malpighian vessels open on the sides of the oval vent, and have no connection with the intestinal canal. Whether this is of morphological import, or is only a secondary adaptation, Koulaguine does not state, his paper being a preliminary abstract.

Wheeler thus sums up our present knowledge regarding the number and homologies of the Malpighian or urinary tubes:

1. It is very probable that the so-called Malpighian vessels of Crustacea and Arachnida are not the homologues of the _vasa Malpighi_ of the Eutracheata (insects and myriopods).

2. The Malpighian vessels of the Eutracheata arise as paired diverticula of the hind-gut and are, therefore, ectodermal.

3. In no insect embryo are more than six vessels known to occur; although frequently only four are developed.

4. The number six occurs either during embryonic or post-embryonic life in members of the following groups: Apterygota, Orthoptera, Corrodentia; Neuroptera, Panorpata, Trichoptera, Coleoptera, Lepidoptera, and Hymenoptera.

5. The number four seems to be typical for the Corrodentia, Thysanoptera, Aphaniptera, Rhynchota, Diptera, and Hymenoptera.

6. The embryonic number in Dermaptera, Ephemeridea, Plecoptera, and Odonata has not been ascertained, but will probably be found to be either four or six.

7. There is evidence that in at least one case (Melolontha), the tetranephric is ontogenetically derived from the hexanephric condition by the suppression of one pair of tubules.

8. It is probable that the insects which never develop more than four Malpighian vessels have lost a pair during their phylogeny.

9. The post-embryonic increase in the number of Malpighian vessels in some orders (Orthoptera, Odonata, Hymenoptera) is secondary and has apparently arisen to supply a demand for greater excreting surface.[56]

LITERATURE ON THE EXCRETORY (URINARY) ORGANS

=Malpighi, M.= Dissertatio epistolica de Bombyce, Societati regiæ Londini ad scientiam naturalem promovendam institutæ dicata. (Londini, 1669, 12 Pls.)

=Herold, M. J. D.= Entwicklungeschichte der Schmetterlinge. 1815.

=Rengger, J. R.= Physiologische Untersuchungen über den tierischen Haushalt der Insekten. Tübingen, 1817, pp. 82.

=Wurzer.= Chemische Untersuchungen des Stoffes in den Gallgefässen von Bombyx mori. (Meckel’s Archiv f. Physiol., iv, 1818, pp. 213–215.)

=Gaede, H. M.= Physiologische Bemerkungen über die sogenannten Gallgefässe der Insekten. (Nova Acta Acad. Caes. Leopold.-Carolin., 1821, x, Pars II, pp. 186–196.)

=Meckel, J. F.= Ueber die Gallen- und Harnorgane der Insekten. (Meckel’s Archiv, i, 1826, pp. 21–36.)

=Audouin, J. V.= Calculs trouvés dans les canaux biliaires d’un cerf volant. (Ann. sc. nat., 2 Sér., 1836, v, pp. 129–137.)

=Frey und Leuckart.= Anatomie und Physiologie der Wirbellosen. 1843.

=Dufour, L.= Mémoire sur les vaisseaux biliaires ou le foie des Insectes. (Ann. sc. nat., 1848, Sér. 2, xix, pp. 145–182, 4 Pls.)

=Karsten, H.= Harnorgane von Brachinus complanatus. (Müller’s Archiv f. Anat. und Physiol., 1848, pp. 367–374.)

=Fabre, J. L.= Étude sur l’instinct et les metamorphoses des Sphégiens. (Ann. d. sc. nat., 4 Sér., 1856, vi, pp. 137–189.)

—— Étude sur le rôle du tissu adipeux dans la sécrétion urinaire chez les Insectes. (Ibid., 4 Sér., xix, pp. 351–382.)

=Schlossberger, J. E.= Untersuchungen über das chemische Verhalten der Krystalle in den Malpighischen Gefässen der Raupen. (Archiv f. Anat. und Physiol., 1857, pp. 61–62.)

=Leydig, F.= Lehrbuch der Histiologie. 1857.

=Sirodot, S.= Recherches sur les sécrétions chez les Insectes. (Ann. sc. nat., 4 Sér., Zool., 1858, x, pp. 141–189, 251–334, 12 Pls.)

=Kölliker, A.= Zur feineren Anatomie der Insekten (Ueber die Harnorgane, u.s.w.) (Verhandl. d. Physikal.-medizin. Gesellsch. in Würzburg, viii, 1858, pp. 225–235.)

=Schindler, E.= Beitrage zur Kenntnis der Malpighischen Gefässe der Insekten. 3 Taf. (Zeitschr. f. wiss. Zool., xxx, 1878, pp. 587–660.)

=Chatin, G.= Note sur la structure du noyau dans les cellules marginales des tubes de Malpighi chez les Insectes et les Myriapodes. (Ann. d. sc. nat., 6 Sér., xiv., 1882, pp. 7, 1 Pl.)

=Witlaczil, E.= Zur Anatomie der Aphiden. (Arbeiten a. d. Zool. Instit. d. Univers. Wien., iv, 1882, pp. 397–441, 3 Taf.)

=Cholodkowsky, N.= Sur les vaisseaux de Malpighi chez les Lépidoptères. (Compt. rend. Acad. d. Sc., Paris, xcix, 1884, pp. 631–633.)

—— Sur la morphologie de l’appareil urinaire des Lépidoptères. (Archives de Biologie, 1887, vi, pp. 497–514, 1 Pl.)

=Loman, J. C. C.= Ueber die morphologische Bedeutung der sogenannten Malpighischen Gefässe der echten Spinnen. (Tijdschr. Nederl. Dierk. Ver. (2) Deel 1, 1887, pp. 109–113, 4 Fig.)

=Marchal, P.= Contribution à l’étude de la désassimilation de l’azote. L’acide urique et la fonction rénale chez les Invertébrés. (Mém. Soc. Zool. de France, 1889, iii, pp. 42–57.)

=Kowalevsky, A. O.= Ein Beitrag zur Kenntnis der Exkretionsorgane. (Biol. Centralbl., ix, 1889–90, pp. 33–47, 65–76, 127–128.)

—— Sur les organes excréteurs chez les arthropodes terrestres. (Congrès international de Zool., 2^{me} Session à Moscou, 1892, Pt. I, pp. 186–235, 4 Pls.)

=Griffiths, A. B.= On the Malpighian tubules of Libellula depressa. (Proc. Roy. Soc., Edinburgh, 1889, xv, pp. 401–403, Figs.)

=Grandis, V.= Sulle modificazioni degli epitelii ghiandolari durante la secrezione. (Atti Accad. Torino, 1890, xxv, pp. 765–789, 1 Pl.; Archiv Ital. Biol., 1890, xiv, pp. 160–182, 1 Pl.)

=Koulaguine, N.= Notice pour servire à l’histoire du développement des hyménoptères parasites. (Congrès internat. de Zool., 2^{me} Session à Moscou, 1892, Pt. I, pp. 253–277.)

=Sograff, Nicolas.= Note sur l’origine et les parentés des Arthropodes, principalement des Arthropodes trachéates. (Congrès internat. de Zool., 2^{me} Session à Moscou, 1892, Pt. I, pp. 278–302.)

=Giard, Alfred.= (Note on the urinary tubes of larval Cecidomyia. Annals Ent. Soc., France, lxii, 1893, pp. lxxx-lxxxiv, 1 Fig.)

=Wheeler, William M.= The primitive number of Malpighian vessels in insects. (Psyche, vi, May-December, 1893, Parts 1–6, pp. 457–460, 485–486, 497–498, 509–510, 539–541, 545–547, 561–564.)

=Metalnikoff, C. K.= Organes excréteurs des insectes. (Bull. Acad. imp. Sci. St. Pétersbourg, 1896, iv, pp. 57–72, in Russian, 1 Pl.)

See also the works of Straus-Dürckheim, Will (Müller’s Archiv. 1848, p. 502), Brugnatelli, Leidy, Dufour, Ramdohr, Basch, Davy, Grassi, Minot, Berlese, Adlerz, Marchal (Bull. Ent. Soc. France, 1896, p. 257); Bordas (Appareil glandulaire des Hyménoptères, 1894), also C. R. Acad. Sc. Paris, 1897.

e. Poison-glands

Poison-glands are mainly confined to the stinging Hymenoptera, i.e. certain ants, and the wasps and bees, but also occur in the mosquito, while many, if not most bugs, seem to instil a drop of poison into the punctured wounds they make.

In the honey and other bees the poison apparatus consists of two poison-glands whose secretion passes by a single more or less convoluted efferential duct into the large poison-sac, and thence by the excretory duct, which is enlarged at the base of the sting (Figs. 194, 195), out through the sting by the same passage as the eggs. According to Carlet, the poison apparatus of bees consists of two kinds of glandular organs, of which one kind secretes a feebly alkaline fluid, the other an acid product. The poison is only effective when both fluids are mixed. The resultant venom is always acid. The action of this venom upon some animals, as rabbits, frogs, and certain beetles, is slight; but the domestic fly and the flesh-fly are immediately killed by it. The inoculation of a fly with the secretion of one of the glands does not produce death until after a considerable time, but death follows very quickly if the same fly is subjected to a second inoculation, this time with the secretion of the other gland. The alkaline glands are in bees and all poisonous Hymenoptera strongly developed, but become vestigial in those forms which sting their prey to serve as food for their larvæ. The poison which the solitary sand and wood wasps and Pompilidæ inject into their victims only paralyzes them.

Bordas has found both the alkaline gland (gland of Dufour) and the acid gland to occur in a hundred species of Hymenoptera, including not only Aculeata, but also Ichneumonidæ (Fig. 348), Tenthredinidæ, and they may be safely said to be of general occurrence. The acid gland consists of three parts, the glandular portion, the reservoir for the poison, and the secretory canal. The alkaline gland is an irregular tube, with a striated surface and without a reservoir. In most Hymenoptera there is still a third gland, which is unpaired, granular, rectangular or lanceolate, with a short filamentous duct which opens beside the orifice of the alkaline glands.

The poison in ants, wasps, and bees consists of two substances, i.e. formic acid and a whitish, fatty, bitter residue in the secretion of the glands; the corroding active formic acid is the essential part of the poison. (Will.)

In Melipona the sting and poison-glands are aborted; in certain ants (Formica, Lasius, etc.) the sting is wanting, but the poison-sac is extraordinarily large.

Bordas finds in various species of Ichneumon three kinds of glands opening into the base of the sting. The first two correspond to the acid (Fig. 348, G.A) and alkaline (G.A) glands of bees and wasps (Vespidæ, etc.), and the third (G.ac) is situated between the two lateral muscular bundles which attach the base of the sting to the last abdominal segment. The poison-reservoir (Fig. 348, V) is recognized by its yellow color and diaphanous and striated appearance. It is situated on the left of the hind-intestine, a little in front of the rectum. The tubular gland (Ga) or alkaline gland of aculeate Hymenoptera is remarkably large; it is situated on the left side of the body. The accessory gland (G.A) is elongated, triangular, flat, its duct opening at the base of the alkaline gland; it is formed of small spherical cells. Bordas has met with well-developed poison-glands in forty species belonging to the Terebrantia, including that of Tenthredo, Emphytus, as well as various genera of Ichneumonidæ, but in all these species the accessory gland was wanting.

Under the name of cephalic glands (Fig. 349), Locy describes a pair of glands in the head of Nepidæ. The epithelial or secreting cells are 8–sided (Fig. 350). “When these insects are irritated,” he says, “a secretion is freely thrown out around the base of the beak, which produces death very quickly when introduced on a needle point into the body of an insect.” He infers that the cephalic glands may be the source of this poisonous secretion. The poisonous salivary fluid of the larva of Dyticus is referred to on p. 324.

That the mosquito injects poison into the wound it makes has been proved by Macloskie, who discovering fine droplets of a yellow oily-looking fluid escaping from the end of the hypopharynx, afterwards detected the poison-glands. It appears that the two salivary glands are subdivided, each into three lobes, the middle of which (Fig. 351, pg) differs from the others in having evenly granulated contents and staining more deeply than the others. Having examined the preparations, we agree with the discoverer that these lobes secrete the poison. The poison is diluted by the secretion of the salivary lobes, and the two efferent ducts, one from each set of glands, “carry forward and commingle the venomo-salivary products in the main duct; and the stream is then carried by the main duct to the reservoir at the base of the hypopharynx.”

f. Adhesive or cement-glands

Dewitz has discovered in ants and bees, in close connection with the poison-glands, and like them discharging their secretion through the sting, cement-glands. They arise by budding at the base of the poison-glands.

The two glands in these Hymenoptera correspond to the tubular glands of the Orthoptera, which open at the base of the inner sheath of the ovipositor (Fig. 299, sb), so that the secretion flows out through it as the poison of bees, etc., out of the sting. The use of the secretion of these glands is either to glue the eggs together, or to afford material for the egg-case of cockroaches and Mantidæ and the gummy egg-case of the locusts, etc. The contents of the cement-glands serves for the fixture of the eggs after deposition. In the stinging Hymenoptera one of the cement-glands is an accessory gland; the other becomes the poison-sac. The cement-glands are in the Hemiptera only short blind sacs, in the Lepidoptera and Diptera long convoluted tubes, tubular and branched in the Coleoptera, or richly branched in the Ichneumonidæ and Tenthredinidæ. In the cockroach there are two cement-glands, but the right one is probably of no functional importance. The left one is filled with a milky substance, containing many crystals and a coagulable fluid, out of which the egg-capsule (oötheca) is formed. (Miall and Denny.) In the locusts the sebific or cement-gland (Fig. 298, sb) secretes a copious supply of a sticky fluid, which is poured out as the eggs pass out of the oviduct and agglutinates the eggs into a mass, forming a thin coating around each egg, which from the mutual pressure of the eggs causes the tough coating to be pitted hexagonally. In other insects also (Trichoptera, Chrysopidæ, Lepidoptera, etc.) there are similar secretions for the protection or fastening of the eggs when laid.[57] The Trichoptera lay their eggs either in or on the surface of the water in bunches or in strings or in annular gelatinous masses on stones or on plants. This jelly-like substance is secreted by two highly developed paired anal glands. (Weltner, in Kolbe, p. 621.) Also in certain dragon-flies (Libellula, Diplax, and Epitheca) the eggs are laid in jelly-like masses.

With a similar secretion, spun from the end of the abdomen, the Psocidæ cover their little bunches of eggs laid on the under side of leaves; and the silk thread forming the egg-sac of the great water-beetle (Hydrophilus) is secreted from such anal glands.

g. The wax-glands

Besides the honey-bee, which secretes wax in little scales on the under side of the abdomen, the bodies of many other insects, such as the plant and bark lice, as well as the Psyllidæ, Cicadidæ (especially Flata and Lystra), are covered with a waxy powder, or as in Chermes, Schizoneura, Flata, etc., with wool-like filaments of wax.

The wax is secreted by minute unicellular dermal glands, which in the lower insects (Hemiptera) are distributed nearly all over the body, but in the bees are restricted either to the under (Apis, Fig. 352) or upper side (Trigona) of the end of the abdomen.

The wax-glands of Pemphigus, Chermes, etc., lie under the little warts, seen in Lachnus strobi, the white-pine aphis, to be distributed in transverse lines across the back and sides of the abdominal segments (Fig. 353). These warts are surrounded by a chitinous ring, and divided into delicately marked areas. Through the delicate numerous pits in the chitinous membrane of these areas the little waxen threads project, since under each area ends a duct leading from a large glandular cell, which is a specially modified hypodermis cell (Claus). The wax threads are hollow, and all those arising from a single glued cell form a bundle, whose threads separate from each other and form a white woolly down or bloom covering the body. Witlaczil also shows that gall-forming Aphids secrete a wax-like substance, which, during the movements of the insects in the gall, is rubbed off, becoming a watery layer mixed with the fluid excrement, which forms a spherical impervious layer lining the gall, and thus rendering possible the mode of life of the gall-lice.

In the Psyllidæ Witlaczil has discovered wax-glands which also secrete slender waxen threads. They are situated in groups of two or three at the end of the abdomen near the anus, and arise from hypodermis cells. The wax threads surround the liquid excrement as it passes out of the vent, covering it with a continuous layer of wax. The excrement accordingly is discharged very slowly and gradually, in sausage-shaped masses slightly strung together and rolled into close spirals. The body becomes unavoidably smeared with the sticky excrement, since it is not entirely covered by the waxy layer. Moreover, in the larvæ of many Psyllidæ waxen threads are formed on the upper side of the abdomen; they are for the most part tightly curled or frizzly, like wool, and form, though partly torn, a waxen coat, chiefly on the side and back of the thorax and abdomen. The insects appear therefore as if covered with dust. The mature animals of many species are also covered with a waxen down. The wax threads rapidly dissolve and disappear in alcohol. From a wax-like substance more or less easily dissolved in alcohol arise peculiar hair-like structures which, in the larvæ of Psyllidæ, are situated on the side and end of the body and also on the rudiments of the wings. They are readily distinguished from ordinary hairs, as they arise from glandular cells, and are of very different lengths, more or less like bristles, but hollow, and very brittle. They are leaf-like in the first nymphal stages of Trioza rhamni, but in following stages become narrow and form a row around the entire periphery of the body.

The waxen dorsal shield which protects the body of bark-lice (Coccidæ) is a similar product.

Witlaczil has described the way it is formed in Aspidiotus and Leucaspis. The freshly hatched nymph shows no signs of a waxy secretion. But eventually waxen threads arise first on the hinder and anterior end of the body, and then over the whole surface. These threads interlace into a sort of felting and thus form the shield, which is usually much larger than the body and lies closely upon it. The shield is formed after the first moult. It is noteworthy that these threads are matted together to form as thick a tissue as that of the shield itself. The shield is whitish or gray and rather thin. On the thinnest part of the edge the single threads may be drawn out. The growth of the shield advances with the increase in size of the nymph around the entire edge, but is greatest behind. The first two larval skins are retained on the back under the shield. Also a very thin waxen pellicle remains on the resting place of the insect when it is raised. The wax-glands open in the pitted fields, and appear as clear brownish cells which are distinguished from the ordinary hypodermis cells by their greater size. (Witlaczil. Compare also Fig. 354.)

The wax-glands in the honey-bee are scale-shaped organs situated on the under side of the four last abdominal segments (Fig. 355). These secrete the wax, which appears as whitish scales, and secretion is only possible when the bees have sufficient honey and pollen. The wax is secreted by the hypodermal cells rather than by glands within the abdominal cavity; the wax traverses the cuticular layer, and accumulates on its outer surface (Carlet). According to Fritz Müller, in the stingless bees (Trigona) which he observed, the wax-glands are situated on the back of the abdomen, but Ihering states that in many species of Trigona and Melipona there are also slightly developed wax-organs on the ventral side.

It has been found that certain caterpillars secrete wax. Thus the cells of the Tortrix of the fir (Retinia resinella) formed of resin are lined with wax, as on dissolving away the resin with alcohol, Dr. Knaggs found a slight film of wax; also a secretion of wax has been detected in the larva of a butterfly (Parnassius apollo). The bodies of certain saw-fly larvæ are covered with a white powdery secretion, while the remarkable larva of a Selandria is clothed with snow-white, long, flocculent, waxy masses, nearly concealing the body (Fig. 356).

h. “Honey-dew” or wax-glands of Aphids

The so-called “honey-dew” of Aphids which oozes from two wart-like tubercles or tubes situated near the end of the body, is secreted by hypodermal unicellular glands which open into a modification of a pore-canal, the tube itself being an outgrowth of the cuticula.

Witlaczil states that both in the “honey” tubes and in the body beneath, the sugary matter exists in cells of the connective tissue in the form of granules. “These large ‘sugar-cells’ in contact with the air undergo destruction, while the sugar crystallizes into needles, and thus each cell is transformed into a radiated crystalline mass.”

“A muscle extends from a horseshoe-shaped place (a valve?) in the middle of the flat terminal plate of the honey tube, through this and down through the abdomen to the ventral surface. By this muscle the honey tube is at times erected, and we then find, as also when we lightly press the body of the insect, lumps of crystallized sugar which have been expressed through the tips of the honey tubes.” (Zool. Anzeiger, 1882, p. 241.)

Busgen, after careful research, denies that this is a sugar, but claims as the result of chemical analysis, that it is more like wax. He observed that on reaching the air the drops issuing from the “nectary” or “honey” tube stiffened almost instantly into a wax-like mass, which was easily crushed between the teeth, and had no taste at all. No sugar-like substance or urea could be detected. He therefore concludes that the secretion in question should be regarded as a wax-like mass, which agrees well with Witlaczil’s anatomical observations, and confirms the statements of previous observers. Thus, as early as 1815, Kyber stated that the Aphides expelled an excrementitious substance through the “sap tubes.” Burmeister states that the tubes give out a fluid which “dries gumlike, but, so far as I have observed, has no peculiar taste.” Réaumur, and also Kaltenbach, state that the “honey” does not issue from the tubes, but from the anus. Lastly, Forel emphatically states that “the two dorsal tubes of Aphides do not secrete a sweet fluid, but a gluey wax, which is not sought by the ants. Moreover the shield-lice and many leaf-lice have no such tubes, but yet are often sought by ants. The drops of sugar which the ants lick up are rather the excrement of the insects in question.” Hence the opinion first stated by Linné, that a sweet fluid is secreted by Aphides, must be abandoned.

On the other hand, Busgen, after careful observations, finds that the use of the sticky, waxen secretion is in reality a protective one, as he observed that when a larval Chrysopa rudely attacks the Aphides, they smear its face with the sticky wax, causing at least a momentary interruption in its attacks. He also observed that Aphides when invaded by coccinellid larvæ set their tubes in motion and besmear their heads and front part of the body. He thus seems to establish the fact that these tubes secrete a protective, sticky fluid.

i. Dermal glands in general

We have seen that certain of the hypodermal cells may be modified or specialized to form secretory unicellular glands. Such are those (trichogens) which secrete chitinous setæ, hairs, and spines, certain setæ in some insects being hollow and containing a poison (p. 187); others secrete wax, certain ones in Aphids “honey-dew”; in some cases dermal glands may excrete protective, sticky, or otherwise offensive matters, or may be depuratory, or facilitate the process of moulting.

There are other minute, unicellular, or compound dermal glands whose function is unknown.

Dermal glands may be segmentally or serially arranged. Thus Verson has detected a series of one or two pairs of unicellular glands near the stigmata in each thoracic, and the first eight abdominal segments of the silkworm (B. mori). In the earliest stages of growth of the caterpillar they give out oxalate of lime, and in later stages uric acid. They thus appear to act interchangeably with the urinary tubes, as excretory organs. They do not, however, carry their products directly outwards, but leave them between the hypodermis and cuticula, in order to facilitate the sloughing off of the latter in the process of moulting.

LITERATURE ON THE SECRETORY GLANDS

a. General

=Sirodot, S.= Recherches sur les sécrétions chez les Insectes. (Ann. sc. nat., 4 Sér., Zool., 1858, x, pp. 141–189, 251–328, 12 Pls.)

=Gazagnaire, G.= Des glandes chez les Insectes. (Compt. rend. Acad. Sc., Paris, 1886, cii, pp. 1501–1503; Annal. Soc. Ent. France, 1886, Bull., pp. 104–106.)

=Leydig, F.= Beitrage zur Anatomie und Histiologie der Insekten. 1887.

=Hanow, Karl.= Ueber Kerfabsonderungen und ihre Benutzung im eigenen Haushalte. (Programm des Realprogymnasiums zu Delitzsch für das Schuljahr 1889, xc; Delitzsch, 1890, pp. 3–22.)

=Verson, E.= Di una serie di nuovi organi escretori scoperti nel filugello. (Publ. R. Stazione Bacologica di Padova, v, 1890, pp. 30. 4 Pls.)

—— Altre cellule glandulari di origine postlarvale. (Ibid., vii, 1892, pp. 16, 1 Pl.)

—— =ed E. Bisson=. Cellule glandulari ipostigmatiche nel Bombyx mori. (Ibid., vi, 1891.)

=Borgert, H.= Die Hautdrüsen der Tracheaten. Jena, 1891, pp. 80.

=Batelli, Andrea.= Di una particolarità nell’ integumento dell’ Aphrophora spumaria. (Monitore Zool. Ital., 1891, Anno ii., pp. 30–32, dermal gland in last segment.)

=Koschewnikow, G. A.= On a new compound dermal gland found in the sting of the bee. (Journal of the Zoological Section of the Society of the Friends of Natural Science. Moscow, ii, Nos. 1, 2, 1892, p. 36. Preliminary notice. In Russian.)

=Willem, V., et H. Salbe.= Le tube ventral et les glandes céphaliques des Sminthurus. (Ann. Soc. Ent. Belg., 1897, xli, pp. 130–132.)

=Henseval, Maurice.= Les glandes à essence der Cossus ligniperda. (La Cellule, 1897, xii, pp. 19–26, 27, 29.)

—— Recherches sur l’essence der Cossus ligniperda. (La Cellule, 1897, xii, pp. 169–181, 183.)

b. Poison-glands

=Macloskie, George.= The poison-apparatus of the mosquito. (Amer. Nat., xxii, 1888, pp. 884–888. 1 Fig. Also in Science, 1887, p. 106.)

=Beyer, Otto W.= Der Giftapparat von Formica rufa, ein reduziertes Organ. (Jena. Zeitschr. f. Wissens, xxv, 1891, pp. 26–112, 2 Taf.)

=Bordas, L.= Sur l’appareil venimeux des Hyménoptères. (Comtes rend., cxviii, 1894, pp. 296–299 and 873–874; also Zool. Anzeiger, xvii Jahrg., 1894, pp. 385–387, Figs.)

—— Appareil glandulaire des Hyménoptères. (Glandes salivaires; Tubes de Malpighi et glandes venimeuses.) Paris, 1894, pp. 362, 11 Pls.

—— Description anatomique et étude histologique des glandes à venin des insectes Hyménoptères. Paris, 1897, pp. 53, 2 Pls.

See Forel (p. 186); also the standard authors, Kolbe, etc.; also Locy (Amer. Nat., xviii, 1884, p. 355), Nagel (p. 324), Fenger.

c. Wax-glands

=Brandt und Ratzeburg.= Medicinische Zoologie, ii, 1830, p. 179. Taf. xxv, Fig. 18.

=Treviranus, George R.= Ueber die Bereitung des Wachses durch die Bienen. (Zeitschrift für Physiologie, etc., iii, 1832, pp. 62, 225.)

=Dufour, L.= Note anatomique sur la question de la production de la cire des abeilles. (Comptes rend. Acad. Sc., Paris, 1843, xvii, pp. 809–813, 1248–1253; Revue Zool. 1843, l’Institut, 1843, xi.)

=Dujardin, F.= Mémoire sur l’étude microscopique de la cire, etc. (Ann. Sc. Nat., xii, 1849, pp. 250–259.)

=Tarzione-Tozzetti, H.= Studii sulle cocciniglie. Milano, 1867, 7 Pls.

—— Sur la cire qu’on peut obtenir de la cochenille du figuier (Coccus caricæ). (Comptes rend. Acad. Sc., Paris, lxv, 1867, pp. 246–247.)

=Claus, C.= Ueber die wachsbereitenden Hautdrüsen der Insekten. (Sitzungsber. Gesells. z. Beförd. d. Gesammt. Naturw. zu Marburg, June, 1867, No. 8, pp. 65–72.)

=Witlaczil, E.= Die Anatomie der Psylliden. (Zeitschr. wissens. Zool., xlii, 1885, pp. 582–586.)

—— Zur Morphologie und Anatomie der Cocciden. (Zeitschr. wissens. Zool., xliii, 1886, pp. 149–174, 1 Taf.)

=Ihering, H. von.= Der Stachel der Meliponen. (Ent. Nachrichten, xii Jahrg., 1886, p. 185.)

=Carlet, G.= Sur les organes sécréteurs et la sécrétion de la cire chez l’Abeille. (Comptes rendus, cx, pp. 361–363, 1890.)

—— La cire et ses organes sécréteurs. (Le Naturaliste, 1890, pp. 149–151, 2 Figs.)

Also the works of Siebold, Cheshire, Kolbe, Howard and Marlatt (Bull., 3, N. S., Div. Ent. U. S. Dept. Agr., 1896, p. 40), Knaggs, Berlese.

d. Wax-like glands of Aphides

=Huber et Forel, A.= Études myrmécologiques, 1875.

=Witlaczil, E.= Zur Anatomie der Aphiden. (Zool. Anzeiger, v Jahrg., 1882, pp. 239–241; Arbeiten a. d. Zool. Institut der Univ. Wien., iv, 1882, pp. 397–441, 3 Taf.)

=Busgen, M. J.= Der Honigtau. Biol. Studien an Pflanzen u. Pflanzenlause. (Jena. Zeitschrift, xxv, 1891, pp. 339–428.)

DEFENSIVE OR REPUGNATORIAL SCENT-GLANDS

While these eversible glands are not found in marine or aquatic arthropods such as Crustacea or Merostomata (Limulus), they are often present in the air-breathing forms, especially insects. In the winged insects they are of frequent occurrence, existing under great variety of form, varying greatly in position, and appearing usually to be in immediate relation with their active volant habits. Their presence is in direct adaptation to the needs and habits of their possessors, and being repellent, warning, or defensive structures, the odors they secrete being often exceedingly nauseous, they appear to have been called into existence in direct response to their biological environment. The fact that these singular organs do not exist in marine or aquatic Crustacea suggests that the air-breathing, aërial, or volant insects by these eversible glands, usually in the form of simple evaginable hypodermic pouches, are enabled to protect themselves by emitting an infinitesimal amount of an offensively odorous fluid or ether-like spray which charges the air throughout an extent of territory which may be practically illimitable to the senses of their enemies. The principle is the same as in the mephitic sulphuretted oil ejected by the skunks, the slight quantity these creatures give out readily mixing with and charging the atmosphere within a radius of many miles of what we may call the centre of distribution.

As is now well known, the very delicate, attenuated highly volatile odors exhaled are perceived by insects with extreme ease and rapidity, the degree of sensitiveness to such scents being enormously greater than in vertebrates, their organs of sense being developed in a corresponding degree. Professors Fischer and Penzoldt, of Erlangen, have recently established the fact that the sense of smell is by far the most delicate of the senses. They find that the olfactory nerve is able to detect the presence of 1⁄2,760,000,000 of a grain of mercaptan.[58] The smallest particle of matter that can be detected by the eye is sodium, when observed by the spectroscope, and this particle is 250 times coarser than the particle of mercaptan which can be detected by the human nose.

In those Arachnida which are provided with poison-glands, these scent-glands are absent, but in certain Acarina and Linguatulidæ, which have no poison-glands, there are various oil-glands, stigmatic glands, as well as scent-glands, and in seizing a Thelyphonus with the forceps we have observed it to send out from each side of the body a jet of offensive spray.

We not infrequently find in myriopods (Polydesmidæ, Julidæ, and Glomeris) repugnatorial or the so-called cyanogenic glands, which are either paired, opening on the sides of the body, or form a single row along the median line of the under side of the body. Leidy describes and figures the spherical glands of Julus marginatus, of which there are 50 pairs. These glands have been regarded as modified nephridia, but are more probably coxal glands, and the homologues of the parapodial glands of annelid worms.

=Eversible coxal glands.=—True coxal glands occur in Scolopendrella immaculata on the 2d to 11th segment, on the inner side of the base of the legs (Fig. 15, c.g.). Homologous glands also occur in the same position in Campodea staphylinus (also in C. cookei and C. mexicana) on the 1st to 8th abdominal segments, and Oudemans has described a pair of eversible sacs on each side of segments one to seven of Machilis. These eversible sacs in the synapterous insects are evidently modified coxal glands, and are probably repugnatorial as well as respiratory in function.

The apparatus consists of an eversible gland, composed of hypodermic cells, usually retracted by a slender muscle and with an efferent passage, but the glands vary greatly in shape and structure in different insects. In some cases these fœtid glands appear not to be the homologues of the coxal glands, but simply dermal glands.

These repugnatorial glands are of not infrequent occurrence in the lower or more generalized winged insects, and in situation and appearance are evidently the homologues of the coxal glands of the Symphyla and Synaptera.

Fœtid glands of Orthoptera.—In the ear-wigs (Forficula and Chelidura) Meinert has detected a pair of what he calls fœtid glands at the posterior margin of the dorsal plates of the 2d and 3d abdominal segments.

Vosseler also describes the same glands as consisting of a retort-shaped sac, in whose walls are numerous small hypodermal cells and large single glandular cells provided with an efferent passage, the fluid being forced out by the pressure of the dermal muscles, one acting specially to retract the gland. The creature can squirt to a distance of 5 and even 10 cm. (4 inches) a yellowish-brown liquid or emulsion with the odor of a mixture of carbolic acid and creosote.

The large eversible dorsal glands of the Blattidæ, since they contain numerous hairs, which, when everted, are fan-like or like tufts, serve, as in the spraying or scent apparatus, to disseminate the odor, and might be classified with the alluring unicellular scent-glands or duftapparat of other insects, as they are by some authors; but as the glands are large and compound they may prove to be the homologues of the coxal glands rather than of the dermal glands.

Evaginable organs in the Blattids were first observed by Gerstæcker in both sexes of Corydia; they are yellowish white, covered with hairs, and are thrust out from between the dorsal and ventral plates of the 1st and 2d abdominal segments.

In the cockroach (P. orientalis) Minchin detected two pouch-like invaginations of the cuticle, lying close on each side of the middle line of the body between the 5th and 6th tergites of the abdomen. They are lined by a continuation of the cuticle, which forms, within the pouches, numerous stiff, branched, finely pointed bristles, beneath which are a number of glandular epithelial cells. In the male nymph of P. decorata he also found beside these glandular pouches “an additional gland, opening by a tubular duct under the intersegmental membrane between the 5th and 6th terga above the glandular pouch of each side, and extending forward into the body cavity. The gland and its duct are proliferations of the hypodermis, and there is no invagination of the cuticle.” These eversible glands are most complicated in Phyllodromia germanica. While it is absent in the female, in the male it is relatively of enormous size, extending over the 6th and 7th somites, as well as projecting far into the body cavity (Minchin). Haase states that these glands become everted by blood-pressure and give out the well-known disagreeable smell of these insects. He states that in the male of P. germanica the dorsal glands in the 6th and 7th abdominal segments are without hairs and produce an oily secretion; they function as odoriferous organs in sexual union.

In the male of another Blattid (Aphlebia bivittata) of the Canary Islands, Krauss has detected two yellowish dorsal sacs 1.5 mm. in length, opening out on the 7th abdominal segment, and filled full of long yellowish hairs, the ends directed towards the opening, where they form a thick tuft. These eversible glands lined with hairs appear to be closely similar to the long slender eversible hairy appendages or scent organs of certain Arctian and Syntomid moths. (Fig. 359.)

We have found the external median wart with lateral lids or flaps in between the 5th and 6th tergites of Platyzosteria ingens Scudder, a large wingless Blattid living under the leaf scars of the cocoanut tree in Southern Florida (Fig. 360), but were unable to detect them in Polyzosteria or in the common Blabera of Cuba, or in another genus from Cordova, Mexico.

In another group of Orthoptera, the Phasmidæ, occur a pair of dorsal prothoracic glands, each opening by a pore and present in both sexes. In the walking-stick, Anisomorpha buprestoides, ♂ and ♀, these openings are situated on each side of the prothorax at its upper anterior extremity, situated at the bottom of a large deep pit. When seized it discharged a “milky white fluid from the pores of the thorax, diffusing a strong odor, in a great measure like that of the common Gnaphalium or life everlasting” (Peale in Say’s American Entomology, i, p. 84). Boll states that the females when captured “spurt from the prothorax, somewhat after the manner of bombardier beetles, a strong vapor, which slightly burnt the skin; when the females were seized by the males a thick fluid oozed from the same spot.” Scudder describes these glands in another Phasmid (Autolyca pallidicornis) as two straight, flattened, ribbon-like bodies, with thick walls, broadly rounded at the end, lying side by side and extending to the hinder end of the mesothorax. In Anisomorpha buprestoides the glands are of the same size and shape (Scudder). In Diapheromera femorata the repugnatorial foramina are very minute, and the apparatus within consists of a pair of small obovate or subfusiform sacs, one on each side of the prothorax, about 1 mm. in length, with a short and very slender duct opening externally at the bottom of the pit (Scudder).

In the Mantidæ these seem to be genuine coxal glands, as there is a pair situated between the coxæ of the first pair of legs. An evaginable organ like a wart, with a glandular appearance, occurs on the hind femora of the Acrydiidæ in a furrow on the under side, into which the tibia fits, about one-fourth from the base (Psyche, iii, p. 32).

In the male cricket, the anal odoriferous glands are small lobes opening into a reservoir on each side of the rectum (Dufour). Homologous glands also occur in the Coleoptera (Fig. 302, l and 317, s).

Most Hemiptera or bugs send out a fœtid or nauseous odor due to a fluid secreted by a single or double yellow or red pear-shaped gland, situated in the middle of the mesothoracic segment, and opening between the hinder or third pair of coxæ. In Belostoma Leidy describes these glands as consisting of two rather long cœcal tubes situated in the metathorax, beneath the other viscera, extending backwards into the abdomen, and opening between the coxæ of the third pair of legs. Locy states that the smell arising from these glands is pleasant, resembling that of well ripened pears or bananas. Other bugs, moreover, emit an agreeable odor, that of Syromastes resembling that of a fine bergamot pear. (Siebold.) The fluid given out by the European fire-bug (Pyrrhocoris apterus) has a sweetish smell, like ether. In the nymph there are three pairs of dorsal glands, on abdominal segments 2–5, which are atrophied in the mature insect. In the bed-bug, the nymph has three odoriferous glands each with paired openings in the three basal abdominal segments respectively, and situated on the median dorsal line, being arranged transversely at the edge of the tergites; but after the last moult these are aborted, and replaced by the sternal metathoracic glands (Künckel). Gissler has detected a pair of glands in Lachnus strobi (Fig. 361).

=Anal glands of beetles.=—Certain beetles are endowed with eversible repugnatorial glands. Eleodes gigantea and E. dentipes of both sexes are said by Gissler to possess these glands. When teased “they stand on their anterior and middle legs, holding the abdomen high up and spurting the contents of the glands right and left.” The glands (Fig. 366, 1) are two reddish brown, somewhat bilobed sacs, and extend from the base of the last up to the middle of the 2d abdominal segment, with an average length of 6.5 mm. The liquid stains the human skin, has an acid reaction, with a peculiar, “intensely penetrant odor, causing the eye to lachrymate. It is soluble in water, alcohol, and ether. Boiled with concentrated sulphuric acid and alcohol an ethereal aromatic vapor is produced, indicating the presence of one or more organic acids, though neither formic or acetic acid could be detected.” Williston has observed the same habits in seven other species of Eleodes, all ejecting a pungent vile-smelling liquid, one species (E. longicollis) ejecting a stream of fluid from the anal gland, backwards sometimes to the distance of 10 cm. or more, and he regards these beetles as “the veritable skunks of their order.” Leidy briefly describes the odoriferous glands of Upis pennsylvanica.

The anal glands consist, according to Meckel and also Dufour, of two long, simple, flexuous cœca with reservoirs having two short excretory ducts situated near the anus (Siebold).

Glands like those of Eleodes found in Blaps mortisaga are described in detail by Gilson (Fig. 366, 2). They form two pouches or cuticular invaginations situated in the end of the abdomen on the sides of the end of the intestine and unite on the median line underneath the genital organs, forming a very short tube with a chitinous wall, continuous with the cuticle of the last abdominal segment. Into each pouch open a large number of fine slender lobules varying in shape, giving a villous aspect to the surface. These lobules are composed of as many as fifty unicellular glands, each of which is composed of four parts: (1) A radiated vesicle, (2) a central sac, giving rise (3) to a fine excretory tube, and (4) a sheath near the origin of the excretory tube. These are all modifications of the cytoplasm of the cell with its reticulum; the nucleus with its chromosomes is also present, but situated on one side of the central sac. The fine excretory tubules form a bundle passing down into the mouth of each lobule.

Similar glands, though usually smaller, which have not been carefully examined, occur in Carabus (Fig. 300, 3) and Cychrus, which eject from the vent a disagreeable fluid containing butyric acid (Pelouse). The bombardier beetle Brachinus, with its anal glands, ejects a jet of bluish vapor accompanied with a considerable explosion, which colors the human skin rust red; it is caustic, smells like nitrous acid, and turns blue paper red. Westwood states that individuals of a large South American Brachinus on being seized “immediately began to play off their artillery, burning and staining the flesh to such a degree that only a few specimens could be captured with the naked hand, leaving a mark which remained for a considerable time.” The fluid ejected by another species, in Tripoli, blackened the fingers of the collector. “It is neither alkaline nor acid, and it is soluble in water and in alcohol.” (Kirby and Spence, iv, p. 149.)

Species of other genera (Agonum, Pheropsophus, Galerita, Helluo, Paussus, Ozæna) are also bombardiers, though less decidedly so than Brachinus. A Paussid beetle (Cerapterus) ejects explosively a fluid containing free iodine (Loman), while Staphylinus, Stenus, Ocypus olens, Lacon, etc., have similar anal fœtid glands, the liquid being more or less corrosive. The secretion of Mormolyce phyllodes is so corrosive that it is said to paralyze the fingers for 24 hours after. (Cuénot.)

The two pairs of remarkably large, soft, eversible, forked, orange-yellow glands of the European genus Malachius, are thrust out from the side of the 1st and the 3d thoracic segments. They are everted by blood-pressure, and retracted by muscles. The larva of Hydrophilus piceus ejects by the anus a black, fœtid fluid.

Claus has shown that the larva of Lina populi and other Chrysomelidæ possess numerous minute, eversible glands in each of the warts on the upper surface of the body, each gland containing a whitish, repellent fluid smelling like the oil of bitter almonds and containing salicylic acid derived from its food-plant, which issues as pearl-like drops. Candèze thinks the fluid may contain prussic acid. The fluid is secreted by a variable number of glandular cells, each provided with an efferent duct. The larvæ of saw-flies, notably Cimbex americana, also eject droplets of a clear fluid from non-evaginable glands situated near each stigma (Chlolodkovsky).

=The blood as a repellent fluid.=—In this connection it may be mentioned that though there are no special glands present, many beetles emit drops of blood from the femoro-tibial joints of their legs as a means of defence. Such are the oil-beetles (Meloë), Cantharis, Lytta. The cantharadine secreted by these beetles, according to Beauregard, is an efficient means of defence, as birds, reptiles, and carnivorous insects will not usually attack them. This substance is formed in the blood and also in the genital organs, and is so extremely caustic that scavenger insects which feed upon their dead bodies will leave untouched the parts containing cantharadine, and if May-beetles or mole-crickets are washed with the blood of Meloë or with cantharidate of potassa, it will for several days render them safe from the attacks of the carabids which usually prey upon them. The eggs even after deposition are strongly vesicant, and are thus free from the attacks of egg-eating insects (Cuénot). The Coccinellidæ are also protected by a yellow, mucilaginous, disagreeable fluid oozing out of the ends of the femora; in our common, two-spotted lady-bird (C. bipunctata) the yellow fluid is disagreeable, smelling like opium. Lutz has found that the blood in Coccinellidæ passes out through a minute opening situated at the end of each femur (Fig. 362). The blood is very repellent to insectivorous animals.

The Dyticidæ eject from the anus a colorless, disagreeable fluid, while these beetles, and especially the Gyrinidæ, when captured send out a milky fluid which appears to issue from the joints of the body. The Silphidæ throw out both from the mouth and vent a fœtid liquid with an ammoniacal odor. They possess but a single anal gland, the reservoir opening on one side of the rectum (Dufour).

Other malodorous insects have not yet been investigated; such are the very persistent odors of lace-winged flies (Chrysopa).

More agreeable secretions, but probably formed by similar glands, is the odor of rose or hyacinth given out by Cicindelæ, or the rose fragrance exhaled by the European Aromia moschata.

=Eversible glands of caddis-worms and caterpillars.=—Gilson, while investigating the segmentally disposed thoracic glands of larval Trichoptera, has found in the larva of Limnophilus flavicornis that the sternal prothoracic tubercle gives exit to an underlying tubular gland. In Phryganea grandis each thoracic sternum affords an exit to an eversible gland. Many caterpillars, as our subjoined list will show, are very well protected by eversible repugnatorial glands situated either in the under or upper side of the body. Since the time of De Geer (1750) the fork-tailed larva of Cerura has been known to throw out a secretion, which was described by Bonnet in 1755 as a true acid, sharp, sour, and biting. This spraying apparatus in Cerura (Harpyia) vinula has been well described by Klemensiewicz (Fig. 366, 4), though Rengger in 1817 noticed the general form of the secretory sac, and that it opens out in two muscular eversible tubes, out of which the secretion is ejected.

The fork-tailed larva of Macrurocampa marthesia, which is much like that of Cerura, when teased sends out a jet of spray to the distance of nearly an inch from each side of the neck. While examining the very gayly-colored and heavily-spined caterpillars of Schizura concinna we observed that when a fully-grown one was roughly seized with the forceps or fingers it sent out a shower of spray from each side of the prothoracic segment, exactly like that of Cerura and Macrurocampa.

In the European Cerura vinula the apparatus consists of a single sac, which opens by a narrow transverse slit on the under side of the neck, out of which is rapidly everted four lateral tubes, two on each side (Fig. 366, 4, t), which are withdrawn within the opening by the contraction of several fine muscles. The apparatus in the American C. multiscripta is as in the European C. vinula. In a living specimen the large secretory sac was seen to be of the same size and shape as in Macrurocampa, and of the color of raw silk. The sac when distended extends back to a little behind the middle pair of legs, and is nearly two-thirds as wide as the body. The caterpillar sent out the fluid when handled, but we could not make it spray.

In the larva of Macrurocampa marthesia the cervical or secretory gland (Fig. 366, 5) is situated in the 1st and 2d thoracic segments, extending to the hinder edge of the latter and lying between the nervous cord and the œsophagus and proventriculus, and when empty the bulk of it lies a little to one side of the median line of the body. It is partly held in place by small tracheæ, one quite large branch being sent to it from near the prothoracic spiracle. The short, large duct, leading from it to the transverse opening in the membrane between the head and prothoracic segment, is a little narrower than this opening, and is kept distended by tænidia, or a series of short, spiral threads which are pale, not honey-yellowish, in color. This duct lies on one side of the prothoracic ganglion, resting just under the commissures passing up to the brain; it is also situated between the two silk ducts.

The very distensible sac (Fig. 366, 5) is rendered elastic by a curious arrangement of the cuticle, the tænidia of the duct itself being represented by very thickly-scattered, irregular, separate, sinuous, chitinous ridges, which stand up from the cuticular lining of the wall of the sac (Fig. 366, 6). The secretory cells of the walls of this sac in Cerura vinula are said by Klemensiewicz to be large hexagonal cells, resembling those of silk-glands, having like them large branched nuclei.

The fluid thrown out is said by Poulton to be formic acid; it causes violent effervescence when allowed to fall upon sodium-bicarbonate, and colors blue litmus paper red. It also appears from the researches of Latter that these creatures in the imago state secrete free potassium hydroxid, a substance for the first time known to exist in the animal kingdom.

In the caterpillar of Astyanax archippus (Limenitis disippus) a dark, bladder-like sac is everted, but the lateral tubes appear to be wanting, and no spray is sent out; it occurs in the larvæ of many Nymphalidæ and other butterflies and moths.

These glands are functionally active in Perophora, but obsolete (at least the external openings) in Lacosoma.

=The osmeterium in Papilio larvæ.=—The caterpillars of the swallowtailed butterflies (Papilio, Doritis, and Thais), as is well known, when irritated thrust out from a transverse slit on the upper part of the prothoracic segment a large orange-yellow V-shaped fleshy tubular process (the osmeterium), from which is diffused a more or less melonlike but disagreeable, in some cases insufferable, odor; the secretion is acid and reddens litmus paper. The mechanism has been described and figured by Klemensiewicz.

When at rest, or retracted, the osmeterium lies in the upper part of the body in the three thoracic segments, and is crossed obliquely by several muscular bundles attached to the walls of the body, and by the action of these muscles the evagination of the osmeterium is strongly promoted. After eversion the tubes are slowly retracted by two slender muscles inserted at the end of each fork or tube, and arising from the sides of the 3d segment behind the head, crossing each other in the median line (Fig. 366, 7 r.m.). The secretion is formed by an oval mass of glandular cells at the base of the forks; in the glandular mass is a furrow-like depression about which the secretory cells are grouped. The secretion collects in very fine drops on the side of each furrow opposite the glandular cells.

According to C. D. Ash the larva of an Australian Notodontid (Danima banksii Lewin) protrudes from the under side of the prothoracic segment a Y-shaped red organ like that of Papilio; no fluid or odor is given out.

=Dorsal and lateral eversible metameric sacs in other larvæ.=—The showy caterpillars of Orgyia and its allies have a conspicuous coral-red tubercle on the back of the 6th and also the 7th abdominal segment, which on irritation are elongated, the end of the tubercles being eversible. When at rest the summit is crateriform, but on eversion the end becomes rounded and conical. These osmeteria are everted by blood pressure, and retracted by a muscle. Fig. 366, 9, represents a section of an osmeterium of Orgyia leucostigma when retracted by the muscle (m); at the bottom of the crater are the secreting or glandular cells (gc), being modified hypodermal cells. These doubtless serve as terrifying organs to ichneumons and other insect enemies, and though we have been unable to detect any odor emanating from the tubercles, yet possibly they give out a scent perceived by and disagreeable to their insect assailants.

In the Hemileucidæ there is a pair of lateral osmeteria, on the 1st and on the 7th abdominal segments, which however, are not highly colored (Figs. 363, 366, 10). In Megalopyge (Lagoa, Fig. 364) there is a lateral row of singular pale permanently everted processes which appear to be the homologues of the osmeteria of larvæ of other lepidopterous families. As these are repeated on seven segments, their metameric arrangement is obvious. The relation of these curious glands to the viscera is seen in Fig. 297, lgp, and their minute structure in Fig. 365.

At A, the lumen (l) is a deep narrow cavity, with the secretion (secr.), collected at the mouth of the cavity, composed of a thin, mucus-like, coagulated fluid, containing granules of varying degrees of fineness, which take the stain readily. Outside of these are collected fine nuclei (bc), stained dark, and enveloped in a slight, transparent, pale, protoplasmic envelope, which may be blood corpuscles. The glandular cells themselves are simply modified hypodermal cells, as seen at C. In some of the nuclei, indistinct nucleoli are seen, and deeply stained granules, especially around the periphery of the nuclei. At B is represented a section on one side of the middle, but still showing the spacious lumen. In the section represented by C, the knife passed through the process still nearer the outer edge, and near the base; at C^1, three of the glandular cells, with their large, deeply stained nuclei, are drawn. A transverse section at D shows the large lumen or cavity (l).

As to the function and homologies of these structures, it is difficult to decide. We have never noticed that they give off any odor, though they may prove to be repugnatorial; they are not visible in the fully grown, living insect, being concealed by the long, dense hairs clothing the body; they are not spraying organs, as they are imperforate at the end, not ending as the lateral, eversible glands of Hyperchiria io, etc., in a crateriform orifice.

They may be permanently everted glands, or osmeteria, which have, by disuse, lost their power of retraction and their crateriform opening, as well as the power of secreting a malodorous fluid.

In certain of the butterflies, the Heliconidæ (Colænis, Heliconius, Euides, and Dione), there is thrust from the end of the abdomen a pair of large, irregular, rounded, eversible glands, which give out a disagreeable odor, and are consequently repellent, and which seem to be the homologues of the odoriferous glands of other butterflies.

The large, soft, rounded, eversible glands, looking like puff-balls or a rounded pudding (Fig. 366, 12), are everted, when the butterflies are roughly seized, from the dorsal side of the penultimate segment of the abdomen. The males possess two smaller tubercles on the inside of the anal claspers or lobes. Müller also detected, in the females of various species of the Heliconidæ enumerated above, a pair of club-shaped processes like the balancers of flies, which are thrust out on each side of and under the odoriferous puff-balls of the hinder edge of the penultimate segment (Fig. 366, 13). The club or head is armed with hairs or bristles, which, in Heliconius, are like the scales of a butterfly.

In the caterpillars of certain blue butterflies (Lycænidæ) is an internal osmeterium, being a very minute sac which is everted from a transverse slit on the top of the 7th abdominal segment. Its function is quite the opposite of those of the caterpillars of other families, since the sac exudes a sweet fluid very attractive to ants, which may be diffused more widely by the delicate spinulose bristles crowning the summit. W. H. Edwards states that in several species of Lycæna, besides that on the 7th abdominal segment, there is on the 8th segment a pair of minute dorsal evaginable tubercles.

A pair of small ramose odoriferous glands are said by Siebold, who regarded them as alluring glands, to occur in Argynnis, Melitæa, and Zygæna, to be situated near the orifice of the oviduct, and Scudder has detected them near the anus of the female pupa of Danais archippus. The appearance of the odoriferous glands in the pupa of Vanessa io is well shown by Jackson (Fig. 366, 14). They develop as two tubular ingrowths of the hypodermis, perfectly distinct one from the other, each having its own separate aperture to the exterior. In Fig. 366, 14 the condition of parts is nearly as in the imago, the glands being situated below the rectum and opening of the oviduct. In both sexes of another Brazilian butterfly (Didonis biblis) on the median line of the abdomen between the 4th and 5th segments are two roundish vesicles covered with short gray hairs, which emit a disagreeable smell.

It is possible that the dark-green fluid in Parnassius, secreted by an evaginable gland, and which is moulded into shape by the scimetar-shaped peraplast (Scudder), is formed by the homologues of the anal glands of other butterflies.

Distribution of repugnatorial or alluring scent-glands in insects[59]

A. LARVAL INSECTS

a. Each thoracic segment; sternal. Phryganea grandis.

b. Prothoracic, sternal, discharging a lateral jet of spray; with a single large internal sack.

LEPIDOPTERA

Family TINEIDÆ

Hyponomeuta evonymella.

Family NOCTUIDÆ

Bryophila, Cucullia formosa, C. scrophulariæ, Habrostola, Cleophana linariæ, Catocala (sp.), Aporia cratægæ, Aplecta nebulosa, Leucania staminea, L. hispanica, L. nonagrioides, Plusia gamma.

Family NOTODONTIDÆ

Pheosia rimosa, Schizura concinna, Danima Banksii (Australia), Macrurocampa marthesia, Heterocampa pulverea, Cerura vinula, C. furcula, C. borealis, C. multiscripta.

Family NYMPHALIDÆ

Probably all the species.

c. Prothoracic, dorsal; sending out a V-shaped odoriferous organ (osmeterium).

Family PAPILIONIDÆ

All the species as a rule.

d. Thoracic sternal, evaginable glands.

Family PEROPHORIDÆ

Lacosoma chirodota, Perophora melsheimerii.

Family NOLIDÆ

In three, and probably in all the species of Nola.

e. Lateral, abdominal, non-eversible glands, one near each spiracle, emitting a clear fluid.

Family TENTHREDINIDÆ

Cræsus septentrionalis, C. varus, Cimbex americana, C. betulæ, Trichiosoma.

f. Lateral, abdominal, partly eversible glands emitting neither moisture nor odor, but flesh-colored.

Family Tineidæ

Phyllocnistis? (eight pairs.)

Family HEMILEUCIDÆ

Hyperchiria io (two pairs, viz. on 1st and 7th segments), H. sp. (Mexico), Hemileuca yavapai, pamina, H. maia, H. artemis, Pseudohazis eglanterina.

g. Lateral, abdominal, permanently everted, metameric glands, not known to secrete a fluid, nor to be odoriferous.

Family MEGALOPYGIDÆ

Megalopyge crispata.

h. Medio-dorsal, partly eversible glands, emitting a spray of liquid but no odor(?), and colored coral-red or orange-yellow (P. auriflua), but usually in the European species yellowish.

Family LIPARIDÆ

All the species except those of Demas.

i. A single, median, abdominal, dorsal gland, emitting a fluid attractive to ants, on 7th segment; with a pair of minute, index glands on the 8th segment.

Family LYCÆNIDÆ

All the species.

j. Protrusile organs near the anus.

Myrmeleon larva (Hagen? Dimmock).

B. NYMPH OF HETEROMETABOLOUS INSECTS

a. Paired, dorsal glands, on abdominal segments 1, 2, and 3.

Cimex lectularius (Künckel).

b. The same on abdominal segment 5.

Lachnus strobi.

C. PUPA OF CERTAIN BOMBYCES

At anterior end of certain pupæ, internal glands to moisten threads of the cocoon for exit of moth.

D. ADULT INSECTS

a. Occurring on the prothorax only; strongly repugnatorial, best developed in ♂.

Anisomorpha buprestoides, Autolyca pallidicornis, Phasma putidum, Phyllium (sp.), Heteropteryx (sp.), Diapheromera femoratum (probably in all the species of the family), Mantis carolina.

b. Occurring on the pro- and mesothorax, and on the middle of the abdomen, orange-yellow, fleshy tubercles or evaginations.

Malachius bipustulatus, Anthocomus equestris, Evæus thoracicus.

c. Segmental, eversible glands, homologues of the coxal glands of other Arthropods, occurring on all, or nearly all, the abdominal segments.

Scolopendrella immaculata (coxal glands on 3d to 11th pair of legs), Campodea staphylinus (a pair of coxal glands on 1st to 8th abdominal segments), Machilis maritima (eversible, coxal glands on segments 1–7).

d. Occurring in the abdomen.

d^1. In the two first abdominal segments.

Corydia carunculigera ♂ and ♀.

d^2. Alluring (?) organs situated on the dorsal side of the abdomen, in the 6th, or 6th and 7th, abdominal segment.

Periplaneta americana ♂, P. orientalis (nymph), P. decorata ♂ (nymph), Ectoblatta germanica ♂, Ectobia lapponica ♂, Phyllodromia ♂, Aphlebia bivittata ♂, Platyzosteria ingens (on seventh segment).

e. At the end of the body.

Colænis julia ♀ (F. Müller), Heliconius apseudes (F. Müller).

LITERATURE ON DEFENSIVE OR REPUGNATORIAL GLANDS

=Aldrovandus, U.= De animalibus insectis libri septem cum singulorum iconibus ad vivum expressis. (Denuo impress: Bonon. apud Clementem Ferronium, 1638, p. 273. The first edition was in 1602.)

=Moufet, T.= Insectorum sive minimorum animalium theatrum, etc. London, 1634, pp. 185, 186.

=Gœdart, J.= Metamorphosis naturalis sive insectorum historia, etc. Amstelodami, 1700, Pars ii, p. 136. (French ed. of 1700, ii, p. 162; Lister’s Latin ed., London, 1685, p. 60.)

=Réaumur, R. A. F.= Mémoires pour servir à l’histoire des insectes, etc. Paris, 1736, ii, pp. 266–269, Pls. 21, 22. [ii, partie ii, pp. 21–23, of the Amsterdam ed. of 1737–1748.]

=De Geer, C.= Observation sur la propriété singulière qu’ont les grandes chenilles à quatorze jambes et à double queue, du saule, de seringuer de la liqueur. (Mém. sav. étrang, Paris, 1750, i, pp. 530, 531, Pl.; Goetze und Bonnet, etc., Auserlesene Abhandlungen, 1774, p. 220.)

=Schaeffer, J. C.= Neuendeckte Theile an Raupen und Zweyfaltern, etc. Regensburg, 1754.

=Sulzer, J. H.= Die Kennzeichen der Insekten, etc. Zürich, 1761, pp. 65–67, Taf. 5, Fig. 34.

=Müller, O. F.= Pile-larven med dobbelt Hale, og dens Phalæne, etc. Kjöbenhavn, 1772, pp. 53–56, Pl. 2, Figs. 3–5.

=Bonnet, C.= Mémoire sur une nouvelle partie commune à plusieurs espèces de chenilles. (Mém. math. d. savants étrang., Paris, 1755, ii, pp. 44–52; Collection complète des œuvres de C. Bonnet, 1779, ii, pp. 3–16.)

—— Mémoire sur la grande chenille à queue fourchue du saule, dans lequel on prouve, que la liqueur que cette chenille fait jaillir, est un véritable acide, et un acide très-actif. (Mém. math, de savants étrang., Paris, 1755, ii, pp. 267–282; Collection complète des œuvres de C. Bonnet, 1779, ii, pp. 17–24.)

=Amoreux, P. J.= Notice des insectes de la France, réputés venimeux, etc. Paris, 1789, pp. 282–285.

=Schwarz, C.= Neuer Raupenkalender. Nürenberg, 1791, Abth. i, p. 59.

=Petzhold, C. P.= Lepidopterologische Beyträge. (L. G. Scriba’s Beiträge zu der Insekten-geschichte, Frankfurt am Main, 1793, Heft 3, pp. 230–251.)

=Nouvelle Dictionnaire d’Hist. Nat.=, xv, p. 487. (Larva of Hydrophilus ejects with a slight noise a fœtid and blackish fluid.)

=Rengger, Johann Rudolph.= Physiologische Untersuchungen über die thierische Haushaltung der Insecten. Tübingen, 1817. (In the chapter entitled Abgesonderte Säfte bei den Raupen, he speaks of the glandular apparatus of the larva of B. vinula, noticing the general form of the secretory sac, that it opens out in two muscular evertible points, out of which the secretion is ejected.)

=Dufour, L.= Mémoire anatomique sur une nouvelle espèce d’insecte du genre Brachinus. (Ann. de mus. d’histoire nat., xviii, 1811, pp. 70–81.)

—— Recherches anatomiques sur les carabiques et sur plusieurs autres coléoptères. (Ann. d. Sci. Nat., 1826, viii, pp. 5–54.)

—— Mémoire sur les métamorphosis et l’anatomie de la Pyrochroa coccinea. Glande odorifique. (Ibid., ii sér. Zoologie, xiii, 1840, pp. 340, 341.)

—— Recherches anatomiques sur les Diptères. 1851, pp. 195, 313. (Alimentary canal of Sepsis contains the seat of a “glande odorifique.”)

=Kirby and Spence.= Introduction to entomology, etc. (2d ed., i, 1815; London, 1818, ii, pp. 238, 239.)

=Lyonet, P.= Recherches sur l’anatomie et les metamorphoses de différentes espèces d’insectes. Ouvrage posthume. Paris, 1832.

=Morren, C.= Mémoire sur l’émigration du puceron du pêcher (Aphis persicæ), et sur les caractères et l’anatomie de cette espèce. (Ann. Sci. Natur. Zool., 1836, vi, pp. 65–93, Pls. 6, 7.)

=Ratzeburg, J. T. C.= Die Forstinsekten, etc. (Theil i, Die Käfer, etc., 1837, p. 246.)

=Aubé, C.= Note sur une sécrétion fétide d’Eumolpus pretiosus. (Ann. Soc. Ent. Fr., 1837, i, vi; Bull., p. 58.)

=Lacordaire, J. S.= Introduction à l’entomologie. 1838, ii, p. 45.

=Meckel, von Hemsbach, Johann Friedrich.= Mikrographie einiger Drüsenapparate der niederen Thiere. (Anat. Phys. u. wiss. Med., 1846, pp. 1–73, Taf. 1–3; p. 46, Müller’s Archiv.)

=Stein, Friedrich.= Vergleichende Anatomie und Physiologie der Insekten. Berlin, 1847.

=Leidy, Joseph.= History and anatomy of the hemipterous genus Belostoma. (Journ. Acad. Nat. Sci. Philadelphia, Ser. 2, 1847, i, Part i, pp. 57–67, Pl. 1.)

—— Odoriferous glands of invertebrata. (Proc. Acad. Philadelphia, 1849, iv, 234–236, 1 Pl.; Ann. and Mag. N. H., Ser. 2, 1850, v, pp. 154–156.)

=Chapuis et Candèze.= Catalogue des larves des coléoptères, etc. (Mém. Soc. Sci. de Liège, 1853, viii, pp. 351–653, Pls. 1–9, pp. 611, 612.)

=Siebold, Carl Theodor.= Lehrbuch der vergleichenden Anatomie der wirbellosen Thiere, 1848. (Burnett’s transl., Boston, 1854.)

=Burnett, Waldo Irving.= Translation of Siebold’s Anatomy of the Invertebrates, 1854. (Note on the osmeteria of Papilio asterias, which he regards as an odoriferous and defensive, rather than tactile, organ, p. 415.)

=Karsten, H.= Bemerkungen über einige shaarfe und brennende Absonderungen verschiedener Raupen. (Müller’s Archiv für Anat. Phys. u. wiss. Med., 1848, pp. 375–382, Taf. 11, 12.) Describes the poison-glands at the base of the spines of Saturnia larvæ.

—— Harnorgane des Brachinus complanatus. (Müller’s Archiv, 1848, pp. 367–376, Taf.)

=Laboulbéne, Alexandre.= Note sur les caroncules thoraciques du Malachius. (Annales de la Soc. Ent. de France, 3^e Sér., vi, 1858, pp. 521–528.)

=Saussure, Henri de.= Recherches zoologiques de l’Amerique centrale et du Mexique. (6^e Partie, Études sur les Myriopodes et les Insectes, Paris, 1860.)

=Gerstaecker, C. E. A.= Ueber das vorkommen von ausstülpbaren Hautanhängen am Hinterleibe an Schaben. (Archiv f. Naturgesch., 1861, xxi, pp. 107–115.)

=Liegel, Hermann.= Ueber den Ausstülpungsapparat von Malachius und verwandten Formen. Inaug. Diss., Göttingen, pp. 31, 1 Taf. (n. d., since 1858 and before 1878.)

=Leydig, F.= Zur Anatomie der Insecten. (Archiv f. Anat. Phys. u. wiss. Med., 1859, pp. 33–89, 149–183, Taf. 2–4, pp. 35 and 38.)

—— Ueber bombardier Käfer. (Biolog. Centralbl., x, 1890, pp. 395, 396.)

=Claus, C.= Ueber die Seitendrüsen der Larve von Chrysomela populi. (Zeits. f. wissens. Zool., xi, 1861, pp. 309–314, Taf. xxv.)

—— Ueber Schutzwassen der Raupen des Gabelschwanzes. (Würzburger Naturw. Zeitschrift, 1862, iii, xiv; Sitz. am., 28 Juni, 1862.)

=Rogenhofer, Alois.= Drei Schmetterlingsmetamorphosen. (Verhandlungen der k. k. zoolog.-bot. Gesellschaft, Wien, xiii, 1862, pp. 1224–1230.)

=Fitch, Asa.= Eighth report on the noxious and other insects of ... New York. (Trans. N. Y. State Agric. Soc., 1862, xxii, pp. 657–684), p. 677. (Separate.)

=Guenée, Achille.= D’une organe particulier que présente une chenille de Lycæna. (Annales Soc. Ent. de France, Sér. 4, 1867, pp. 665–668, Pl. 13.)

=Landois, L.= Anatomie der Bettwanze, Cimex lectularius, mit berücksichtigung verwandter Hemipterengeschlechter. (Zeitsch. f. wissens. Zool., 1868, xvii, pp. 206–224, 218–223, Taf. 11, 12.)

=Studer, Theodor.= Mittheilungen der naturforsch. Gesellschaft in Bern, 1872–1873, No. 792–811, p. 101.

=Candèze, E.= Les moyens d’attaque et de défense chez les insectes. (Bull. Acad. royale de Belgique, 2 Sér., xxxviii, 1874, pp. 787–816.)

=Mayer, Paul.= Anatomie von Pyrrhocoris apterus. (Reichert und du Bois-Reymond’s Archiv f. Anat. Phys., etc., 1874, pp. 313–347, 3 Taf.)

=Scudder, Samuel Hubbard.= Odoriferous glands in Phasmidæ. (Psyche, i, pp. 137–140, Jan. 14, 1876; Amer. Nat., x, p. 256, April, 1876.)

—— Prothoracic tubercles in butterfly caterpillars. (Psyche, i, pp. 64, 168, 1876.)

—— Organs found near the anus of the ♀ pupa of Danais, which recall the odoriferous organs mentioned by Burnett, transl. Siebold’s Comp. Anat. as occurring in Argynnis and other genera. (Psyche, iii, p. 278, 1882, p. 453, note 22.)

—— Glands and extensile organs of larvæ of blue butterflies. (Proc. Bos. Soc. Nat. Hist., xxxiii, pp. 357, 358, 1888.)

—— Butterflies of Eastern United States. i-iii, 1889.

—— New light on the formation of the abdominal pouch in Parnassius. (Trans. Ent. Soc. London for 1892, January, 1893, pp. 249–253.)

=Müller, Fritz.= Die Stinkkölbchen der weiblichen Maracujáfalter. (Zeitschr. f. wissens. Zool., 1877, xxx, pp. 167–170, Taf. 9.)

=Plateau, Félix.= Note sur une sécrétion propre aux coléoptères dytiscides. (Ann. Soc. Ent. Belg., 1876, v, xix, pp. 1–10.)

=Edwards, William H.= Notes on Lycæna pseudargiolus and its larval history. (Can. Ent., x, Jan., 1878, pp. 1–14. Fig.)

—— On the larvæ of Lycæna pseudargiolus and attendant ants. (Can. Ent., x, July, 1878, pp. 131–136.)

—— Butterflies of North America, i-iv. Many Pls. Phil., 1868—.

=Voges, Ernst.= Beiträge zur Kenntniss der Juliden. (Zeitsch. f. wissens. Zoologie, xxxi, p. 127, 1878. The scent-glands are retort-shaped bodies, the necks of which open into foramina repugnatoria.)

=Rye, E. C.= Secretion of water-beetles. (Ent. Month. Mag., xiv, 1877–1878, pp. 232, 233.)

=Forel, A.= Der Giftapparat und die anal Drüsen der Ameisen. (Zeits. f. wissens. Zool., 1878, xxx, Suppl., pp. 28–68, Taf. 3, 4.)

=Saunders, William.= Notes on the larva of Lycæna scudderi. (Can. Ent., x, Jan., 1878, p. 14.)

=Weismann, A.= Ueber Duftschuppen. (Zool. Anzeiger, 26th Aug., 1878, Jahrg., i, pp. 98, 99.)

=Gissler, Carl Friedrich.= On the repugnatorial glands in Eleodes. (Psyche, ii, Feb., 1879, pp. 209, 210.)

—— Odoriferous glands on the 5th abdominal segment in nymph of Lachnus strobi. (Fig. 273, p. 804, of Packard’s Report on Forest and Shade Tree Insects, 1890.)

=Brunner von Wattenwyl, K.= Ueber ein neues Organ bei den Acridiodeen. (Verhandl. k. k. Zool. Bot. Gesells. Wien., 1879, xxix; Sitzungsber., pp. 26, 27.)

—— Verhandl. k. k. Zool. Bot. Gesells. Wien. (A peculiar organ on hind femora of Acridiidæ.)

=Rougement, P.= Observations sur l’organe détonnant du Brachinus crepitans Oliv. (Bull. Soc. Sci. Nat. Neuchâtel, 1879, xi, pp. 471–478, Pl.)

=Goossens, Th.= Sur une organe entre la tête et la première paire de pattes de quelques chenilles. (Ann. Soc. Ent. France, ix, p. 4, 1809; Bull., pp. 60, 61.)

—— Des chenilles vésicantes. (Ann. Ent. Soc. France, vi, pp. 461–464, 1887.)

=Coquillett, D. W.= On the early stages of some moths. (Can. Ent., March, 1880, xii, pp. 43–46.)

=Chambers, Victor Tousey.= Notes upon some Tineid larvæ. (Psyche, iii, July, 1880, p. 67. Certain retractile processes “from the sides of certain segments of the larva.”)

—— Further notes on some Tineid larvæ. (Psyche, iii, p. 135, Feb. 12, 1881. Larva of Phyllocnistis has eight pairs of lateral pseudopodia on first eight abdominal segments.)

=French, G. H.= Larvæ of Cerura occidentalis Lint, and C. borealis Bd. (Can. Ent., July, 1881, xiii, pp. 144, 145.)

=Passerini, N.= Sopra i due tubercoli abdominali della larva della Porthesia chrysorrhœa. (Bull. Soc. Ent. Ital., 1881, xiii, pp. 293–296.)

=Klemensiewicz, Stanislaus.= Zur näheren Kenntniss der Hautdrüsen bei den Raupen und bei Malachius. (Verhandlungen d. Zool. Bot. Gesellsch. Wien., xxxii, pp. 459–474, 1882, 2 Taf.)

=Weber, Max.= Ueber eine Cyanwasserstoffsäure bereitende Drüse. (Archiv für Mikr. Anat., xxi, pp. 468–475, xxiv, 1882.)

=Bertkau, Philip.= Ueber den Stinkapparat von Lacon murinus L. (Archiv f. Naturg., 1882, Jahrg., xlviii, pp. 371–373.)

=Dimmock, George.= Organs, probably defensive in function, in the larva of Hyperchiria varia Walk. (Saturnia io Harris). (Psyche, iii, pp. 352, 353, Aug. 19, 1882. Account of lateral eversible glands on 1st and 7th abdominal segments; they emit neither moisture nor odor.)

—— On some glands which open externally on insects. (Psyche, iii, pp. 387–399, Jan. 15, 1883. Treats of poison-glands, glandular hairs, eversible glands of Cerura, etc.)

=Coleman, N.= Notes on Orgyia leucostigma. (Papilio, November-December, 1882, Jan., 1883, ii, pp. 164–166.)

=Müller, F.= Der Anhang am Hinterleibe der Acræa-weibchen. (Zool. Anzeiger, 6th Aug., 1883, Jahrg., vi, pp. 415, 416.)

=Dewitz, H.= Ueber das durch die Foramina repugnatoria entleerte Secret bei Glomeris. (Biol. Centralblatt, iv, pp. 202, 203, 1884.)

=Williston, S. A.= Protective secretion of Eleodes ejected from anal gland. (Psyche, iv, p. 168, May, 1884.)

=Poulton, Edward Bagnall.= Notes in 1885 upon lepidopterous larvæ and pupæ, including an account of the loss of weight in the freshly-formed lepidopterous pupæ. (Trans. Ent. Soc., London, June, 1886, pp. 156, 157, 159.)

—— Notes in 1886 upon lepidopterous larvæ, etc. (Trans. Ent. Soc., London, Sept., 1887, pp. 295–301.)

—— Notes in 1887 upon lepidopterous larvæ, etc. (Trans. Ent. Soc., London, 1888, p. 597.)

=Künckel-d’Herculais, J.= La punaise de lit et ses appareils odoriférants. (Comptes rendus, ciii, 1886, pp. 81–83; Annals & Mag. Nat. Hist., 5th Ser., xviii, 1886, pp. 167, 168.)

—— Étude comparée des appareils odorifiques dans les differents groupes d’Hemiptères hétéroptères. (Compt. rend. Acad. Sc., Paris, cxx, pp. 1002–1004.)

=Packard, A. S.= The fluid ejected by notodontian caterpillars. (Amer. Nat., 1886, xx, pp. 811, 812.)

—— An eversible “gland” in the larva of Orgyia. (Amer. Nat., 1886, xx, p. 814.)

—— Fifth Rep. U. S. Ent. Comm. Insects injurious to forest and shade trees, p. 136, 1890.

—— Hints on the evolution of the bristles, spines, and tubercles of certain caterpillars. (Proc. Boston Soc. Nat. Hist., xxiv, 1890, p. 551.)

—— Notes on some points in the external structure and phylogeny of lepidopterous larvæ. (Proc. Bost. Soc. Nat. Hist., xxv, 1890, pp. 83–114.)

—— A study of the transformations and anatomy of Lagoa crispata, a bombycine moth. (Proc. Amer. Phil. Soc., Philadelphia, xxxii, 1893, pp. 275–292, 7 Pls.)

—— The eversible repugnatorial scent glands of insects. (Journ., N. Y. Ent. Soc. iii, 1895, pp. 110–127; iv, p. 896; pp. 26–32, 1 Pl.)

=Loman, J. C. C.= Freies Jod als Drüsensecret. (Tijdschr. Neder. Dierk. Ver. Deel 1, 1887, pp. 106–108.)

=Riley, Charles Valentine.= Proc. Ent. Soc., Washington, March 13, 1888, i, pp. 87–89.

—— Notes on the eversible glands of larvæ of Orgyia and Parorgyia leucopæa and P. clintonii (achatina). (See 5th Rep. U. S. Ent. Comm., p. 137.)

=Denham, Ch. S.= The acid secretion of Notodonta concinna. (Insect Life, i, p. 147, 1888; hydrochloric acid.)

=Michin, Edward A.= Note on a new organ, and on the structure of the hypodermis, in Periplaneta orientalis. (Quart. Journ. Micros. Sc., Dec., 1888, xxiv, 1 Pl.)

—— Further observations on the dorsal gland in the abdomen of Periplaneta and its allies. (Zool. Anz., 27 Jan., 1890, pp. 41–44.)

=Maynard, C. L.= The defensive glands of a species of Phasma, Anisomorpha buprestoides. (Contributions to Science, i, April, 1889.)

=Schaeffer, Cæsar.= Beiträge zur Histologie der Insekten. (Zool. Jahrb. Morph. Abth. iii, pp. 611–652, Taf. xxix, xxx, 1889; treats of the ventral glands in prothorax of caterpillars; scales and hairs are secretions from the very greatly enlarged hypodermic cells.)

=Gilson, G.= Les glandes odorifères der Blaps mortisaga et de quelques autres espèces. (La Cellule, v. pp. 1–21, 1 Pl., 1889.)

—— The odoriferous apparatus of Blaps mortisaga. (Rep. 58th Meeting Brit. Assoc. Adv. Sc., 1889, pp. 727, 728.)

=Haase, Erich.= Ueber die Stinkdrüsen der Orthoptera. (Sitzgsber. Ges. Naturf. Freunde, Berlin, pp. 57, 58, 1889.)

—— Zur Anatomie der Blattiden. (Zool. Anz., xii Jahrg., pp. 169–172, 1889.)

=Herbst, Curt.= Anatomische Untersuchungen an Scutigera coleoptrata. Ein Beitrag zur vergleichenden Anatomie der Articulaten. Dissert., Jena, pp. 36 (Hautdrüsen, Coxal-Organ.); p. 1, 1889.

=Wheeler, William M.= Hydrocyanic acid secreted by Polydesmus virginiensis Drury. (Psyche, v, p. 422.)

—— New glands in the hemipterous embryo. (Amer. Nat., Feb. 1890, p. 187; odorous(?) glands.)

=Jackson, W. Hatchett.= Studies in the morphology of the Lepidoptera, Pt. i. (Trans. Linn. Soc., London, 2 Ser., Zoöl., v, May, 1890.)

=Krauss, Hermann.= Die Duftdrüse der Aphlebia bivittata Brullé (Blattidæ) von Teneriffa. (Zool. Anz., xiii Jahrg., 1890, pp. 584–587, 3 Figs.)

=Fernald, H. T.= Rectal glands in Coleoptera. (Amer. Nat., xxiv, pp. 100, 101, Pls. 4, 5, 1890.)

=Verson, E.= Hautdrüsen system bei Bombyciden (Seidenspinner). (Zool. Anzeiger, 1890, pp. 118–120.)

=Vosseler, Julius.= Die Stinkdrüsen der Forficuliden. (Arch. Mikr. Anat., xxxvi, 1890, pp. 565–578, Taf. 29.)

=Carrière, J.= Die Drüsen am ersten Hinterleibsringe der Insektenembryonen. (Biol. Centralblatt, xi, pp. 110–127, 1891.)

=Borgert, Henry.= Die Hautdrüsen der Tracheaten. Inaugural Diss., Jena, 1891, pp. 1–80.

=Lang, Arnold.= Lehrbuch der vergleichende Anatomie, English Trans. by Henry M. and Matilda Bernard, 1891, pp. 458, 459.

=Kennel, J. von.= Die Verwandtschaftverhältnisse der Arthropoden. (Schriften herausgegeben von der Naturforscher Gesellschaft bei der Universität Dorpat, vi, Dorpat, 1891.)

=Patton, W. H.= Scent-glands in the larva of Limacodes. (Can. Ent., xxiii, Feb. 1891, pp. 42, 43; eight pairs of glands with pores along the edges of the back.)

=Batelli, Andrea.= Di una particolarita nell integumento dell’ Aphrophora spumaria. (Monitore Zoöl. Ital. Anno 2, pp. 30–32, 1891. Dermal glands in the hindermost segment.)

=Ash, C. D.= Notes on the larva of Danima banksii Lewin. (Ent. Month. Mag., Sept. 1892, p. 232, Fig.) notodontian larva protrudes from under side of prothoracic segment a Y-shaped, red organ like that of Papilio; no odor or fluid given out.

=Bernard, Henry M.= An endeavor to show that the tracheæ of the Arthropoda arose from setiparous sacs. (Spengel’s Zool., Jahrbuch, 1892, pp. 511–524, 3 Figs.)

=Latter, Oswald.= The secretion of potassium hydroxide by Dicranura vinula, and the emergence of the imago from the cocoon. (Trans. Ent. Soc. London, 1892, 287, also xxxii; Prof. Meldola adds that the larva of D. vinula secretes strong, formic acid, and is the only animal known to secrete a strong, caustic alkali.)

—— Further notes on the secretion of potassium hydroxide by Dicranura vinula (imago), and similar phenomena in other Lepidoptera. (Trans. Ent. Soc. London; Nature, 1895, p. 551, March 20, 1895.)

=Zograff, Nicolas.= Note sur l’origine et les parentes des Arthropodes, principalement des Arthropodes trachèates. (Congrès Internationale de Zoologie, 2^e Session à Moscow, Aug. 1892; Part i, Moscow, 1892, pp. 278–302, 1892; cyanogenic glands in Myriopods, p. 287.)

=Swale, H.= Odor of Olophrum piceum. (Ent. Month. Mag., v, Jan. 1896, pp. 1, 2.)

=Cuénot, L.= Moyens de défense dans la série animale, Paris, n. d. (1892); the ejection of blood as a means of defence by some Coleoptera. (Comptes rendus, Acad. Sc. France, April 16; Nature, April 26, 1894.)

—— Sur la saignée réflexe et les moyens de défense de quelques insectes. (Arch. Zool. expér. (3), 1897, iv, pp. 655, 679, 680.)

=Holmgren, Emil.= Studier öfverhudens och de körtelartade hudorganens morfologi hos skandinaviska macrolepidopterlarver. (K. Svenska Vetenskaps-Akademiens Handlingar, xxvii, No. 4, Stockholm, 4º 1895, pp. 82, 9 Pls.)

=Lutz, K. G.= Das Blut der Coccinelliden. (Zool. Anzeiger, 1895, pp. 244–255, 1 Fig.)

=Gilson, Gustav.= Studies in insect morphology. (Proc. Linn. Soc. London, March 5, 1896; Nature, p. 500.)

—— On segmentally disposed thoracic glands in the larvæ of the Trichoptera. (Journ. Linn. Soc., London, xxv. 1897.)

=Cholodkowsky, N.= Entomotomische Miscellen, v, Ueber die Spritzapparate der Cimbiciden Larven, pp. 135–143, 2 Taf. Ibid., vi. Ueber das Bluten der Cimbiciden Larven, pp. 352–357, 1 Fig. (Horæ Soc. Ent. Rossicæ, xxx, 1897.)

Also the writings of Darwin, Wallace, Poulton, Weir, Beddard, Butler, Busgen, (pp. 365–367), Girard, Kolbe, Locy.

THE ALLURING OR SCENT-GLANDS

It is difficult to draw the line between repelling and alluring glands. Attention was first definitely called to the alluring odors of Lepidoptera by Fritz Müller, who showed that the males of certain butterflies are rendered attractive to the other sex by secreting odorous oils of the ether series. He pointed out that the seat of the odor is the androconia (see p. 199), while either repellent or pleasant odors are exhaled from abdominal glands.

Those of Pieris napi yield a scent like that of citrons, Didonis biblis gives off three different odors from different parts of the body, besides having a distinctly odorous spot on the hind wings. Both sexes have a sac between the fourth and fifth abdominal segments which exhales a very unpleasant (protective) odor, while the males have on the succeeding segment a pair of glands from which proceeds an agreeable odor like that of the heliotrope. Callidryas argante throws off a musky odor. In Prepona laertes the odor is like that of a bat, in Dircenna xantho it is vanilla-like, the androconia being situated on the front edge of the hind wings. In Papilio grayi the odor is said to be as agreeable and intense as in flowers. Certain sphingids are known to exhale a distinct odor, which Müller has traced to a tuft of hair-like scales at the base of the abdomen, and which fits into a groove in the first segment, so as to be ordinarily invisible.

In the noctuid genus, Patula, the costal half of the hind wing is modified to form a large scent-gland, and in consequence the venation has been modified. The still greater distortion of the veins in the allied genus, Argida, was attributed by the author to its once having possessed a similar scent-gland, now become rudimentary by disuse. (Hampson.)

Peculiar white or orange-colored, hairy, thread-like processes have been found protruding from narrow openings near the tip of the abdomen of Arctian moths (Fig. 367), which throw off, according to J. B. Smith, “an intense odor, somewhat like the smell of laudanum.” We have perceived the same unpleasant odor emanating from the males of Spilosoma virginica and Arctia virgo, as well as Leucarctia acræa.

We are informed by C. Dury that similar but longer hairy appendages are thrust out by the male of Haploa clymene. Many glaucopid moths protrude similar glandular processes. Thus Müller tells us that on seizing a glaucopid female by the wings, nearly the whole body became enveloped in a large cloud of snow-white wool which came out of a sort of pouch on the ventral side of the abdomen.

The male of a glaucopid was seen to dart out a pair of long hollow hairy retractile filaments which in some species exceed the whole body in length. The apparatus secretes a peculiar odor, probably serving to allure the female (Nature), and certain Zygænidæ have on the inner side of the paranal lobes (Afterklappen) glands filled with a sweetly scented fluid. Smith has detected a peculiar brush of hair-like scales in a groove between the dorsal and ventral parts of the basal two segments of the abdomen of Schinia marginata (family Noctuidæ), and when removed it exhaled a laudanum-like smell.

The pupa of Citheronia regalis gives out from the end of the abdomen a scent reminding us of laudanum.

Another mode of disseminating pleasant, alluring odors is that of the males of certain moths, which bear pencils and tufts on their fore or hind legs, and in the case of an Indian butterfly on the greatly elongated palpi. Those on the legs are ordinarily concealed in cavities or furrows in the leg, and may be thrust out and expanded so as to widely diffuse their odor. Such are those of the males of Catocala (Fig. 368), which resemble an artist’s fitch brush. In Hepialus hecta, where the arrangements for protecting the tufts are quite abnormal, Bertkau has detected the cells which secrete the odorous fluid. In the male of another Hepialus (H. humuli) a peculiar scent proceeds from the curiously aborted and altered hind tibiæ. (Barrett.) In one case, that of a geometrid moth (Bapata dichroa of New Guinea), these pencils occur on all the legs. (Haase). In many species a distinct odor is perceptible when the leg bearing the pencil or tuft is crushed.

These eversible scent-glands have been supposed to be mostly restricted to the Lepidoptera, and to a single known case in the Trichoptera, but similar alluring male glands also occur in the Orthoptera (Locustidæ). H. Garman has described and figured in the cave cricket (Hadenœcus subterraneus) “a pair of white fleshy appendages protruding from slits between the terga of the 9th and 10th abdominal somites, the nature of which is not clear,” adding, “the slits through which the organs appear are situated one on each side anterior to and a little within the cerci. When fully protruded, the glands are white, cylindrical, a little tapering, and are about one-eighth of an inch long.” He believes that they are protruded during the period of sexual excitement, and suggests that “the sense of smell is certainly the one best calculated to bring the sexes together in the darkness of caves.” We had previously noticed these organs in alcoholic specimens, but supposed that they were fungous growths. On dissecting and making microscopic sections of them, the gland is, when extended (Fig. 369), seen to be a long, ensiform, sharp, band-like process, with numerous retractor muscular fibres. When at rest each gland is folded about five times, forming a bundle lying on each side of the end of the intestine. The walls are formed of a single layer of epithelium, as seen in Fig. 369, B.

In the male of the common wingless cricket, Ceuthophilus maculatus, we have discovered what appears to be a pair of scent-glands lying directly over the last abdominal ganglion. They form two large white sacs situated close together, with a short common duct which passes back and opens externally upwards by a transverse slit on the under side of the last segment of the body.

LITERATURE ON ALLURING GLANDS

=Watson, J.= On the microscopical examination of plumules, etc. (Ent. Month. Mag., ii, 1865, p. 1.)

—— On certain scales of some diurnal Lepidoptera. (Mem. Lit. and Phil. Soc. Manchester, Ser. 3, ii, 1868, p. 63.)

—— On the plumules or battledore scales of Lycænidæ. (Mem. Lit. and Phil. Soc. Manchester, Ser. 3, iii, 1869, p. 128.) Further remarks, etc. (Ibid., p. 259.)

=Anthony, J.= Structure of battledore scales. (Month. Microsc. Journ., vii, 1872, p. 250; see also p. 200.)

=Morrison, Herbert Knowles.= On an appendage of the male Leucarctia acræa. (Psyche, i, pp. 21–22, October, 1874.)

=Müller, Fritz.= The habits of various insects. (Nature, June 11, 1874, pp. 102–103.)

—— Ueber Haarpinsel, Fitzflecke und ähnliche Gebilde auf den Flügeln männlicher Schmetterlinge. (Jena. Zeitschr. f. Naturw., 1877, xi, pp. 99–114.)

—— Beobachtungen an brasilianischen Schmetterlingen, ii. I. Die Duftschuppen der männlichen Maracujáfalter. (Kosmos, 1877, i, pp. 391–395, Figs. 5, 6.) II. Die Duftschuppen des männchens von Dione vanillæ. (Kosmos, ii, 1877, pp. 38–42, 7 Taf.)

—— As maculas sexuaes dos individuos masculinos das especies Danais erippus e D. gilippus. (Arch. Mus. Nac. Rio Janeiro, ii, 1877 (1878), pp. 25–29, 1 Pl.)

—— Die Duftschuppen der Schmetterlinge (nach dem “Kosmos” in Ent. Nachr., 1878, pp. 29–32, 109).

—— Wo hat der Moschusduft der Schwärmer seinen Sitz? (Kosmos, ii Jahrg., 1878, pp. 84, 85.)

—— Os orgaos odoriferos dos especias Epicalia acontius, Lin. e de Myscelia orsis, Dru. (Arch. Mus. Nac. Rio Janeiro, ii, 1879, pp. 31–35.)

—— Os orgaos odoriferos nas pernas de certos Lepidopteres. (Arch. Mus. Nac. Rio Janeiro, ii, 1879, pp. 37–46, 3 Pls.)

—— Os orgaos odoriferos da Antirrhœa archœa. (Arch. Mus. Nac. Rio Janeiro, iii, 1878, pp. 1–7, 1 Pl.)

—— A prega costal das Hesperideas. (Arch. Mus. Nac. Rio Janeiro, iii, 1880, pp. 41–50, 2 Pls.)

=Weismann, August.= Ueber Duftschuppen. (Zool. Anzeiger, i, 1878, pp. 98, 99.)

=Arnhart, L.= Sexundäre Geschlechtscharaktere von Acherontia atropos. (Verh. d. k. k. zool. bot. Ges. Wien, xxix, 1879, p. 54.)

=Bertkau, Philipp.= Duftapparat an Schmetterlingsbeinen. (Ent. Nachrichten, 1879, Jahrg., pp. 223, 224.)

—— Ueber den Duftapparat von Hepialus hecta. (Archiv f. naturg., xlviii Jahrg., 1882, pp. 363–370, Figs.; also in Biol. Centralblatt., ii Jahrg., 1882, pp. 500–502.)

—— Ergänzung (Duftvorrichtungen bei Lepidopteren). (Ent. Nachr., 1880, p. 206.)

—— Entomologische Mizellen. 1. Ueber Duftvorrichtungen einiger Schmetterlinge. (Verh. d. naturhist. Ver. d. preuss. Rheinlande und Westf., 1884, pp. 343–350.)

=Reichenau, W. von.= Der Duftapparat von Sphinx ligustri. (Ent. Nachr., 1880, p. 141; also Kosmos, iv Jahrg., 1880, pp. 387–390.)

=Fügner, R.= Duftapparat bei Sphinx ligustri. (Ent. Nachr., 1880, p. 166.)

=Lelievre, Ernest.= (Note in Le Naturaliste, June 1, 1880. Both sexes of Thais polyxena emit an odorous exhalation. Notes on exhalation from Spilosoma fuliginosa.)

=Hall, C. G.= Peculiar odor emitted by Acherontia atropos. (Entomologist, London, xvi, p. 14.)

=Åurivillius, Christopher.= Ueber secundäre Geschlechtscharactere nordischer Tagfalter. (Stockholm, 1880, Bihang till K. Svensk. Vet. Akad. Handl., v, pp. 56, 3 Taf.)

—— Des caractères sexuels secundaires chez les papillons diurnes. (Ent. Tidskrift, 1880, pp. 163–166.)

—— Anteckningar om några skandinaviska fjärilarter. (Ent. Tidskr., iv, Årg., 1884, pp. 33–37; Résumé (French), ibid., pp. 55–57.)

=Kirby, W. F.= Fans on the fore legs of Catocala fraxini. (Papilio, ii, p. 84, 1882.)

=Bailey, James S.= Femoral tufts or pencils of hair in certain Catocalæ. (Papilio, ii, 1882, pp. 51, 52, 146; also in Stettin Ent. Zeitung, xliii, p. 392.)

=Edwards, Henry.= Fans on the feet of Catocaline moths. (Papilio, ii, p. 146, 1882.)

=Stretch, R. H.= Anal appendages of Leucarctia acræa. (Papilio, iii, pp. 41, 42, 1883, 1 Fig.)

=Weed, Clarence M.= Appendages of Leucarctia. (Papilio, iii, 1883, p. 84.)

=Grote, Aug. R.= Appendages of Leucarctia acræa. (Papilio, iii, 1883, p. 84.)

=Haase, Erich.= Ueber sexuelle Charactere bei Schmetterlingen. (Zeitschr. f. Ent., Breslau, N. F., 1885, pp. 15–19, 36–44; also Ent. Nachr., xi Jahrg., pp. 332, 333.)

—— Duftapparate indo-australischer Schmetterlinge. (Corresp. Blatt. Ent. Ver. Iris, Dresden, 1886, pp. 92–107, 1 Taf.; ibid., 1887, pp. 159–178; ibid., 1888, pp. 281–336.)

—— Ueber Duftapparate bei Schmetterlingen. (Sitzgsber. Nat. Ges. Iris, Dresden, 1886, pp. 9–10; Abstr. in Journ. R. Micr. Soc., vi, pp. 969–970, 1886.)

—— Der Duftapparate von Acherontia. (Zeitschr. f. Ent., Breslau, N. F., 1887, pp. 5–6.)

—— Dufteinrichtung indischer Schmetterlinge. (Zool. Anzeiger, 1888, pp. 475–481.)

=Dalla Torre, K. W. von.= Die Duftapparate der Schmetterlinge. (Kosmos, 1885, ii, pp. 354–364, 410–423; Abstr. by J. B. Smith in Proc. Ent. Soc., Washington, i, pp. 38, 1888.)

=Smith, John B.= Cosmosoma omphale. (Entomologica Americana, i, pp. 181–185, 1886. Describes and figures cavities in under side of 2d–4th abdominal segments of male, filled with a silky substance. This may be for display to attract ♀, as the whole mass must be very conspicuous when protruded. No odor noticed.)

—— Scent organs in some Bombycid moths. (Entomologica Americana, ii, No. 4, pp. 79–80, 1886. Describes and figures long, slender, forked hairy, orange or white, eversible glands, everted from between 7th and 8th segments of abdomen of ♂ of Leucarctia acræa, Pyrrharctia isabella, Scepsis fulvicollis, and Cosmosoma omphale.)

—— [Notes on odors and odoriferous structures of various moths and a note by L. O. Howard on odor of Dynastes.] (Proc. Ent. Soc., Washington, i, pp. 40, 55, 56.)

=Müller, W.= Duftorgane der Phryganiden. (Archiv f. Naturgesch., 1887, Jahrg. liii, pp. 95–97.)

=Pollack, W.= Duftapparate der Hadena atriplicis und Litargyria. (xv Jahrb. Westphäl. Prov. Ver. Münster, 1887, p. 16.)

=Patton, W. H.= Scent-glands in the larva of Limacodes. (Can. Ent., 1891, xxiii, pp. 42, 43.)

=Garman, H.= On a singular gland possessed by the male Hadenœcus subterraneus. (Psyche, 1891, p. 105, 1 Fig.)

=Barrett, C. G.= Scent of the male Hepialus humuli. (Ent. Month. Mag., Ser. 2, iii, 1892, p. 217. Arises from the curiously aborted and altered hind tibiæ.)

Also the writings of Baillif, Duponchel, F. Müller, Scudder (Psyche, iii, p. 278, 1881), Burgess, Keferstein, Alpheraky, Plateau, Marshall and Nicéville, Wood-Mason, White, Hampson.

THE ORGANS OF CIRCULATION

Although Malpighi was the first to discover the heart in the young silkworm, it was not until 1826 that Carus proved that there was a circulation of blood in insects, which he saw flowing along each side of the body, and coursing through the wings, antennæ, and legs of the transparent larva of Ephemera, though three years earlier Herold demonstrated that the dorsal vessel of an insect is a true heart, pulsating and impelling a current of blood towards the head. This discovery was extended by Straus-Dürckheim, who discovered the contractile and valvular structures of the heart. It is noteworthy that both Cuvier and Dufour denied that any circulation, except of air, existed in insects; and so great an anatomist as Lyonet doubted whether the dorsal vessel was a genuine heart, though he pointed out the fact that there are no arteries and veins connected with this vessel. Another French anatomist, Marcel de Serres, thought that the dorsal vessel was merely the secreting organ of the fat-body.

The so-called peritracheal circulation claimed by Blanchard and by Agassiz has been shown by McLeod to be an anatomical impossibility, the view having first been refuted by Joly in 1849.

Except the aorta-like continuation in the thorax and head which divides into two short branches, there are, with slight exceptions (p. 405), no distinct arteries, such as are to be found in the lobster and other Crustacea, and no great collective veins, such as exist in Crustacea and in Limulus. This is probably the result of a reduction by disuse in the circulatory system, since in myriopods (Julidæ and Scolopendridæ) lateral arteries are said to diverge near the ostia.

a. The heart

The heart or “dorsal vessel” is a delicate, pulsating tube, situated just under the integument of the back, in the median line of the body, and above the digestive canal. It can be partially seen without dissection in caterpillars. It is covered externally and lined within by membranes which are probably elastic; and between these two membranes extends a system of delicate muscular fibres, which generally have a circular course, but sometimes cross each other. The heart is divided by constrictions into chambers, separated by valvular folds. The internal lining membrane referred to forms the valvular folds separating the chambers. Each of these chambers has, at the anterior end, on each side, a valvular orifice (Fig. 370, ostium, i) which can be inwardly closed.

Miall and Denny thus describe the different layers of the wall of the heart of the cockroach:

“There are: (1) a transparent, structureless intima, only visible when thrown into folds; (2) a partial endocardium, of scattered, nucleated cells, which passes into the interventricular valves; (3) a muscular layer, consisting of close-set, annular, and distant, longitudinal fibres. The annular muscles are slightly interrupted at regular and frequent intervals, and are imperfectly joined along the middle line above and below, so as to indicate (what has been independently proved) that the heart arises as two half-tubes, which afterwards join along the middle. Elongate nuclei are to be seen here and there among the muscles. The adventitia (4), or connective tissue layer, is but slightly developed in the adult cockroach.”

Graber says that the heart of insects may be regarded not as an organ de novo, but only as the somewhat modified contractile dorsal vessel of the annelids, in which, however, the transverse arteries arising on each side became, with the gradual development of the tracheæ, superfluous and finally abortive. He describes it as a muscular tube composed of very delicate annular fibres, which within and without is covered by a relatively homogeneous, strong, elastic membrane.

The division into separate chambers is effected by means of a folding inwards and forwards of the entire muscular wall. “A portion of each side of the heart is first extended inwards so as very nearly to meet a corresponding portion from the opposite side, and then, being reflected backwards, forms, according to Straus (Consid., etc., p. 356), the interventricular valve which separates each chamber from that which follows it. Posteriorly to this valve, at the anterior part of each chamber, is a transverse opening or slit (Fig. 371, b), the auriculo-ventricular orifice, through which the blood passes into each chamber, and immediately behind it is a second, but much smaller, semilunar valve (c), which, like the first, is directed forwards into the chamber. It is between these two valves on each side that the blood passes into the heart, and is prevented from returning by the closing of the semilunar valve. When the blood is passing into the chamber, the interventricular valve is thrown back against the side of the cavity, but is closed when, by the contraction of the transverse fibres, the diameter of each chamber is narrowed, and the blood is forced along into the next chamber.” (Newport.)

According to Müller, there is but a single pair of ostia in Phasma, and, in the larva of Corethra, the heart is a simple, unjointed tube, not divided into chambers, and Viallanes states that, in the very young larva of Musca, there are no ostia (Kolbe). In the larva of Ptychoptera, Grobben found a short oval heart, with one pair of ostia situated in the 6th abdominal segment; a long aorta proceeds from it, the thoracic portion of which pulsates; from behind the heart arises a pulsating pouch, which connects with the hinder aorta, which does not pulsate, and ends at the base of two tracheal gills. Burmeister was able to find only four pairs of openings in the larva of Calosoma. Newport states that, while Straus figures nine chambers in Melolontha, and, consequently, eight pairs of openings, he has not been able to observe more than seven pairs of openings in Lucanus cervus. He has invariably found eight pairs of openings both in the larva and imago of Sphinx ligustri, as well as in other Lepidoptera. According to Béla-Dezso, the number of pairs of ostia corresponds to that of the pairs of stigmata.

There also occur, on each side of each chamber, two so-called pear-shaped bodies which are separated from the tubular portion of the heart itself, but, by means of muscular fibres, are united with the chamber and with their valves. These pyriform bodies appear as vesicles or cells with granular contents, besides some nuclei with nucleoli. They are of very small size. According to the measurements of Dogiel, in the larva of Corethra plumicornis, they are 0.02 to 0.1 mm. long, and 0.06 to 0.08 mm. broad. He regards these peculiar bodies as apolar nerve-cells of the heart. (Kolbe.)

Besides the venous openings of the heart which open into the pericardial region, Kowalevsky has discovered, in the heart of some Orthoptera (Caloptenus, Locusta, etc.), five pairs of openings by which the cardiac chambers receive the blood of the peri-intestinal region. Graber had divided the cœlom of insects into three regions (pericardial, peri-intestinal, and perineural regions), and hitherto only a union of the heart with the pericardial region by slit-like openings was known. These openings are symmetrically distributed on five abdominal segments; each section of the heart in this region has, therefore, four openings, which are all of a truly venous nature. These openings, called cardio-cœlomic apertures, are visible to the naked eye, being situated on conical papillæ of the walls of the heart. These papillæ pass through the outer diaphragm, and open into the peri-intestinal part of the cœlom, in the Acrydiidæ directly, in the Locustidæ through special canals. The cells of the papillæ are spongy, possessing large nuclei, and similar, as a whole, to glandular cells. (Comptes rendus, cxix, 1894.)

The mechanism by which the ostia are closed consists, according to Graber, of an ∞-shaped muscle passing around the two openings, and which, being interlaced, is sufficient to close the openings. But this is not all. The fore and hinder edge of the ostia project, leaf-like, into the cavity of the heart, and thus form, with the outer walls, two valves which, during the systole, filled with the blood rushing in, not only hermetically close the lateral openings, but also, by the simultaneous closure of the entire chamber by the circular muscles in the middle of the same, the two valves, simultaneously approaching each other, so nearly touch that they form a transverse partition wall in the chamber. But, for the last purpose, i.e. for the separation of the chambers from one another, there is a very special contrivance. In the May beetle, we find, besides a valve (Fig. 373, B, e), opening into the middle of the chambers, a large, stalked cell (d), which, in the diastole, i.e. in the expansion of the heart, hangs down free on the walls of the heart; but, in the systole or contraction, like a cork, closes the middle of the valve, but does not wholly close the cavity. He has observed, in the larva of Corethra, formal, interventricular valves, which also are not in the middle, but are separated from one another in the interlaced ends. They consist of two longitudinally membranous flaps which move against each other like two valves (Fig. 373, B, b).

“But what is the necessity for such a complicated mechanism? All the blood from behind passes into the heart, and, for its propulsion a simple muscular tube, whose circular fibres would draw together and contract it, would be thought to be sufficient. But the heart, except in some larvæ, ends posteriorly in a blind sac, and the blood can only pass into it by a series of pairs of lateral openings. Now, as regards the reception and the propulsion of the blood forwards, two modes are conceivable. The simplest way would be that the tubular heart should, along its whole length, contract or expand; that, moreover, the blood should be simultaneously sucked in through all the openings, and that then, also, the contraction, or systole, should take place in every part of the heart at the same moment. But this would, plainly, in so long and thin-walled a vessel, be highly impracticable, since, through such a manipulation, the mass of blood enclosed in the heart would be crowded together rather than really impelled forwards. Only the second case could be admissible, and that is this, that each chamber pulsates, one after another, from behind forwards. But, then, each segmental heart must be separated from the others by a valve. To make the matter wholly clear, we may observe an insect heart pulsating, and this is best seen in one of its middle chambers. This chamber expands (simply by the relaxation of its circular muscles), the ostia, also, consequently open, and a given quantity of blood is drawn in from the pericardial cavity. What now would happen after the succeeding contraction if there were no valves between? The blood would not flow forwards, but seek a way out backwards.

“But, in fact, the valve of the hinder chamber, at this time, closes itself, while, by the simultaneous expansion of the anterior ones, their door opens, and this section of the heart, at the same time, causes a sucking in of the contents of the posterior chamber. This phenomenon is repeated, in the same way, from chamber to chamber, which also acts alternately as ventricle and auricle, or by a sucking and pumping action. One is involuntarily reminded of the ingenious manipulation by which, by the alternate opening and shutting of the flood-gates, a vessel is carried along a canal.

“This wave-like motion of an insect’s heart also has the advantage that, just before a pulse-wave has reached the chambers farthest in front, the hinder ones are already prepared for the production of a second, for, as a matter of fact, often 60, and even 100, and, in very agile insects, 150, waves pass, in a single minute, through the series of chambers, which make it very difficult to follow the flowing of their waves.” (Graber.)

=The propulsatory apparatus.=—But the heart itself is only a part of the entire propulsatorial apparatus to which belongs the following contrivance, the nature of which has been worked out by Graber.

Under the dorsal vessel is stretched a sort of roof-like diaphragm, i.e. a membrane, arched like the dorsal wall of the hind-body which is attached, in a peculiar way, to the sides of the body. The best idea can be gained by a cross-section through the entire body (Fig. 374): H is the true dorsal vessel; S, the diaphragm. A surface view is seen at 373, C, b, where it appears as a plate with the edge regularly curved outwards on each side. Its precise mode of working is thus: from each dorsal band of the sides of the abdomen arises a pair of muscles spreading out fan-like, and extending to the heart, so that the fibres of one side pass directly over to those of the other, often splitting apart, or, between the two, extends outwards a perforated, thin web, like an elastic, fibrous sheet (Fig. 373, A, a), with numerous perforations, forming a diaphragm.

Graber has thus explained the action of the pericardial diaphragm and chamber, as freely translated by Miall and Denny: “When the alary muscles contract, they depress the diaphragm, which is arched upwards when at rest. A rush of blood towards the heart is thereby set up, and the blood streams through the perforated diaphragm into the pericardial chamber. Here it bathes a spongy or cavernous tissue (the fat-cells), which is largely supplied with air-tubes, and having been thus aerated, passes immediately forwards to the heart, entering it at the moment of diastole, which is simultaneous with the sinking of the diaphragm.”

In the cockroach, however, Miall and Denny think that the facts of structure do not altogether justify this explanation: “The fenestræ of the diaphragm are mere openings without valves. The descent of a perforated non-valvular plate can bring no pressure to bear upon the blood, for it is not contended that the alary muscles are powerful enough to change the figure of the abdominal rings.... The diaphragm appears to give mechanical support to the heart, resisting pressure from a distended alimentary canal, while the sheets of fat-cells, in addition to their proper physiological office, may equalize small local pressures, and prevent displacement. The movement of the blood towards the heart must (we think) depend, not upon the alary muscles, but upon the far more powerful muscles of the abdominal wall, and upon the pumping action of the heart itself.”

“The peculiar office,” says Graber, “performed by the heart has already been stated. It is nothing more than a regulator; than an organ for directing the blood in a determinate course in order that this may not wholly stagnate, or only be the plaything of a force acting in another way, as, for example, through that afforded by the body-cavity and the inner digestive canal. At regular intervals a portion of the blood is sucked through the same, and then by means of the anterior supply tube it is pushed onward into the head, whence it passes into the cavities of the tissues. The different conditions of tension under which the mass of blood stands in the different regions of the body then causes a farther circulation. Besides this, the blood passes through separate smaller pumping apparatuses, and through vessel-like modifications of cavities, also through hollow spaces between the muscles, as, for example, in the appendages where a regular backward and forward flow of the blood, especially in the limbs, wings, antennæ, and certain abdominal appendages takes place. Here and there may occasionally occur a narrow place where the flow of blood is obstructed by the accumulation of the blood corpuscles, causing a considerable stagnation.” (Graber.)

=The supraspinal vessel.=—In many insects there is a ventral heart acting on the heart’s blood as an aspirator, or more correctly a ventral sinus lying on the nervous cord, and closed by a pulsating diaphragm. This was discovered by Réaumur in the larva of a fly, and by Graber in the dragon-fly and locusts (Acrydiidæ). A glance at Figs. 375 and 376 will save a long description. The ventral wall forms a furrow, and between its borders (Fig. 377, e) extends the diaphragm. During the contraction of the muscles—and this, here, acts from before backwards—the membrane rises up and makes a cavity for the blood, which passes backwards over the nervous cord. The dorsal and ventral sinuses together thus bring about a closed circulation.

It thus appears that the insects are well provided with the means of distribution of their nutritive fluid, and that the blood is kept continually fresh and rich in oxygen. (Graber.)

=The aorta.=—While the heart is mostly situated within the abdomen, it is continued into the thorax and the head as a simple, non-pulsating tube, called the aorta. In Sphinx the aorta, as described by Newport, begins at the anterior part of the 1st abdominal segment, where it bends downwards to pass under the metaphragma and enter the thorax; it then ascends again between the great longitudinal dorsal muscles of the wings, and passes onwards until it arrives at the posterior margin of the pronotum; it then again descends and continues its course along the upper surface of the œsophagus, with which it passes beneath the brain, in front of which and immediately above the pharynx, it divides into two branches, each of which subdivides. Newport, however, overlooked a thoracic enlargement of the aorta called by Burgess the “aortal chamber” (Fig. 310, a, c).

“In Sphinx and Vanessa urticæ, immediately after the aorta has passed beneath the cerebrum, it gives off laterally two large trunks, which are each equal in capacity to about one-third of the main vessel. These pass one on each side of the head, and are divided into three branches which are directed backwards, but have not been traced farther in consequence of their extreme delicacy. Anterior to these trunks are two smaller ones which appear to be given to the parts of the mouth and antennæ, and nearer the median line are two others which are the continuations of the aorta. These pass upwards, and are lost in the integument. The whole of these parts are so exceedingly delicate that we have not, as yet, been able to follow them beyond their origin at the termination of the aorta, but believe them to be continuous, with very delicate, circulatory passages along the course of the tracheal vessels. It is in the head alone that the aorta is divided into branches, since, throughout its whole course from the abdomen, it is one continuous vessel, neither giving off branches, nor possessing lateral muscles, auricular orifices, or separate chambers.” (Newport, art. Insecta, p. 978.)

Dogiel observed in the transparent larva of Corethra plumicornis that the aorta extends only to the hinder border of the brain. Here it divides into two lamellæ, each of which independently extends farther on. One lamella is seen under the brain and under the eye, the other reaches near the eye. The lamellæ are tied to the integument by threads. At the point of division of the aorta is an opening. (Kolbe.)

True blood-vessels appear to exist in the caudal appendages of the May-flies, as the heart appears to divide and pass directly into them (Fig. 378). The last chamber of the heart diminishes in size at the end of the body, and then divides into three delicate tubular vessels which pass into the three caudal appendages, and extend to the end of each one, along the upper side. While the valves of the heart, in all insects, are directed anteriorly because the blood flows from behind, in the larva of the Ephemeridæ the valves of the last chamber of the heart are directed backwards, because from this chamber the blood flows in the opposite direction, i.e. into the caudal appendages. During the contraction of the heart, the elongated section of the same in the last abdominal segment receives a part of the mass of blood contained in the last chamber, which is driven by independent contractions into the caudal appendages. These vessels have openings before the end through which the blood enters into the cavity of the appendages, and can also pass back, in order to be taken up by the body cavity. It is possible that these blood-vessels stand in direct relation to respiration. (Zimmermann, Creutzburg, in Kolbe, p. 544.)

=The pericardial cells.=—Along the heart, on both sides, occur the so-called pericardial cells, which differ from the fat-cells, and also the peritracheal cells of Frenzel, and are mostly arranged in linear series, which have a close relation to the circulation of the blood. In the larva of Chironomus, they lie in groups; in that of Culex, they are arranged segmentally. In caterpillars, these pericardial cells are not situated in the region of the heart, but are arranged linearly on the side, and form a network of granulated cells situated between the fat-bodies. Other rows of these cells are situated near the stigmata and the main lateral tracheæ. (Kolbe.)

According to Kowalevsky, the pericardial cells, and the garland-shaped, cellular cord consist of cells, whose function it is to purify the blood, and to remove the foreign or injurious matters mingled with the blood.

=Ampulla-like blood circulation in the head.=—In the head of the cockroach occurs, according to Pawlowa, a contractile vascular sac at the base of each antenna. The cavity has a valvular communication with the blood space below and in front of the brain, and muscle-fibres effect systole and diastole. Each sac is beyond doubt an independently active part of the circulatory system. These organs also occur in Locusta and other Acrydiidæ, and Selvatico has described similar structures in Bombyx mori and certain other Lepidoptera.

=Pulsatile organs of the legs.=—Accessory to the circulation is a special system of pulsatile organs in the three pairs of legs of Nepidæ, generally situated in the tibia just below its articulation with the femur, but in the fore legs of Ranatra, in the clasp-joint or tarsus, just below its articulation with the tibia. First observed by Behn (1835), Locy has studied the organ (Fig. 380) in Corixa, Notonecta, Gerris, besides the Nepidæ. It is a whip-like structure attached at both ends, with fibres extending upward and backward to the integument of the leg, separate from the muscular fibres and does not involve them in its motions, and is not affected by the muscles themselves. “As the blood-corpuscles flow near the pulsating body they move faster, and around the organ itself there is a whirlpool of motion.” The beating of these organs aids the circulation in both directions, and when the motion ceases, the blood-currents in the legs stop; the rate of the pulsating organ is always faster than that of the heart, and the action is automatic.

b. The blood

The blood of insects, as in other invertebrates, differs from that of the higher animals in having no red corpuscles. It is a thin fluid, a mixture of blood (serum) and chyle, usually colorless, but sometimes yellowish or reddish, which contains pale amœboid corpuscles corresponding to the white corpuscles (leucocytes) of the vertebrates, though they are relatively less numerous in the blood of insects. The yellow fluid expelled from the joints of certain beetles (Coccinella, Timarcha, and the Meloidæ) is, according to Leydig, only the serum of the blood. In phytophagous insects the blood is colored greenish by the chlorophyll set free during digestion. The blood of Deilephila euphorbia is colored an intense olive-green, and that of Cossus ligniperda is pale yellow. (Urech.) The blood of case-worms (Trichoptera) is greenish. In some insects it is brownish or violet. The serum is the principal bearer of the coloring material, yet Graber has shown that in certain insects the corpuscles are more or less beset with bright yellow or red fat-globules, so as to give the same hue to the blood.

=The leucocytes.=—The corpuscles are usually elongated, oval, or flattened oat-shaped, with a rounded nucleus, or are often amœbiform; and they are occasionally seen undergoing self-division. When about to die the corpuscles become amœbiform or star-shaped. (Cattaneo.) Their number varies with the developmental stage of the insect, and in larvæ increases as they grow, becoming most abundant shortly before pupation. The blood diminishes in quantity in the pupal stage, and becomes still less abundant in the imago. (Landois.) The quantity also varies with the nutrition of the insect, and after a few days’ starvation nearly all the blood is absorbed. Crystals may be obtained by evaporating a drop of the blood without pressure; they form radiating clusters of pointed needles. The freshly drawn blood is slightly alkaline. (Miall and Denny.)

The size of the corpuscles has been ascertained by Graber, who found that the diameter of the circular blood-disks of the leaf-beetle, Lina populi, is 0.006 mm.; of Cetonia aurata and Zabrus gibbus, 0.008 to 0.01 mm.; and those of certain Orthoptera (Decticus verrucivorus, Ephippiger vitium and Œdipoda cœrulescens), 0.011 to 0.014 mm. The longest diameter of the elongated corpuscles of Carabus cancellatus is 0.008 mm.; of Gryllus campestris, Locusta viridissima, Cossus ligniperda, Sphinx ligustri (pupa), and others, 0.008 to 0.01 mm.; of Caloptenus italicus, Saturnia pyri, Anax formosus, and others, 0.011 to 0.014 mm.; of Ephippiger vitium, Œdipoda cœrulescens, Pezotettix mendax, Zabrus gibbus, Phryganea, and others, 0.012 to 0.022 mm.; in Stenobothrus donatus and variabilis, 0.012 to 0.035 mm. The largest known are those of Melolontha vulgaris, which measure from 0.027 to 0.03 mm.

As regards the nature of the corpuscles, Graber concludes that they are more like the cells of the fat-bodies than genuine cells. That they are not true cells is shown by the fact that after remaining in their normal condition a long time they finally coalesce and form cords. After shrivelling, or after the blood has been subjected to different kinds of treatment, the nucleus is clearly brought out (Fig. 381).

Besides the blood corpuscles there have been detected in the blood round bodies which are regarded as fat-cells. They are circular, and for the most part larger than the blood corpuscles, have a sharp, even, dark outline, and an invariably circular nucleus. (Kolbe.)

The blood of Meloe, besides the amœboid corpuscles, according to Cuénot, contains abundant fibrinogen, which forms a clot; a pigment (uranidine), which is oxidized and precipitated when exposed to the air; a dissolved albuminoid (hæmoxanthine), which has both a respiratory and nutritive function; and, finally, dissolved cantharidine.

The corpuscles arise from tissues which are very similar to the fat-bodies, and which, at given times, separate into cells. The position of these tissues is not always the same in different insects. In caterpillars, they occur in the thorax, near the germs of the wings; in the saw-flies (Lyda), in all parts of the thorax and abdomen; in larval flies (Musca), in the end of the abdomen, just in front of the large terminal stigmata. The place where the blood corpuscles are formed is usually near, or in relation with, the fat-bodies. But while the fat-bodies mostly serve as the material for the formation of the blood-building tissues, in caterpillars the tracheal matrix also, and, in dipterous larvæ, the hypodermis serve this purpose. (Cæsar Schaeffer in Kolbe. See also Wielowiejski, Ueber das Blutgewebe.)

Other substances occur in the blood of insects. Landois (1864) demonstrated the existence of egg albumen, globulin, fibrin, and iron in the blood of caterpillars. Poulton found that the blood of caterpillars often contained chlorophyll and xanthophyll derived from their food plants. A. G. Mayer has recently found that the blood (hæmolymph) of the pupæ of Saturniidæ (Callosamia promethea) contains egg albumin, globulin, fibrin, xanthophyll, and orthophosphoric acid, and Oenslager has determined that iron, potassium, and sodium are also present. (Mayer.)

c. The circulation of the blood

Every part of the body and its appendages is bathed by the blood, which circulates in the wings of insects freshly emerged from the nymph or pupal state, and even courses through the scales of Lepidoptera, as discovered by Jaeger (Isis, 1837).

In describing the mechanism of the heart we have already considered in a general way the mode of circulation of the blood.

The heart pumping the blood into the aorta, the nutritive fluid passes out and returns along each side of the body; distinct, smaller streams passing into the antennæ, the legs, wings (of certain insects), and into the abdominal appendages when they are present. All this may readily be observed in transparent aquatic insects, such as larval Ephemeræ, dragon-flies, etc., kept alive for the purpose under the microscope in the animalcule box.

Carus, in 1827, first discovered the fact of a complete circulation of the blood, in the larva of Ephemera. He saw the blood issuing in several streams from the end of the aorta in the head and returning in currents which entered the base of the antennæ and limbs in which it formed loops, and then flowing into the abdomen, entered the posterior end of the heart. Wagner (Isis, 1832) confirmed these observations, adding one of his own, that the blood flows backward in two venous currents, one at the sides of the body and intestine, and the other alongside of the heart itself, and that the blood not only entered at the end of the heart, but also at the sides of each segment, at the position of the valves discovered by Straus-Dürckheim.

Newport maintains that the course of the blood is in any part of the body, as well as in the wings, almost invariably in immediate connection with the course of the tracheæ, for the reason that “the currents of blood in the body of an insect are often in the vicinity of the great tracheal vessels, both in their longitudinal and transverse direction across the segments.”

The circulation of the blood in the wings directly after the exuviation of the nymph or pupa skin, and before they become dry, has been proved by several observers. As stated by Newport, the so-called “veins” or “nervures” of the wings consist of tracheæ lying in a hollow cavity, the peritracheal space being situated chiefly under and on each side of the trachea.

Newport gives the following summary of the observations of the early observers, to which we add the observations of Moseley. “A motion of the fluids has been seen by Carus in wings of recently developed Libellulidæ, Ephemera lutea and E. marginata, and Chrysopa perla; among the Coleoptera, in the elytra and wings of Lampyris italica and L. splendidula, Melolontha solstitialis and Dytiscus.” Ehrenberg saw it in Mantis, and Wagner in the young of Nepa cinerea and Cimex lectularius. Carus detected a circulation in the pupal wings of some Lepidoptera, and Bowerbank witnessed it in a Noctuid (Phlogophora meticulosa); Burmeister observed it in Eristalis tenax and E. nemorum, and Mr. Tyrrel in Musca domestica, but it has not been observed in the wings of Hymenoptera.

Bowerbank observed that in the lower wing of Chrysopa perla the blood passes from the base of the wing along the costal, post-costal, and externo-medial veins, outwards to the apex of the wing, giving off smaller currents in its course, and that it returns along the anal vein to the thorax. He found that the larger veins, 1⁄408 in. in diameter, contained tracheæ which only measured 1⁄2222 of an inch in diameter; but in others the tracheæ measured 1⁄1340, while the cavity measured only 1⁄500 of an inch. He states, also, that the tracheæ very rarely give off branches while passing along the main veins, and that they lie along the canals in a tortuous course. (Newport, art. Insecta, p. 980.)

Bowerbank, also, in his observations on the circulation in the wings of Chrysopa, “used every endeavor to discover, if possible, whether the blood has proper vessels, or only occupied the internal cavities of the canals; and that he is convinced that the latter is the case, as he could frequently perceive the particles not only surrounding all parts of the tracheæ, and occupying the whole of the internal diameter of the canals, but that it frequently happens that globules experienced a momentary stoppage in their progress, occasioned by their friction against the curved surface of the tracheæ, which sometimes gave them a rotatory motion.”

Moseley found, owing to the large size and number of the corpuscles, that the circulation of the blood in the wings of insects is most easily observed in the cockroach, especially the hind wings. As seen in Moseley’s figure, the blood flows outward from the body through the larger veins (I and II) of the front edge of the wings, which he calls the main arteries of the wings, and more generally returns to the body through the veins in the middle of the wing; the blood also flows out from the body through the inner longitudinal veins (those behind vein IV), and the blood is also seen to flow through some of the small cross-veins. Fig. 383 shows one of the main trunks during active circulation. The corpuscles change their form readily, “the spindle-shaped ones doubling up in order to pass crossways through a narrow aperture.... In the irregularly formed corpuscles, which seem to represent leucocytes amœboid movements were observed.... Corpuscles pass freely above and under the tracheæ, showing that these latter lie free in the vessels.” The hypodermis lining the vessels is best seen in the small transverse veins.

The pulse or heart-beat of insects varies in rapidity in different insects, rising at times of excitement, as Newport noticed in Anthophora retusa, to 142 beats in a minute.

When an insect, as, for example, a tineid caterpillar, has been enclosed in a tight box for a day or more, the pulsations of the heart are very languid and slow, but soon, on giving it air, the pulsations will, as we have observed, rise in frequency to about 60 a minute, Herold observed 30 to 40 in a minute in a fully-grown silkworm, and from 46 to 48 in a much younger one. Suckow observed but 30 a minute in a full-grown caterpillar of Gastropacha pini, and 18 only in its pupa.

In a series of observations made by Newport on Sphinx ligustri from the fourth day after hatching from the egg until the perfect insect was developed, he found that before the larva cast its first skin the mean number of pulsations, in a state of moderate activity and quietude, was about 82 or 83 a minute; before the second moult 89, while before the third casting it had sunk down to 63; and before its fourth to 45, while, before leaving its fourth stage, and before it had ceased to feed, preparatory to pupating, the pulse was not more than 39. “Thus the number gradually decreases during the growing larva state, but the force of the circulation is very much augmented. Now when the insect is in a state of perfect rest, previously to changing its skin, the number is pretty nearly equal at each period, being about 30. When the insect has passed into the pupa state it sinks down to 22, and subsequently to 10 or 12, and after that, during the period of hibernation, it almost entirely ceases. But when the same insect which we had watched from its earliest condition was developed into the perfect state in May of the following spring, the number of pulsations, after the insect had been for some time excited in flight around the room, amounted to from 110 to 139; and when the same insect was in a state of repose, to from 41 to 50. When, however, the great business of life, the continuation of the species, has been accomplished, or when the insect is exhausted, and perishing through want of food or other causes, the number of pulsations gradually diminishes, until the motions of the heart are almost imperceptible.” Insects, then, he remarks, do not deviate from other animals in regard to their vital phenomena, though it has been wrongly imagined that the nutrient and circulatory functions are less active in the perfect than in the larval condition.

The heart of a larval Gastrus equi taken the day previous from a horse’s stomach beat from 40 to 44 times a minute (Scheiber); while Schröder van der Kolk observed only 30 beats in the same kind of maggot.

In the larva of Corethra, while at rest, the heart contracts from 12 to 16 or 18 times a minute, but when active the number rises to 22. The systole and diastole last from 5 to 6 minutes. (Dogiel.)

Temperature also affects the pulsations, as they increase in frequency with a rise and decrease with a fall in temperature.

=Influence of electricity.=—The influence of electricity on the action of the insect’s heart, from Dogiel’s experiments, is such as to cause an acceleration in the frequency of the beats, while an increase in the strength of the electric currents either diminishes the frequency of the beats or entirely stops the heart’s action. A violent excitation with the induction current causes a systole when the heart’s action has stopped for a long time; and if the excitation lasts uninterruptedly, then the contractions after a while become noticeable, according to the strength of the current. In such a case there are, however, interruptions in the regularity, strength, and order of the contractions. (Kolbe.)

=Effects of poisons on the pulsations.=—Dogiel has also experimented on the influence of poisons in the form of vapor or as liquid solutions on the pulsations of insects, which is much as in vertebrates. The application of carbonic oxide to the larva of Corethra, whose heart one minute previous to the poisoning beat 15 times a minute, accelerated the heart-beats in about 55 minutes to 25 pulsations in a minute. Afterwards there was a retardation in the pulse to the normal beat. Carbonic acid had a similar effect.

The following results obtained by Dogiel are somewhat as tabulated by Kolbe:—

I. Substances which cause the pulsations of the heart to accelerate.

a. An induction current of electricity, acting feebly. b. Ammonia, acting feebly. c. Ethyl ether, acting feebly. d. Oxalic acid, acting feebly. e. Carbolic acid, acting feebly. f. Potassium nitrate, acting feebly. g. Aconite, acting feebly.

II. Substances retarding the heart’s action.

a. An induction current of electricity, acting energetically. b. Ammonia, acting energetically. c. Ethyl ether, acting energetically. d. Oxalic acid, acting energetically. e. Carbolic acid, acting energetically. f. Veratrine, acting energetically. g. Atropine, acting energetically. h. Aconitine, acting energetically. i. Potassium nitrate, acting energetically. g. Ethyl alcohol. h. Chloroform. i. Carbonic oxide. j. Carbonic acid. k. Sulphuretted hydrogen.

III. Substances whose action is indifferent.

1. Muscarine. 2. Curare. 3. Atropine, acting slowly. 4. Strychnine.

The above-named substances comprise those which in the vertebrates effect a change in the activity of the motor nerve-ganglia of the heart and the muscular fibres. Hence it follows that the heart of the larval Corethra consists of muscular fibres provided with ganglia, and that the contractions of the muscular fibres are provoked through the agency of the ganglia. But since muscarine, atropine, and curare, whose influence in stopping the heart’s action of vertebrates is known, in insects either have no action or only make the pulsations slower; it seems to follow that the heart of the larval Corethra possesses no similar apparatus for lessening the heart’s action, and this is also confirmed by anatomical studies. On the contrary, aconite acts, as we must from observations conclude, exclusively on the motor centres and the muscles, but not on the apparatus for lessening the heart’s action, which, as has been remarked, is not present in the larval Corethra. (Kolbe ex Dogiel.)

Dewitz has discovered an onward movement of the blood corpuscles, somewhat independent of the general circulation. This independent motion of the blood corpuscles is not only a creeping one like the amœboid motion of the white corpuscles of vertebrates, but they have besides a peculiar swimming movement. Dewitz noticed this in the hind wings of a recently emerged meal-worm beetle (Tenebrio molitor), still white and soft, after they had been cut off. The tissues forming the matrix within the wings constitute a network filled with blood. The current of blood within the wing thus cut off may be stopped flowing by a tap on the firmly clamped object-bearer on which the wing is placed, or by drawing it by an apparatus described by the same author, to incite in one way or another the blood corpuscles to swim forwards. When a corpuscle is disposed to move, we see it first stirring restlessly, or wabbling about, in this way changing its form; then it moves forwards, and does not come to a standstill. If it remains still there, after a while, by tapping, it begins again its movements.

“Should one yet doubt the fact of this spontaneous movement of the blood corpuscles, he will surely be convinced of its correctness by observing the so-to-speak reluctantly springing motion of a blood corpuscle in the wing of Tenebrio molitor with the simultaneous change of appearance and shape of the corpuscle.”

This spontaneous or independent motion of the blood corpuscles is also produced by the heating apparatus. As soon as the corpuscles lie still in the severed wing and they are warmed, the corpuscles begin to pass through the meshes of the tissue. When cooled, the motion ceases, but as soon as the temperature rises to a certain grade, the corpuscles again move onwards.

To explain this independent motion Dewitz thinks that they take up and then expel the blood-fluid, and in this way cause their motion. This independent motion is necessitated, in order that the stream of blood may become so regulated, that the blood corpuscles shall not be arrested in their course, but even turn back again out of the farther end of the antennæ and limbs. The chief mechanical power for the blood circulation must go on independently of the propulsatorial apparatus and of the heart. (Kolbe.)

LITERATURE ON THE HEART AND ON THE CIRCULATION OF THE BLOOD

a. Anatomy of the organs

=Meckel, J. F.= Ueber das Rückengefäss der Insekten. (Meckel’s Archiv, i, 1815, pp. 469–476.)

=Müller, J. G.= De vasi dorsali Insectorum. Berolini, 1816, pp. 22.

=Serres, P. Marcel de.= Observations sur les usages du vaisseau dorsal ou sur l’influence que le cœur exerce dans l’organisation des animaux articulés, etc. (Ann. du Mus. d’hist. nat., 1818, iv, pp. 149–192, 313–380, 2 Pls.; v, 1819, pp. 59–147, 1 Pl.)

=Herold.= Physiologische Untersuchungen über das Rückengefäss der Insekten. (Schriften d. Gesellsch. z. Beförderung d. Naturk. in Marburg, 1823, i, pp. 41–107.)

=Carus, C. G.= Entdeckung eines einfachen vom Herzen aus beschleunigten Blutkreislaufes in den Larven netzilügliger Insekten. Leipzig, 1827, pp. 40, 3 Taf.

—— Fernere Untersuchungen über Blutlauf in Kerfen. (Acta Acad. Leopold. Carol., 1831, xv, pp. 1–18, 1 Taf.)

=Stadelmayr, L.= Ansichten vom Blutlauf nebst Beobachtungen über das Rückengefäss der Insekten. Diss. München, 1829, pp. 24.

=Berthold.= Beitrage zur Anatomie, Zoologie und Physiologie. Göttingen, 1831.

=Treviranus, G. R.= Ueber das Herz der Insekten, dessen Verbindung mit den Eierstocken und ein Bauchgefäss der Lepidopteren. (Zeitschr. f. d. Physiologie, von F. Tiedemann. G. R. u. L. C. Treviranus, 1832, iv, pp. 181–184, 1 Taf.)

—— Beobachtungen aus der Zootomie und Physiologie. Bremen, 1839.

=Wagner, R.= Beobachtungen über Kreislauf des Blutes und den Bau des Rückengefässes bei den Insekten. (Isis, 1832, iii, p. 30; vii, pp. 320–331, 778–783, Fig.)

=Bowerbank, J. S.= Observations on the circulation of the blood in insects. (Ent. Mag., 1833, i, pp. 239–244, 1 Pl; also in Müller’s Archiv f. Physiolog., 1834, i, pp. 119–120.)

—— Observations on the circulation of the blood and the distribution of the tracheæ in the wing of Chrysopa perla. (Ent. Mag., 1837, iv, pp. 179–185.)

=Jaeger.= Ueber die Entdeckung von einer Bewegung in den Schuppen des Schmetterlingsflügel. (Isis, 1837, v, p. 512.)

=Behn, W.= Découverte d’une circulation de fluide nutritif dans les pattes de plusieurs insectes hémipteres. (Ann. Sc. nat., 1835, Sér. 2, iv, pp. 1–12.)

=Newport, G.= Insecta, in Todd’s Cyclopædia of Anatomy and Physiology, 1839. London, pp. 853–994. On the circulation of the blood, p. 976, Figs.

=Duvernoy, G. L.= Résumé sur le fluide nourricier, ses réservoirs et son mouvement dans tout règne animal. (Ann. Sc. nat., 1839, Sér. 2, xii, pp. 300–346.)

=Dufour, L.= Études anatomiques et physiologiques sur une mouche dans le but d’eclairer l’histoire des metamorphoses et de la prétendue circulation dans les insectes. (Ann. Sc. nat. Zool., Sér. 2, 1841, xvi, pp. 5–14.)

—— Note sur la prétendus circulation dans les insectes. (Compt. rend. Acad., Paris, 1844, xix, pp. 188–189.)

—— Études anatomiques et physiologiques sur une mouche, dans le but d’éclaircir l’histoire des metamorphoses et de prétendue circulation des insectes. (Mém. mathémat. des Savants étrangers, Paris, 1846, ix, pp. 545–628, 1 Pl.)

—— Sur la circulation dans les insectes. Bordeaux, 1849, 8º, pp. 40. (Compt. rend. Acad. Sci., Paris, 1849, xxviii, pp. 28–33, 101–104, 163–170.)

—— De la circulation du sang et de la nutrition chez les insectes. Bordeaux, 1851. (Act. Soc. Linn., Bordeaux, 1851, xvii, p. 9.)

—— Études anatomiques et physiologiques et observations sur les larves des Libellules, Appareil circulatoire. (Annal. Sci. nat., Sér. 3, Zool., xvii, 1852, pp. 98–101, 1 Pl.)

=Schröder van der Kolk, J. S. C.= Mémoire sur l’anatomie et physiologie du Gastrus equi. (N. Verhandl. Kl. Nederl. Instit., 11, 1845, pp. 1–155, 13 Pl.)

=Nicolet, H.= Note sur la circulation du sang chez les Coléoptères. (Ann. Sc. nat., 1847, Sér. 3, vii, pp. 60–64.)

=Verloren, C.= Mémoire en réponse à la question suivante: éclaircir par des observations nouvelles le phénomène de la circulation dans les insectes, en recherchant si on peut la reconnaître dans les larves de différents ordres de ces animaux. (Mém. couronn. et Mém. d. savants étrang. de l’Acad. Roy. Belgique, xix, 1847.)

=Blanchard, E.= De la circulation dans les insectes. (Ann. Sc. nat., 1848, Sér. 3, ix, pp. 359–398, 5 Pls.)

—— Sur la circulation du sang chez les insectes et sur la nutrition. (Compt. rend. Acad. Sc., Paris, 1849, xxviii, pp. 76–78; 1851, xxxiii, pp. 367–370.)

—— Nouvelles observations sur la circulation du sang et la nutrition chez les insectes. (Ibid., pp. 371–376.)

=Joly, N.= Mémoire sur l’existence supposée d’une circulation péritrachéenne chez les insectes. (Ann. Sc. nat. Zool., Sér. 3, 1849, xii, pp. 306–316.)

=Bassi, C. A.= Rapporto alla sezione di zoologia, anatomia comparata e fisiologia del congresso di Venezia, sul passagio delle materie ingerite nel sistema tracheale degli insetti. (Gazette di Milano, 1847, vi; also Ann. Sc. nat. Zool., Sér. 3, 1851, xv., 362–371.)

=Agassiz, Louis.= On the circulation of the fluids in insects. (Proceed. Amer. Assoc. Adv. Sc., 1849, pp. 140–143; Ann. Sc. nat. Zool., Sér. 3, xv, 1851, pp. 358–362.)

=Leydig, F.= Anatomisches und Histiologisches über die Larve von Corethra plumicornis. (Zeitschr. f. wissen. Zool., iii, 1851, pp. 435–451.)

=Wedl, C.= Ueber das Herz von Menopon pallidum. (Sitzungsber. der k. Akad. d. Wissensch. Wien., 1855, xvii, pp. 173–180.)

=Scheiber, S. H.= Vergleichende Anatomie und Physiologie der Oestriden Larven. (Sitzungsber. d. k. Akad. d. Wiss. Wien. Math.-naturwiss. Cl., xli, 1860, pp. 409–496, 2 Taf.; The circulatory system, pp. 463–490.)

=Brauer, Fr.= Beitrag zur Kenntnis des Baues und der Funktion der Stigmenplatten der Gastrus-Larven. (Verhdl. d. k. k. zool.-bot. Gesellsch. Wien., xiii, 1863, pp. 133–136.)

=Moseley, H. N.= On the circulation in the wings of Blatta orientalis and other insects, and on a new method of injecting the vessels of insects. (Quart. Jour. of Micr. Science, xi, n. s., pp. 389–395, 1871, 1 P1.)

=Graber, V.= Ueber die Blatkörperchen der Insekten. (Sitzber. Akad. Wien. Math.-naturw. Classe, lxiv, 1871, pp. 9–44, 1 Taf.)

—— Vorläufiger Bericht über den propulsatorischen Apparat der Insekten. (Sitzber. d. k. Ak. d. Wiss. Wien., lxv, 1872, pp. 16, 1 Taf.)

—— Ueber den propulsatorischen Apparat der Insekten. (Archiv f. mikroskop. Anatomie, ix, 1873, pp. 129–196, 3 Taf.)

—— Ueber den pulsierenden Bauchsinus der Insekten. (Archiv f. mikroskop. Anat., xii, 1876, pp. 575–582, 1 Taf.)

=Grobben, Carl.= Über bläschenförmige Sinnesorgane und eine eigenthümliche Herzbildung der Larve von Ptychoptera contaminata L. (Sitzb. k. Akad. Wissensch. Wien., 1875, lxxii, p. 22, 1 Taf.)

=Liebe, Otto.= Ueber die Respiration der Tracheaten, besonders über den Mechanismus derselben und über die Menge der ausgeatmeten Kohlensäure. Inaug.-Diss. Chemnitz, 1872, pp. 28.

=Dogiel, John.= Anatomie und Physiologie des Herzens der Larve von Corethra plumicornis. (Mém. Acad. imp. St. Petersbourg, 7 Sér., xxiv, 1877, Nr. 10, pp. 37, 2 Pls.) Separate, Leipzig, Voss.

=Bütschli, O.= Ein Beitrag zur Kenntnis des Stoffwechsels, insbesondere der Respiration bei den Insekten. (Reichert’s und Du Bois-Reymond’s Archiv f. Anatomie u. Physiologie, 1874, pp. 348–361.)

=Béla-Dezso.= Ueber den Zusammenhang des Kreislaufs und der respiratorischen Organe bei den Arthropoden. (Zool. Anzeiger, i Jahrg., 1878, p. 274.)

=Plateau, F.= Communication préliminaire sur les mouvements et l’innervation de l’organe central de la circulation chez les animaux articulés. (Bull. Acad. roy. de Belgique, Sér. 2, xlvi, 1878, pp. 203–212.)

=Jaworovski, Ant.= Ueber die Entwicklung des Rückengefässes und speziell der Muskulatur bei Chironomus und einigen anderen Insekten. (Sitzgsber. d. k. Akad. d. Wissensch. Wien. Math.-naturwiss. Cl., lxxx, 1879, pp. 238–258.)

=Zimmermann, O.= Ueber eine eigentümliche Bildung des Rückengefässes bei einigen Ephemeridenlarven. (Zeitschr. f. wissens. Zool., 1880, xxxiv, pp. 404–406.)

=Burgess, E.= Note on the aorta in lepidopterous insects. (Proc. Bost. Soc. Nat. Hist., xxi, 1881, pp. 153–156, Figs.)

—— Contributions to the anatomy of the milk-weed butterfly, Danais Archippus F. (Anniversary Memoirs Boston Soc. Nat. Hist., 1880, pp. 16, 2 Pls.)

=Vayssière, A.= Recherches sur l’organisation des larves des Éphémérines. (Ann. Sc. nat. Zool., Sér. 6, xiii, 1882, pp. 1–137, 11 Pls.)

=Viallanes, H.= Recherches sur l’histologie des insectes et sur les phénomènes histologiques qui accompagnent le développement post-embryonnaire de ces animaux. (Ann. Sc. nat., Sér. 6, xiv, 1882, pp. 1–348, 18 Pls.)

=Schimkewitsch, W.= Ueber die Identität der Herzbildung bei den wirbel und wirbellosen Tieren. (Zool. Anzeiger, 1885, viii Jahrg., pp. 37–40, Fig.)

—— Noch etwas über die Identität der Herzbildung bei den Metazoen. (Zool. Anzeiger, 1885, pp. 384–386.)

=Creutzburg, N.= Ueber den Kreislauf der Ephemerenlarven. (Zool. Anzeiger, 1885, pp. 246–248.)

=Poletajewa, Olga.= Du cœur des insectes. (Zool. Anzeiger, 1886, ix Jahrg., pp. 13–15.)

=Selvatico, S.= L’aorta nel corsaletto e nel capo della farfalla del bombice del gelso. (Padova, 1887, p. 19, 2 Pls.)

—— Die Aorta im Brustkasten und im Kopfe des Schmetterlings von Bombyx mori. (Zool. Anzeiger, 1887, x Jahrg., pp. 562–563.)

=Kowalevsky, A.= Ein Beitrag zur Kenntnis der Excretionsorgane. (Biol. Centralbl., 1889, ix, pp. 33–47, 65–76, 127–128.)

=Tosi, Alessandro.= Osservazioni sulla valvola del cardias in varii generi della famiglia delle Apidi. (Ricerche Lab. Anat. R. Univ. Roma, v, 1895, pp. 5–26, 16 Figs., 3 Pls.)

=Pawlowa, Mary.= Ueber ampullenartige Blutcirculationsorgane im Kopfe verschiedener Orthopteren. (Zool. Anzeiger, xviii Jahrg., 1895, pp. 7–13, 1 Fig.)

Also the writings of Kolbe.

b. The blood, blood corpuscles, leucocytes, and blood tissue

=Wagner, R.= Ueber Blutkörperchen bei Regenwürmern, Blutegeln und Dipterenlarven. (Müller’s Archiv f. Anatomie u. Physiologie, 1835, pp. 311–313.)

—— Nachtrage zur vergleichenden Physiologie des Blutes. (Archiv f. Anat. u. Physiologie, 1838.)

=Newport, G.= On the structure and development of the blood. First series. The development of the blood corpuscle in insects and other invertebrata, and its comparison with that of man and the vertebrata. (Abstr. of the paper in Roy. Soc., 1845, v, pp. 544–546: also in Ann. Mag. Nat. Hist., Ser. 3, 1845, iii, pp. 364–367.)

=Landois, H.= Beobachtungen über das Blut der Insekten. (Zeitschr. f. wissens. Zool., xiv, 1864, pp. 55–70, 3 Pl.)

——, =and L. Landois.= Ueber die numerische Entwicklung der histiologischen Elemente d. Insektenkörpers. (Ibid., xv, 1865, pp. 307–327.)

=Rollett, A.= Zur Kenntnis der Verbreitung des Hämatins. (Sitzgsber. d. k. Akad. d. Wiss. Wien., lxiv, 1871.)

=Wielowiejski, H. v.= Ueber das Blutgewebe der Insekten. Eine vorläufige Mitteilung. (Zeitschr. f. wissens. Zool., 1886, xliii, pp. 512–536.)

=MacMunn, C. A.= Researches on myohæmatin and the histohæmatins. (Proc. Roy. Soc. London, 1886, xxxix, pp. 248–252.)

=Peyron, J.= Sur l’atmosphere interne des insectes comparée à celle des feuilles. (Compt. rend. Acad. Sc., Paris, 1886, cii, pp. 1339–1341.)

=Cuénot, L.= Études sur le sang, son rôle et sa formation dans la série animale. Part 2, invertébrés; note préliminaire. (Arch. Zool. Expériment., 1888, Sér. 2, v, pp. xliii-xlvii. See also ibid., Sér. 3, 1897, pp. 655, 679–680.)

=Dewitz, H.= Die selbstandige Fortbewegung der Blutkörperchen der Gliedertiere. (Naturwiss. Rundschau. Braunschweig, 1889, iv Jahrg., pp. 221–222.)

—— Eigenthätige Schwimmbewegung der Blutkörperchen der Gliedertiere. (Zool. Anzeiger, 1889, xii Jahrg., pp. 457–464, Fig.)

=Schäffer, C.= Beitrage zur Histiologie der Insekten. II, Ueber Blutbildungsherde bei Insektenlarven. (Spengel’s Zool. Jahrbücher Abt. f. Anat. u. Ontogenie, iii, 1889, pp. 626–636, 1 Taf.)

=Cattaneo, G.= Sulla morfologia delle cellule ameboidi dei Molluschi e Artropodi. (Boll. Sc. Pavia. Anno 11, 1889, p. 59, 2 Pls.)

=Wagner, W. A.= Ueber die Form der körperlichen Elemente des Blutes bei Arthropoden, Würmern und Echinodermen. (Biolog. Centralblatt, 1890, x, p. 428.)

=Preyer, W.= Zur Physiologie des Protoplasma. II, Die Funktionen des Stoffwechsels. Die Saftströmung. (Patanie’s Naturwiss. Wochenschr., 1891, vi, pp. 1–5.)

=Cholodkowsky, N.= Ueber das Bluten der Cimbiciden-Larven. (Entomologische Miscellen, vi, Horæ Soc. Ent. St. Petersburg, 1897, pp. 352–357, 1 Fig. The fluid thrown out through pores or fissures in the skin is the blood.)

With the writings of Korotaiev, Tichomeroff, Pékarsky, Balbiani, Korotneff, Cuénot, and others.

c. The fat-bodies

=Dufour, L.= Recherches anatomiques sur les Carabiques et sur plusieurs autres insectes Coléoptères. Du tissu adipeux splanchnique. (Ann. Sc. nat., viii, 1826, pp. 29–35.)

—— Histoire comparative des métamorphoses et de l’anatomie des Cetonia aurata et Dorcus parallelepipedus. Tissu adipeux splanchnique. (Ann. Sc. nat., Zoologie, Sér. 2, 1842, xviii, pp. 178–179.)

=Meyer, H.= Ueber die Entwicklung des Fettkörpers, der Tracheen und der keimbereitenden Geschlechtsteile bei den Lepidopteren. (Zeitschr. f. wissens. Zool., 1849, i, pp. 175–179, 4 Taf.)

=Fabre, J. H.= Étude sur le rôle du tissu adipeux dans la sécretion urinaire chez les insectes. (Ann. Sc. nat., Sér. 4, xix, 1862, pp. 351–382.)

=Leydig, Fr.= Einige Worte über Fettkörper der Arthropoden. (Reichert, u. du Bois-Reymond’s Archiv f. Anat., 1863, pp. 192–203.)

=Lindemann, K.= Zoologische Skizzen. 1. Struktur des Fettkörpers. (Bull. Soc. Imp. d. Natural. Moscou, 1864, pp. 521–526, 1 Pl.)

=Landois, Leonh.= Ueber die Funktion des Fettkörpers. (Zeitschr. f. wissens. Zoologie, xv, 1865, pp. 371–372.)

=Wielowiejski, H. v.= Ueber den Fettkörper von Corethra plumicornis und seine Entwicklung. (Zool. Anzeiger, 1883, vi Jahrg., pp. 318–322.)

—— Ueber das Blutgewebe der Insekten. (Zeitschr. f. wissens. Zool., 1886, xliii, pp. 512–536.)

=Kowalevsky, A. O.= Sur les organes excréteurs chez les arthropodes terrestres. (Congrès internat. Zool., 2^{me} Sess., pp. 196–205, Moscou, 1892.)

THE BLOOD TISSUE

Under this name Wielowiejski has included several important tissues or cellular bodies intimately concerned with the nutrition of the insect. These are:—

1. The blood corpuscles. (See p. 407, leucocytes and phagocytes.) 2. The fat-body proper (Corpus adiposum). 3. The pericardial fat-body (pericardial cells). 4. The œnocytes. 5. The garland-shaped cord of muscid larvæ. 6. The subœsophageal body, a peculiar organ found by Wheeler in the embryos and young larvæ of Blatta and Xiphidium. 7. The phosphorescent organs.

a. The fat-body

In the body cavity of winged insects and of their larvæ occur yellowish masses of large cells filled with small drops of fat, and forming the “fat-body.” It is of various shapes, more or less lobulated or net-like, and covers or envelops parts of the viscera, also forming a layer under the integument (Fig. 143). The tracheal endings are usually enveloped by the fat-body. It is larger in the larvæ than in the adults, especially in Lepidoptera, in them forming a reserve of nutrition, used during metamorphosis and during the formation and ripening of the eggs and male cells.

Wielowiejski has shown that there is a regular arrangement of the fat-body in the general cavity of the body. For example, in the larva of Chironomus occur the following forms of this tissue. Around the periphery, on each side of the body cavity, is a loose network of lobes with large meshes constituting the peripheral layer or external lobular fat-body; these lobular masses are segmentally arranged.

Within these segmental lobes, on each side of and along the digestive tract, extending along through almost the entire body, is an unbroken strand of this tissue, forming the internal fat-body cords. From the first larval stage, and even before hatching, its cells are so unusually large, being filled with large, clear, mostly colorless fat-drops, that their limits cannot be defined, and their nuclei can only with great difficulty be detected. Only in some large larvæ of Chironomus has Wielowiejski found clearly defined cells; the protoplasm of these cells contain almost no fat-drops.

The fat-body is of mesodermal origin, and as Wheeler insists, is not derived from the œnocytes, as supposed by Graber. Formed from the mesoderm, it is a differentiation of portions of the cœlomic walls, and therefore metameric in origin. That the fat-body gives origin to the blood corpuscles Wheeler is doubtful.

The fat-cells are distinct, spherical, and as a rule possess only one nucleus, though in those of Apis and Melophagus there are two nuclei, and in Musca several. Sometimes the cells contain a substance like the white of an egg, and concretions of uric acid, or these take the place of the fat-drops. The presence of uric acid shows that a very active metabolism goes on in the fat-body. “In some cases it has been proved that the fat-body in the larva is rich in fat and poor in concretions of uric acid, while in the imago it is poor in fat and rich in concretions of uric acid” (Lang).

Leydig, in 1857 (Lehrbuch der Histiologie), spoke of the presence of dark concretions in the fat-body, and afterwards (1864) showed that there was a wide distribution of uric acid salts and concretions. Witlaczil, also, has detected concretions in the fat-body of the Psyllidæ, in larval Cecidomyiidæ, in the larvæ and pupæ of ants, and in the pupa of Musca.

The physiological processes which take place in the fat-bodies are obscure. Graber regarded the whole system of the fat-bodies as “a single, many-lobed lung,” while before him Landois, taking into account the intimate relation existing between the finer tracheal branches and the fat-body, considered that the latter was concerned in respiration. Marchal thinks that the fat-body is a urinary organ, as the urates are formed within the cells of this body.

Moreover, Schäffer maintains that a special kind of fat-body cell has the important function of taking up and giving out nutritious matters during the internal processes of metamorphosis, while he also believes that there is a genetic connection between the fat-body and the blood corpuscles—a view combated by Wheeler.

Kowalevsky finds that the fat-body remains absolutely insensible to the action of the substances which stained the Malpighian tubes (p. 352). So long as the cells are healthy and living they are not stained and do not absorb the colors in question; and this insensibility persists, even when the cells are of a different nature, as those of the fly (adipose and “intercalary” cells).

b. The pericardial fat-body or pericardial cells

We have already, on p. 405, called attention to these organs, but they also have an intimate relation to the fat-body.

Kowalevsky (1892) remarks that the disposition of these cells varies much in different insects and even in the same animal. Thus, in the Diptera and the ordinary flies there are found around the lower part of the dorsal vessel 13 pairs of large pericardial cells which lie next to a crowded bed of small cells forming a compact mass around the anterior part of the dorsal vessel. In caterpillars, notably silkworms, from the compact layer of pericardial cells which surround the heart, pass off trunks which are directed towards the lateral walls of the body, also forming close networks around the tracheæ and then passing down into the abdominal cavity of the body of the larva.

In the larvæ of certain Hymenoptera, the trunks which pass off from the pericardial region form a loose cord, a sort of fatty tissue covering the entire body cavity.

This tissue, adds Kowalevsky, entirely differs from œnocytes, or from the so-called glandular body whose formation in Gryllotalpa has been described by Korotaiev, and in Bombyx mori by Tichomiroff. In a recent work wherein has been collected everything known regarding these last-named cells, Pékarsky proves that they are unique in nature and cannot be regarded either as fat-cells, or as pericardial cells, or even as formative leucocytes.

As to the structure of the pericardial cells, Kowalevsky adds that they are always attached to muscular fibres passing off from the heart, and that they lie, so to speak, upon them. In the locusts the muscular fibres supporting the pericardial cells appear distinctly like little staves or sticks. The attachment of the pericardial cells to the muscular fibres has been observed by Cuénot and reproduced by him in his work, but his description somewhat differs from that observed by Kowalevsky in the locust (Acrydium migratorium).

As to the nature of the acid excretions which are formed in the pericardial cells, in spite of his attempts to solve the problem, Kowalevsky has been unsuccessful. The only observations in this direction are those of Letellier on the pericardial glands of lamellibranch molluscs, which he found to contain hypouric acid, and it is probable, says Kowalevsky, that the acidity of the pericardial cells in insects is due to the presence of the same acid.

=Leucocytes or phagocytes in connection with the pericardial cells.=—It is thought by Schäffer that the leucocytes or phagocytes may be free or wandering fat-body cells. They play an important part in metamorphosis, while they absorb or feed upon the remains of the larval organs, and thus prove of use in the building up of the organs of the adult insects.

While the faculty of phagocytosis is wanting in the urinary tubes, Balbiani and more recently Cuénot have expressed the opinion that the pericardial cells of insects may have the power of absorbing hard bodies, “acting as a phagocytic gland.” This, however, is called in question by Kowalevsky, from studies made on different insects. On introducing powdered carmine into the body of an insect it has not been absorbed by the pericardial cells, as they have not been colored red. It is the leucocytes which absorb the grains of carmine, and which, after having dissolved them, transmit them to the pericardial cells. Hence, then, the pericardial cells have not the phagocytic power of which Cuénot speaks.

Returning to his own observations on hard bodies introduced into insects, or large globules introduced under the form of a milk emulsion, Kowalevsky has found that these bodies were absorbed in the first place by the free-swimming leucocytes, and in the second place by whole groups or nests of leucocytes situated in different parts of the body, principally on the threads of the adipose body. In the Orthoptera the absorption is immediately effected by means of the cells of the membrane which separates the pericardium from the cavity of the body underneath the heart. The regions where the hard bodies are absorbed in great number coincide with the regions of formation of the blood corpuscles. In his researches on the larvæ of Hyponomeuta and other Lepidoptera, Schäffer describes these regions as forming a sort of island. The nests where the blood globules are formed are the most active centres of phagocytosis.

Balbiani, and also Cuénot, have supposed that the formation of the blood corpuscles takes place in the pericardial cells, but Kowalevsky insists that these cells cannot form the leucocytes, which “are probably formed in different parts of the body, notably in the special nests [Herde of Jäger] situated near the heart, but outside of the pericardial cells.”

In Fig. 384, where the nests of leucocytes (l) are shown, it is evident that they are formed where observed, and “could not have come from the pericardial cells, which have their own structure and their special function,” these cells being very large and characteristic.

In Kowalevsky’s preparations of Truxalis, the pericardial cells with deposits of carmine and the groups of leucocytes (Fig. 385, l and l′) stained with India ink, we have to deal with elements absolutely different. If the formation of leucocytes was caused by the pericardial cells, these last would be obliged to free themselves from their contents and to modify their essential nature.

c. The œnocytes

These cells (Fig. 386), with the exception of the eggs, are the largest in the body, and occur in most if not all winged insects. They were called œnocytes (oinos, wine; kustis, cyst), by Wielowiejski in allusion to their wine-yellow color. These cells are arranged segmentally (Fig. 387) in clusters, held in place by tracheæ, and are situated mostly on each side of the abdomen, rarely being found in the adjoining parts of the thorax. They are more or less intimately associated with the blood and fat-body. Unlike the fat-body, however, they arise in embryonic life from the ectoderm, either by delamination or by immigration, just behind the tracheal involutions.

The separate cells of each cluster are usually separate, but in rare cases may fuse in pairs or form smaller clusters. In shape they are round or oval, often sending out pseudopodia-like processes, by which they are attached to the tracheal twigs or to each other. “The cytoplasm, which is very abundant, is full of yellowish granules and is sometimes radially situated towards its periphery. The large spherical or oval nucleus contains a densely wound and delicate chromatic filament.” (Wheeler.)

Graber first pointed out the identity of these clusters of cells with certain metameric cell-masses in insect embryos, observed by Tichomiroff in those of the silkworm, and by Korotneff in the embryo mole-cricket.

Although they resemble the blood corpuscles in some insects, they are always much larger, and do not seem to be amœboid, while they are never seen to undergo self-division, or to exhibit any appearance of giving rise to the blood-cells (Wheeler). They have not yet been detected in Thysanura (Synaptera) or in Myriopoda.

d. The phosphorescent organs

Phosphorescence is not infrequent in the Protozoa, cœlenterates, worms, and has been observed in the bivalve Pholas, in a few abyssal Crustacea, in myriopods (Geophilus), in an ascidian, Pyrosoma, and in certain deep-sea fishes.

In insects luminosity is mostly confined to a few Coleoptera, and besides the well-known fireflies, an Indian Buprestid (Buprestis ocelata) is said to be phosphorescent; also a telephorid larva. Other luminous insects are the Poduran Anurophorus, Fulgora, certain Diptera (Culex, Chironomus[60] and Tyreophora), and an ant (Orya).

The seat of the light is the intensely luminous areas situated either in the head (Fulgora), in the abdomen (Lampyridæ), or in the thorax (in a few Elateridæ of the genus Pyrophorus). The luminous or photogenic organ is regarded by Wielowiejski and also by Emery as morphologically a specialized portion of the fat-body, being a plate consisting of polygonal cells, situated directly under the integument, and supplied with nerves and fine tracheal branches.

In Luciola as well as in other fireflies, including Pyrophorus, the phosphorescent organ or plate consists, as first stated by Kölliker, of two layers lying one over the other, a dorsal one (Fig. 388, d) which is opaque, chalky white, and non-photogenic, and a lower one (v), the active photogenic layer, which is transparent. Through the upper or opaque layer and on its dorsal surface extend large tracheæ and their horizontal branches, from which arise numerous very fine branches which pass down perpendicularly into the transparent or photogenic layer of the organ. Each tracheal stem, together with its short branches, is enveloped by a cylindrical mass of transparent tissue, so that only the short terminal branches or very fine tracheal capillaries project on the upper part of the cylinder. These finest tracheal capillaries are not in Luciola filled with air, but with a colorless fluid, as was also found by Wielowiejski and others in Lampyris.

These transparent cylinders, with the tracheæ within, forming longitudinal axes, resemble lobules. These lobules are so distributed that they appear on a surface section of this plate as numerous round areas in which circular periphery the tracheal capillaries are arranged with the axially disposed tracheal end-cells. These “tracheal end-cells” are only membranous enlargements at the base of the tracheal capillaries (Wielowiejski). The cylindrical lobules are separated from each other by a substance consisting of abundant large granular cells (parenchym cells) among which project the tracheal capillaries. The cylindrical lobules extend to the hypodermis and come in contact only by their lateral faces with the parenchym.

The structure of the upper opaque chalky white layer of the phosphorescent organ is, compared with that of the photogenic lower portion, very simple. In its loose, pappose, mass are no cellular elements, but when treated with different reagents it is seen to be filled with countless urate granules (guanine) swimming in the fluid it contains, the cell plasma appearing to be dissolved, the cells having lost their cohesion.

In comparing the phosphorescent plate or organ of Luciola with that of Lampyris, the general structure, including the clear cell elements of the cylindrical lobules, which envelop the perpendicular tracheal twigs and their branches, and also the granular parenchymatous cells are alike in both, though the arrangement and distribution of the elements in Luciola is more regular, in Lampyris the tracheal stems being irregularly scattered through the parenchym.

Wielowiejski found in the larval and female Lampyris a higher degree of differentiation than in the male, and Luciola has a more differentiated photogenic organ than Lampyris, as seen in the more regular structure of the lobules.

As regards the light-apparatus of Pyrophorus, or the cucujo, Heinemann shows that, as in the Lampyridæ, it consists of distinct cells, and may be regarded as a glandular structure. It is rich in tracheæ and the other parts already described. In still later researches on a Brazilian Pyrophorus Wielowiejski shows that the phosphorescent plate consists of two layers, the upper usually being filled with crystalline urate concretions, and entirely like those of the Lampyridæ, consisting of distinct polygonal cells, among which are numerous tracheal stems, with tænidia, coursing in different directions, when freshly filled with air, and sending capillaries into the underlying photogenic layer. The latter shows in its structure a striking difference in the cellular arrangement from that of Lampyrids. In the upper or non-photogenic layer are tracheal capillaries which pass down into the underlying cellular plate and which are in the closest possible relations with the single cells—a point overlooked by Heinemann.

=Physiology of the phosphorescence.=—As is well known, the phosphorescence of animals is a scintillating or glowing light emitted by various forms, the greenish light or luminous appearance thus produced being photogenic, i.e. without sensible heat.

Langley rates the light of the firefly at an efficiency of 100 per cent, all its radiations lying within the limits of the visible spectrum. “Langley has shown that while only 2.4 per cent of luminous waves are contained in the radiation of a gas-flame, only 10 per cent in that of the electric arc, and only 35 per cent in that of the sun, the radiation of the firefly (Pyrophorus noctilucus) consists wholly of visible wave-frequencies.” (Barker’s Physics, p. 385.)

The spectrum of the light of the cucujo was found by Pasteur to be continuous. (C. R. French Acad. Sc. 1864, ii, p. 509.) A later examination by Aubert and Dubois showed that the spectrum of the light examined by the spectroscope is very beautiful, but destitute of dark bands. When, however, the intensity diminishes, the red and orange disappear, and the green and yellow only remain.

Heinemann studied the cucujo at Vera Cruz, Mexico. At night in a dark room it radiates a pale green light which shows a blue tone to the exclusion of any other light. The more gas or lamp light there is present, the more apparent becomes the yellowish green hue, which in clear daylight changes to an almost pure very light yellow with a very slight mixture of green. “In the morning and evening twilight, more constantly and clearly in the former, the cucujo light, at least to my eyes, is an intensely brilliant yellow with a slight mixture of red. In a dark room lighted with a sodium light the yellow tone entirely disappears; on the other hand, the blue strikingly increases.” As regards the spectrum he found that almost exactly half of the blue end is wanting and that the red part is also a little narrower than in the spectrum of the petroleum flame.

Professor C. A. Young states that the spectrum given by our common firefly (Photinus?) is perfectly continuous, without trace of lines either bright or dark. “It extends from a little above Fraunhofer’s line C, in the scarlet, to about F in the blue, gradually fading out at the extremities. It is noticeable that precisely this portion of the spectrum is composed of rays which, while they more powerfully than any others affect the organs of vision, produce hardly any thermal or actinic effect. In other words, very little of the energy expended in the flash of the fire is wasted. It is quite different with our artificial methods of illumination. In the case of an ordinary gaslight the best experiments show that not more than one or two per cent of the radiant energy consists of visible rays; the rest is either invisible heat or actinism; that is to say, over 98 per cent of the gas is wasted in producing rays that do not help in making objects visible.”

Panceri also remarks that while in the spectroscope the light of some Chætopteri, Beroë, and Pyrosoma exhibit one broad band like that given by monochromatic light, that of Lampyris and Luciola is polychromatic. (Amer. Nat., vii, 1873, p. 314.)

The filtered rays of Lampyris pass (like Röntgen and uranium rays) through aluminium (Muraoka).

The physiology of insect phosphorescence is thus briefly stated by Lang: “The cells of this luminous organ secrete, under the control of the nervous system, a substance which is burnt during the appearance of the light; this combustion takes place by means of the oxygen conveyed to the cells of the luminous body by the tracheæ, which branch profusely in it and break up into capillaries.”

Emery states that the males of Luciola display their light in two ways. When at night time they are active or flying, the light is given out at short and regular intervals, causing the well-known sparkling or scintillating light. If we catch a flying Luciola or pull apart one resting in the day time, or cut off its hind body, it gives out a tolerably strong light, though not nearly reaching the intensity of the light waves of the sparkling light. In this case the light is constant, yet we notice, especially in the wounded insect, that the phosphorescent plate in its whole extent is not luminous, but glows at different places as if phosphorescent clouds passed over it.

It is self-evident that a microscopic observation of the light of the glow-worm or firefly is not possible, but an animal while giving out its light, or a separated abdomen, may readily be placed under the microscope and observed under tolerably high powers. By making the experiment in a rather dark room Emery saw clear shining rings on a dark background. “All the rings are not equally lighted. Comparing this with the results of anatomical investigation, it is seen that the rings of light correspond with the previously described circular tracheal capillaries, i.e. the limits between the tracheal-cell cylinder and the parenchym-cells. The parenchym-cells are never stained of a deep brown; this proves that its plasma may be the seat of the light-producing oxidation. Hence this process of oxidation takes place in the upper surface of the parenchym-cells, but outside of their own substance. The parenchym-cells in reality secrete the luminous matter; this is taken up by the tracheal end-cells and burnt or oxidized by means of the oxygen present in the tracheal capillaries. Such a combustion can only take place when the chitinous membrane of the tracheæ is extraordinarily fine and easily penetrable, as is the case in the capillaries of the photogenic plate; therefore the plasma of the tracheal cells only oxidizes at the forking of the terminal tracheal twigs and in the capillaries.” (Emery.)

The color of the light of Luciola is identical in the two sexes, and the intensity is much the same, though that of the female is more restricted. The rhythm of the flashes of light given out by the male is more rapid, and the flashes briefer, while those of the female are longer, more tremulous, and appear at longer intervals.

Emery then asks: What is the use of this luminosity? Is it only to allure the females of Luciola, which are so much rarer than the males? Contrary to the general view that it is an alluring act, he thinks that phosphorescence is a means of defence, or a warning or danger-signal against insectivorous nocturnal animals. If we dissect or crush a Luciola, it gives out a disagreeable cabbage-like smell, and perhaps this is sufficient to render it inedible to bats or other nocturnal animals. An acrid taste they certainly do not possess.

It has long been known that the eggs of fireflies, both Lampyridæ and Pyrophorus, are luminous. Both Newport and more recently Wielowiejski attributes the luminosity not to the contents of the egg, but to the portions of the fat-body cells or fluid covering on the outside of the eggs, due to ruptures of the parts within the body of the female during oviposition. The larvæ at different ages are also luminous.

The position of the luminous organs changes with age. In the larvæ of Pyrophorus before moulting, according to Dubois, the luminous organs are situated only on the ventral side of the head and prothoracic segment. In larvæ of the second stage there are added three shining spots on each of the first eight abdominal segments, and a single luminous spot on the last segment. These spots are arranged in a linear series and thus form three luminous cords. In the adult beetles there is a luminous spot in the middle of the first abdominal sternite, but the greatest amount of light is produced by the two vesicles on the hinder part of the prothorax, the position of which varies according to the species.

LITERATURE ON PHOSPHORESCENCE

=Peters, W.= Ueber das Leuchten der Lampyris italica. (Müller’s Archiv f. Anatomie, 1841, pp. 229–233.)

=Kölliker, A.= Die Leuchtorgane von Lampyris, eine vorläufige Mittheilung. (Verhandl. d. phys. medizin. Gesellsch. Würzburg, 1857, viii, pp. 217–224.)

=Schultze, Max.= Ueber den Bau der Leuchtorgane der Männchen von Lampyris splendidula. (Sitzber. d. niederrhein. Gesellsch. f. Natur. u. Heilkunde zu Bonn, 1864, Sep. p. 7.)

—— Zur Kenntniss der Leuchtorgane von Lampyris splendidula. (Archiv f. mikroskop. Anat., 1865, i, pp. 124–137, 2 Taf.)

=Wielowiejski, H. Ritter von.= Studien über die Lampyriden. (Zeits. f. wissens. Zool., 1882, xxxvii, pp. 354–428, 2 Taf.)

=Emery, Carlo.= Untersuchungen über Luciola italica L. (Zeits. f. wissens. Zool., 1884, xl, pp. 338–355, 1 Taf.)

—— La luce della Luciola italica osservata col microscopio. (Bull. Soc. Ent. Ital. Anno xvii, 1885, pp. 351–355, 1 Taf.)

=Wheeler, William Morton.= Concerning the blood tissue of the Insecta, III; Psyche, vi, 1892, p. 255. (Structure of the light organ of Photuris pennsylvanica.)

=Heinemann, C.= Zur Anatomie und Physiologie der Leuchtorgane Mexikanischer Cucujos, Pyrophorus. (Archiv f. mikroskop. Anat., 1886, xxvii, pp. 296–383.)

=Dubois, R.= Contribution à l’étude de la production de la lumière par les êtres vivants. Les Elaterides lumineux. (Bull. Soc. Zool. France, 1886, Année ii, pp. 1–275, 9 Pls.)

=Wielowiejski, H. Ritter von.= Beiträge zur Kenntniss der Leuchtorgane der Insekten. (Zool. Anzeiger, 1889, Jahrg. xii, pp. 594–600.)

=Hudson, G. V.= The habits and life-history of the New Zealand glowworms. (Trans. N. Zealand Inst., xviii, 1890, pp. 43–49, 1 P1.)

=Dubois, R.= Sur le mechanisme de la production de la lumière chez l’Orya barbarica d’Algérie. (Comptes rend. Acad. Sc. France, 1892, cxvii, pp. 184–186. Also in Ann. and Mag. Nat. Hist., 1893 (6), xii, pp. 415–416.)

=Schmidt, P.= Ueber das Leuchten der Zuckmücken (Chironomidæ). (Zool. Jahrb. Morph., Abth. viii, 1894, pp. 191–216, 2 Taf. Also Ann. and Mag. Nat. Hist., xv, pp. 133–141, 1895. The light is due to bacteria. Nature, 1897.)

=Chun, C.= Leuchtorgane und Facettenaugen. (Biol. Centralbl., 1896, pp. 315–320.)

=Muraoka, H.= (On the filtered rays of Lampyris, Wiedemann’s Annalen, Dec., 1896.)

See also the writings of Audouin, Carus, D. Turner, Thompson.

THE RESPIRATORY SYSTEM

While land vertebrates breathe by inhaling the air through the mouth into the lungs, insects respire by internal air-tubes (tracheæ), which ramify throughout every part of the body and its appendages. The air enters these tubes through a few openings, called spiracles or stigmata, arranged segmentally in the sides of the body. These tracheæ are everywhere bathed by the blood, and thus the latter is constantly aërated or kept fresh; the blood not, as in vertebrates or as in molluscs, seeking the lungs or gills, or any specialized respiratory portion of the body where the oxygen combines with the hæmoglobin, but the respiratory tubes, so to speak, themselves seek out the blood and the blood-tissue in every part of the insect body, penetrating to the tips of the antennæ and of the legs, entering the most delicate tissues, even perhaps passing through the walls of epithelial cells. As Lang remarks, the want of an arterial vascular system is compensated for as well as conditioned by the extremely profuse branching of the tracheæ.

The aquatic larvæ of certain dragon-flies (Agrionidæ), may-flies, case-worms, etc., respire by means of tracheal gills or branchiæ, which are either filamental or leaf-like appendages containing tracheæ. Somewhat similar structures appended to the thorax of pupal aquatic Diptera, as in the mosquito and its allies, enable them to breathe while stationed a little beneath the surface of the water. Other larvæ, as the rat-tail larva of Eristalis, etc., lying at the bottom of shallow pools or in ditches, etc., can breathe by raising slightly above the surface a long appendage with two spiracles at the end, through which the air enters the tracheal system. (See p. 461.)

Although Aristotle, as well as the natural philosophers of the Middle Ages, supposed that insects did not breathe, one can easily see that they do by holding a grasshopper or dragon-fly in one’s hand and observing the rhythmical rise and fall of the upper and lower walls of the abdomen, during which the air enters and passes out of the air-openings or spiracles on each side of the body.

It is plain that insects consume very little air, since caterpillars may be confined in very small, almost air-tight tin boxes, and continue to eat and undergo their transformations without suffering from the confinement. According to H. Müller an insect placed in a small, confined space absorbs all the oxygen. Insects can survive for many hours when placed in an exhausted receiver, or in certain irrespirable gases. “Cockroaches in carbonic acid speedily become insensible, but after twelve hours’ exposure to the pure gas they survive and appear none the worse.” (Miall and Denny, p. 165.) Insects of the swiftest flight breathe most rapidly, their great muscular activity requiring the absorption of an abundance of oxygen.

Warmth, plenty of food, besides muscular activity, increases the demand for oxygen and the quantity of carbonic acid exhaled.

a. The tracheæ

It will much simplify our conception of the nature of the air-tubes when we learn that they originate in the embryo as tubular ingrowths of the integument (ectoderm), these branching and finally reaching every part of the interior of the body. They are elastic tubes, and being filled with air are silvery in color, though at their origin near the spiracles they are reddish or violet bluish; or, in the larva of Æschna, reddish brown, this tint being due to a finely granular pigment situated in the peritoneal membrane.

In their essential structure the tracheæ consist of the chitinous intima, which is a continuation of the cuticle of the integument, and of a cellular membrane or outer layer of cells (a continuation of the hypodermis) called the peritoneal membrane, or ectotrachea (Figs. 392, 393).

Leydig discovered that the spiral filaments are not distinct and separate, but intimately connected with the inner membrane (intima), and he detected the outer or peritoneal membrane, which Chun afterwards found to be epithelial in its nature, Minot stating that it is a true pavement epithelium.

Figure 393 represents a longitudinal section of a large trachea of Hydrophilus, showing the peritoneal membrane (ectotrachea, ep) and the intima or endotrachea, divided into the cuticula (cu), with the darker colored inner layer, in which are embedded the dark-colored tænidia (f).

=Distribution of the tracheæ.=—The distribution of the air-tubes, as Lubbock and also Minot state, depends first upon the shape of the organs, and upon the size of those whose size is variable. Around the large, hollow organs (digestive canal, sexual organs) the tracheæ ramify in all directions, forking so that the branches diverge at a wide angle. In the organs which have muscular walls, like the oviduct, the tracheæ run straight when the walls are distended, but have a sinuous course when the walls are contracted. (Minot.)

“Around the organs of more elongated form the branches of the tracheæ run more longitudinally, as is shown by the air-tubes of the muscles, which also present some peculiarities worthy of especial notice.

“A short, thick trunk arrives at the muscular bundle, and dividing very rapidly, breaks up into a large number of delicate tubes, which penetrate between the muscular fibres, then terminating in tubes of exceeding fineness, which at first sight seem to form a network that might well be called a rete mirabile. A closer examination, however, reveals that it is not a real network, but rather an interlacing confusing to the eye. The longitudinal direction of the tracheæ of the muscles presents a striking contrast to the system of divarication represented in Figs. 13 and 14. The course of the tracheæ of the Malpighian tubes is also very curious. There is one large trachea which winds around the tube in a long spiral, giving off numerous small branches which run to the surface of the tube, upon which they form delicate ramifications. Each tube has but a single main trachea, and I think the trachea continues the whole length of the tube, but of this last point I am not quite sure.” (Minot.)

While in the nymphs of Orthoptera the tracheæ very closely resemble those of the adult, in larvæ of insects with a complete metamorphosis the tracheæ differ very much in distribution from those of the adult. The larval tracheæ are also more generalized and more like those of the original type than the tracheæ of perfect insects. (Lubbock.)

In general there are two main tracheæ, one passing along each side of the body, near the digestive canal, connected with its mate by a few transverse anastomosing branches, and sending off a branch to each spiracle, this arrangement being most simple and apparent in the maggots of Diptera. From these two main branches smaller twigs branch off into every part of the body with its appendages, passing among the different organs, often serving as cables to hold them loosely in place; they also penetrate into the component parts of the organ themselves, passing into the fat-bodies, and among the fibres of muscles, where they become finely attenuated and refined like the capillaries of the vascular system of vertebrates. (Figs. 395, 396.)

In the youngest larva of Corethra plumicornis Weismann ascertained the thickness of the longitudinal stem to be 0.0017 mm. That of the finest tracheal endings in the silk-glands of the silkworm was found by Von Wistinghausen to be 0.0016 mm. (Zeits. f. Wiss. Zool. xlix, 1890, p. 575.) Weismann states that in the larvæ of Corethra and Chironomus the tracheal system is only incompletely developed; the tracheæ are not united with each other, and in the youngest larvæ they do not contain air.

Each of the two main tracheæ, as Kolbe states, sends off into each segment of the body three branches.

1. An upper or dorsal branch, which supplies the muscles of the dorsal region.

2. A middle (visceral) branch, whose twigs pass to the digestive canal and back to the organs of reproduction.

3. A lower (ventral) branch, whose twigs are distributed to the ganglia and to the muscles of the ventral region.

In certain Thysanura, as a species of Machilis (Fig. 397), we probably have the primitive condition of the tracheal system, the longitudinal and transverse anastomoses being absent, but in other Thysanura (Japyx, Nicoletia, Lepisma, and a few species of Machilis) they are present.

As Kolbe remarks, whether the fine ends of the tracheæ are closed or open, whether after the analogy of the blood capillaries of vertebrates they anastomose with each other, whether the ends of the air-tubes pass between the cells or penetrate into them, these questions are not fully settled. According to Leydig’s[61] latest views the tracheæ penetrate into the cells and unite with the hyaloplasma. Hence the process of respiration in the last instance takes place in the hyaloplasma. This assumption accords with the fact that in the tracheate Arthropods the terminations of the tracheæ carry the atmospheric air into the space bounded by the cellular network, also to the hyaloplasma filling the spaces. Leydig[62] also thinks that the finest tracheal endings penetrate into the muscular tissue and unite with the primitive muscular fibres.

Kupffer is likewise of the opinion that the fine tracheæ penetrate into the cells, and Lidth de Jeude asserts that they enter the epithelial cells, “each cell containing several branches.” Kölliker, Emery, etc., maintain, however, that the tracheal endings lie between the cells. Wielowiejski,[63] in describing the line tracheæ of the phosphorescent organs, thinks that the tracheal endings (tracheal capillaries) rarely end blindly, but anastomose with one another, forming an irregular network. The latest observer, Gilson (1893), asserts that tracheal twigs penetrate deeply into the epithelial cells of the silk glands of larval Trichoptera as well as of caterpillars, passing through their protoplasm.

A late investigator, C. von Wistinghausen, finds in the tracheæ of the spinning-glands of caterpillars a completely formed network between the terminal branches of two or several tracheal groups. The tracheal tubes of this series of terminal branches pass into this network, which he calls the tracheal capillary end-network (Figs. 398, 400). This last varies in thickness and spreads out under the membrana propria of the glandular mass over the entire surface of the large gland-cells and on a level with the tracheal capillaries. The tracheal endings do not penetrate into the cells, but are separated from the plasma of the cells by a thin membrane. The tracheal capillary end-network appears as a system of fine tubes like the tracheal capillaries, consisting of a peritoneal layer and a chitinous intima (Fig. 400). The walls of these tubes are homogeneous, not porous, though readily permeable by the parenchymatous fluid. The interchange of gases consequently may go on easier and more vigorously in a system of richly anastomosing tubules of the net-like mass of tracheal capillaries, than in tubes ending blindly.

While the diameter of the tracheal capillaries is 0.0016 mm. or 1 µ, that of the tubules composing the tracheal capillary end-network is scarcely measurable, but is less than 1 µ.

These tracheal capillaries also occur on the seminal and other sexual tubes, on the intestine, on the urinary tubes, on the fat-bodies, but are most easily detected on the silk-glands.

The latest researches are those of E. Holmgren, who has studied the branching of the tracheæ in the spinning-glands of caterpillars. He prefers to call the end-cells “transition cells,” as they lead from the tracheal tubes proper to the capillary network. This latter is formed by slender nucleated cells, often with an intracellular lumen, and, according to the author, probably constituting a respiratory epithelium. He finds that both large and small tracheæ may penetrate the gland-cells. (Anat. Anzeiger, xi, 1895, pp. 340–6, 3 figs.; Jour. Roy. Micr. Soc., 1896, p. 182.)

b. The spiracles or stigmata

The spiracles are segmentally arranged openings in the sides of the thorax and abdomen, through which the air passes into the air-tubes. In its essential structure a spiracle, or stigma, is a slit-like opening surrounded by a chitinous ring, the lips or edges of the opening being membranous and closed by a movable valve of the spiracle attached by its lower edge, which is closed by an occlusor muscle (Fig. 401). The aperture when open forms a narrow oval slit; and in most insects the slit is within guarded by a row of projecting spines or setæ, which form a lattice work or grate to keep out dust, dirt, fluids, etc.

Krancher[64] has described five leading types of stigmata, not, however, taking into account those of the Synaptera.

I. Stigmata without lips (Primitive or generalized stigmata).

a. The simplest stigma is an aperture which is kept open by a chitinous ring (Acanthia). The opening may be round or elliptical. There are no lips nor any movement of the edges to be observed. Such air-holes occur in the abdomen of bugs (Hemiptera) and beetles (Coleoptera); within the opening of the stigmata in the same insects is a funnel-like contraction. Also in the Diptera the abdominal stigmata are of the same type.[65] The stigmata of the Pulicidae (Siphonaptera) are more complicated, as the edges of the openings are provided with setæ (Fig. 402).

b. The stigma consists of a series of minute single stigmata, which are usually surmounted by a common chitinous ring, and whose tubular continuations unite within in a common trachea, so that the single tubes pass off from the stigma like the fingers on the hand. This form is found in the larvæ and puparia of Diptera.

II. Stigmata with lips (Secondary more specialized stigmata).

c. The lips are represented by a single chitinous ring, with sparse spines. One side of the stigma is a little higher, and partly overlaps the other posteriorly; this form is peculiar to the Orthoptera and Libellulidae.

d. The lips are roof-like, bent inwards and densely hairy, forming a peculiar kind of felting. The setæ of the lips are in most beetles and many Lepidoptera separate, and more or less branched. In caterpillars, the setæ are so finely branched as to form a loose felt, or sieve-like arrangement.

e. The stigmata are round, with a very broad border and a concentric middle portion, the structure being complicated. The concentric middle portion is pouch-like and bears the occlusor muscle. This form occurs in the larvæ of lamellicorn beetles, and can be seen with the naked eye, or with a lens, in Oryctes, Cetonia, and Melolontha (Fig. 403).

f. Over the outer opening of the spiracle is an incurved chitinous projection, on one side of which the trachea takes its origin. It is thus in the Hymenoptera.

The remarkable grate-like stigma of the lamellicorn larvæ has the appearance as if the outer closing plate or valve were impenetrable. The earlier observers considered these stigmata to be open, but Meinert regards them as closed; Schiödte, however, has observed by pressing a preserved specimen of a Melolontha larva the alcohol within passing out in drops, through the grate-like plate, and hence he considers this a proof that the stigma is permeable (Kolbe).

More recently (1893) Boas has examined the same structure in the same species of larva as examined by Schiödte, and he finds it to be open only during the process of moulting. He finds that on each side of the larva there are nine short and wide stigmatic branches, each of which is shut off from the exterior by a brown plate; this consists of a reniform sieve-plate, and of a curved bulla which fits into the cavity of the plate. The stigmatic branch, however, is provided with a large external opening, which is homologous with the stigma, but which is usually closed by the plate and bulla, and is only open during the moulting; at first it is circular, but later becomes a cleft. A transverse section shows that the bulla is a simple tegumentary fold, the outer chitinous layer of which has become especially firm. The plate forms a horizontal half-roof, which springs from one side of the tracheal orifice, and is supported by obliquely set bases, which spring from the adjoining part of the inner side of the tracheæ. The plate and bars are purely cuticular structures. (Zool. Anz., 1893; also Journ. Roy. Micr. Soc., p. 54.)

The tracheal system of libellulid nymphs is not closed; on the other hand, in the fully-grown nymphs the anterior stigmata occurring on the dorsal side are large, and the tracheæ arising from them are thick. These stigmata are permeable by the air. In half-grown and still younger stages of Æschna the two anterior thoracic stigmata are undeveloped. In order to breathe, the fully-grown nymph either rises up on the upper side and elevates the end of the body to the surface in order to take the air into the rectum, or it rests with the back of the thorax at the surface in order to breathe through the large stigmata. The young nymphs take in air only through the rectum. The young nymphs of Libellula and its allies, on the other hand, possess large thoracic stigmata, but they prefer to breathe through the rectum. The fully-grown nymphs of Agrion breathe through the thoracic stigmata. (Dewitz, in Kolbe.)

=The position and number of pairs of stigmata.=—The spiracles are usually situated in the soft membrane between the tergites and pleurites, but their exact position varies in different groups. In the Coleoptera they occupy on the thorax a more ventral position, and on the abdomen are placed near the edge of the dorsal side, under the elytra. In the dragon-flies, the first pair is situated much more dorsally than the second and third pairs; the following seven pairs are almost wholly ventral and lie concealed in the membranous fold near the external plate. In the Hemiptera, also, the abdominal stigmata, though entirely free and visible, are situated ventrally.

Primarily, in the embryo a pair of stigmata appear on each segment of the thorax and abdomen, except the 10th and 11th, and even possibly in the head, for a pair of stigmata are said to occur in the head of Podurids (Smynthurus) (Lubbock), though this statement needs confirmation. Scolopendrella, however, is known to possess a pair of cephalic spiracles.

From the foregoing statement it will be seen that while in existing winged insects no more than 10 (in Japyx 11) pairs of stigmata are to be found in any one species, yet that 12 segments of the body, in different groups taken collectively, bear them. The primitive number of pairs of spiracles, therefore, in winged insects, was 12, i.e. a pair in each thoracic segment, and a pair in each of the first nine abdominal segments. Insects were originally all holopneustic, and gradually as the type became differentiated into the different orders they became peripneustic or amphipneustic, and, in certain aquatic forms, apneustic. (See pp. 459, 461.)

In the still more primitive, probably wingless, ancestors of insects there was a larger number of stigmata. Hatschek, in 1877, discovered a pair of tracheal invaginations in each of the three posterior head-segments of the embryo of a moth, with stigmatal openings in the 1st and 2d maxillary segments.

Thus early in embryonic life every segment of the body, except those bearing the eyes and the last abdominal, bore a pair of stigmata, so that the primitive insect had at least 15, and perhaps more, pairs of stigmata.

The position of the stigmata is subject to much variation, the result of adaptation to this or that mode of life. Examples are those insects which live in dusty situations or usually more or less concealed in the earth, as in most beetles, and in the Hymenoptera. In such beetles, the stigmata are situated in the thin membrane between the segments; in the Hymenoptera, on the upper edge of the segments. In the Siphonaptera, Pediculina, bed-bug, and similar forms, which breathe an air freer from dust, the spiracles lie free on the outside of the body.

“When the stigmata are free and without any protection on the abdomen, there are other ways by which the entrance of foreign bodies into the tracheæ is prevented. In such cases the body is covered with dense hairs, as in most Diptera and Neuroptera, as well as many Lepidoptera; or there is situated in front of the stigma either a small fissure which is covered over by a number of hairs arising from the edge, as in many Orthoptera; or, as in most insects, a luxurious growth of hairs on the inside of the stigma forms a thick filter for the air. Thus we see that also in this respect each species of insect is completely adapted to its surroundings.” (Krancher.)

=The closing apparatus of the stigma.=—Whether the external opening of the stigma is permanently open or closed, communication with the tracheæ may be cut off at pleasure during respiration by an internal apparatus of elastic chitinous bands and rods and the occlusor muscle.

The parts concerned in this operation are: 1. The closing bow; 2. The closing lever or peg; 3. The closing band; 4. The occlusor muscle (Figs. 405, 406).

“The first three parts are chitinized; they form a ring around the stigmatic opening, and are united to each other by joints. The bow is usually crescentic and as a rule surrounds one-half of the trachea. On the other side is the closing band which, by different contrivances, representing the closing lever or peg, becomes closely pressed against the closing bow. This lever is usually of the shape of a slender chitinous rod, which causes the closure; but it can also bend rectangularly, become converted into a typical lever as in the Lepidoptera, or it may assume the form of two peg-like processes, which press with their base against the closing bow.” (Krancher.)

“The closure of the spiracular opening is effected by the contraction of the muscles, while the opening is due to the elasticity of the chitinous parts. When at rest the spiracle is naturally open, so that the air in the trachea can directly communicate with the external air. Usually one end of the muscle is attached to the closing peg, and the other end to the closing bow. Where, as in Melolontha, the closing apparatus is provided with two levers, then naturally the muscle binds these two together and brings about by powerful contractions a firm closure of the trachea”; but, remarks Krancher, “this is not the only kind; there are numerous modifications. Besides the form just described, the levers assume the form of valves (Sirex), or of a brush (Pulex); or of a ring (larvæ of Diptera) with a circular muscle attached to it; or of a ring which simply becomes compressed (thoracic stigmata of Diptera).”

c. Morphology and homologies of the tracheal system

As first shown by Bütschli, the tracheal system is a series of segmentally arranged tubular invaginations of the ectoderm; a pair of stigmata primitively occurring on every segment of the body except perhaps the most anterior, and the last two or last one, a reduction in their number having since taken place, until in the Podurans none have survived. In the supposed ancestor of myriopods and insects, Peripatus, there are tracheæ; but they are very fine, simple, not-branched chitinous tubes which are united into tufts at the base of a flask-shaped depression of the integument, the outer aperture of which depression is regarded as a stigma. In one species (P. edwardsii) these tufts and their openings are scattered irregularly over the body; but in another kind (P. capensis) some of the stigmata at least show traces of a serial arrangement, being disposed in longitudinal rows—two on each side, one dorsally and one ventrally, those of each row, however, being more numerous than the pairs of legs. (See p. 9 and Fig. 4, D.)

It should be observed that in Peripatus, which does not possess urinary tubes, the segmental organs or nephridia are well developed, hence the tracheal tubes coexisting with them cannot be their homologues. We are therefore compelled to regard the tracheal system as of independent origin, arising in the earliest terrestrial air-breathing arthropod, and not indebted for its origin to any structure found in worms, unless perhaps, as both Kennell and Lang suggest, to dermal glands, since, according to Kennell, certain Hirudinea and many Turbellarian worms possess long, mostly unicellular, glands which spread far through the parenchyma of the body. (Kennell.)

Thus Kennell supposes that the ancestors of the Tracheates had spiracles on every segment of the body where the internal organization allowed them to exist. “The reduction of the breathing holes to a smaller number, and their restriction of a pair only to a single segment, was brought about partly by adaptation to a peculiar mode of life,—as insect larvæ especially teach us,—partly also—I may say mechanically—as a result of the obstruction to their development made by the growth or excessive development of other organs.” Among these he reckons the thick, dense cuticula of the integument, the internal fusion of several segments to form body-regions, and the arrangement and great development of the muscles in the head and thorax, etc. (p. 29.)

Kennell has suggested the origin of the tracheæ of Peripatus from the unicellular dermal glands of annelidan ancestors, since he has found glands in certain land-leaches of tropical America, which are provided with enormously long tubular passages united into bundles and opening externally, these tubes appearing to be slightly chitinized. Fig. 407 will show the appearance of a bundle of fine tracheal tubes of Peripatus ending at the bottom of a follicle formed by a deep invagination of the integument, which may be regarded as a primitive spiracle. (See Kennell, Ueber einige Landblutegel des tropical America, Zool. Jahrb. ii, 1886; also Die Verwandtschaftsverhältnisse der Arthropoden, 1891, p. 25.) We may add that Carrière supposes from his study of the embryology of the wall-bee (Chalicodoma muraria), published in 1890, that not only the salivary glands, but also the tentorium, are homologues of the tracheæ, while other structures than tracheæ may have evolved from unicellular dermal glands, which are widely distributed. It may in this connection be observed that some authors derive the book-lungs or book-leaf tracheæ of Arachnida from the gills of Limulus; hence if those of Arachnida arose from quite different and more specialized organs than dermal glands, it is not impossible that the tracheæ of Peripatus, Myriopods, and insects arose de novo, and then we need not look for any primitive structures in worms from which they arose.

Although Bütschli in 1870 in his embryology of the honey-bee called attention to the “great similarity which the eleven pairs of invaginations in the eleven first trunk-segments in their first indication (anlage) have with the spinning-glands, and also with the segmental organs of Annelids,” he did not go further than this, and it is now known that in the 2d maxillary segment open not only spinning-glands, but in the embryo a pair of stigmata.

Paul Mayer, however, regarded the tracheæ and urinary tubes as homodynamous structures, and this view was advocated by Grassi (1885) for the reason that while in the embryo honey-bee there are ten pairs of stigmata, the first thoracic and two last abdominal segments wanting them, the germs of the urinary tubes arise in a corresponding situation on the two last abdominal segments. To this view Emery (Biol. Centralb., 1886, p. 692) objects that in Peripatus the nephridia and tracheæ “have nothing to do with the segmental organs,” as Peripatus besides nephridia possesses both coxal glands and tracheæ.

Both Kennell and Lang derive the coxal glands of Arthropoda from the setiparous or parapodial glands of annelid worms, and the recent endeavor of Bernard to show that the tracheæ arose from setiparous glands seems to be disproved by the fact that in insects as well as in other Arthropoda coxal glands with their outlets exist in the same segments as those bearing stigmata. Reasoning by exclusion, we are led to regard Kennell’s original view as the soundest.

Patten, however, regards the tracheæ as modified ends of nephridia, remarking: “Since in Acilius some of the abdominal tracheæ at first communicate with the cavities of the mesoblastic somites, it is probable that all the tracheæ represent the ectodermic portions of the nephridia.” (Origin of Vertebrates from Arachnids, p. 355.)

It is probable, therefore, that the tracheæ first arose as modifications of dermal glands, as in mites and Peripatus, and that at first they were not provided with tænidia (as in Chilopoda), while in later forms tænidia were developed. In the earliest tracheate forms the stigmata were not segmentally arranged, probably appearing irregularly anywhere in the body, but afterwards in the myriopods and insects became serially arranged.

d. The spiral threads or tænidia

It is generally supposed that the so-called “spiral thread” forms a continuous thread from one end of a tracheal branch to the other. This was first shown not to be the case by Platner in 1844. Minot has proved that “there is not a single spiral thread, but several, which run parallel to one another and end after making a few turns around the trachea.”

The tænidia we have found to be in some cases separate, independent, solid rings, though when there is more than one turn the thread necessarily becomes spiral. The tænidia of a main branch stop at the origin of the smaller branches, and a new set begins at the origin of each branch. The tænidia at the origin of the branch do not pass entirely around the inside of the peritoneal membrane; in the axils they are short, separate, spindle-shaped bands (Fig. 409).

At one point in the main trachea of the larva of Datana the tænidia were seen to end singly on one side (at a considerable distance from any branch or axil) at intervals, with a tænidium situated between them, making four or five turns; then there is only one band situated between two ends; this band or thread is succeeded by a set with five turns between the two ends, this set being succeeded by one complete ring situated between two ends; in all cases the ends vary in length, some threads being short and others long, so that they apparently end anywhere along the circumference of the trachea, and this arrangement is seen to apparently extend along the whole length of the trachea. Hence it is seen that as a rule the tænidia vary much in length, and never, as generally supposed, pass continuously from one end to another of a tracheal branch, for there are many spirals in a branch, each making only from one to five turns, most usually four turns. Fig. 408, part of a trachea of Dyticus marginatus, shows that at a slight bend in a trachea the tænidia is interrupted, and short, incomplete, wedge-shaped tænidia (e) are interpolated; at A, d is seen a split in one of the tænidia (compare also MacLeod, Pl. 1, Fig. 9). The threads are quite irregular in width. In the axils of the branches there is, as seen in Fig. 409, a basketwork of independent, short, often spindle-shaped tænidia; these are succeeded by longer ones, until we have threads passing entirely around near the base of each new branch; these being succeeded by others which make from two to five spiral turns.

The shape of the tænidia appears to vary to a great extent. In lepidopterous insects we have observed them to be in their general shape rather flat and slightly concavo-convex, the hollow looking towards the centre of the trachea. Minot’s section (Fig. 393) shows that in Hydrophilus they are cylindrical and solid, and Chun states that those of Stratiomys are round, while in Eristalis they are round, with a ridge projecting into the cavity of the trachea; in Æschna the thread is quadrangular. MacLeod states that sometimes it is cylindrical, in other cases flat, likewise prismatic; Macloskie believes that the spiral threads of the centipede are “fine tubules, externally opening by a fissure along their course.”

Stokes confirms Macloskie’s statements, stating that in the hemipterous Zaitha fluminea “the tænidia are fissured tubules formed within and from chitinized folds of the intima, the convexity of the folds looking towards the lumen of the tracheæ.” In Fig. 414, 1, are represented portions of several tænidia showing the fissure, which is sometimes interrupted; at 2 are seen “the formation of what may be called apertures in a chitinous bridge.” Stokes regards the tænidia as “inwardly directed folds of the membrane.” Near the spiracles the tracheal membrane is externally studded with minute papillæ, as shown at 3, where are represented three broad and incomplete tænidia, with the tapering end, or the beginning, of another. Stokes adds, “Here they are only broad grooves, with no appearance of the narrow fissure of the completed tænidium. At 4 is figured a portion of the internal surface of a large trachea near the external orifice, the tænidia being in an incipient stage, evidently forming more or less of a network, as is usually the case next to the stigma” (compare p. 451, and Fig. 414).

The tracheæ of chilopod myriopods appear to be like those of insects. A number of authors have failed to detect the spiral threads in the Julidæ. As to the Arachnida, several observers, including Menge and Bertkau, have denied the existence of the spiral thread in the spiders with the exception of the Attidæ; and MacLeod finds them “scarcely visible” in Argyroneta.

Besides the tracheæ, the salivary duct is kept permanently distended by tænidia, which, however, are not spiral. They usually form incomplete rings, as in Stomoxys, arranged as shown in Fig. 410.

The labella (proboscis) of flies are supported by incomplete chitinous tubes or “pseudo-tracheæ,” the ends of which form the scraping teeth, this being, according to Dimmock, their primary function. Dimmock describes them as cylindrical channels opening on the surface in zigzag slits. These channels are held open by incomplete rings, one end of which is forked. “These rings are apparently arranged so that one has its fork on one side of the opening of the channel, the next ring the fork on the opposite side of the channel, and so on, in alternation. Their true structure is revealed when flattened out.”

The use of the elastic tænidia is to render the tracheæ elastic, and to keep them permanently open, as is the case with the parallel rings of the trachea of the higher vertebrates. The tracheæ are thus rendered firm and solid, at the least expense of chitinous material. The spiral thread, as MacLeod remarks, “is the realization in nature of what engineers call a form of the greatest resistance.”

The tænidia are wanting in the fine endings of the tracheæ (tracheal capillaries); also in the cockroach, according to Miall and Denny, they are not developed in the large tracheæ close to the spiracles, and the intima or wall of the tube has a tessellated instead of a spiral marking (Fig. 411). The same structure is seen in the Perlidæ (Nemoura, Gerstaecker, Zeit. f. wissen. Zool. xxiv, Taf. xxiii, Figs. 5 and 7); also in Æschna (Hagen, Zool. Anz. 1880, p. 159). In certain fine tracheæ of the eyes of the fly no spiral threads are developed. (Hickson.) The air-sacs or dilated tracheæ are also without tænidia.

While in the living insect the main and smaller tracheæ are filled, with air, it is stated by Von Wistinghausen that the fine capillary ends contain a fluid.

e. Origin of the tracheæ and of the “spiral thread”

While we owe to Bütschli the discovery of the mode of origin and morphology of the tracheæ, which as he has shown[66] arise by invaginations of the ectoblast; there being originally a single layer of epiblastic cells concerned in the formation of the tracheæ; we are indebted to Weismann[67] for the discovery of the mode of origin of the “intima,” from the epiblastic layer of cells forming the primitive foundation of the tracheal structure.

Weismann did not observe the earliest steps in the process of formation of the stigma and main trunk of the tracheæ, which Bütschli afterwards clearly described and figured.

Weismann, however, thus describes the mode of development of the intima; after describing the cells destined to form the peritoneal membrane, he says: “The lumen is filled with a clear fluid and already shows a definite border in a slight thickening of the cell-wall next to it.

“Very soon this thickening forms a thin, structureless intima, which passes as a delicate double line along the cells, and shows its dependence on the cells by a sort of adherence to the rounded sides of the cells (Taf. vii, 97 A, a b c). Throughout the mass, as the intima thickens, the cells lose their independence, their walls pressing together and coalescing, and soon the considerably enlarged hollow cylinder of the intima is surrounded by a homogeneous layer of a tissue, whose origin from cells is recognized only by the regular position of the rounded nuclei (Taf. vii, Fig. 97, B).

“Then as soon as the wavy bands of the intima entirely disappear, and it forms a straight, cylindrical tube, a fine pale cross-striation becomes noticeable (vii, 97, B, int), which forms the well-known ‘spiral thread,’ a structure which, as Leydig has shown, possesses no independence, but arises merely from a partial thickening of the originally homogeneous intima.

“Meyer’s idea that the spiral threads are fissures in the intima produced by the entrance of air is disproved by the fact that the spiral threads are present long before the air enters. Hence the correctness of Leydig’s view, based on the histological structure of the tracheæ, is confirmed by the embryological development, and the old idea of three membranes, which both Meyer and Milne-Edwards maintain, must be given up.”

Weismann also contends that the elastic membrane bearing the “spiral thread” is in no sense a primary membrane, not corresponding histologically to a cellular membrane. On the contrary, the “peritoneal membrane comprises the primary element of the trachea; it is nowhere absent, but envelops the smallest branches, as well as the largest trunks, only varying in thickness, which in the embryo and the young larva of Musca stands in relation to the thickness of the lumen.”

The trachea, then, consists primarily of an epithelial layer, the “peritoneal membrane,” or the invaginated epiblast; from this layer an intima is secreted, just as the skin or cuticle is secreted by the hypodermis. We may call the peritoneal membrane the ectotrachea, the intima or inner layer derived from the ectotrachea the endotrachea. The so-called “spiral threads” are a thickening of the endotracheal membrane, sometimes arranged in a spiral manner. For these chitinous bands we have proposed the name tænidia (Greek, little bands).

As to the origin of the spiral thread our observations[68] have been made on the caterpillar of a species of Datana, which was placed in alcohol, just before pupation, when the larva was in a semipupal condition, and the larval skin could be readily stripped off. At this time the ectotrachea of the larva had undergone histolysis, nothing remaining but the moulted endotrachea, represented by the tænidia, which lay loosely within the cavity of the trachea. The ectotrachea or peritoneal membrane of the pupa is meanwhile in process of formation; the nuclear origin of the tænidia is now very apparent.

Fig. 412 represents a longitudinal section through a secondary tracheal branch, showing the origin of the chitinous bands, or tænidia. At t′ are pieces of six tænidia which have been moulted; ectr indicates the nuclei forming the outer cellular layer, the ectotrachea or peritoneal membrane. These nuclei send long slender prolongations around the inside of the peritoneal membrane; these prolongations, as may be seen by the figure, become the tænidia. The tænidia, being closely approximate, grow together more or less, and a thin endotracheal membrane is thus produced, of which the tænidia are the thickened band-like portions. The endotracheal membrane is thus derived from the ectotrachea, or primitive tracheal membrane, and the so-called “spiral thread” is formed by thickenings of the nuclei composing the secondary layer of nuclei, and which become filled with the chitin secreted by these elongated nuclei. The middle portion of the tænidia, immediately after the moult, is clear and transparent, with obscure minute granules, while the nuclear base of the cell is filled as usual with abundant granules, and contains a distinct nucleolus.

The origin of the tænidia is also well shown by Fig. 413, which is likewise a longitudinal section of a trachea at the point of origin of a branch. The peritracheal membrane or ectotrachea (ectr) is composed of large granulated nuclei; and within are the more transparent endotracheal cells; at t′ are fragments of the moulted tænidia. The new tænidia are in process of development at t; at base they are seen to be granulated nuclei, with often a distinct nucleolus, each sending a long, slender, transparent, pointed process along the inside of the trachea. These unite to form the chitinous bands or spiral threads.

=Internal hair-like bodies.=—In the large tracheæ of Lampyris very fine chitinous bristles project free into the cavity of the tube (Gerstaecher), while according to Leydig there are similar chitinous points in the tracheæ of the Carabid beetle Procrustes. Dugardin had previously (1849) called attention to such hairs, giving a list of the insects in which he observed them. Emery figures a section of the tracheæ of Luciola, “in wendig behaart.”[69] Stokes has described those of Zaitha fluminea (Fig. 414) as “internal chitinous, hair-like bodies arising from the fold of the tænidia and projecting into the lumen of the tubes.” They are hollow, their minute cavity distinctly communicating with that of the tænidium, from which they arise by an enlarged base. They end in an exceedingly fine point which is occasionally bifid or trifid. In Fig. 414, 4, several are shown attached to the wrinkles of the tracheæ near a spiracle, and at 5 is represented a transverse section of a trachea with three hairs projecting into its cavity.[70]

Stokes has also described “certain minute, elliptical bodies in the tænidia, each with an internal, presumably glandular, appendage, to all appearance forming part of the tænidium from which it springs.” These are shown in Fig. 414, at 1, 3, and, more in detail, at 6; those at 7, whose thickness is about 1⁄8000 of an inch, appear as collections of exceedingly minute, rounded apertures in a cushion-like mass. Although not commonly occurring on the tracheal membrane between the tænidia, they may be found there, as at 4.

f. The mechanism of respiration and the respiratory movements of insects

By holding a locust in the hand one may observe the ordinary mode of breathing in insects. During this act the portion of the side of the body between the stigmata and the pleurum contracts and expands; the contraction of this region causes the spiracles to open. The general movement is caused by the sternal moving much more decidedly than the tergal portion of the abdomen. When the pleural portion of the abdomen is forced out, the soft pleural membranous region under the fore and hind wings contracts, as does the tympanum, or ear, and the membranous portions at the base of the hind legs. When the tergum or dorsal portion of the abdomen falls, and the pleurum contracts, the spiracles open; their opening is nearly but not always exactly coördinated with the contractions of the pleurum, but as a rule they are. There were 65 contractions in a minute in a locust which had been held between the fingers about ten minutes. It was noticed that when the abdomen expanded, the air-sacs in the first abdominal ring contracted.

For expanding the abdomen no special muscles are required, since it expands by the elasticity of the parts. For contracting its walls there are two sets of muscles, viz., special vertical expiratory muscles serving to compress or flatten the abdomen (Figs. 415–418), and other muscles which draw together or telescope the segments.

It was formerly supposed that when the abdomen contracted the air was expelled from the body and the tracheæ emptied; that, when the abdomen again expanded by its own elasticity, the air-tubes were refilled, and that no other mechanism was needed. But Landois insisted that this was not enough; as Miall and Denny state: “Air must be forced into the furthest recesses of the tracheal system, where the exchange of oxygen and carbonic acid is effected more readily than in tubes lined by a dense intima. But in these fine and intricate passages the resistance to the passage of air is considerable, and the renewal of the air could, to all appearance, hardly be effected at all if the inlets remained open. Landois accordingly searched for some means of closing the outlets, and found an elastic ring or spiral, which surrounds the tracheal tube within the spiracle.” By means of the occlusor muscle this ring compresses the tube, “like a spring clip upon a flexible gas-pipe.” “When the muscle contracts, the passage is closed, and the abdominal muscles can then, it is supposed, bring any needful pressure to bear upon the tracheal tubes, much in the same way as with ourselves, when we close the mouth and nostrils, and then, by forcible contraction of the diaphragm and abdominal walls, distend the cheeks or pharynx.”

Thus an important point in the respiration of tracheate animals, whether insects, myriopods, or arachnids, is, as Landois claimed, the closure of the spiracles, in order that pressure may be brought upon the air in the tubes, so that it may pass onward into the finest terminations.

The injection of air by muscular pressure into a system of very fine tubes may, as Miall and Denny remark, appear extremely difficult or even impossible. Graham (Researches, p. 44) applies the law of diffusion of gases to explain the respiration of insects, but until physical experiments have been made, we may, with Miall and Denny, “be satisfied that an appreciable quantity of air may be made by muscular pressure to flow along even the finer air-passages of an insect.”

As to the respiratory movements of insects, Plateau is the principal authority, and the following account of the process is taken from his elaborate memoir, and from the statements afterwards contributed by him to Miall and Denny’s “The Cockroach.”

Although many observers have superficially described the respiratory movements of various insects, Rathke was the first one to state precise views as to the mechanism of respiration. His posthumous work, treating of the respiratory movements of the movable chitinous plates of the abdomen, and of the respiratory muscles characteristic of all the principal groups, filled an important blank in our knowledge. But, notwithstanding the skill displayed in this research, many questions still remain unanswered which require more exact methods than mere observations with the naked eye or the simple lens.

Plateau, who was followed a year later by Langendorff, conceived the idea of studying, by such graphic methods as are now familiar, the respiratory movements of perfect insects.

“He has made use of two modes of investigation. The first, or graphic method, in the strict sense of the term, consisted in recording, upon a revolving cylinder of smoked paper, the respiratory movements, transmitted by means of very light levers of Bristol board attached to any part of the insect’s exoskeleton. Unfortunately, this plan is only applicable to insects of more than average size. A second method, that of projection, consisted in introducing the insect, carried upon a small support, into a large magic lantern fitted with a good petroleum lamp. When the amplification does not exceed 12 diameters, a sharp profile may be obtained, upon which the actual displacements may be measured, true to the fraction of a millimetre. Placing a sheet of white paper upon the lantern screen, the outlines of the profile are carefully traced in pencil so as to give two superposed figures, representing the phases of inspiration and expiration respectively. By altering the position of the insect so as to obtain profiles of transverse sections, or of the different parts of the body, and, further, by gluing very small paper slips to parts whose movements are hard to observe, the successive positions of the slips being then drawn, complete information is at last obtained of every detail of the respiratory movements; nothing is lost.”

“This method, similar to that employed by the English physiologist, Hutchinson,[71] is valuable, because it enables us, with a little practice, to investigate readily the respiratory movements of very small arthropods, such as flies or lady-birds. It has this advantage over all others, that it leaves no room for errors of interpretation.”

“Not satisfied with mere observation by such means as these, of the respiratory movements of insects, the writer has also studied the muscles concerned, and, in common with other physiologists (Faivre, Barlow, Luchsinger, Dönhoff, and Langendorff), has examined the action of the various nervous centres upon the respiratory organs. The result at which he has arrived may be summarized as follows:—

“1. There is no close relation between the character of the respiratory movements of an insect and its systematic position. Respiratory movements are similar only when the arrangement of the abdominal segments, and especially when the disposition of the attached muscles, are almost identical. Thus, for example, the respiratory movements of the cockroach are different from those of other Orthoptera, resembling those of the heteropterous Hemiptera. Those of the Trichoptera are like those of the aculeate Hymenoptera, while the Locustidæ ally themselves in respect to these movements with the Neuroptera and Lepidoptera.

“2. The respiratory movements of insects, when at rest, are localized in the abdomen. As graphically stated by Graber, in insects the chest is placed at the hinder end of the body. If thoracic respiratory movements exist, they do not depend on the action of special muscles.

“3. In most cases the thoracic segments do not share in the respiratory movements of an insect at rest. The respiratory displacements of the posterior segments of the thorax are, however, less rare than Rathke believed. Plateau has observed them in certain Coleoptera (Staphylinus, Chlorophanus, Corymbites), and they are more feebly manifested in Hydrophilus, Carabus, and Tenebrio. Among the singular exceptions to this rule is the cockroach (Periplaneta orientalis), in which the terga of the meso- and metathoracic segments perform movements exactly opposite in direction to those of the abdomen (Fig. 419).

“4. Leaving out of account all details and all exceptions, the respiratory movements of insects may be said to consist of the alternate contraction and recovery of the figure of the abdomen in two dimensions, viz. vertical and transverse. During expiration both diameters are reduced, while during inspiration they revert to their previous amounts. The transverse expiratory contraction is often slight, and may be imperceptible. On the other hand, the vertical expiratory contraction is never absent, and usually marked. In the cockroach (P. orientalis) it amounts to one-eighth of the depth of the abdomen (between segments 2 and 3); in Eristalis tenax to one-ninth (at the 2d segment).

“5. Three principal types of respiratory mechanism occur in insects, and these admit of further subdivision:

“a. Sterna usually short and very convex, yielding but little. Terga mobile, rising and sinking appreciably. To this class belong all Coleoptera, heteropterous Hemiptera, and Blattina (Fig. 420).

“In the cockroach (Periplaneta), the sterna are slightly raised during expiration (Fig. 421).

“b. Terga well developed, overlapping the sterna on the sides of the body, and usually concealing the pleural membrane, which forms a sunken fold. The terga and sterna approach and recede alternately, the sterna being almost always the more mobile. To this type belong Odonata, Diptera, aculeate Hymenoptera, and acrydian Orthoptera (Fig. 422).

“c. The pleural membrane, connecting the terga with the sterna, is well developed and exposed on the sides of the body. The terga and sterna approach and recede alternately, while the pleural zone simultaneously becomes depressed, or returns to its original figure. To this type, Plateau assigns the Locustidæ, Lepidoptera, and the true Neuroptera (excluding Trichoptera) (Fig. 423).

“6. Contrary to the opinion once general, changes in length of the abdomen, involving protrusion of the segments and subsequent retraction, are rare in the normal respiration of insects. Such longitudinal movements extend throughout one entire group only, viz. the aculeate Hymenoptera. Isolated examples occur, however, in other zoölogical groups.

“7. Among insects, such as large beetles, Locustidæ, dragon-flies, etc., sufficiently powerful to give good graphic tracings, it can be shown that the inspiratory movement is slower than the expiratory, and that the latter is often sudden.

“8. In most insects, contrary to what obtains in mammals, only the expiratory movement is active; inspiration is passive, and effected by the elasticity of the body-wall.

“9. Most insects possess expiratory muscles only. Certain Diptera (Calliphora vomitoria and Eristalis tenax) afford the simplest arrangement of the expiratory muscles. In these types, they form a muscular sheet of vertical fibres, connecting the terga with the sterna, and underlying the soft, elastic membrane which unites the hard parts of the somites. One of the most frequent complications arises by the differentiations of this sheet of vertical fibres into distinct muscles, repeated in every segment, and becoming more and more separated as the sterna increase in length. Special inspiratory muscles occur in Hymenoptera, Acridiidæ, and Trichoptera.

“10. The abdominal, respiratory movements of insects are wholly reflex. Like other physiologists who have examined this side of the question, Plateau finds that the respiratory movements persist in a decapitated insect, as also after destruction of the cerebral ganglia or œsophageal connectives; further, that in insects whose nervous system is not highly concentrated (e.g. Acridiidæ and dragon-flies), the respiratory movements persist in the completely detached abdomen; while all external influences which promote an increased respiratory activity in the uninjured animal, have precisely the same action upon insects in which the anterior, nervous centres have been removed, upon the detached abdomen, and even upon isolated sections of the abdomen.

“The view formerly advocated by Faivre, that the metathoracic ganglia play the part of special, respiratory centres, must be entirely abandoned. All carefully performed experiments on the nervous system of Arthropoda have shown that each ganglion of the ventral chain is a motor centre, and, in insects, a respiratory centre, for the somite to which it belongs. This is what Barlow calls the ‘self-sufficiency’ of the ganglia.” (Miall and Denny.)

Plateau has made similar observations upon the respiration of spiders and scorpions; but, to his great surprise, he was unable, either by direct observation, or by the graphic method, or by projection, to discover the slightest respiratory movement of the exterior of the body. This can only be explained by supposing that inspiration and expiration in pulmonate Arachnida are “intrapulmonary,” and affect only the proper, respiratory organs. The fact is less surprising because of the wide zoölogical separation between Arachnida and insects.

g. The air-sacs

In flying insects the tracheæ are in certain parts of the body enlarged into sacs of various sizes. These air-sacs were first observed by Swammerdam in a beetle (Geotrupes) and afterwards by Sir John Hunter in the bee, Sprengel subsequently discovering them in other insects. Those of the cockroach were described and illustrated in a very elaborate and detailed way by Straus-Dürckheim (Figs. 424 and 425). These vesicles are without tænidia. In the locust (M. femur-rubrum) there is a pair of very large vesicles in the prothorax (Fig. 396). The five pairs of large abdominal air-sacs arise, independently of the main tracheæ, directly from branches originating from the spiracles. All these large sacs are superficial, lying directly beneath the hypodermis, while the smaller ones are buried among the muscles. We have detected 53 of these vesicles in the head.

In the honey-bee (Fig. 426) and humble bee (Fig. 427) as well as the flies there are two enormous air-sacs at the base of the abdomen. In larval and wingless insects these sacs are entirely absent.

=The use of the air-sacs.=—It was supposed by Hunter as well as by Newport, and the view has been generally held, that the use of these sacs is to lighten the weight, i.e. lessen the specific gravity of the body during flight. It has, however, been suggested to us by A. A. Packard that this view from the standpoint of physics is incorrect. It is evident that the wings have to support just as much weight when the insect is flying, whether the tracheæ and vesicles are filled with air or not, the body of the insect during flight not being lightened by the air in the sacs. The use of these numerous sacs, some of them very spacious, is to afford a greater supply of air or oxygen than that contained in the air-tubes alone, and thus to afford a greater breathing capacity. The sacs are largest in dragon-flies, moths, flies, and bees, which are swift of flight. When we compare the active movements of these insects on the wing with those of a caterpillar or maggot, it will be seen that the far greater muscular exertions of the volant insect create a demand for a sudden and abundant supply of air to correspond to the increased rapidity of respiration; and the enlargements of the air-tubes, rapidly filled with air at each inspiration, render it possible to supply the demand.

The case is thus seen to be very different from that of those fishes which, having a swimming-bladder, can in the water change the specific gravity of their bodies. The case of insects is almost exactly paralleled by that of birds, where, as stated by Wiedersheim, the air-sacs appear to form integral parts of the respiratory apparatus: “a greater amount of air can by their means pass in and out during inspiration and expiration, especially through the larger bronchi, and consequently there is less necessity for the expansion of the lung parenchyma.” In other words, the supply of air in these sacs, as in insects, increases the breathing capacity of the bird during flight. Wiedersheim’s retention of the old idea that the specific gravity of the body is lessened (p. 262) seems, however, to be incorrect, as the weight of the bird’s body is not diminished by the air contained in the sacs.

h. The closed or partly closed tracheal system

There are two chief morphological tracheal systems: 1. The open or normal and primitive (holopneustic) type, and 2. The closed, or secondary and adaptive, i.e. apneustic, type. The open system is characterized by the presence of the stigmata. Through them the air directly enters into the tracheal tubes, whose delicate walls allow the exchange of gases in the blood. This type occurs in all sexually mature individuals, and also in the greater number of larvæ.

The closed or apneustic tracheal system is distinguished either by the want of stigmata, or, if present, they are not open, and do not function, so that the tracheæ cannot communicate with the air. In such cases the direct oxygenation of the blood is effected through the delicate integument, especially over the surface of the body in general, or in certain specialized places where the gill-like expansions of the skin are rich in tracheæ; such outgrowths, generally tubular or leaf-like, are called by Palmén tracheal gills.

This closed form of the tracheal system only occurs in the larval stage of aquatic or parasitic insects, as in the Plectoptera (Ephemeridæ), Perlidæ, Odonata, and Trichoptera, besides single genera of other orders, i.e. among Coleoptera, Gyrinus, Pelobius, Cnemidotus, and the young larva of Elmis; in the aquatic caterpillar of Paraponyx; in certain Diptera (Corethra, Chironomus, etc.), and some of the parasitic Hymenoptera (Microgaster).

Palmén has discovered that in the nymphs of Ephemeridæ, Perlidæ, Odonata, and the larvæ of most Trichoptera the tracheal branch (stigmatal branch) sent from the longitudinal trachea to where the thoracic stigmata would be situated if present, or where their vestiges only exist, are aborted, becoming simple solid cords not filled with air (Fig. 436, vf, and 447, f, funiculus or stigmatic cord). In the imago, however, they resume their function, connecting with the open functional stigmata. In Corethra, in its earliest stages, the entire tracheal system is, like the stigmatic branch, a system of solid cords and empty of air. (Palmén.)

Embryology shows that these stigmatal branches are well developed, and are formed at the same time as the stigmata. It was also shown by Dewitz, in a posthumous paper (1890), that in the young larval stage of the Odonata and Ephemeridæ the tracheal system is at first an open one, and in some of the families (Libellulidæ, Agrionidæ, and Ephemeridæ) thoracic stigmata are seen at a very early stage. From numerous experiments Dewitz concludes that in the young stages of Odonata and Ephemeridæ there is an open tracheal system; certainly in very young nymphs the thoracic spiracles allow the air to pass out. Fully grown nymphs of Æschnidæ, Libellulidæ, and Agrionidæ are capable not only of forcing the air out, but also, like the perfect insect, of inhaling it. Moreover, he proved that the gills of Ephemeridæ and Agrionidæ are not indispensable for the maintenance of life, as the insects can live without them, breathing either through the skin or by the rectum, or in both ways. It would seem that while in freshly hatched or very young larvæ of aquatic insects of different orders the skin is so delicate as to allow of dermal respiration, in after life, when the skin becomes thicker and denser, these expansions (gills), provided with a very thin and delicate skin, of a necessity grow out from the walls of the body.

It thus appears that the closure and total or partial abolition of the stigmata are in adaptation to aquatic life, and that such insects have descended from terrestrial air-breathing winged forms. This is an important argument against the view that the wings are modified tracheal gills.

In this connection may be noticed the closure of the 2d and 3d thoracic stigmata in holopneustic insects. We have found on laying open the body of a Sphinx larva that a large number of tracheal branches are seen to arise from the prothoracic and from the first pair of abdominal stigmata. Now between these points there are no spiracles or any external signs of them, there being in Lepidoptera no mesothoracic or metathoracic spiracles. Yet the main lateral trachea between the prothoracic and first abdominal segments deviates from its course and bends down to send off a small shrivelled stigmatal branch or cord to a place where, did a spiracle exist, we should look for it. In the larva of Platysamia cecropia, a similar vestigial stigmata branch is present.

In the larva of Corydalus, also, a trachea as large as the main longitudinal one takes its origin and passes directly under the main trachea. Now both tracheæ send a stigmatal branch opposite to where the mesothoracic stigma should be, if present, i.e. on the hind edge of the segment.

Verson, moreover, has found in the freshly hatched silkworm vestiges of meso- and metathoracic stigmata, each consisting of a circle of high hypodermal cells radially arranged around a common centre. The stigmatal branch is long, but shrivelled; its peritoneum is widened out into several berry-like saccules filled with cell-elements. In profile these rudimentary stigmata appear as a series of high hypodermal cells, which form the basis of a short blind tube.

After the second moult there begins a peculiar transformation of the rudimentary stigmata. The stigmatal branch connected with them sends off at various points thick tufts of capillary tracheæ which press against the base of the blind tube. Gradually lengthening, they form a fold which continues to increase in length. The numerous tufts of tracheal capillaries extend beyond the inner surface of the two layers of which the developing wing consists, the berry-like saccules are drawn into the wing and converted into more or less thick tubes, which finally form the “veins.” It is clear, therefore, says Verson, as Landois claimed, that the wings of Lepidoptera must be regarded as in the fullest sense organs of respiration. (Zool. Anz., 1890, p. 116.)

The number of pairs of stigmata varies, especially in maggots or larval Diptera, in adaptation to their varied modes of life. The larvæ of most flies (Muscidæ) have a pair of peculiarly shaped processes on the prothoracic segment bearing spiracular openings, and two anal spiracles, while in Ctenophora atrata L. only the anal pair are present. In the rat-tailed maggots (Eristalis) the long caudal process ends in two stigmata forming a respiratory tube, which can be thrust out of the water for the reception of air. In the larval mosquito (Fig. 433) and its ally, Mochlonyx, a short thick dorsal tube arises from the penultimate segment of the body, in which the two main tracheæ end, opening outward by a single spiracular aperture. Other dipterous larvæ (Simulium, Tanypus, and Ceratopogon) possess no spiracles, the tracheal system being a closed one.

The larvæ of most water beetles (Dyticidæ, Hydrophilidæ) possess but two spiracles, which, as in maggots, are situated at the end of the body. The aquatic larva of Amphizoa, according to Hubbard, breathes much as in the Dyticidæ, by means of two large valvular spiracles placed close together at the end of the body; “closed or rudimentary stigmata also occur on the mesothorax and on abdominal segments one to seven inclusive.”

Hubbard adds: “The larva of Pelobius is wholly aquatic and breathes by branchiæ, but the obsolete stigmata are indicated precisely as in Amphizoa, with the exception of the last pair, which in Amphizoa are open spiracles, but in Pelobius are suppressed; the terminal eight segments being prolonged in a swimming stylet.”

From a review of the distribution of spiracles, and their atrophy, partial or total, it will be seen that there are intermediate stages between the open (holopneustic) and closed (apneustic) systems. These, following Schiner, Brauer, and Palmén, may be defined thus:

1. Metapneustic type.—The larvæ possess only a single pair of open stigmata situated at the end of the body. (The dipterous Eristalis, Tipula, Culex, Ptychoptera, Bittacomorpha (Plate I.) with certain Tachinidæ, and in Coleoptera, the larvæ of Dyticus, and allies of Hydrophilus and Cyphon.)

2. Propneustic type.—The pupæ of Corethra, Culex, etc., in which only the most anterior pair of spiracles are open.

3. Amphipneustic type.—Larvæ with a pair of open spiracles situated at each end of the body, the intermediate spiracles being closed. (Most dipterous larvæ, Musca, after the first moult, Œstridæ, Asilidæ, and Syrphus.)

4. Peripneustic type; with prothoracic and abdominal spiracles, the mesothoracic pair atrophied or closed. (The larvæ of Neuroptera, Mecoptera, Trichoptera, Lepidoptera, of most Coleoptera,[72] of most Diptera, and of most of the Hymenoptera.[73])

These differences in the number of functional spiracles are in direct relation with the surroundings of the insects, the physical conditions of existence evidently determining the position of the active functional open spiracles and the closure of those useless to the organism.

i. The rectal tracheal gills, and rectal respiration of larval Odonata and other insects

The remarkable mode of respiration by tracheal gills situated within the intestine of the nymphs of dragon-flies was first described by Swammerdam and afterwards by Réaumur. The most complete and best illustrated modern account is that of Oustalet. In these insects the large rectum is lined with six double longitudinal ridges, in Æschna bearing numerous delicate tubes or papillæ, each of which contains very numerous (by estimate 24,000) tracheal branches (Fig. 431); while in Libellula the gills are lamellate (Fig. 432). The tracheæ arise both from the main dorsal and visceral longitudinal trunks, which give rise to secondary branches passing into the walls of the rectum and sending into the branchial papillæ fine twigs, which, extending to the distal end of the papilla or lamella, recurve and anastomose with the efferent twigs.

The anal opening is externally protected by the suranal and lateral triangular chitinous plates, three to five in all. When open, the water passes into the rectum and bathes the rectal gills, where it may be forcibly expelled as if shot out from a syringe, thus propelling the insect forward. In Libellula the anus affords direct access to the intestinal cavity, but in Æschna Oustalet describes “a sort of vestibule separated from the rectum by a circular valvule.” He also states that the inspiration and the repulsion of water is produced at irregular intervals, and rather by the movements of the dorsal and sternal arches of the abdomen than by the contractions of the rectum, since the walls of this organ are less muscular than is supposed.

The nymph of Calopteryx (and probably of all the group Calopteryginæ) possesses rectal gills besides external caudal tracheal gills. There are three double rectal longitudinal folds or ridges, interpenetrated by tracheal twigs. (Dufour, denied by Poletaiew, but confirmed by Hagen.)

Dewitz claims that the caudal gills of the Agrionidæ are not their sole means of respiration, since he cut off the caudal tracheal gills of an Agrionid nymph, which continued to live for a week. Hence he thinks that there may be a rectal respiration, since under the microscope he saw a stream of water pass in and out of the end of the intestine.

Dewitz’ experiments prove that in young Ephemerids there may be besides branchial, both rectal and skin respiration. He saw under the microscope the anus for a while opened and then closed, causing the rectum to move; powdered carmine mixed with water was drawn into and then expelled from the rectum. There was, however, no enlargement and contraction of the abdomen as in the rectal respiration of Æschna. (Zool. Anz. 1890, p. 500.)

Eaton states that there is a rectal respiration in the nymphs of may-flies, and Palmén observed in young larvæ of Bætis and Cloëon that the rectum took in “by gulps” water colored by carmine and expelled the whole of it at once, in order to fill it again in the same way. “This rectal respiration therefore corresponds to that of Libellulid larvæ.”

Besides breathing by spiracles, by tracheal gills, as well as through the integument, the larva of Culex has been observed by Raschke to have a rectal respiration. At the anterior end of the rectum arises a countless number of fine tracheæ, which pass through the walls and, subdividing, end in numberless very fine twigs in the papilla-like folds situated within the rectum. The supply of tracheal twigs is greatest where the papillæ are largest. (Figs. 433, 434.)

j. Tracheal gills of the larvæ of insects

In many aquatic insects respiration is carried on by tracheal gills. These are delicate, hollow, leaf-like or tubular outgrowths of the integument usually attached to the sides or end of the hind-body, and containing a trachea which usually sends off numerous minute branches, so that the exchange of gases readily takes place in them.

Palmén has shown that these tracheal gills, as he calls them, are not developed on the same segments as the stigmata, and that the two structures have no genetic connection with each other. It is evident that these gills are secondary, adaptive organs.

In some cases (see p. 475) the tracheæ are wanting, but as such gills are filled with blood, the air contained in the water must pass in through their delicate walls.

In the Plectoptera (Ephemeridæ) the tracheal gills are either foliaceous or filamentous; when foliaceous they form simple or double leaves, with or without branches, or with a fringe of tubules, or under the leaf-like cover-bearing tufts of filaments. They are situated on the (usually) basal seven abdominal segments, at their hinder edge (Figs. 435, 436). In Oligoneuria and Jolia a pair occurs on the under side of the head, attached to the maxillæ, while in Jolia there is a pair on the under side of the first thoracic segment at the insertion of each of the legs. In certain genera (Heptagenia, Oligoneuria, and Jolia), they are in the form of a flat cover, under which lies a tuft of respiratory tubes, or (Ephemerella) a small bifid cluster of very delicate leaves (Fig. 437, A). In Cœnis and Tricorythus the tracheal gills of the second pair are modified to form plates covering all the succeeding pairs, those of the first pair being nearly atrophied and well-nigh functionless. (Fig. 437, B.)

Finally, in the highly modified forms Bætisca and Prosopistoma the tracheal gills are entirely concealed and protected by mesothoracic projections so as to form a true respiratory chamber, to which the water has access either by an opening behind, as in Bætisca, or by three openings, two ventral and one dorsal (Fig. 441), as in Prosopistoma.

The slender cylindrical tracheal gills of Heptagenia in the third or fourth nymphal stage are 2–jointed, and the first abdominal pair in Cænis are said by Palmén to be finger-shaped and 2–jointed. In Polymitarcys virgo the gills do not appear until the eighth or tenth day after hatching.

Dewitz found that young nymphs of Ephemerids will well endure the amputation of their gills, while fully grown ones die. Amputation of the lateral gills hastens ecdysis. After the change of skin, the gills are smaller than before, and at first contain no tracheæ, but in a few weeks they develop as completely as in normal individuals. The caudal gills were also renewed.

In the nymphs of Perlidæ the tracheal gills are usually present, and are either foliaceous (Nemoura) or more commonly filamentous in shape (Fig. 442). They are situated either on the prosternum (Nemoura and Pteronarcys), or on each side of the thorax, or on the sides of the abdomen, or are restricted to a tuft on each side of the anus at the base of the caudal stylets (Pteronarcys and Perla). Unlike the Ephemeridæ the gills persist in certain genera throughout life.

The larvæ of the aquatic Neuroptera, Sisyra, Sialis, and Corydalus possess lateral pointed bristle-like tracheal gills, which in Sisyra are 2–jointed; those of Sialis are, in the living larva, curved upwards and backwards (Fig. 444). Corydalus is also provided with a ventral tuft of delicate filamentous gills, which, however, according to Riley, do not appear until after the first moult.

While the nymphs of Agrionidæ (which have rectal gills) respire chiefly by the large caudal foliaceous gills (Fig. 445), there are, according to Hagen, two genera of the Calopteryginæ (Euphæa, Fig. 445, and Anisopleura) whose nymphs possess seven pairs of external lateral tracheal gills, in shape like those of Sialis, besides three caudal and three rectal tracheal gills.[74]

Hagen has also detected in the under side of the 5th abdominal segment of Epitheca and Libellula a pair of sacs of the shape of a Phrygian bonnet, each of which contains a smaller sac lined with epithelium,—as in Æschna they occur in the 5th and 6th, and in Gomphus in the 4th, 5th, and 6th segments. This serial arrangement appears to confirm Hagen’s suggestion that they are survivals of abdominal gills, which in Euphæa are completely evaginated.

In the Trichoptera, all of which, except Enoicyla, are apneustic, and most of which have tracheal gills, the latter are filamentous, and arise either from the dorsal and ventral sides of the abdominal segment, or they grow out from the sides; while in certain genera (Neuronia, Phryganea, etc.) the gills are represented by conical hooks on the sides of the 1st abdominal segment, which are evidently respiratory, as they contain numerous tracheæ. The tracheal gills are either single or more rarely form tufts (Figs. 447, 448).

In Hydropsyche (Fig. 448) the tracheal gills persist throughout life, while in other genera they only last through the pupal stage. When first hatched, the larva of Phryganea lacks gills. The larvæ of most of the Hydropsychidæ, Rhyacophilidæ, and Hydroptilidæ have no gills, though they appear well developed in the pupal stage. (Klapálek.)

The only lepidopterous larva known to be provided with tracheal gills is that of the pyralid genus Paraponyx. Its thread-like gills, arranged in tufts of three or four, arise from a common tubercle situated on the sides of nearly all the segments. Wood-Mason describes the East Indian P. oryzalis as “covered with a perfect forest of soft and delicate white filaments,” arranged in tufts disposed in four longitudinal rows. “The stigmata of the 2d, 3d, and 4th abdominal somites only are clearly discernible.” The caterpillar crawls “free and uncovered” over the submerged leaves of the rice plant “in the very midst of the water.” In a Brazilian species of Paraponyx described as Cataclysta pyropalis, by W. Müller, the tufts are reduced to simple unbranched filaments, and the case is more complex than in the European species (Fig. 449).

Of coleopterous larvæ breathing by tracheal gills there are but few. The larva of Gyrinus (Fig. 454) respires by 10 pairs of slender, hairy abdominal gills similar to those of Corydalus, and the stigmata are entirely wanting. Somewhat similar are the tracheal gills of Hydrocharis caraboides. Hydrobius has shorter setose gills, our American species having seven pairs of short setose gills. It has two spiracles at the end of the body, through which the air is taken by thrusting the body out of the water. The larvæ of two other aquatic coleopterous genera, Pelobius and Cnemidotus, also have gills; those of the former situated at the base of the coxæ, and brush-like, but containing no tracheæ, though filled with blood, while those of Cnemidotus are very long, bristle-like, jointed, and arising from the dorsal side of the thoracic and abdominal segments. The stigmata are wanting. (Schiödte.)

The larva of the dipterous genus Tanypus respires by two caudal papilliform processes, in each of which a trachea ramifies.

Certain larvæ with both stigmata and tracheal gills are enabled either to live in or out of water or on the surface, as in the case of certain beetles (Cyphonidæ, Elmidæ, Hydrophilidæ, Fig. 452), or the larval mosquito and Psychodes (Fig. 455); also the nymphs of dragon-flies.

The larvæ of the Cyphonidæ (Helodes, Cyphon, Hydrocyphon) possess but a single pair of stigmata, situated in the penultimate abdominal segment, while at the end of the abdomen are delicate tracheal gills. The two main tracheal trunks are much swollen. When on the surface of the water the larva breathes through the stigmata situated near the end of the abdomen; when floating in the water, the larva, like that of Gyrinus, carries along at the end of its body a bubble of air. The gills are only of use, as Rolph thinks, when the insect is compelled to remain a long time under water.

The larva of our native Prionocyphon discoideus (Say) is described by Walsh as “vibrating vigorously up and down a pencil of hairs proceeding from a horizontal slit in the tail”; this pencil is composed “of three pairs of filaments, each beautifully bipectinate. I presume it is used to extract air from the water.” When the larva is at the surface the pencil of hairs touches the surface of the water, and occasionally a bubble of air is discharged from the tail. “The general habit is to crawl on decayed wood beneath the surface, occasionally swimming to the surface, probably for a fresh supply of air.” (Proc. Ent. Soc. Phil., i, p. 117.)

The larvæ of the small water beetles of the family Elmidæ (Elmis, Potamophilus, Macronychus, and Psephenus) have similar habits. That of Elmis has ten dorsally situated pairs of spiracles, and on the end of the body bushy gills which are protruded at pleasure. The young larva is without spiracles, its tracheal system being closed. Macronychus and Potamophilus have similar habits. In the larva of the latter genus, which has nine pairs of spiracles, there are at the end of the body on each side three tufts of thread-like gills which are connected with the two main horizontal tracheæ, while the branches of the abdominal tracheæ are dilated into numerous (64) bladder-like sacs. The larva usually breathes through the caudal gills. When the water is low or dried up, the air is inhaled directly through the spiracles. (Kolbe.)

The larva of Psephenus lecontei, by its broad hemispherical body, is adapted to adhere to the smooth surface of rounded stones, in which situation we have found it. Although it is said by Rolph to have two pairs of spiracles, one pair on the mesothoracic and the other on the 1st abdominal segment, it probably rarely rises to the surface to breathe the air direct.

It possesses five pairs of gills on the under side of the 2d to the 6th abdominal segments. Each gill has finger-shaped processes on its hinder edge, which are “from their constant motion evidently connected with respiration.” Tracheæ may be seen, according to H. J. Clark, entering the gills, and “the circulation of water among the branchiæ is kept up by the flapping of the tail-pieces.” The larva of Helichus fastigiatus is said by Leconte to be “very nearly allied, while the remotely allied Stenelmis crenatus has no external branchiæ.”[75]

The larva of the mosquito also has two modes of respiration, breathing either at the surface of the water through the two spiracles situated on the projection (siphon) at the hinder end of the body which is thrust out into the air; or when at the bottom respiring by tracheal gills. The pupa also has a double mode of respiration, either taking in air at the surface by the two thoracic horns with stigmatic openings, or when submerged using its tracheal gills.

Besides its long caudal tracheal air-tubes, the larval Eristalis is said by Chun to thrust out from the anus a number (20) of short tracheal filaments which float about in the water and serve to absorb the air.

An aquatic Brazilian larva of the family Psychodidæ has been found by Fritz Müller to take down under the water a large bubble of air (Fig. 455, C), the main tracheal trunk ending each in an opening at the end of the body (A, B); besides this, while at the bottom it breathes by three digitiform tracheal gills; another species having two pairs (C, a).

The remarkable larvæ of the Blepharoceridæ (represented in the United States by Blepharocera capitata), which live permanently in swift streams, attached by median suckers to stones, are apneustic, and breathe solely by leaf-like tracheal gills (Fig. 456, br) attached to the under side of the second to sixth abdominal segments. Those of the European Liponeura are said by Wierzejski to be branched, tree-like. Also immediately in front of the anus and behind the last sucker are four membranous sacs provided with tracheæ, but which are not capable of being withdrawn. These are said by Müller to be the same as what Dewitz states to serve as gills, and by Wierzejski they are homologized with the four anal gills of Chironomus.

The double mode of respiration in the larva of the horse bot-fly has been described by Scheiber. On the hinder end of the body are the stigmatic plates, which contain two lateral gill-plates and the middle stigmatal leaf. Besides this there is a pair of slightly developed prothoracic spiracles. The embryo and also freshly hatched larva of Gastrophilus equi do not possess these gill-plates, but on the end of the body are, according to Joli, two long thread-like gills. The freshly hatched larva of the allied Cephenomyia rufibarbis bears two caudal projections. (Kolbe.) As in shrimps and other Crustacea the gills are kept in constant motion, the water being driven over them by the rapid movements of the telson, so in the larval may-flies, and in the case-worm (Macronema), the gills move more or less rapidly. In case-worms as well as larval Perlidæ, Sialidæ, Paraponyx, and Hydrophilidæ the abdominal region is constantly moved to promote respiration. (Kolbe.)

=Blood-gills.=—Fritz Müller describes in trichopterous larvæ certain delicate anal tubular processes into which the blood flows, and which do not as a rule contain tracheæ, though occasionally very fine tracheal branches. Müller compares them with the gills of crabs and of shrimps. They are eversible finger-like tubules. They are used when the tracheal gills are temporarily not available. Their number varies even in the same genus. There are six in certain Rhyacophilidæ; five in different Hydropsychidæ; in Macronema there are four, and they are green when filled with the green blood of that insect, the tracheal gills being whitish. In the freshly hatched larva, while the tracheal gills are present, no anal blood-gills are visible. Similar blood-gills also occur in the pupæ of certain caddis-flies. (Pictet.)

Similar anal gills filled with blood occur in the larvæ of the fireflies (Lampyris, etc.), and perhaps, Kolbe thinks, serve for respiration, though other authors believe them to be adhesive organs.

The larva of Pelobius has true blood-gills. (Schiödte. See p. 461.)

The eversible ventral segmental sacs of Scolopendrella, Campodea, and Machilis, as well as the ventral tube (collophore) of Podura, Smynthurus, etc., may, as Oudemans and Haase have suggested, serve a respiratory purpose, though they lack tracheæ, and differ from blood-gills in containing no gases; yet the blood is forced into them, causing their eversion. Oudemans observed that Machilis everted its sacs when the vessel in which it was put was filled with warm, damp air. The sacs are only thrust out when the creature is completely at rest.

Structures referable to blood-gills also occur temporarily in the embryo of Orthoptera; Rathke observed them in the mole-cricket; Ayres observed them in Œcanthus niveus, where they form two stalked broad oval appendages on the first abdominal appendages, which he regarded as gills. Patten observed them in Phyllodromia germanica, as pear-shaped structures occurring in the same situation, but regarded them as sense-organs, as did Cholodkowsky. Graber found these structures in the embryo of the May-beetle, which looked like the other embryonic limbs, but survived after the disappearance of the latter, being longer and broader and unjointed. These disappeared shortly before birth. In Hydrophilus they remain, Graber states, after birth. Nussbaum has seen them in Meloë.

Finally, Wheeler has discussed at length these embryonic organs, which he regards as glandular structures, and calls pleuropodia, their primitive function having been that of limbs. He has detected them in the embryo of Periplaneta orientalis, Mantis carolina, Xiphidium ensiferum (Fig. 387); also in the Hemiptera (Cicada septemdecim, Zaitha fluminea), and in Sialis infumata. He discards the view that they were once gills or sense-organs, and concludes that they were glands. But, as we have suggested, their function once that of gills, and still respiratory in Synaptera, has perhaps become in the winged insects glandular and repugnatorial. Instead, then, of being modified abdominal limbs afterwards serving as glands, as Wheeler claims, we are inclined to believe that they functioned as blood-gills.

k. Tracheal gills of adult insects

Tracheal gills are known to be retained by a few insects in the imago stage, the nymphs in all stages breathing by them. The most notable example is the perlid genus Pteronarcys, in which, as Newport states, there are eight sets, comprising 13 pairs of branchial tufts distributed over the under surface of the thoracic and first two abdominal segments.

The first set, consisting of three pairs of tufts, partly encircling the neck like a ruff, arises from the soft membrane connecting the head and prosternum. The thoracic tufts originate between and behind the coxæ, as well as on the front margin of the meso- and metathoracic segments. The number of filaments in each tuft varies from about 20 to 50 or more, the densest tufts being those of the two hinder thoracic segments. Each filament is usually simple, though in a few cases they are branched (Fig. 457, A).

The adult Pteronarcys is nocturnal, flying only at dewfall or in the night, and Mr. Barnston observed it when on the wing, “constantly dipping on the surface of the water”; by day it hides “in crevices of rocks which are constantly wetted by the spray of falling water, under stones and in other damp places.” It may thus be compared with the Amphibians, Necturus and Proteus, whose gills are retained in adult life. A similar large Chilian Perlid (Diamphipnoa lichenalis Gerst.) differs in completely lacking the thoracic gills, though there are four pairs on the abdomen, i.e. a pair on each of the first four segments. In this form the number of individual filaments in the largest tufts may amount to about 200.

Another Perlid (Dictyopteryx signata) is said by Hagen to have two pairs of gill-tufts on the under side of the head; the first pair situated on the base of the submentum, the second on the membrane connecting the head and prosternum.

Kolbe states that in the imagines of Perla marginata and P. cephalotes on the hinder edge of the thoracic stigmata arise three very small chitinous plates, which, on their under side and on the edges are beset with numerous short white filaments. These completely correspond to the filaments of the tuft-like larval gills. Persistent anal gills also occur in the imagines of Perla.

In Nemoura lateralis and cinerea the tracheal gills are differently disposed. On each side of the anterior edge of the prosternum arise delicate tightly twisted filaments, like those of the larva. (Einführung, p. 536.)

Hagen also states that in the dragon-fly, Euphæa, the gills of the nymphs are retained in the imago, and Palmén remarks that in Æschna the rectal gills of the nymph persist in the imago, though not used for respiration.

Palmén gives an instance of a caddis-fly (Hydropsyche, Fig. 448) retaining its gills through the imago stage, but they are unfit for respiration, as they are minute and shrunken.

A walking-stick (Prisopus flabelliformis) found in the mountains of Brazil has the remarkable habit, according to Murray, of spending “the whole of the day under water, in a stream or rivulet, fixed firmly to a stone in the rapid part of the stream,” with its head turned up stream; but leaving the water at dark. The under side of the body, including the head, is hollowed so that the creature may adhere, sucker-like, to smooth stones; the claws, claspers, and flaps on the legs aid in retaining its hold, while the outer margin of the legs is dentate and thickly fringed with hair to repel the water.

Another form, closely related to Prisopus, from Borneo (Cotylosoma dipneusticum) is said by Wood-Mason to be even more profoundly modified for an aquatic life, since it has not only spiracles, but also, as he claims, tracheal gills. From each side of the body, in fact along the lower margins of the sides of the metathorax, there stand straight out five equal, small, but conspicuous ciliated oval plates, “which, when the insect is submerged and its stigmata are closed, doubtless serve for respiration.” The author did not note the actual presence of tracheæ in these plates.

LITERATURE ON THE ORGANS AND PHYSIOLOGY OF RESPIRATION

a. On the tracheal system in general

=Lyonet, P.= Traité anatomique de la chenille qui ronge le bois du saule. (La Haye, 1760; 2d edit., La Haye, 1762, pp. 616, 18 Pls.)

=Treviranus, G. R.= Beiträge zur Anatomie und Physiologie der Tiere und Pflanzen, 1816.

—— Das organische Leben. Bremen, 1831.

=Rengger, J. R.= Physiologische Untersuchungen über die tierische Haushaltung der Insekten. Tübingen, 1817. (Germar’s Mag. f. Ent., 1818, iii, pp. 410–413.)

=Dufour, L.= Recherches anatomiques sur les Carabiques et sur plusieurs autres insectes Coléoptères. Organes de la respiration. (Ann. Sc. nat., viii, 1826, pp. 19–27, 2 Pls.)

—— Études anatomiques et physiologiques sur les insectes Diptères de la famille des Pupipares. Appareil respiratoire. (Ann. Sc. nat. Zool., Sér. 3, iii, 1845, pp. 56–64, 1 Pl.)

—— Description et anatomie d’une larve à branchies externes d’Hydropsche. (Ann. Sc. nat, Zool., Sér 3, 1847, viii, pp. 341–354.)

—— Études anatomiques et physiologiques, et observations sur les larves des Libellules. Appareil respiratoire. (Ann. Sc. nat. Zool., Sér. 3, xvii, 1852, pp. 76–97, 3 Pls.)

—— Recherches anatomiques sur les Hyménoptères de la famille des Urocerates. Appareil respiratoire. (Ann. Sc. nat. Zool., Sér. 4, i, 1854, pp. 203–209, 1 Pl.; see also p. 344.)

=Burmeister, H.= Handbuch der Entomologie, i, 1832, pp. 169–194, 416–436.

=Kirby, W., and W. Spence.= Introduction to entomology, 1833, iv, pp. 35–81.

=Bowerbank, J. S.= Observations on the circulation of blood and the distribution of the tracheæ in the wing of Chrysopa perla. (Ent. Mag., iv, 1837, pp. 179–185.)

=Platner, E. A.= Mitteilungen über die Respirationsorgane in der Haut bei der Seidenraupe. (Müller’s Archiv f. Physiol., 1844, pp. 38–49.)

=Filippi, F. de.= Alcuni osservazioni anatomico-fisiologische sugl’ Insetti in generale, ed in particulare sul Bombice del Gelso. (Ann. R. Acad. d’ Agricoltura di Torino, 1850, ii, p. 25, 1 Pl.; Transl. by C. A. Dohrn, Stettin, Ent. Zeit., 1852, xiii, pp. 258–267; xiv, pp. 124–132, 1 Pl.)

=Newport, G.= On the formation and the use of the air-sacs and dilated tracheæ in insects. (Trans. Linn. Soc. London, 1851, xx, pp. 419–423.)

=Lubbock, J.= Distribution of tracheæ in insects. (Trans. Linn. Soc. London, 1860, xxiii, pp. 23–50.)

=Landois, L.= Anatomie des Phthirius inguinalis Leach. (Zeitschr. f. wissens. Zool., xiv, 1864, pp. 1–26, 5 Taf.)

—— Anatomie des Pediculus vestimenti Nitzsch. (Ibid., xv, 1865, pp. 32–55, 3 Taf.)

—— Anatomie des Hundeflohs (Pulex canis). (Nova Acta Leop.-Carol. Akad. der Naturf., Dresden, 1866, xxxiii, 1867, pp. 67, 7 Taf.)

—— Anatomie der Bettwanze (Cimex lectularius L.). (Ibid., xviii, 1868, pp. 206–224, xix, 1869, pp. 206–233, 4 Taf.)

=Meinert, Fr.= Campodeæ: en familie af Thysanurernes orden. (Naturhistorisk Tidsskr., 3 Raek., iii, 1864–65, pp. 400–440, 1 Pl.)

=Reinhardt, H.= Zur Entwicklungsgeschichte des Tracheensystems der Hymenopteren mit besonderer Bezeichung auf dessen morphologische Bedeutung. (Berlin. Ent. Zeitschr., 1865, ix, pp. 187–218, 2 Taf.)

=Gerstaecker, A.= Bronn’s Klassen und Ordnungen des Tierreichs, v, 1866–1879. Organs of respiration, pp. 119–131.

=Pouchet, G.= Développement du système trachéen de l’Anophèle (Corethra plumicornis). (Archiv zool. expérimentale, i, 1872, pp. 217–232, 1 Fig.)

=Graber, V.= Ueber eine Art fibrilloiden Bindegewebes der Insektenhaut und seine lokale Bedeutung als Trachealsuspensorium. (Archiv f. Mikroskop. Anat. x, 1874, pp. 124–144, 1 Taf.)

—— Die Insekten; München, 1877. Organs of respiration, pp. 346–369.

=Packard, A. S.= On the distribution and primitive number of spiracles in insects. (Amer. Naturalist, viii, 1874, pp. 531–534.)

—— On the nature and origin of the so-called “spiral thread” of tracheæ. (Amer. Naturalist, xx, 1886, pp. 438–442, 2 Figs., p. 558.)

=Wolff, O. J. B.= Das Riechorgan der Biene nebst einer Beschreibung des Respirationswerkes der Hymenopteren, des Saugrüssels und Geschmacksorganes der Blumenwespen. (Nova Acta d. kais. Leop-Carol. Akad. der Naturf., xxxviii, 1876, pp. 1–251, 8 Taf.)

=Palmén, J. A.= Zur Morphologie des Tracheensystems. Leipzig, 1877, pp. 140, 2 Taf.

=Moseley, H. N.= Origin of tracheæ in Arthropoda. (Nature, xvii, 1878, p. 340.)

=Poletajew, Olga.= Quelques mots sur les organes respiratoires des larves des Odonates. (Horæ Soc. Ent. Ross., xv, 1880, pp. 436–452, 2 Pls.)

=Viallanes, H.= Sur l’appareil respiratoire de quelques larves de Diptères. (Compt. rend. Acad. Sc., Paris, 1880, pp. 1180–1182.)

=MacLeod, J.= La structure des trachées et la circulation peritrachéenne. Bruxelles, 1880, pp. 70, 4 Pls.

=Hagen, H. A.= Beitrag zur Kenntnis des Tracheensystems der Libellenlarven. (Zool. Anzeiger, 1880, pp. 157–162.)

—— Einwürfe gegen Palmens Ansicht von der Entstehung des geschlossenen Tracheensystems. (Ibid., 1881, pp. 404–406.)

=Macloskie, G.= The structure of the tracheæ of insects. (Amer. Naturalist, 1884, xviii, pp. 567–573, Fig.)

=Haase, E.= Das Respirationssystem der Chilopoden und Symphylen (Scolopendrellen) vergleichen mit dem der Hexapoden. (Zeitschr. f. Ent. N. F., ix, Breslau, 1884.)

—— Das Respirationssystem der Symphylen und Chilopoden. (Zool. Beiträge, von A. Schneider, i, 1884, pp. 65–95, 3 Taf.; Zool. Anzeiger, 1883, pp. 15–17.)

=Grassi, B.= I progenitori degli Insetti e dei Miriapodi. L’Japyx e la Campodea. (Atti d. Accad. Gioenia d. Sc. Nat. Catania, 1885, Sér. 3, xix, pp. 83, 5 Pls.)

—— I progenitori dei Miriapodi e degli Insetti. Anatomia comparata dei Tisanuri. (Reale Accad. d. Lincei di Roma, Anno 284, 1887.)

=Meinert, Fr.= De eucephale Myggelarver. Sur les larves eucephales des Diptères. (Vidensk. Selsk. Skrifter., 6 Raekke, naturvid. og mathem. Afd. Kjöbenhavn, 1886, iv, pp. 369–493, 4 Pls.)

=Cajal, S. R.= Coloration par la méthode de Golgi des terminaisons des trachées et des nerfs dans les muscles des ailes des insectes. (Zeitschr. f. wiss. Microscopie, 1890, vii, pp. 332–342, 1 Pl.)

=Wistinghausen, C. v.= Ueber Tracheenendigungen in den Sericterien der Raupen. (Zeitschr. wissensch. Zool., xlix, 1890, pp. 565–582, 1 Taf.)

=Stokes, Alfred C.= The structure of insect tracheæ, etc. (Science, 1893, pp. 44–46, 7 Figs.)

=Sadones, J.= L’appareil digestif et respiratoire larvaire des Odonates. (La Cellule, xi, 1895, pp. 271–325, 3 Pls.)

=Holmgren, Emil.= Über das respiratorische Epithel der Tracheen bei Raupen. (Festschrift Lilljeborg. Upsala, 1896, pp. 76–79, 2 Taf.) See also p. 437.

Also Gegenbaur’s Comparative Anatomy, Engl. Trans.

b. On the Stigmata

=Loewe, C. L. W.= De partibus quibus insecta spiritus ducunt. Diss. inaug. Halæ, 1814, pp. 28.

=Sprengel, C.= Commentarius de partibus quibus Insecta spiritus ducunt. Lipsiæ, 1815, pp. 38, 3 Taf.

=Dufour, L.= Recherches anatomiques sur l’Hippobosque des chevaux. (Ann. Sc. nat., 1825, vi, pp. 299–322, 1 Pl.)

—— Nouvelles observations sur la situation des stigmates thoraciques dans les larves des Bupresticides. (Ann. Soc. Ent. France, Sér. 2, 1844, ii, p. 203.)

=Landois, H.= Der Tracheenverschluss bei Tenebrio molitor. (Reichert u. Dubois-Reymond’s Archiv f. Anat., 1866, pp. 391–397, 1 Taf.)

—— =und W. Thelen.= Der Tracheenverschluss bei den Insekten. (Zeitschr. wissensch. Zool., xvii, 1867, pp. 187–214, 1 Taf.)

—— Der Stigmenverschluss bei den Lepidopteren. (Reichert u. Dubois-Reymond’s Archiv f. Anat., 1886, pp. 41–49, 1 Taf.)

=Hagen, H. A.= Beitrag zur Kenntnis des Tracheensystems der Libellen-Larven. (Zool. Anzeiger, 1880, pp. 157–162.)

—— Einwürfe gegen Palmén’s Ansicht von der Entstehung des geschlossenen Tracheensystems. (Ibid., 1881, pp. 404–406.)

=Krancher, O.= Der Bau der Stigmen bei den Insekten. (Zeitschr. wissensch. Zool., xxxv, 1881, pp. 505–574, 2 Taf.; Zool. Anz., 1880, pp. 584–588.)

=Meinert, Fr.= Spirakelpladen hos Scarabæ-Larverne. (Vid. Meddel. Nat. For. Kjöbenhavn (4), Aarg. iii, 1882, pp. 289–292.)

—— Noget mere om Spiracula cribraria og Os clausum. En Replik. (Ibid. (4), Aarg. v, 1884, pp. 68–91, Fig.)

=Schiödte, J. G.= Spiracula cribraria—os clausum: lidt om naturvidenskabelig Methode og Kritik. (Nat. Tidsskrift (3), xiii, 1883, pp. 427–473; also Jahresber. Neapel, 1883, p. 105.)

=Verson, E.= Il meccanismo di chiusura negli stimmati di Bombix mori. (Atti Istit. Veneto. Sc., 1887, p. 9, Pl.)

—— Der Bau der Stigmen von Bombyx mori. (Zool. Anzeiger, 1887, x Jahrg., pp. 561, 562.) See also Zool. Anzeiger, 1890, p. 116.

=Haase, E.= Die Stigmen der Scolopendriden. (Zool. Anzeiger, 1887, x Jahrg., pp. 140–142.)

—— Holopneustie bei Käfern. (Biolog. Centralbl., 1887, vii, pp. 50–53.)

=Carlet, G.= Note sur un nouveau mode de fermeture des trachées, “fermeture operculaire” chez les insectes. (Comp. rend. Acad. Sci. Paris, 1888, cvii, pp. 755–757.)

=De Meijere, J. C. H.= Über zusammengesetzte Stigmen bei Dipterenlarven [etc.]. (Tijd. Ent., xxxviii, 1895, pp. 65–100, 33 Figs.)

Also the other writings of Palmén, Dufour, Dewitz, Boas, Verson.

c. On tracheal gills and tracheal respiration

=Pictet, F. J.= Mémoires sur les larves des Némoures. (Annal. Sc. nat., 1832, xxvi, pp. 369–391, 2 Pls.)

—— Recherches pour servir à l’histoire et à l’anatomie des Phryganides. Genève, pp. 235, 20 Pls.

—— Histoire naturelle générale et particulière, des insectes Neuroptères. I, Monographie: Famille des Perlides, Genève, 1841, 1842, pp. 423, 53 Pls.

—— Histoire naturelle générale et particulière des insectes Neuroptères. II, Monographie: Famille des Ephémérines. Genève, 1843–1845, pp. 300, 47 Pls.

=Dufour, L.= Recherches anatomiques et considerations entomologiques sur les insectes Coléoptères des genres Macronychus et Elmis. (Ann. Sc. nat. Zool., Sér. 2, 1835, iii, pp. 151–174, 1 Pl.)

—— Description et anatomie d’une larve à branchies externes d’Hydropsyche. (Ibid., Sér. 3, 1847, viii, pp. 341–354, Fig.)

—— Recherches anatomiques sur la larve à branchies extérieures du Sialis lutarius. (Ibid., Sér. 3, 1848, ix, pp. 91–99, Fig.)

—— De diverses modes de respiration aquatique chez les insectes. (Compt. rend. Acad. d. Sc. Paris, 1849, xxix, pp. 763–770; Ann. and Mag. Nat. Hist., Sér. 2, 1850, vi, pp. 112–118.)

—— Études sur la larve du Potamophilus. (Ann. Sc. nat., Sér. 4, xvii, 1862, pp. 162–173, 1 Pl., Bericht v. Gerstaecker f. 1862, pp. 16, 17.)

=Grube, A. E.= Beschreibung einer auffallenden an Süsswasser-schwammen lebenden Larve (Sisyra). (Wiegmanns Archiv f. Naturgesch., 1843, ix, pp. 331–337, Fig.)

=Schröder van der Kolk, J. L. G.= Mémoire sur l’anatomie et physiologie de Gastrus equi. (Nieuwe Verhandl. d. K. Nederl. Instit. Amsterdam, 1845, ix, pp. 1–155, 13 Pls.; Erichson’s Bericht. f. 1845, p. 109.)

=Scheiber, S. H.= Vergleichende Anatomie und Physiologie der Œstriden-Larven. Respirationssystem. (Sitzungsber. Akad. Wissensch. Wien. Math.-naturw. Cl., 1862, xlv, pp. 7–39.)

=Lubbock, J.= On the development of Chloëon dimidiatum. (Trans. Linn. Soc. London, I, 1868, xxiv, pp. 61–78, 2 Pls.; II, 1866, xxv, pp. 477–492.)

=Oustalet, E.= Note sur la respiration chez les nymphes des Libellules. (Ann. Sc. nat. Zool., Sér. 5, xi, 1869, pp. 370–386, 3 Pls.)

=Rolph, W. H.= Beitrag zur Kenntnis eininger Insektenlarven. 1 Taf. Inaug. Dissertat. Bonn, 1873.

=Chun, C.= Ueber den Bau, die Entwicklung und physiologische Bedeutung der Rektaldrüsen bei den Insekten. Frankfurt a. M., 1875.

=Haller, G.= Die Stechmückenlarve. Kleinere Bruchstücke zur vergleichenden Anatomie der Arthropoden. I. Ueber das Atmungsorgan der Stechmückenlarven. (Archiv f. Naturgesch., xliv, 1878, pp. 91–96, 1 Taf.)

=Vayssière, A.= Recherches sur l’organisation des larves des Ephémérines. (Ann. d. Sc. nat. Zool., Sér. 6, xiii, 1882, pp. 1–137, 11 Pls.)

—— Monographie zoologique et anatomique du genre Prosopistoma Latr. (Ibid., Sér. 7, ix, 1890, pp. 19–87, 4 Pls.)

=Eaton, A. E.= Notes on some species of Cloëon. (Ann. Mag. Nat. Hist, Ser. 3, xviii, 1866, pp. 145–148.)

—— A revisional monograph of recent Ephemeridæ or may-flies. (Trans. Linn. Soc. London, 1883–1887, Ser. 2, iii, 63 Pls.)

=Müller, Wilh.= Ueber einige im Wasser lebende Schmetterlingsraupen Brasiliens. (Archiv f. Naturgesch., 1884, i Jahrg., pp. 194–212, 1 Taf.)

=Vogler.= Die Tracheenkiemen der Simulien-Puppen. (Mitteil. Schweiz Ent. Gesellsch., 1887, vii, pp. 277–282.)

=Raschke, E. W.= Die Larve von Culex nemorosus. Ein Beitrag zur Kenntnis der Insekten-Anatomie und Histiologie. (Archiv für Naturgesch., 1887, liii Jahrg., pp. 133–163, 2 Taf.; Zool. Anz., 1887, x Jahrg., pp. 18, 19.)

=Klapálek, Fr.= Untersuchungen über die Fauna der Gewässer Böhmens. I. Metamorphose der Trichopteren. (Archiv f. naturwissensch. Landesdurchforschung von Böhmen, Prag, 1888, vi, No. 5, pp. 63; No. 6, 1893, pp. 145, Figs.)

=Müller, Fritz.= Larven von Mücken und Haarflüglern mit zweierlei abwechselnd thätigen Atemwerkzeugen. (Ent. Nachr., 1888, xiv Jahrg., pp. 273–277; also Zool. Anzeiger, iv, 1881, pp. 499–502.)

=Kolbe, H. J.= Ueber den Kranzförmigen Laich einer Phryganea. (Sitzungsber. d. Gesellsch. naturforsch. Freunde in Berlin, 1888, pp. 22–26.)

=Haase, Erich.= Die Abdominalanhänge der Insekten mit Berücksichtigung der Myriopoden. (Morpholog. Jahrbuch, 1889, xv, pp. 331–435, 2 Taf.)

=Dewitz, H.= Einiger Beobachtungen, betreffend das geschlossene Tracheensystem bei Insectenlarven. (Zool. Anzeiger, xiii, 1890, pp. 500–504, 525–531.)

=Miall, L. C.= Some difficulties in the life of aquatic insects. (Nature, xliv, London, 1891, pp. 456–462.)

—— Natural History of aquatic insects, 1895, 116 Figs., pp. 1–395.

=Weltner, W.= (Note on Sisyra.) (Ent. Nachr., p. 145, 1894.)

Also papers by Hagen, Dewitz, Williams, Tömösváry (1884).

d. Literature on rectal respiration

=Suckow, F. W. L.= Respiration der Insekten, insbesondere über die Darmrespiration der Æschna grandis. (Zeitschrift f. d. organ. Physik, von Heusinger, 1828, ii, pp. 24–29, 4 Taf.)

=Dufour, L.= Sur la respiration branchiale des larves des grandes Libellules comparée à celle des poissons. (Compt. rend. de l’Acad. Sc. Paris, 1848, xxvi, pp. 301–303.)

—— Études anatomiques et physiologiques et observations sur les larves des Libellules. (Ann. Sc. nat. Zool., Sér. 3, 1852, xvii, pp. 76–97, 3 Pls.)

=Gilson, G. and J. Sadones.= Larval gills of Odonates. (Journ. Linn. Soc. London, 1897.)

e. Physiology of Respiration

=Bonnet, Ch.= Recherches sur la respiration des chenilles. (Mém. Math. des Savants Étrangers, Paris, 1768, v, pp. 276–303.)

=Treviranus, G. R.= Biologie, oder Philosophie der lebenden Natur, für Naturforscher und Aerzte. 6 Bände, Göttingen, 1802–1822. (Atmung, in Bd. iv.)

—— Die Erscheinungen und Gesetze des organischen Lebens. 2 Bände, Bremen, 1831–1833. (Atmung der Insekten in Bd. i.)

—— Versuche über das Atemholen der niederen Tiere. (Zeitschrift f. d. Physiologie, von F. Tiedemann, G. R. u. L. C. Treviranus, 1832, iv, pp. 1–39.)

=Hausmann, J. F. L.= De animalium exsanguinum respiratione commentatio. Hannover, 1803, vi, p. 70.

=Spallanzani, L.= Memoirs on respiration. London, 1804.

=Sorg, F. L. A. W.= Disquisitiones physiologicæ circa respirationem insectorum et vermium. Rudolstadt, 1805, Part II, p. 146.

=Nitzsch, C. L.= Commentatio de respiratione animalium. Vitebergæ, 1808, 4º, pp. 56.

—— Ueber das Atmen der Hydrophilen. (Reil’s Archiv f. Physiologie, 1811, x, pp. 440–458.)

=Reimarus, J. A. H.= Ueber das Atmen, besonders über das Atmen der Vögel und Insekten. (Reil u. Autenrieth, Archiv f. Physiologie, 1812, xi, pp. 229–236.)

=Dufour, L.= Anatomie de la Ranatre linéaire et de la Nèpe cendrée. (Annal. génér. Scienc. phys., Bruxelles, 1821, vii, pp. 194–213, 1 Pl.)

—— Mémoire pour servir à l’histoire du genre Ocyptera. (Annal. Scienc. natur., 1827, x, pp. 248–261, 1 Pl.)

—— Recherches sur quelques entozoaires et larves parasites des insectes Orthoptères et Hyménoptères. (Ann. Sc. nat. Zool., Sér. 2, 1836, vi, p. 55; Sér. 2, 1837, vii, pp. 5–20.)

—— Note sur le parasitisme. (Compt. rend. Acad. Sc. Paris, 1851, xxxiii, pp. 135–139; Rev. et Mag. de Zool., 1851, pp. 408–412.)

=Dutrochet, R. J. H.= Du mecanisme de la respiration des Insectes. (Ann. Sc. nat., 1833, xxviii, pp. 31–44; Mém. Acad. Sc. Paris, 1838, xiv, pp. 81–93.)

=Newport, G.= On the respiration of insects. (Phil. Trans. Roy. Soc., London, 1836, cxxvi, pp. 529–566.)

=Coquerel, Ch.= Note pour servir à l’histoire de l’Æpus robini. (Ann. Soc. Ent. France, Sér. 2, 1850, viii, pp. 529–532.)

=Davy, J.= On the effects of certain agents on insects. (Trans. Ent. Soc. London, 1851, pp. 195–212.)

=Barlow, W. F.= Observations of the respiratory movements of insects. (Phil. Trans. Roy. Soc. London, cxlv, 1855, pp. 139–148.)

=Rathke, H.= Anatomisch-physiologische Untersuchungen über den Atmungsprozess der Insekten. (Schriften d. k. phys.-ökon. Ges. Königsberg, i Jahrg., 1860, pp. 99–138, 1 Taf.)

=Lubbock, J.= On two aquatic Hymenoptera, one of which uses its wings in swimming. (Trans. Linn. Soc. London, xxiv, 1863, pp. 135–142, 1 Pl.)

=Boyle, R.= New pneumatical experiments about respiration. (Phil. Trans., 1870, v, No. 63, pp. 2051–2056.)

=Lambrecht. A.= Das Atmungsgeschäft der Bienen. (Bienenwirtschaftl. Centralblatt, vii Jahrg., 1871, pp. 20–25.)

—— Luftverbrauch eines Biens und die damit zusammenhängenden Lebensprozesse der Glieder desselben. (Ibid., 1871, pp. 115–120.)

=Monnier.= Sur la rôle des organes respiratoires chez les larves aquatiques. (Compt. rend. Acad. Sc. Paris, lxxiv, 1872, p. 235.)

=Liebe, Otto.= Ueber die Respiration der Tracheaten, besonders über den Mechanismus derselben und über die Menge der ausgeatmenten Kohlensäure. (Inaug. Diss. Chemnitz, 1872, pp. 28.)

=Plateau, F.= Recherches physico-chimiques sur les articulés aquatiques. (Bull. Acad. Roy. Belg., Sér. 2, xxxiv, 1872, pp. 271–321.)

—— Recherches expérimentales sur les mouvements respiratoires des Insectes. (Mem. Acad. Belg., 1884, xlv, pp. 219, 7 Pls., 56 Figs.)

—— Recherches physico-chimiques sur les articulés aquatiques. Part I. Action de sels en dissolution dans l’eau. Influence de l’eau de mer sur les articulés aquatiques d’eau douce. Influence de l’eau douce sur les Crustacés marines. (Mém. cour. et Mém. des savants étrang. de Belgique, xxxvi, 1871, pp. 68.) Part II. Résistance à l’asphyxie par submersion, action du froid, action de la chaleur, temperature maximum. (Bull. Acad. Roy. de Belgique, Sér. 2, xxxiv, 1872, pp. 271–321.)

—— Les Myriopodes marins et la résistance des Arthropodes à respiration aërienne à la submersion. (Journ. de l’anatomie et de la physiologie, 1890, xxvi, pp. 236–269.)

=Bütschli, O.= Ein Beitrag zur Kenntnis des Stoffwechsels, insbesondere die Respiration bei den Insekten. (Reichert und Dubois-Reymond’s Archiv f. Anatomie, 1874, pp. 348–361.)

=Ritsema Cz., C.= Acentropus niveus Oliv., in Zijne levenswijze en verschillende toestanden. (Tijdschr. voor Entom, 1876, xxi, separate, pp. 34, 2 Taf.)

=Pott, Rob.= Chemical experiments on the respiration of insects. (Psyche, ii, 1878.)

=Sharp, D.= Observations on the respiratory action of the carnivorous water-beetles. (Journ. Linn. Soc. London, xiii, Zoology, 1878, pp. 161–183.)

=Krancher, O.= Das Atmen der Biene. (Deutscher Bienenfreund., xvi Jahrg., 1880, pp. 49–51.)

=Gissler, C. F.= Sub-elytral air-passages in Coleoptera. (Proc. Amer. Assoc. Advanc. Sc. 29 Meet. (1881), 1881, pp. 667–669.)

=Langendorff, O.= Studien über die Innervation der Atembewegungen. 6. Das Atmungszentrum der Insekten. (Archiv f. Anatomie u. Physiol., Physiol. Abteil., 1883, pp. 80–87.)

=Macloskie, G.= Pneumatic functions of insects. (Psyche, iii, 1883, pp. 375.)

=Chalande, J.= Recherches sur le mecanisme de la respiration chez les Myriopodes. (Compt. rend. Acad. Sc. Paris, civ, 1887, pp. 126, 127.)

=Comstock, J. H.= Note on respiration of aquatic bugs. (Amer. Naturalist, 1887, xxi, pp. 577, 578.)

=Fricken, W. v.= Ueber Entwicklung, Atmung und Lebensweise der Gattung Hydrophilus. (Tagebl., 60, Versamml. deutscher Naturf. u. Aerzte, 1887, pp. 114, 115.)

=Schmidt, E.= Ueber Atmung der Larven und Puppen von Donacia crassipes. (Berlin. Ent. Zeitschr., 1887, xxxi Jahrg., pp. 325–334, 1 Taf.)

=Dewitz, H.= Entnehmen die Larven der Donacien vermittelst Stigmen oder Atemrohren den Luftraumen der Pflanzen die sauerstoffhaltige Luft? (Ibid., 1888, xxxii Jahrg., pp. 5, 6, Fig.)

=Müller, G. W.= Ueber Agriotypus armatus. (Spengel’s Zoolog. Jahrbücher. Abt. f. Systematik, etc., iv, 1889, pp. 1132–1134.)

—— Noch einmal Agriotypus armatus. (Ibid., v, 1890, pp. 689–691.)

=Devaux, H.= Vom Ersticken durch Ertrinken bei den Tieren und Pflanzen. (Naturwiss. Rundschau, vi Jahrg., 1891, p. 231; Compt. rend. Soc. de Biol., 1891, Ser. 9, iii, p. 43.)

See also Dewitz, p. 482; Kolbe, p. 482.

THE ORGANS OF REPRODUCTION

Insects are without exception unisexual, the male and female organs existing in different individuals, no insects being normally hermaphroditic. The reproductive organs are situated in the hind-body or abdomen, especially near the end, the genital glands opening externally either in the space between the 7th and 8th, or 8th and 9th, or 9th and 10th abdominal segments, but as a rule between the 8th and 9th segments (Fig. 299).

The primary or essential male organs are the testes, those of the female being the ovaries. As we shall see, the primitive number of seminal ducts and oviducts was two, this number being still retained in Lepisma and the Ephemeridæ. The reproductive organs of both sexes are at first, in their embryonic condition, of the same shape and structure, becoming differentiated in form and function before sexual maturity. These glands and ducts have a paired mesodermal genital rudiment, the ends of the ducts being often connected with corresponding ectodermal invaginations of the cuticle.

The secondary sexual organs mainly comprise the external genital armature of the male, and the egg-laying organs, or ovipositor of the female. Besides these structures there are other more superficial secondary sexual characters, such as differences in the size and ornamentation as well as coloring of the body, or of parts of it.

The primary sexual organs of insects have been conveniently tabulated by Kolbe, thus:—

I. Male reproductive organs.

1. Two testes, with testicular follicles. 2. Seminal ducts (vasa deferentia). 3. Seminal vesicle. 4. Accessory glands. 5. The common seminal outlet, with the penis. 6. The copulatory apparatus.

II. Female reproductive organs.

1. Two ovaries, with the egg-tubes. 2. Two oviducts. 3. Receptaculum seminis; bursa copulatrix. 4. Accessory sac. 5. The common oviduct, vagina, uterus. 6. The ovipositor.

The ducts of the sexual glands in Peripatus being transformed nephridia or segmental organs, it has been inferred that this is also the case with those of insects, though, as Lang states, there is a considerable difference in the two cases, as the greater part of the ducts in Peripatus arises out of the ectoderm, while in the Myriopoda and insects they come from the mesoderm; but he adds that in the Annelids the greater part of the nephridial duct is of mesodermal origin.

While in insects there is but a single pair of genital outlets, the serial arrangement of the testicular (Fig. 458) and egg-tubes (Fig. 459) in some Thysanura (Campodea, Japyx, and Lepisma), where the tubes (5 to 7 on each side) open singly one behind the other in segmental succession, indicates that in their ancestors these egg-tubes opened out on different segments situated one behind the other. Each egg-tube independently opens into one of the two oviducts, which extend through the abdomen as straight canals. The two oviducts open externally by a short unpaired terminal portion, which in Machilis is said to be wanting, only the outer aperture of the two oviducts being in this case common to both. In Campodea and in the Collembola the ovaries and testes on each side are simply tubes. It is to be observed that in the young Lepisma Nassonow found that the external openings of the two ejaculatory ducts are paired (Fig. 458 B, ed.).

In the Stylopidæ, also, though this may be the result of adaptation to the singular parasitic habits of the females whose bodies are mostly situated in the abdomen of their host, the ends of the oviducts are formed by the invagination of the integument of the 2d, 3d, and 4th abdominal segments. In the 2d to 5th segments are situated tubes which open in the cavity of the body with funnel-like ends, so that the ducts have a close resemblance to the segmental organs of worms. (Nassonow.)

Among the winged insects the reproductive organs of the cricket (Fig. 466) are perhaps as simple as any. The testes are separate, and the vasa deferentia very long. The seminal vesicles bear numerous large and short utricles (utriculi majores and breviores), the penis being simple and dilated at the end; while in Phyllodromia germanica the testes are functional throughout life, and consist of four lobes each. In the common cockroach (P. orientalis) (Fig. 461) the testes are functional only in the young male; they afterwards shrivel and are functionally replaced by the vesiculæ seminales and their appendages, when the later transformations of the sperm-cells are effected. The accessory glands are numerous and differ both in function and insertion. Two sets of these glands (utriculi majores and breviores) are attached to the vesiculæ seminales and the fore end of the ejaculatory duct, while another appendage, called by Miall and Denny the conglobate gland, opens separately on the exterior upon a double hook, which forms a part of the external genital armature. The so-called penis is long, slender, and dilated at the end, but is not perforated.

In the locusts (Acrydiidæ) the testes are, unlike those of most other Orthoptera, closely united to each other so as to form a single mass of tubular glands into which penetrate both simple and dilated tracheæ; the entire mass is situated in the 3d, 4th, and 5th abdominal segments, and above the intestine. The anterior end of the testicular mass is rounded and held in place by a broad, thin band, one on each side; two similar bands are situated a little behind the middle of the mass. From the under side, and a little in advance of the middle of the mass, two straight small ducts, as long as the testicular mass, pass obliquely to the sides of the body, at the posterior end of the 7th segment of the abdomen; these are the vasa deferentia. Each vas deferens, with its mate, forms a convoluted mass of tubes, comprising twenty folded bundles (epididymis of Dufour), and two single, long, convoluted tubes, the vesiculæ seminales, which are lobed in the 6th and 7th segments of the abdomen. The two vesiculæ unite over the 5th abdominal ganglion, forming a thick, very short canal (ductus ejaculatorius), which passes into a large spherical muscular mass (præputium), behind which is the large intromittent organ (penis), which forms a short chitinous cylinder, quite complicated in structure, being armed with hooks and projections and affording excellent specific characters. It can be seen in place without dissection by drawing back the orbicular convex piece called the velum penis.

In the Hymenoptera the reproductive system is quite simple, as seen in Fig. 462.

The general shape and relations of the female reproductive organs are seen in Fig. 298, of the locust (Acrydiidæ). The ovaries consist of two large bundles of tubes, each bundle tied to the other by slight bands, with air-sacs and tracheæ ramifying among them. These tubes extend along the intestine, passing into the prothorax. The ovarian tubes opening into the oviducts unite to form the vagina, which lies on the floor of the abdomen. (In the cockroach the vagina has a muscular wall and chitinous lining.) Above the opening of the duct, and directly communicating with it, is the copulatory pouch (bursa copulatrix), a capacious pocket lined within with several narrow, longitudinal, chitinous bands. Behind the bursa copulatrix lies, partly resting under the fifth abdominal ganglion, the sebific, cement, or colleterial gland (colleterium; compare Fig. 299, sb), which is flattened, pear-shaped, a little over half as long as a ripe egg of the same insect. From the under side, a little in advance of the middle, arises the sebific duct, which, after making three tight coils next to the ganglion, passes back and empties into the upper side of the bursa copulatrix, dilating slightly before its junction with the latter.

The most primitive type of reproductive organs observed in insects is that of the young Lepisma and the Ephemeridæ, in which the outlets of the oviducts and of the vasa deferentia respectively are double or paired, showing that insects have probably inherited these structures from the segmental organs of their vermian ancestors.

Réaumur had already observed the process of oviposition and seen that the female Ephemera had two openings near the end of the “6th” abdominal segment, from which he saw two masses of eggs pass out at a time (Fig. 463). Eaton afterwards (1871) referred to the oviducts as terminating between the 7th and 8th segments of the abdomen, and after him Joly; but for a detailed monograph on the subject we are indebted to Palmén. He found that the outlets of the sexual glands are paired, not only in the larvæ of all stages, but also in the imagines, and in both sexes. In the males the vasa deferentia pass on the ventral side of the 9th segment through two external appendages, both reproductive organs, at whose tips or sides the openings are situated. In the larvæ the female openings are not formed until after the last moult. In the females the two oviducts open on the ventral side of the hind-body between the 7th and 8th segments.

Palmén suggests that the Ephemerids represent, in respect to the reproductive system among insects, a very primitive type of organization, and he concludes that the inner sexual organs of insects are built up of two different morphological elements; i.e. (a) internal primitive paired structures (testes with vasa deferentia, ovaria with oviducts), and (b) integumental structures, such as the ductus ejaculatorius and vagina.

In the younger larvæ the vasa deferentia form slender cords along which are situated the seminal glands; these cords are inserted in the integument on the hinder edge of the 9th sternite, where afterwards, during the last moult, the copulatory organs grow out. In the older larvæ the sperm collects in the cavities of these cords. Their walls become expanded, and this section then functions as vesiculæ seminales. The ends of the cords remain contracted and act as ductus ejaculatorii. Common unpaired glandular structures are not present. At the last moult the copulatory organs reach their complete development, and the ducts become open externally.

The oviducts in the larva are at first slender, string-like, and bear the egg-follicles. As soon as the eggs pass out of the follicles and collect in the oviducts, the walls of the latter become stretched, and this portion forms two uterus-like structures. The terminal division of the two passages forms their vaginal portions. But since there is no common vagina, there are no unpaired glands and no receptaculum seminis. The two ducts become open after the last shedding of the skin.

Palmén adds that this paired or double nature of the sexual glands and their external ducts in this group of insects occurs in some Myriopoda (Fig. 3, E, F) and a few Arachnida (Fig. 3, C, D, the outlets being in this class unpaired), numerous Crustacea, and most worms; and as already stated it is very marked in Limulus, where the paired outlets are in both sexes very simple and wide apart (Fig. 3, A). In the worms the paired genital ducts are modified segmental organs. As we have seen, in the young male Lepisma there are two male genital openings. Hence this double nature of the genital passages in the may-flies seems to be very primitive.

In the Dermaptera, also, the genus Labidura was found by Meinert to have two independent ductus ejaculatorii, opening externally in double external slit-like processes (penes). The two ducts arise from a single seminal vesicle, which is either paired (L. advena), or forms a common passage (L. gigantea). In Forficula (Fig. 464, B) only one ejaculatory duct persists, the other is obliterated, and one of the penes is atrophied, the other assuming a position in the middle line of the body. Thus the single ejaculatory duct and seminal vesicle arise from the primitive vasa deferentia, and not from the integument of the body, as is the case in the following examples.

According to the researches of Dufour, Loew, etc., most species of Orthoptera (Œdipoda), Libellula, Perla, Panorpa, Rhaphidia, Myrmeleon, Sialis, and Trichoptera (Hydropsyche) have double vasa deferentia and seminal vesicles, and two ejaculatory ducts. The male genital passages of Rhaphidia have a double opening, Loew describing “the two seminal vesicles as lying near each other and at last uniting in a common passage, with an external opening, which, however, must be very short, since I could only once clearly observe it.” This opening is a deep invagination of the external integument, at the bottom of which the two ducts open independently of each other. In such insects, Palmén states that the single ejaculatory duct morphologically arises by an invagination of the integument.

In another group, forming, as regards the genital apparatus, a step next above the Ephemeridæ, viz. the Perlidæ, the oviducts open near each other at the bottom of a median single “vagina,” situated between the 7th and 8th abdominal segment; it is covered beneath by a valve-like, enlarged sternite of the preceding segment, and Palmén homologizes it with the ovi-valvula of some Ephemeridæ. He regards this bell-shaped vagina as a cup-like, deep, intersegmental fold, which projects into the body-cavity and there receives the two ducts.

This differentiation in the Perlidæ may be regarded as the type for several groups of insects. But in others occur a complication which in some degree modifies the type. Thus the invagination arises out from one segment alone, but several segments during metamorphosis may become so reduced that the ventral portions of all may be invaginated to form the vagina. Thus in the larva of Corethra, according to Leydig, and also Weismann, the two testes are attached by two cords to the integument; the hinder ones are inserted independently, and share in the development of the outlets.

Graber has observed the same relations in the pupa of Chironomus, the efferent genital tubes in both sexes being separate, so that there are two vaginal passages and two penes present. Palmén comments on these relations in the dipterous insects, remarking that during metamorphosis certain parts of the terminal abdominal segments are reduced, while others are hypertrophied; hence the points of insertion of the cords referred to becoming the openings of the vasa are carried within the abdomen; and this part of the integument becomes an unpaired section. In these insects, also, there is an unpaired vesicula seminalis, but its morphological nature (whether formed from the integumental duct or the fused vasa deferentia) can only be settled after special investigation.

In the Lepidoptera, also, it has been shown by Herold, Suckow, Bessels, and recently with full details by Jackson, that the paired larval oviducts are at first solid, but become tubular early in pupal life. A little later, their cavities open into that of the azygos or unpaired oviduct. The paired oviducts open in the female caterpillars on the hind edge of the 7th abdominal segment, afterwards uniting with the unpaired vagina of the 8th segment, which is developed from the hypodermis.

Jackson adds that there are three stages traceable in the evolution of the genital ducts of Lepidoptera: “an ephemeridal stage, which ends towards the close of larval life; an orthopteran stage, indicated during the quiescent period preceding pupation; and a lepidopteran stage, which begins with the commencement of pupal life.”

As a summary of these results it appears that the genital organs of insects consist of two morphologically different elements: 1. the primitive internal paired structures (testes with the vasa deferentia; ovaries with the ovarian tubes), and 2. integumental structures (Fig. 464). In the most primitive winged insects (Ephemeridæ) the latter structures are only represented by the two external sexual openings, the entire reproductive system being paired. The paired parts become in the more highly differentiated forms united into single parts, while, a, a common integumental division, grows in, forming the ductus ejaculatorius, or the vagina; or, b, the inner passages anastomose together, i.e. the openings fuse together; or, c, both of these cases occur at once; or, finally, we have d, where the superfluous paired parts by reduction become single.

The male ducts open behind the 9th, the female passages of Ephemerids behind the 7th abdominal segment, those of other insects behind the 8th, except in the Stylopidæ (Strepsiptera), in which they open much in front.

Figure 464 graphically shows their relation. In the Odonata (F) the chitinous lining or integumental invagination extends inwards where the two oviducts begin, in the Coleoptera (E) the vagina, bursa copulatrix, and receptaculum seminis being lined by a thick chitinous layer. While in Perla the two seminal ducts pass directly into the copulatory organ, in the Coleoptera they open into the unpaired ductus ejaculatorius at a distance from the copulatory organ.

The morphological results obtained by Palmén, and for the Lepidoptera by Jackson, were apparently confirmed from an embryological point of view by Nusbaum, from observations on the development of the sexual passages in two genera of Pediculidæ, and are as follows:—

1. The prevalent impression that the larval ducts unite with each other and give origin to the whole system of sexual ducts is incorrect; they form only the vasa deferentia or the oviducts.

2. All other parts of the efferent apparatus (uterus, vagina, receptaculum seminis, ductus ejaculatorius, penis, and appended glands) develop from the hypodermis.

3. The connective tissue and the musculature of the efferent apparatus are derived from mesoblast cells present in the body-cavity.

4. The efferent ducts originate as paired rudiments. All unpaired (azygos) parts (uterus, penis, receptaculum seminis, unpaired glands, etc.) are at first paired. The unpaired efferent apparatus of insects must therefore be regarded as morphologically a secondary and more complicated form.[76]

5. The male and female efferent ducts are strictly homologous.

6. The cavities of the oviducts, uterus, vagina in the female, of the vasa deferentia, appended organs, and ductus ejaculatorius of the male arise independently, and come into connection secondarily.

The presence of two genital openings, viz. a bursa copulatrix or copulatory pouch, and of the primitive oviducal orifice behind the 9th segment, is peculiar to Lepidoptera, and the inquiry naturally arises whether they represent the outlets of two pairs of segmental organs. The question has been fully set at rest, however, by Jackson, who shows that the copulatory pouch is a secondary invagination of the ectoderm, being derived from the hypodermis, while the second aperture is a special adaptation. It is, however, the partial homologue of the vaginal orifice in other orders of insects. It opens behind the sternite of the 8th abdominal segment, the typical position of the vaginal aperture as shown by Lacaze-Duthiers. The lateral position of the bursa and its separation from the azygos oviduct are probably late features in the phylogenetic history of the Lepidoptera, subsequent even to the closure of the furrow.

“The existence of a second or posterior aperture is probably to be attributed to the advantage gained by a terminal position for the aperture through which the ova are laid. The remarkable way in which this aperture shifts backwards seems to point very distinctly to this explanation, especially as the Lepidoptera are entirely devoid of the outgrowths which form the ovipositor in some orders; e.g. most Orthoptera.”

The original condition of things appears to have been retained in a moth, Nematois metallicus, which, according to Cholodkowsky, possesses but a single external aperture, the bursa opening into the dorsal wall of the unpaired oviduct.

a. The male organs of reproduction

Bearing in mind that the testes with their efferent ducts are, like the ovaries and egg-tubes, primitive structures, there are various secondary or adaptive structures which are either due (1) to modifications of the male efferent ducts, or of the ovarian tubes, or (2) to various accessory organs, mostly glandular, resulting from the invagination of the ectoderm.

The male organs are, then, the following:—

1. Two testes (Figs. 465–469, t, H, ho).

2. The two seminal ducts (vasa deferentia, v, sl, SL), whose lower or outer (distal) division becomes enlarged and acts as a seminal vesicle (vesicula seminalis; Figs. 467–469, bl, SB).

3. The common ejaculatory duct (ductus ejaculatorius), with the penis (Figs. 467–469, ag, uSG).

4. Accessory glands at the base of the vasa deferentia (glandulæ mucosæ, Figs. 465–469, a. g., dr, D), whose secretion mixes with the semen or serves for the formation of the seminal packets (sematophores).

In his paper on the internal male organs of beetles, Escherich states that those of the Carabidæ illustrate the simplest, most primitive condition (Fig. 465). A simple blind tube on each side produces spermatozoa, stores the elements, and secretes mucus. Each of these tubes opens into a somewhat larger duct, and the two unite in a common ejaculatory canal. The terminal portion in these beetles is lined with chitin, and is therefore ectodermal, and not the result of the union of the mesodermic vasa deferentia. The region corresponding to the testes, vasa deferentia, and seminal vesicles are mesodermic. Blaps (Fig. 465, B) is intermediate between the Carabidæ and Hydrophilus (Fig. 465, C). The accessory glands (a. g.) are developed, and the seminal vesicles are situated in the middle, and not at the lower end of the vasa deferentia, as in Hydrophilus.

=The testes.=—Each testis is composed of follicles or corresponding parts, which according to the group of insects in which they occur are united in different ways; or each testis consists of a single hank or skein-like blind tube which is enveloped by a membrane, as in the Carabidæ, Dyticidæ, or Lucanidæ.

The number of testicular tubes is small in most Hemiptera, but very great in the Cicadidæ, Orthoptera, Coleoptera, and many Hymenoptera. Although the testes are usually separated from each other, they are closely united in certain Orthoptera (Gryllotalpa, Ephippigera), Coleoptera (Galerucella), in many Lepidoptera, and in a number of Hymenoptera (Scolia, Pompilus, Crabro, and others).

The two testes of most Lepidoptera are so closely grown together or coalesced into a single body that one might regard them as a single testis. But in the different families there occur all grades, from the unpaired testes of most Lepidoptera to Hepialus with separate testes. Cholodkowsky therefore distinguishes four types:—

1. The embryonal or primitive type, with two testes, whose seminal follicles are entirely separate. (Brandt.) These testes are contained, as in all other Lepidoptera, in a well-developed thick chitinous membrane or scrotum, analogous to that of the higher vertebrates, which envelops each separate seminal follicle (Hepialus humuli).

2. The larval type, with two testes, whose four follicles are enclosed by a common scrotal membrane (Bombyx mori, Gastropacha quercifolia, Ichthyura anachoreta and anastomosis, Saturnia pyri, Aglia tau).

3. The pupal type (since it first occurs in the pupa state), with a single testis, which possesses an external median lace-like covering. (Adela, Lycæna.)

4. The imaginal type, with a single testis enveloped by a lace-like scrotum, within which the follicles are wound around the longitudinal axis of the testis. (Most Lepidoptera.)

In Nematois there are twenty seminal follicles, the number of ovarian tubes being the same. (Cholodkowsky.)

In many insects the testes are not composed of tubes (follicles), but of button-like bodies, each of which has its own duct.

The color of the testes is usually white, but they may be orange (Decticus), yellowish green (Locusta viridissima), or deep yellow (Chrysopa).

The testes of Asilid flies are enveloped by a common dark-red membrane rich in tracheæ, like that in Lepidoptera which clothes the separate testicular follicles. The two testes of Calliphora are enveloped by an orange-yellow capsule, outside of which is a special membrane formed by the fat-body. (Cholodkowsky.)

In the honey-bee the testis has two envelopes, the outer of which is formed by the fat-body, the inner coat of connective tissue. The entire testis corresponds to a portion only of that of Bombyx mori.

=The seminal ducts.=—The vasa deferentia are fine tubes, which vary much in length; being short in many beetles and locusts, very short in many Diptera (Syrphidæ, etc.), very long in Cicada and many beetles; according to Burmeister, being in Dyticus about five times, in Necrophorus and Blaps eight to ten times, in Cicada 14 times, in Cetonia aurata 30 times, as long as the body. They either resemble a skein of silk, or form a tangled mass.

The distal or lower end of the vasa is in many insects dilated into a sac or seminal vesicle, which serves for the reception and storage of the seminal fluid after it passes through the vasa deferentia. In the honey-bee the vas deferens is given off from the reservoir, forms loops in and outside of the testis, and passes to the seminal vesicle. The canal into which the vesicle narrows does not open into the ductus ejaculatorius, but into the glandulæ mucosæ; its epithelial cells are much vacuolated, and have, therefore, a spongy appearance. (Koschewnikoff.)

=The ejaculatory duct= during coition conducts the sperm into the copulatory pouch of the female. In consequence of the stretching of the integumental membrane the end of the duct can be erected and again withdrawn. For this purpose the end of the duct is thickened and is said to be provided with powerful muscles. The evaginable terminal portion is covered by a strong chitinous membrane forming the penis or intromittent organ (Fig: 462, h), which is externally enveloped by a pair of chitinous lobes, which in many beetles are converted into a capsule. The ductus ejaculatorius of the honey-bee is inserted by two chitinous branches into the point of union of the two glandulæ mucosæ; it and the entire copulatory apparatus are devoid of muscles, though it is, however, well developed beneath the mucous glands. (Koschewnikoff.)

=The accessory glands= of the vasa deferentia are tubes whose secretions either directly mix with the semen, or in many cases form seminal packets (spermatophores). In Coleoptera, Lepidoptera, and Diptera there is usually one pair. In many insects there are several pairs, as in Hydrophilidæ and Elateridæ; they are branched in Hemiptera, and in Orthoptera bushy. The single glandular tubules are very long, and form a skein-like mass. In Orthoptera, in the larger number of accessory glands, two forms may be distinguished, which differ from each other in their contents (Siebold). In the cockroach (Fig. 461) these glands form the “mushroom” shaped gland of Huxley, which was at first regarded as the testis.

=The spermatozoa.=—These very minute bodies, the sexual homologues of the eggs, abound in the seminal fluid, and are formed in the follicles of the testes from a germinal layer or epithelium, as are the eggs. They are hair- or thread-like, usually consisting of a head, a body or middle-piece, and a long, thread-like tail (flagellum), which vibrates rapidly, causing the spermatozoön to move actively forwards (Fig. 470).

In beetles, according to Ballowitz, there are two main types of spermatozoa, connected, however, by intermediate forms. There is a double-tailed type, already described by Bütschli and v. la Valette St. George, and there are others which are single-tailed. Bütschli showed that in the double spermatozoön one tail-filament is straight and stiff, the other being undulating and contractile. Ballowitz describes this type in Calathus (Fig. 470, B), Chrysomela, and Hylobius, etc., and shows that the straight or supporting portion of the tail is elastic, but somewhat stiff, resistant to reagents, and without any fibrillar structure, while the contractile fringe consists of an extremely complicated system of fibrils (Fig. 470). The single-tailed type of spermatozoön, as seen, e.g., in Melolontha and Hydrophilus, has no supporting fibres. The tail is twisted in a spiral, corresponds to the contractile fringe of the double type, and exhibits a complicated fibrillar structure. The fringed type works its way ahead like the screw of a steamer.

Each spermatozoön is a modified but complete cell, and the nucleus contains the chromatin, a deeply staining substance of the nuclear network and of the chromosomes and the supposed bearer of heredity.

=Formation of the spermatozoön.=—It arises from a primordial germcell called spermatogonium. This cell contains a large, pale nucleus and a dark body, the accessory nucleus of Bütschli. The spermatogonia subdivide, but at a certain period pause in their subdivisions, and undergo considerable growth. “Each spermatogonium is thus converted into a spermatocyte, which, by two rapidly succeeding divisions gives rise to four spermatozoa, as follows: The primary spermatocyte first divides to form two daughter-cells, known as spermatocytes of the second order, or sperm mother-cells. Each of these divides again—as a rule without pausing, and without the reconstruction of the daughter-nuclei—to form two spermatids or sperm-cells. Each of the four spermatids is then directly transformed into a single spermatozoön; its nucleus becoming very small and compact, its cytoplasm giving rise to the tail and to certain other structures.... As the spermatid develops into the spermatozoön, it assumes an elongated form, the nucleus lying at one end, while the cytoplasm is drawn out to form the flagellum at the opposite end.” (Wilson’s The Cell, from La Valette St. George.)

Henking finds that the primordial sperm-cells correspond to the primordial ova, both forms of cells in the insect he studied containing the characteristic number of twenty-four chromosomes.

The spermatogenesis of Laphria, according to Cholodkowsky, is very peculiar, and strongly resembles that described by Verson in Bombyx mori. In the blind end of the testicular tubes lies a colossal cell visible to the naked eye, the spermatogone, from which the entire contents of the testes originate. In Bombyx this spermatogone appears in the larva state. Such colossal spermatogones also occur in Lepidoptera of different families (Hyponomeuta, Vanessa, and in the pupa of Chareas graminis), in Trichoptera, and in Hemiptera (Syromastes); and Cholodkowsky inquires whether they may not be typical of insects. Toyama has observed these colossal cells not only in the testes but also in the ovaries of the silkworm. He regards them as supporting cells.

The spermatozoa are inclined to remain in bundles, and in this state are expelled during copulation. These bundles are either root-like, bushy, string-like, sinuous, or worm-like.

Auerbach has observed the spermatozoa of Dyticus marginalis in their passage through the convoluted seminal vesicles. All those arising from one testicular tube are united in a bundle. Each has a very complex structure, bilateral but unsymmetrical. The right side of the head is concave, the left convex; the whole head is longitudinally curved to right or left; and on the posterior half of the right side there is a projecting ridge bearing a hook-shaped cyanophilous “anchor,” at the free end of which an erythrophilous spherule appears. The most remarkable fact is that the spermatozoa unite in pairs in a perfectly definite way, opposed and crossed in a manner somewhat suggestive of a pair of scissors, with the right sides of the heads in contact. During this conjugation, or “dejugation” as Auerbach calls it, the anchors change their shape, and the little spherules are lost. Hundreds of these double spermatozoa are found together in little balls. The conjugation is a temporary one, but it may permit a molecular exchange of substance, perhaps with the result of mixing the hereditary qualities and limiting variability. (Journ. Roy. Micr. Soc., 1893, p. 622.)

In many insects which lack a true penis, the bundle of spermatozoa are united in the ejaculatory duct, forming packets which are enveloped by the secretion of the accessory glands which stiffens into a hard case. These packets are called spermatophores. They are either introduced into the vagina of the female or simply remain outside. Graber has repeatedly observed that the male crickets, in the absence of the female, let their spermatophores fall to the earth; whether it is afterwards made available is not known, because hitherto no case is reported that females seeking impregnation search, as in the case of the Isopod crustacean, Porcellio, for the spermatophores.

In the Gryllidæ and Locustidæ the spermatophore lies in a cup-like cavity under the penis. This is called the “spermatophore cup” (Chadima, 1871), into which the ejaculatory duct of the testis opens.

According to the views of Schneider, the spermatophores, with their capsule, usually consist solely of seminal filaments, which stick closely to each other, and only exceptionally have a capsule formed by a glandular secretion. In Locusta, however, and perhaps also in Gryllus, the sperm is enveloped by the secretion of the accessory glands of the seminal ducts; the spermatophores pass, still fluid, out of the sexual opening of the male into that of the female, but become chilled on the outer surface, so that the sperm, without coming in contact with the air, passes into the receptaculum seminis.

The mode of grouping of the spermatozoa of the Locustidæ as they occur in the spermatheca of the female is remarkable. Their heads lie so close to each other that they form a long shaft, while the numerous threads are arranged so as to look like the two vanes of a feather, the entire mass being like a very long heron’s feather. (Siebold.)

In the honey-bee the spermatophore is likewise enveloped by the secretion of the accessory glands, and thereby becomes a sort of seminal cartridge. This is a peculiar oval body which is carried during the marriage-flight into the air within the upper part of the penis, the so-called penis-bulb. (Leuckart.)

b. The female organs of reproduction

The different parts of the female reproductive organs are the following:

1. The two ovaries.

2. The two oviducts.

3. The common egg-passage in nearly all insects (its distal or hindermost part forming the uterus or vagina).

4. The receptaculum seminis, or spermatheca.

5. The bursa copulatrix, or copulatory pouch.

6. The accessory glands (cement, sebific, or colleterial glands, or “oil reservoirs,” glandulæ sebaceæ, coleterium).

=The ovaries and the ovarian tubes.=—As in the testes, so each ovary consists of a variable number of ovarian tubes, by some called ovarioles, united by a thread at the distal end, and at the lower or hinder end opening into the oviduct. Each ovarian or egg tube is divided into three sections: (1) the terminal thread; (2) the terminal chamber, and (3) the actual ovarian tube, or chambered main division, this forming the longest part of the egg-tube.

The slender terminal thread serves to attach or suspend each egg-tube near the dorsal vessel (not directly to the heart, as formerly supposed), becoming lost in the fat-body.

The terminal chamber contains undifferentiated cell elements, supposed to be the remains of the ovarian rudiments. From these arise (either in the embryo or larva) first, the follicle epithelium of the ovarian tubes; and, second, the material for the formation of the new eggs, and nutritive cells. “In the terminal chamber these cell-elements remain undifferentiated, excepting when required for the removal of the follicle epithelium, eggs, and nutritive cells in the adult insect.” (Lang.) This portion of the ovariole is called the germarium. In Blatta it is filled with protoplasm in which numerous small nuclei are imbedded. (Wheeler.) The chambered main division of the egg-tube contains the ripening eggs, one in each compartment, the tube appearing like a string of beads.

The egg-tubes are of two types: (1) those without, and (2) those with nutritive cells, the first kind being the simplest, and occurring in the Synaptera (except Campodea) and in Orthoptera. As an example may be cited that of the cockroach (Fig. 473), where in each tube there is a simple continuous row of eggs from the terminal chamber to the oviduct. The tube being constricted between these consecutive eggs, gives it a beaded appearance.

In the cockroach (Periplaneta orientalis) each egg-tube has a beaded appearance. Its wall consists of a transparent elastic membrane, lined by epithelium, with an external peritoneal layer of connective tissue. The terminal filament (tf) is filled with a clear protoplasm, with a few nuclei. In the terminal chamber (tc) are large nucleated cells, with separate nuclei, both entangled in a network of protoplasm. In the third, or egg-chamber (ec), are about twenty ripening eggs, arranged in a single row. “Between and around the eggs the nuclei gradually arrange themselves into one-layered follicles, which are attached, not to the wall of the tubes, but to the eggs, and travel downwards with them. As the eggs descend, the yolk which they contain increases rapidly, and the germinal vesicle and spot (nucleus and nucleolus), which were at first plain, disappear. A vitelline membrane is secreted by the inner surface and a chitinous chorion by the outer surface of the egg-follicle.

“The lowest egg in an ovarian tube is nearly or altogether of the full size; it is of elongate-oval figure, and slightly curved, the convexity being turned towards the uterus. It is filled with a clear albuminous fluid, which mainly consists of yolk. The chorion now forms a transparent yellowish capsule, which, under the microscope, appears to be divided up into very many polygonal areas, defined by rows of fine dots. These areas probably correspond to as many follicular cells.” (Brandt, from Miall and Denny.)

In the second type, i.e. those egg-tubes with nutritive cells, there are two kinds. In the first the egg-chambers and yolk- or nutritive chambers alternate, each of the latter containing one or more nutritive cells, which serve for the nourishment of the ripening egg contained in the neighboring chamber. “The egg- and yolk-chambers may be distinctly separated externally by constrictions (Hymenoptera and many Coleoptera), or one nutritive and one egg-chamber may lie in each section of the ovarian tube, which is externally visible as a swelling (Lepidoptera, Diptera).”

In the second kind with nutritive cells, the actual tube consists (Fig. 474, C) of ovarian chambers only; the nutritive cells here remain massed together in the large terminal chamber. The single egg in the tube is united with the terminal chamber by connective strands (d. s.), which convey the nutritive material to the eggs. (Lang.)

Egg-cells, nutritive cells, and the cells of the follicle-epithelium (epithelium of the chambers of the ovarian tubes) are, says Lang, according to their origin, similar elements, like the egg and yolk-cells of the flat worms (Platodes); division of labor leads to their later differentiation. Only a few of the numerous egg-germs develop into eggs, the rest serving as envelopes and as food for these few.

Korschelt considers that all the chief elements of the egg-tubes, viz. egg, nutritive, and epithelial cells, arise by a direct transformation of the elements of the terminal chamber, and that the last may be traced to the indifferent elements of the terminal thread, the elements in question originating from the nuclear elements by a breaking down of the syncytium (or masses of protoplasm with nuclei scattered through it) composing it (Fig. 475).

The latest work is that of Wielowiejski (Zoologische Anzeiger, ix, 1886, p. 132), whose observations are based on a study of the ovarian tubes and the growing eggs of the Hemiptera (Pyrrhocoris), the Coleoptera (Telephorus, Saperda, Cetonia and Melolontha, Carabidæ, and Hydradephaga), etc.

Wielowiejski divides the ovaries of insects into three groups:—

1. Comprising such ovaries in the ends of whose egg-tubes (terminal filament) the embryonal cells in the early stages are accumulated, and are transformed into egg-, yolk-, and epithelial cells respectively. (Ovaries of Orthoptera, geodephagous and hydradephagous Coleoptera, Lepidoptera, Diptera, and Hymenoptera).

2. Comprising ovaries whose ends above the egg-cells and egg-germs (Eianlagen) possess throughout life a more or less voluminous solid accumulation of cells (terminal chamber), but which stand in no close relation with the first. (Ovaries of Coleoptera, with the exception of the Geodephaga and Hydradephaga, and Aphidæ in part.)

3. Comprising ovaries whose ends above the egg-germs contain a well-developed mass of cells functioning as a yolk-forming organ, between whose special elements grow root-like offshoots of nearly ripe egg-cells. (Hemiptera.)

When the egg is ripe the food-chamber disappears because its contents have served for the formation of the egg below it. In Lepidoptera especially, the egg-tubes resemble strings of pearls because most of the numerous eggs ripen simultaneously and are likewise deposited at the same period, which is naturally not the case in those insects whose eggs gradually ripen (Fig. 477). In other cases the egg- or food-compartments are transformed into each other, but only one egg- and one food-compartment can be situated in the same dilatation of the ovarian tube. Finally, there are insects in whose egg-tubes the egg-compartments are arranged in a single row, while the capacious terminal chamber contains a large mass of food-cells.

Egg-cells, nutritive cells, as well as the cells of the follicle epithelium (epithelium of the chambers of the ovarian tubes), originate as similar or homologous elements, division of labor leading to their later differentiation. Only a few of the numerous egg-germs develop into eggs, the rest serving as envelopes and also as food for these few. (Lang.)

In many insects the egg-tubes open into an egg-calyx (Fig. 478, c), in which the ripe eggs collect before passing into the oviduct (ov).

As the result of his investigations on the origin of the cellular elements of the ovaries of insects Korschelt concludes:—

1. The different cell-elements of the egg-tubes, eggs, nutritive cells, and epithelium arise from identical undifferentiated elements situated in the contents of the earliest germ of the egg-tubes.

2. The first formation of the cellular elements present, and the differentiation of the individual compartments of the egg-tube, occur during embryonic and larval life.

3. The undifferentiated elements of the terminal chamber correspond to the embryonic condition, while in post-embryonic time, and even during imaginal life, a new formation of the different kinds of cells takes place.

4. The mode of origin of the different kinds of cells from the undifferentiated elements varies greatly in different insects.

5. From their histological nature, and from the mode of origin of their elements, the most complex egg-tubes and those provided with nutritive compartments are phylogenetically derived from those without such nutritive compartments.

6. The nutritive cells in certain cases originate in the same way and at the same time as the germ-cells, and are therefore to be regarded as germ-cells which have abandoned the function of egg-making, and exchanged it for the production of nutritive material.

7. In the egg-tubes with numerous nutritive compartments the nutritive cells can originate at the same place as the egg-cells, and they afterwards still lie intermingled with these in the beginning or upper part of the egg-tubes.

8. While the capability of egg-making of the germ-cells originally situated in the extremity of the terminal chamber gradually becomes transferred to those at the base of the terminal chamber, and the first transform into nutritive cells, egg-tubes with nutritive compartments at the base may be found.

9. The nutritive cells of certain forms arise independently of the germ-cells and therefore could not have previously originated from them.

10. The epithelium has in all forms nearly the same mode of formation; it everywhere shows a close similarity to the undifferentiated elements of the terminal chamber, out of which it directly develops. As to the fact of formation of epithelium through the germ-vesicles (Keimblaschen), nutritive-cell nuclei, or the so-called “oöblasts,” I could not feel certain.

11. Neither the eggs of Hemiptera or of other insects arise through the agency of “oöblasts,” but like the epithelial and nutritive cells arise by a gradual differentiation from the indifferent elements of the ovarian tubes.

12. The different elements of the egg-tubes, also the eggs, have the morphological value of cells.

=Origin of incipient eggs in the germ of the testes.=—Heymons has detected in the germ of the testes of the male larvæ of Phyllodromia germanica 7 mm. in length, young or incipient eggs, similar to those seen in the ovarian tubes of the female larva of the same size. In another male larva of the same size also occurred short cylindrical tubes each with a terminal thread, which had the appearance of rudimentary egg-tubes. Hence he thinks that every part of the genital germs (Anlagen) in the male, which are not concerned in the formation of testicular follicles, represents the germ of a female genital gland. As is well known, no insects are hermaphroditic, but this case of the practical origin of eggs and egg-tubes in the lowest division of the male efferent passage, which is homologous with the egg-producing division of the female ovarian tubes, points back to hermaphroditic ancestors. And Heymons suggests that the frequent occurrence of hermaphroditism in insects probably confirms this view.

=The bursa copulatrix.=—The copulatory pouch in most insects is a special cup-shaped appendage of the vagina adapted for the reception of the male organ during sexual union. Its mode of formation in the cockroach is thus described by Haase:—

“By the retreat of the female sexual aperture, situated in the 8th ventral plate, a considerable space, the genital pouch, is produced; this is formed chiefly by the extended connective membrane between the elongated 7th and 8th ventral plates. This serves for the development of the egg-cocoon, which is retained by the internal appendages of the posterior gonapophyses.”

The fertilization of the female takes place once for all a long time previous to oviposition; the semen in the receptaculum seminis passes out as the eggs slip down the egg-passage, and a spermatozoön gains entrance into the interior of the egg through the micropyle. In Œcanthus, according to Ayers, fecundation probably takes place while the egg is passing into the vagina, “since it is hardly possible that the male element could gain access to the follicles before the chorion is secreted.”

In the Lepidoptera, as has been stated, the copulatory pouch opens separately from the opening of the oviduct (vagina), but a slender canal connects the pouch with the vagina (Fig. 310, bc). The outlet (“vagina” of Burgess) of the copulatory pouch opens between the 7th and 8th segments, that of the oviduct (vagina) on the 9th segment being “situated immediately below the anus and hardly separated from it, between the lappets of the 9th segment.” (Burgess.) The opening of the copulatory pouch is, as we have seen, the genuine or primitive sexual opening.

=The spermatheca.=—This is a sac or pouch for the reception and storage or preservation of the semen. While in most of the higher insects it opens into the dorsal wall of the vagina (Fig. 472, f), in the cockroach, locusts, and grasshoppers it opens into the bursa; but in other European Orthoptera, as in most insects, it lies upon the dorsal wall of the vagina. (Berlese.) In the cockroach, it is a short tube dilated at the end and wound into a spiral of about one turn. “From the tube a cœcal process is given off, which may correspond with the accessory gland attached to the duct of the spermatheca in many insects (e.g. Coleoptera, Hymenoptera, and some Lepidoptera). The spermatheca is filled during copulation, and is always found to contain spermatozoa in the fertile female. The spermatozoa are no doubt passed into the genital pouch from time to time, and there fertilize the eggs descending from the ovarian tubes.” In Meloë the spermatheca is exceedingly large. (Miall and Denny, pp. 170, 171.)

=The colleterial glands.=—We have already briefly referred to these glands. Those of the cockroaches form a number of long blind tubes opening into the vagina. They furnish the material for the egg-capsule or oötheca, viz. chitin and large crystals of oxalate of lime.

In Phyllodromia germanica “these glands are glistening white till the time of oviposition approaches, when they assume a yellow tint, and the octahedral crystals are seen imbedded in a viscid substance which fills their lumina. This viscid substance is soluble in potassium hydrate, and is consequently not chitin. When excreted to form the oötheca, it slowly hardens, deepens in color, and becomes insoluble in potassium hydrate. Light has nothing to do with this change, which is possibly produced by the oxygen in the air. It is the same change which is undergone by the cuticula of the insect itself immediately after ecdysis.” (Wheeler.)

=The vagina or uterus.=—This is simply the end of the common oviduct, which, when dilated, is called the vagina, and, in the pupiparous forms, the uterus.

In the cockroach the vagina opens by a median vertical slit situated in the 8th sternite, into the genital pouch or bursa, upon the dorsal wall of which the orifice of the spermatheca is situated. In the sheep-tick the oviduct is enlarged to form the so-called uterus, which furnishes a milk-like secretion for the nourishment of the larva during its intra-uterine life.

In insects in general, the external opening of the vagina is simple, the chitinous structures (valves) at the opening being adapted to receive the male intromittent organ.

When the eggs are to be deposited deep below the surface of the earth, or in wood, or in wood-boring larvæ, or in the body of caterpillars, etc., they are inserted by the ovipositor (see p. 167).

Signs of copulation in insects.—Leydig has collected, partly from his own observations and partly from those of others, a number of cases in which female insects bear traces of having had sexual union, in the form of tags or plates attached to the body, and apparently formed from material secreted by the male. Such probably is the “pouch” on the abdomen of Parnassius apollo, and a somewhat similar structure in Fulgora laternaria, and such is the plate which is found on the hinder part of the abdomen of Dyticus latissimus and D. marginalis. Leydig compares these structures with the white plate in Astacus fluviatilis, and with the little white lid on the spider Argenna, and finds analogues among vertebrates. (Arbeit. Zool. Zoot. Inst. Wurzburg, x, 1891, pp. 37–55, 2 Figs.)

LITERATURE ON THE ORGANS OF REPRODUCTION

a. General

=Hunter, J.= Observations of bees. (Phil. Trans. Roy. Soc. London, 1792, lxxxii, pp. 128–195.)

=Hegetschweiler, J. J.= Dissertatio inauguralis zootomica de insectorum genitalibus. Turici, 1820, pp. 28, 1 Pl.

=Audouin, V.= Recherches anatomiques sur la femelle du Drile jaunatre et sur le male de cette espèce. (Ann. Sc. nat., ii, 1824, pp. 443–462, 1 Pl.)

=Müller, Johannes.= Ueber die Entwickelung der Eier im Eierstock bei den Gespenstheuschrecken. (Nova Acta Acad. Leop.-Carol., xii, 1825, pp. 555–672, 6 Taf.)

=Dufour, L.= Recherches anatomiques sur les Carabiques et sur plusieurs autres insectes coléoptères. Organes de la génération (Ann. Sc. nat., vi, 1825, pp. 150–206, 6 Pls.; pp. 427–468, 4 Pls.).

—— Recherches anatomiques sur l’Hippobosque des cheveaux. (Ibid., 1825, vi, pp. 299–322, 1 Pl.)

—— Recherches anatomiques sur les Labidoures. Appareil de la génération. (Ibid., 1828, xiii, pp. 354–359, 2 Pls.)

—— Recherches anatomiques et considérations entomologiques sur quelques insectes coléoptères, compris dans les familles des Dermestins, des Byrrhiens, des Acanthopodes et des Leptodactyles. Appareil génital. (Ibid., Sér. 2, Zool. i, 1834, pp. 76–82, 2 Pls.)

—— Résumé des recherches anatomiques et physiologiques sur les Hémiptères. (Ibid., Sér. 2, i, pp. 232–239.)

—— Mémoire sur les métamorphoses et l’anatomie de la Pyrochroa coccinea. Appareil génital. (Ibid., Sér. 2, Zool., xiii, pp. 337–339, 1 Pl.)

—— Histoire des métamorphoses et de l’anatomie des Mordelles. (Ibid., Sér. 2, xiv, pp. 235–238, 1 Pl.)

—— Anatomie générale des Diptères. Appareil génital. (Ibid., Sér. 3, Zool., i, 1844, pp. 250–264.)

—— Histoire des métamorphoses et de l’anatomie du Piophila petasionis. Appareil génital. (Ibid., Sér. 3, Zool., i, 1844, pp. 378–386.)

—— Études anatomiques et physiologiques sur les insectes diptères de la famille des Pupipares. Appareil génital. (Ann. Sc. nat., Sér. 3, Zool., iii, 1845, pp. 73–93, 2 Pls.)

—— Recherches sur l’anatomie et l’histoire naturelle de l’Osmylus maculatus. Appareil génital. (Ibid., Sér. 3, Zool., ix, 1848, pp. 349–356, 1 Pl.)

—— Recherches anatomiques sur les Hyménoptères de la famille des Urocerates. Appareil génital. (Ibid., Sér. 4, Zool., i, 1854, pp. 216–234, 1 Pl.)

—— Fragments d’anatomie entomologique. Sur les ovaires du Nemoptera lusitanica. (Ibid., Sér. 4, viii, 1857, pp. 9–10, 1 Pl.)

—— Fragments anatomiques sur quelques Élatérides. (Ibid., Sér. 4, viii, 1857, pp. 365–372, 1 Pl.)

—— Recherches anatomiques et considérations entomologiques sur les Hémiptères du genre Leptopus. Appareil génital. (Ibid., Sér. 10, 1858, pp. 356–362, 1 Pl.)

—— Recherches anatomiques sur l’Ascalaphus meridionalis. Appareil génital. (Ibid., Sér. 4, xiii, 1860, pp. 203–206, 1 Pl.)

—— Sur l’appareil génital male du Coræbus bifasciatus. (Thomson’s Archiv Ent., 1857, i, pp. 378–381.)

=Suckow, F. W. L.= Geschlechtsorgane der Insekten. (Heusinger’s Zeitschr. f. organ. Physik., 1828, ii, pp. 231–264, 1 Taf.)

=Rathke, M. H.= Miscellanea anatomico-physiologica. Fasc. 1. De Libellarum partibus genitalibus. Regiomonti, 1832, p. 38, 3 Pls.

=Dutrochet, R. J. H.= Observations sur les organes de la génération chez les pucerons. (Ann. Sc. nat., 1833, xxx, pp. 204–209.)

=Doyère, L.= Observations anatomiques sur les organes de la génération chez la Cigale femelle. (Ann. Sc. nat., 1837, vii, pp. 200–206, Fig.)

=Siebold, C. Th. E. von.= Ueber die weiblichen Geschlechtsorgane der Tachinen. (Wiegmann’s Archiv f. Naturgesch., 1838, iv, pp. 191–201.)

—— Ueber die inneren Geschlechtswerkzeuge der viviparen und oviparen Blattläuse. (Froriep’s Notizen, 1839, xii, pp. 305–308.)

—— Ueber das Receptaculum seminis der Hymenopteren-Weibchen. (Germar’s Zeitschr. f. Ent., 1843, iv, pp. 362–388, 1 Taf.)

=Loew, H.= Beitrag zur anatomischen Kenntniss der inneren Geschlechtsteile der zweiflügligen Insekten. (Germar’s Zeitschr. f. Ent., 1841, iii, pp. 386–406, 1 Taf.)

—— Horæ anatomicæ, Abth. I. Entomotomien. Heft i-iii, Posen, 1841.

—— Beiträge zur Kenntniss d. inneren Geschlechtstheile der zweifl. Insecten. (Germar’s Zeitschr. f. Entomologie, iii, 1841, pp. 386–406, 1 Taf.)

=Stein, F.= Vergleichende Anatomie und Physiologie der Insekten. I, Monographie. Ueber die Geschlechtsorgane und den Bau des Hinterleibes bei den weiblichen Käfern, Berlin, 1847, i, pp. 139, 9 Taf.

=Brauer, F.= Beitrag zur Kenntniss des inneren Baues und der Verwandlung der Neuropteren. (Verhandl. d. zool. botan., Vereins in Wien, 1855, pp. 1–26, 5 Taf.)

=Haliday, A. H.= Note on a peculiar form of the ovaries observed in a hymenopterous insect, constituting a new genus and species of the family Diapriadæ. (Nat. Hist. Review, 1857, iv, pp. 166–174, 1 Pl.)

=Laboulbène, A.= Recherches sur les appareil de la digestion et de la reproduction du Buprestis manca. (Thomson’s Archiv Ent., 1857, i, pp. 204–236, 2 Pls.)

=Lubbock, John.= On the ova and pseudova of insects. (Phil. Trans. Roy. Soc., London, cxlix, 1860, pp. 341–369.)

=Landois, H.= Ueber die Verbindung der Hoden mit dem Rückengefäss bei den Insekten. (Zeitschr. f. wissens. Zool., xiii, 1863, pp. 316–318, 1 Taf.)

=Leydig, F.= Der Eierstock und die Samentasche der Insekten. (Nova Acta Acad. Leop.-Carol., xxxiii, 1867, pp. 88, 5 Taf.)

—— Beiträge zur Kenntniss des thierischen Eies im unbefruchteten Zustande. (Spengel’s Zool. Jahrbücher, 1889. Abth. f. Anat., iii, pp. 287–432, 7 Taf.)

=Bessels, E.= Studien über die Entwicklung der sexual Drüsen bei den Lepidopteren. (Zeitschr. wissens. Zool., xvii, 1867, pp. 545–563.)

=Rajewsky.= Ueber die Geschlechtsorgane von Blatta orientalis, etc. (Nachr. d. k. Gesellschaft d. Moskauer Universität, xvi, 1875. Testes of cockroach. In Russian; for abstract, see Hoffmann u. Schwalbe, Jahresbericht, 1875, p. 425.)

=Brehm, Siegfr.= Comparative structure of the reproductive organs in Blatta germanica and Periplaneta orientalis. (Horæ Ent. Soc. Rossicæ, St. Petersburg, viii, 1880.) (In Russian, male organs only.)

=Cholodkowsky, N. A.= Ueber die Hoden der Schmetterlinge. (Zool. Anzeiger, iii Jahrg., 1880, pp. 115–117.)

—— Ueber den Bau der Testikel bei Schmetterlingen. (Zool. Anzeiger, 1880, iii, pp. 214–215.)

—— Ueber die Hoden der Lepidopteren. (Zool. Anzeiger, 1884, pp. 564–568.)

—— Ueber den Geschlechtsapparat von Nematois metallicus. (Zeitschr. f. wissens. Zool., xliii, pp. 559–568, 1885.)

=Tichomirow, A.= Ueber den Bau der Sexualdrüsen und die Entwickelung der Sexualprodukte bei Bombyx mori. (Zool. Anzeiger, iii, 1880, pp. 235–237.)

=Nusbaum, F.= Zur Entwicklungsgeschichte der Ausführungsgange der Sexualdrüsen bei den Insekten. (Zool. Anzeiger, v, 1882, pp. 637–643.)

——On the developmental history of the efferent passages of the sexual glands in insects. Lemberg, 1884 (in Czech).

=Berlese, Ant.= Ricerde sugli organi genitali degli ortotteri. (Atti della R. Acad. dei Lincei. Ser 3, xi, 1882.) (Genital organs of European Orthoptera.)

=Balbiani, G.= Le Phylloxera du chêne et le Phylloxera de la vigne. Paris, 1884, pp. 45, 11 Pls.

—— Contribution à l’étude de la formation des organes sexuel chez les insectes. (Recueil Zool. Suisse, 1885.)

=Schneider, Anton.= Die Entwicklung der Geschlechtsorgane bei den Insekten. Zool. Beiträge, Breslau, i, 1885.

=Beauregard, H.= Recherches sur les insectes vésicants. Suite. (Journal Anat. Phys., Paris, 1887, xxii Année, pp. 528–548, 1 Pl.; xxiii Année, pp. 124–163, 6 Pls.)

—— Les insectes vésicants. Paris, 1890. Chap. v, Appareil de la génération, pp. 103–159, Pls. 10–12.

=Nassonow, N.= Études morphologiques sur les Lepisma, Campodea et Podura. (Mém. Soc. Imp. Anthropologie et d’Ethn. Moscou, iii, 1887, pp. 85, 2 Pls., 68 Figs.)

—— Xenos rossii; seine Anatomie und Entwicklungsgeschichte. (Bull. de l’Université de Varsovie, 1892, pp. 74, 2 Taf.) (In Russian.)

—— Position des Strepsiptères dans le systeme selon les données du développement post-embryonal et de l’anatomie, p. 11, Warsaw, 1892.

=Grassi, B. J.= Progenitori dei Miriapodi e degli Insetti. Memoria vii, Anatomia comparata dei Tisanuri. (Atti d. R. Acad. de’ Lincei, Cl. scienc. e fis., Serie 4, iv, 1888, pp. 435–606, 5 Pls.)

=Heymons, R.= Ueber die hermaphroditsche Anlage der Sexualdrüsen beim Mannchen von Phyllodromia germanica. (Zool. Anzeiger, 1890, pp. 451–457.)

=Koschewnikoff, G.= Zur Anatomie der männlichen Geschlechtsorgane der Honigbiene. (Zool. Anzeiger, xiv, 1891, pp. 393–396.)

=Verhoeff, C.= (See p. 186.)

=Ingenitzky, J.= Zur Kenntniss der Begattungsorgane der Libelluliden. (Zool. Anzeiger, 1893, xvi Jahrg., pp. 405–407, 2 Figs.)

—— On the fauna and organization of dragon-flies of Russian Poland, 1893, 1 Pl. (In Russian.)

=Escherich, K.= Anatomische Studien über das männliche Genitalsystem der Coleopteren. (Zeitschr. f. wissens. Zool., lvii, pp. 620–641, 1894.)

=Kluge, Max H. E.= Das männliche Geschlechtsorgan von Vespa germanica. Inaug. Diss. Leipzig, 1895, pp. 1–45, 1 Taf.

=Verson, E.= La borsa copulatrice nei Lepidotteri (Atti e Mem. Accad. Sc. Lett. ed Arti, Padova, 1896, xii, pp. 369–372, 4 Pls.).

=Klapálek, Fr.= Über die Geschlechtstheile der Plecopteren, mit besonderer Rücksicht auf die Morphologie der Genitalanhänge. (Sitzungsb. k. Akad. Wissens. Wien. Math.-Naturw. Cl., cv, 1896, pp. 56, 5 Taf.)

=Fenard, A.= Recherches sur les organes complémentaires internes de l’appareil génital des Orthoptères. (Bull. Sc. France Belg., xxix, 1897, pp. 390–527, 528–533, 5 Pls.)

Consult also the Works of Ayres, Balfour, Burgess, Burmeister, Bütschli, Claus, Dzierzon, Gensch, Henking, Honert, Huxley, Kluge, Kramer, Landois, Leuckart (art. Zeugung), Leydig, Ludwig, Metschnikoff, H. Meyer, Minot, Müller, Pfitzner, Schneider, Seeliger, Scholz, Siebold, Suckow, Swammerdam, Tichomiroff, Wagner, Waldeyer, Weismann, Wheeler, v. Wielowiejski, Will, Witlaczil, and Ziegler.

b. Formation of the egg (oögenesis)

=Claus, C.= Beobachtungen über die Bildung des Insekteneies. (Zeitschr. wissens. Zool., xiv, 1864, pp. 42–54.)

=Brandt, A.= Ueber die Eiröhren der Blatta orientalis. (Mém. Acad. Imp. Scienc. de St. Petersbourg, Sér. 7, xxi, 1874, p. 30.)

—— Vergleichende Untersuchungen über die Eiröhren und die Eier der Insekten. (Nachr. d. Gesellsch. Freunde d. naturwiss. Moskau, xxiii, 1876; also xxiv, 1877, pp. 77–79.)

—— Das Ei und seine Bildungsstatte. Ein vergleichenden-morphologischer Versuch mit Zugrundelegung der Insecteneies. Leipzig, 1878.

=Kadyi, H.= Beiträge zur Vorgänge beim Eierlegen der Blatta orientalis. Vorläufige Mittheilung. (Zool. Anzeiger, 1879, pp. 632–636.) (Formation of the egg-capsules of cockroach.)

=Brass, Arn.= Das Ovarium und der Eibildung und der ersten Entwicklungsstadien bei viviparen Aphiden. Halle, 1883. (Zeits. f. Naturwiss. in Halle, Jahrg. 1882.)

—— Zur Kenntniss der männlichen Geschlechtsorgane der Dipteren. (Zool. Anzeiger, 1892, pp. 178–180.)

=Will, Ludvig.= Zur Bildung der Eies und des Blastoderms bei den viviparen Aphiden. (Arbeiten Zool. Inst. Univ. Würzburg, vi, 1882, pp. 217–258.)

=Korschelt, E.= Zur Frage nach dem Ursprung der verschiedenen Zellenelemente der Insectenovarien. (Zool. Anzeiger, 1885, pp. 581–586, 599–605.)

=Wielowiejski, H. V.= Zur Morphologie des Insektenovariums. (Zool. Anzeiger, 1886, ix Jahrgang, pp. 132–139.)

—— Zur Kenntniss der Eibildung bei der Feuerwanze (Pyrrhocoris apterus). (Zool. Anzeiger, 1885, pp. 369–375.)

=Blochmann, F.= Ueber die Richtungskörper bei Insekteneiern. (Morph. Jahrb., 1887, xii, ix 544.)

Also the writings of Leydig (p. 509).

c. On the spermatozoa

=Treviranus, G. R.= Ueber die organischen Körper des tierischen Samens und deren Analogie mit dem Pollen der Pflanzen. (Zeitschr. f. d. Physiologie, von F. Tiedemann, G. R. und L. C. Treviranus, 1835, v, pp. 136–153, 2 Taf.)

=Siebold, C. Th. E. von.= Ueber die Spermatozoen der Crustaceen, Insekten, Gasteropoden und einiger anderer wirbellosen Tiere. (Müller’s Archiv f. Anatomie, 1836, pp. 13–52, 2 Taf.)

—— Fernere Beobachtungen über die Spermatozoen der wirbellosen Tiere. (Ibid., 1836, p. 232; 1837, pp. 381–432, 1 Taf.)

—— Ueber die Spermatozoen der wirbellosen Tiere, iv. (Ibid. 1837, pp. 392–433.)

—— Lange Lebensdauer der Spermatozoen bei Vespa rufa. (Wiegmann’s Archiv f. Naturgesch., 1839, v, pp. 107, 108.)

—— Ueber die Spermatozoiden der Locustinen. (Nova Acta Acad. Leop.-Carol., 1845, xxi, pp. 249–274, 1 Pl.)

=Kölliker, A.= Beiträge zur Kenntnis der Geschlechtsverhältnisse und der Samenflüssigkeit wirbelloser Tiere, nebst einem Versuch über das Wesen und die Bedeutung der sogenannten Samentiere, Berlin, 1841, pp. 88, 3 Taf.

—— Die Bildung der Samenfaden in Bläschen als allgemeines Bildungsgesetz. (Neue Denkschr. d. allg. Schweiz. Ges., viii, 1847, pp. 28, 3 Taf.)

—— Physiologische Studien über die Samenflüssigkeit. (Zeitschr. f. wissens. Zool., vii, 1856, pp. 201–272, 1 Taf.)

=Yersin, A.= Observations sur le Gryllus campestris. (Bull. Soc. Vaudoise sc. nat., 1853, iii, pp. 128.)

=Lespès, Ch.= Mémoire sur les spermatophores des grillons. (Ann. Sc. nat., Sér. 4, iii, 1855, pp. 366–377, 1 Pl.; iv, pp. 244–249, 1 Pl.)

=Landois, H.= Entwicklung der büschelförmigen Spermatozoiden bei den Lepidopteren. (Schultze’s Archiv f. Anat. u. Physiol., 1866, pp. 50–58, 1 Taf.)

=Bütschli, O.= Vorlaufige Mitteilungen über Bau und Entwicklung der Samenfaden bei Insekten und Crustaceen. (Zeitschr. f. wissens. Zool., xxi, 1871, pp. 402–415.)

=Bütschli, O.= Nähere Mitteilungen über die Entwicklung und den Bau der Samenfaden der Insekten. (Ibid., xxi, 1871, pp. 526–534, 2 Taf.)

=La Vallette St. George, A. V.= Ueber die Genese der Samenkörper, III. Mitteilung. (Archiv f. Mikroscop. Anat., 1874, x, pp. 495–504, 1 Taf.)

—— Spermatologische Beitrage: II. Mitteilung (Ibid., 1886, xxvii, pp. 1–122 Taf.). IV. Mitteilung (Ibid., 1886, xxviii, pp. 1–13, 4 Taf.) V. Mitteilung (Ibid., 1887, xxx, pp. 426–434, 1 Taf.)

=Schneider, A.= Das Ei und seine Befruchtung, pp. 88, 10 Taf.; Arthropoden, pp. 57–68 und 79. Taf. 8–10, 1883.

=Wielowiejski, H. de.= Observations sur la spermatogénèse des Arthropodes. (Archiv Slav. de Biologie, 1886, ii, pp. 28–36.)

=Ballowitz, E.= Zur Lehre von der Struktur der Spermatozoen. (Anat. Anzeiger, i Jahrg., 1886, pp. 363–376.)

—— Untersuchungen über die Struktur der Spermatozoen, zugleiche in Beitrag zur Lehre von feineren Bau der kontraktilen Elemente. Die Spermatozoen der Insekten. I. Coleopteren. (Zeitschr. f. wissensch. Zool., 1, 1890, pp. 317–407, 4 Taf.)

—— Zu der Mittheilung des Herrn Professor L. Auerbach in Breslau über merkwürdiger Vorgänger am Sperma von Dytiscus marginalis. (Anat. Anzeiger, 1893, viii Jahrg., pp. 505–506.)

—— Die Doppelspermatozoen der Dyticiden. (Zeitschr. f. wissensch. Zool., lxvi, 1895, pp. 458–499, 5 Taf.)

=Beauregard, H.= Note sur la spermatogénèse chez la cantharide. (Compt.-rend. Soc. Biol., Paris, 1888, iv, pp. 331–333.)

=Gilson, G.= Étude comparée de la spermatogénèse chez les Arthropodes, in La Cellule, Recueil de Cytologie et d’Histologie gén., i, 1888, 8 Pl.

=Verson, E.= Zur Spermatogenesis. (Zool. Anzeiger, xii Jahrg., 1889, pp. 100–103, Fig.)

—— La spermatogenesi nel Bombyx mori. (Padova, 1889, 25 pp. und 3 Taf.)

—— Zur spermatogenesis. (Zool. Anzeiger, xii, 1889, pp. 100–103.)

—— Zur spermatogenesis bei der Seidenraupe. (Zeitschr. f. wissens. Zool., lviii, 1894, pp. 303–313, 1 Taf.)

=Henking, H.= Ueber Reductionsteilung der Chromosomen in den Samenzellen von Insekten. (Internat. Monatsschr. f. Anat. und Phys., 1890, vii, pp. 243–248.)

—— I. Untersuchungen über die erste Entwicklungsorgänge in der Eiern der Insekten. II. Ueber Spermatogenese und deren Beziehung zur Eientwickelung bei Pyrrhocoris apterus. (Zeits. wissens. Zool., xlix, 1890, pp. 503–564; li, 1891, pp. 685–736.) III. Specielles und Allgemeines. (Ibid., 1892, liv, pp. 1–274, 12 Taf.)

=Sabatier, A.= De la spermatogénèse chez les Locustides. (Comptes rend. Acad. Paris, 1890, cxi, p. 797.)

=Cholodkowsky, N.= Zur Frage über die Anfangsstadien der Spermatogenese bei den Insecten. (Zool. Anzeiger, 1894, pp. 302–304.) See also Zool. Anzeiger, 1892, p. 179.

=Auerbach, Leopold.= Ueber merkwürdige Vorgänge am Sperma von Dytiscus marginalis. (Sitz. Ber. Akad., Berlin, 1893, pp. 185–203, 2 Figs.)

—— Zu dem Bemerkungen des Herrn Dr. Ballowitz betreffend das Sperma von Dytiscus marginalis. (Anat. Anzeiger, viii Jahrg., 1893, pp. 627–630.)

=Toyama, K.= On the spermatogenesis of the silkworm. (Bull, ii, No. 3, Coll. Agric. Imp. Univ. Tokyo, pp. 125–157, 1894, 2 Pls.)

=Wilcox, E. V.= Spermatogenesis of Caloptenus femur-rubrum and Cicada tibicen. (Bull. Mus. Comp. Zool., xxvii, 1895, pp. 32, 5 Pls.)

—— Further studies on the spermatogenesis of Caloptenus femur-rubrum. (Ibid., xxix, 1896, pp. 193–202, 3 Pls.)

=Wilson, Edmund B.= The cell in development and inheritance. (New York, 1896.) Also the writings of Platner, Waldeyer.

d. On the paired genital efferent passages

=Loew, H.= Abbildungen und Bemerkungen zur Anatomie einiger Neuropterengattungen. (Linnæa Ent., 1848, pp. 345–385, 6 Taf.)

=Meinert, F.= Anatomia Forficularum, i, Kjöbenhavn, 1863, 1 Pl.

—— Om dobbelte Saedgange hos Insecter. (Naturhist. Tidsskrift, 3 Raekke, v, 1868, pp. 278–294.)

=Palmén, J. A.= Zur vergleichenden Anatomie der Ausführungsgänge der Sexualorgane bei den Insekten. (Morph. Jahrb., ix, 1883, pp. 169–176.)

—— Ueber paarige Ausführungsgänge der Geschlechtsorgane bei Insekten. Eine morphologische Untersuchung. Helsingfors, 1884, pp. 108 und 5 Taf.

=Nusbaum, J.= Zur Entwicklungsgeschichte der Ausführungsgänge der Sexualdrüsen bei Insekten. (Kosmos, Lemberg, 1884, ix Jahrg., pp. 256–266, 393–408, 462–474, 2 Taf. In Polish with résumé in German.)

=Spichardt, C.= Beitrag zur Entwickelung der männlichen Genitalien und ihrer Ausführgänge bei Lepidopteren. (Verhandl. d. naturwiss. Vereins zu Bonn, 1886, xliii Jahrg., pp. 1–34, 1 Taf.)

=Jackson, W. H.= Studies in the morphology of the Lepidoptera. I. (Zool. Anzeiger, xii Jahrg., 1889, pp. 622–626.) (See p. 389.)

=Lowne, B. Th.= On the structure and development of the ovaries and their appendages in the blow-fly (Calliphora erythrocephala). (Journ. Linn. Soc. London, 1889, xx, pp. 418–442, 1 Pl.)

See also Meinert (1897), Heymons (1897).

END OF PART I

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