Opening…

Letter XXXVII

_INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS._

SENSATION.

Having given you this full account of the external parts of insects, and their most remarkable variations; I must next direct your attention to such discoveries as have been made with regard to their Internal Anatomy and Physiology: a subject still more fertile, if possible, than the former in wonderful manifestations of the POWER, WISDOM and GOODNESS of the CREATOR.

The vital system of these little creatures, in all its great features, is perfectly analogous to that of the vertebrate animals. Sensation and perception are by the means of nerves and a common sensorium; the respiration of air is evident, being received and expelled by a particular apparatus; nutrition is effected through a stomach and intestines; the analogue of the blood prepared by these organs pervades every part of the body, and from it are secreted various peculiar substances; generation takes place, and an intercourse between the sexes, by means of appropriate organs; and lastly, motion is the result of the action of muscles. Some of these functions are, however, exercised in a mode apparently so dissimilar from what obtains in the higher animals, that upon a first view we are inclined to pronounce them the effect of processes altogether peculiar. Thus, though insects respire air, they do not receive it by the mouth, but through little orifices in the sides of the body; and instead of lungs, they are furnished with a system of air-vessels, ramified ad infinitum, and penetrating to every part and organ of their frame; and though they are nourished by a fluid prepared from the food received into the stomach, this fluid, unlike the blood of vertebrate animals, is white, and the mode in which it is distributed to the different parts of the system, except in the case of the true Arachnida, in which a circulation in the ordinary way has been detected, is altogether obscure.

In order that you may more clearly understand the variations that occur in insects, and in what respects they differ amongst themselves, and from the higher animals, in the vital functions and their organs, I shall consider them as to their organs of sensation, respiration, circulation, nutrition, generation, secretion, and muscular motion.

* * * * *

Organs of Sensation.--The nervous system of animals is one of the most wonderful and mysterious works of the CREATOR. Its pulpy substance is the visible medium by which the governing principle[1] transmits its commands to the various organs of the body, and they move instantaneously--yet this appears to be but the conductor of some higher principle, which can be more immediately acted upon by the mind and by the will. This principle, however, whatever it be, whether we call it the nervous fluid, or the nervous power[2], has not been detected, and is known only by its effects. The system of which we are speaking may therefore be deemed the foundation and root of the animal, the centre from which emanate all its powers and functions.

Comparative anatomists have considered the nervous system of animals as formed upon four primary types, which may be called the molecular, the filamentous, the ganglionic, and cerebro-spinal[3]. The first is where invisible nervous molecules are dispersed in a gelatinous body, the existence of which has only been ascertained by the nervous irritability of such bodies, their fine sense of touch, their perceiving the movements of the waters in which they reside, and from their perfect sense of the degrees of light and heat[4]. Of this description are the infusory animals, and the Polypi. The nervous molecules in these are conjectured to constitute so many ganglions, or centres of sensation and vitality[5]. The second, the filamentous, is where the nervous system consists of nervous threads radiating from the mouth, as in the Radiata, or star-fish and sea-urchins[6]. The third, the ganglionic, is where the nervous system consists of a series of ganglions connected by nervous threads or a medullary chord, placed, except the first ganglion, below the intestines, from which proceed nerves to the various parts of the body. This system may be considered as divisible into two--the proper ganglionic, in which it is ganglionic with the ganglions arranged in a series with a double spinal chord. This prevails in the classes Insecta, Crustacea, Arachnida, &c., and the improper ganglionic, in which it is ganglionic with the ganglions dispersed irregularly, but connected by nervous threads, as in the Mollusca[7]. In the fourth, the cerebro-spinal, the nervous tree may be said to be double, or to consist of two systems--the first taking its origin in a brain formed of two hemispheres contained in the cavity of the head, from which posteriorly proceeds a spinal marrow, included in a dorsal vertebral column. These send forth numerous nerves to the organs of the senses and the muscles of the limbs. The second consists of two principal ventral chords, which by their ganglions, but without any direct communication, anastomose with the spinal nerves and some of those of the brain, and run one on each side from the base of the skull to the extremity of the sacrum. This system consists of an assemblage of nervous filaments bearing numerous ganglions, from which nervous threads are distributed to the organs of nutrition and reproduction[8]. Its chords are called the great sympathetic, the intercostal, or trisplanchnic nerves[9]. While the first of these two systems is the messenger of the will, by means of the organs of the senses connects us with the external world, and is subject to have its agency interrupted by sleep or disease[10]; the latter is altogether independent of the will and of the intellect, is confined to the internal organic life, its agency continues uninterrupted during sleep, and is subject to no paralysis. While the former is the seat of the intellectual powers, the latter has no relation to them, but is the focus from whence instincts exclusively emanate: from it proceed spontaneous impulses and sympathies, and those passions and affections that excite the agent to acts in which the will and the judgement have no concern[11].

It is probable, though the above appear to exhibit the primary types of nervous systems, that others exist of an intermediate nature, with which future investigators may render us better acquainted[12]: but as our business is solely with that upon which insects in this respect have been modelled, without expatiating further in this interesting field, I shall therefore now confine myself to them.

We have before seen[13] that the nervous system of insects belongs to the ganglionic type: but it requires a more full description, and this is the place for it. It originates in a small brain placed in the head, and consisting almost universally of two lobes, sometimes extremely distinct. It is placed over or upon the œsophagus or gullet, and from its posterior part proceeds a double nervous chord, which embracing that organ as a collar dips below the intestines, and proceeds towards the anus, forming knots or ganglions at intervals, in many cases corresponding in number with the segments of the body, and sending forth nerves in pairs, the ramifications of which are distributed to every part of the frame. In the perfect insect the bilobed ganglion of the head or the brain is usually of greater volume than in the larva, and the ganglions of the spinal chord are fewer, which gives a more decided character of centricity to the whole nervous system[14]. This may be considered more particularly with respect to its substance and colour; its tunics, and parts.

I. Substance and Colour.--The nervous apparatus of insects is stated by those who have examined it most narrowly, though consisting of a cortical and medullary part, the latter more delicate and transparent than the former, to be less tender and less easy to separate than the human brain[15]. It has a degree of tenacity, and does not break without considerable tension; in general, it is clammy and flabby, and under a microscope a number of minute grains are discoverable in it, and when left to dry upon glass, it appears to contain a good deal of oil, which does not dry with the rest[16]. That of the ganglions differs from the substance of the rest of the spinal chord, in being filled with very fine aërial vessels, which are not discoverable in the latter[17]. With regard to colour, Lyonet states that the chords of the spinal marrow in the larva of the great goat-moth are of a blueish gray, and have some transparence[18]; Malpighi and Swammerdam observed that the cortical part of the ganglions of that of the silk-worm and the hive-bee had a reddish hue, while the medullary part was white[19]; Cuvier relates that the brain and the third ganglion in Hypogymna dispar, with us a scarce moth, differed in colour from all the rest, being quite white, while the others were more or less tinted, and examined under a lens appeared variegated by reddish sinuous markings, resembling blood vessels as they are seen in injected glands[20].

II. Tunics.--The coats that inclose the various branches of the nervous system in insects seem analogous to those of vertebrate animals. The first thing that strikes the eye, when these parts in a recent subject are submitted to a microscope, is a tissue of very delicate vessels, which ramify beyond the reach of the assisted sight; these are merely air-vessels or bronchiæ derived originally from the tracheæ of the animal: but besides these is an exterior and an interior tunic; the first corresponding with the dura mater of anatomists; and the other, which is the most delicate and incloses the cortical and medullary parts, with the pia mater[21].

III. Parts.--The nervous system of insects consists of the brain; the spinal marrow and its ganglions; and the nerves.

i. Brain.[22] Linné denied the existence of a brain in insects, and most modern physiologists seem to be of the same opinion. A part however, analogous to this important organ--at least in its situation, and in its emission of nerves to the principal organs of the senses, in which respect it certainly differs very materially from the upper cervical ganglion, which Dr. Virey regards as its analogue[23]--is certainly to be found in them; and as Messrs. Cuvier and Lamarck distinguish this part by the name of brain, we may continue to call it by that name without impropriety. The brain of insects, then, is distinguished from the succeeding ganglions of the spinal chord by its situation in the head, the middle of the internal cavity of which it occupies, and by being the only ganglion above the œsophagus. It is usually small, though in some cases larger than they are[24]. It consists of two lobes, more or less distinct and generally of a spherical form. In Oryctes nasicornis and Pontia Brassicæ the lobes are separated both before and behind[25]; while in the larva of Dytiscus marginalis, but not in the imago, in which there are two large hemispheres separated by a furrow, the brain is undivided[26]. Cuvier mentions the larva of a saw-fly in which this part is formed of four nearly equal spherical bulbs[27]: in the Scorpion (to judge by the figure of Treviranus[28]) the two lobes represent an equilateral triangle, the exterior angle of which terminates in several lesser spherical bulbs; in Acrida viridissima, Nepa cinerea, Clubiona atrox, and the common Louse, the lobes are pear-shaped[29].

ii. The spinal marrow and its ganglions[30]. From the posterior part of the brain of insects, but in the ground and water beetles (_Eutrech_in_a_ and _Eunech_in_a_) from its sides below[31], issue two chords which diverging embrace the œsophagus, and dipping below it and the intestines,--a situation they maintain to the end of their course,--and in their further progress uniting at intervals and dilating into several knots or ganglions, compose their spinal marrow. This part is so named, from a supposed analogy to the spinal marrow of vertebrate animals, which however admits of some degree of doubt; yet, since it mixes the functions of that organ with those of the great sympathetic nerves, the denomination is not wholly improper, and may be retained. Though this chord is usually double when it first proceeds from the brain, and surrounds the œsophagus like a collar, yet in some insects it may be called a single chord. This is the case with that of the common louse, in which Swammerdam could perceive no opening for the transmission of the part just named[32]; if he was not mistaken in this, the brain, as well as the rest of the spinal marrow in that animal, would be below the intestines; from the figures of Treviranus it should seem that the spiders, at least Clubiona atrox, are similarly circumstanced[33]; in the cheese-maggot, which turns to a two-winged fly (Tyrophaga Casei), the chord is also single, but it has a small orifice through which the gullet passes[34]. At the union of the chords in other cases below that organ, a knot or ganglion is usually formed, and an alternate succession of internodes and ganglions commonly follows to the end. The internodes also may generally be stated to consist of a double chord, though in many cases the two chords unite and become one, or are distinguished only by a longitudinal furrow, and even where they are really distinct and separable, in the body of the insect they lie close together[35]. In the rhinoceros beetle (Oryctes nasicornis) and Acrida viridissima &c. all the internodes consist of a double chord[36]; but in many other insects numerous variations in this respect occur.--Thus in the stag-beetle the last internode is single[37]; in the caterpillar of the cabbage butterfly (Pontia Brassicæ) the five first are double, and the six last single[38]; in that of the great goat-moth (Cossus ligniperda) the three first only are double, but the others terminate in a fork[39]; in the cockroaches (Blatta) the four first, in Hydrophilus piceus the three first, and in Eristalis tenax the two first only are double, the rest being all single[40]. A singular variation takes place in Hypogymna dispar; all the internodes are single, except the second, the chords of which at first are separate, and afterwards united[41]; and, to name no more, in Clubiona atrox there is only one internode, which is single, with a longitudinal furrow[42]. In some, as in the louse, the grub of Oryctes nasicornis, and the cheese-maggot, there are no internodes, the spinal marrow being formed of knots separated only by slight or deep constrictions[43].

I must next say something of the ganglions[44]. Lyonet has observed that, in the caterpillar of the great goat-moth, these in one respect differ remarkably from the chords that connect them; in the latter the air-vessels or bronchiæ only cover the outside of the tunic, while in the former they enter the substance of the ganglion, which is quite filled with their delicate and numberless branches[45]. Every ganglion may be regarded in some degree as a centre of vitality or little brain[46], and in many cases, as well as the brain, they are formed of two lobes[47]. I shall now consider them more particularly as to their station, number, and shape.

1. With regard to the first head, their station, they are most commonly divided between the trunk and abdomen; but in some cases, as in Hydrophilus piceus and Acrida viridissima, the first ganglion is in the head[48]; in others, as in the louse, the water-scorpion, and the grub of the rhinoceros-beetle, they are confined to the trunk, their functions in the abdomen being supplied by numerous radiating nerves[49]; in others again, as in the scorpion, they are all abdominal. The ganglions vary also in their situation with respect to each other. Thus in some, as in the larva of the Chamæleon-fly (Stratyomis Chamæleon), they are so near as to appear like a string of beads[50]; in that of the ant-lion (Myrmeleon) the two ganglions of the trunk are separated by an interval from those of the abdomen, which are so contiguous as to resemble the rattle of the rattle-snake[51]. In others the internodes are longer, and the ganglions occur at nearly equal intervals, as in the larva of the Ephemeræ[52]; but in the majority they are unequal in length: thus in the scorpion the three first ganglions are the most distant[53]; in the hive-bee the third and fourth[54]; and in the spider the last[55].

2. The ganglions also in different species, and often in the same insect in its different states, vary in their number. Thus in the grub of the rhinoceros-beetle the whole spinal marrow appears like a single ganglion divided only by transverse furrows[56]; in the water-scorpion there are two[57]; in the louse there are three[58]; in the rhinoceros-beetle there are four[59]; five in the stag-beetle[60]; seven in the hive-bee and some Lepidoptera[61]; eight in the grub of the stag-beetle[62]; nine in the great Hydrophilus[63]; ten in Dytiscus[64]; eleven in the grub of the great Hydrophilus[65]; twelve in the grub of Dytiscus and the caterpillars of Lepidoptera[66]; thirteen in the larva of Æshna[67]; and twenty-four in Scolopendra morsitans[68]. You must observe that, generally speaking, the number of ganglions is less in the imago than in the larva. With regard to the distribution of these knots to the different primary parts of the body, the following table will exhibit it, as far as I am acquainted with it, at one view. I omit those in which the ganglions are only in one of these parts.

Head. Trunk. Abdomen.

Acrida viridissima 1 3 6[69]

Hydrophilus piceus 1 6 2

Clubiona atrox 0 2 1

Gryllotalpa vulgaris 0 2 7[70]

Myrmeleon, Larva 0 2 8[71]

Eristalis tenax 0 3 2[72]

Apis mellifica 0 3 4

Ephemera, Larva 0 3 7

Æshna, Larva 0 6 7

3. I am next to say a few words upon the shape of the ganglions. Most commonly it approaches to a spherical figure, but in many instances, as I said before, they, as well as the brain, consist of two lobes: they are, however, seldom all precisely of the same shape. In the Dytisci, and Carabi, the last is marked with a transverse furrow, which seems to indicate the reunion of two[73]; in the stag-beetle, the first ganglion is oval or elliptical, the second hexagonal; the third and fourth shaped like a crescent, and the last like an olive[74]; in the caterpillar of the great goat-moth the first is oblong and constricted in the middle, and the seven last are rhomboidal[75]; in the great Hydrophilus the second, and in the silk-worm all the ganglions are quadrangular[76]; in Hypogymna dispar the third is heart-shaped[77]; the great ganglion which forms the spinal marrow of the cheese-maggot is pear-shaped[78]; that of the grub of the rhinoceros-beetle is fusiform[79]; and in the scorpion all the ganglions are lenticular[80]. But the most remarkable in this respect are those of a spider (Clubiona atrox): in this insect the brain sits upon a bilobed ganglion of the ordinary form, which is immediately followed without any internode by another bilobed one, terminating on each side in four pear-shaped processes or fingers, which give it a very singular appearance[81].

iii. The nerves[82] of insects, as of other animals, are white filaments running from the brain and spinal marrow to every part of the body which they are destined to animate; and their numerous ramifications, when delineated, form no unpleasing picture[83]. In the caterpillar of the goat-moth the accurate Lyonet counted forty-five pairs of them, and two single ones, making in all ninety-two nerves; whereas in the human body anatomists count only seventy-eight[84]. From the brain issue several pairs, which go to the eyes, antennæ, palpi, and other parts of the mouth: sometimes those that render to the mandibles issue from the first ganglion, as in the larva of Dytiscus marginalis, the stag-beetle, &c.[85]; those both of mandibles and palpi in the great Hydrophilus[86]; and in Blatta some which act also upon the antennæ[87].

The optic are usually the most conspicuous and remarkable of the nerves. In some insects with large eyes, as many Neuroptera, Hymenoptera, and Diptera, their size is considerable; in the hive-bee they present the appearance of a pair of kidney-shaped lobes, larger than the brain[88]; in the dragon-flies, whose brain consists of two very minute lobes, these nerves dilate into two large plates of a similar shape, which line all the inner surface of the eyes[89]; in the stag-beetle they are pear-shaped, and terminate in a bulb, from which issue an infinity of minute nerves[90]; it is probable that this takes place in all cases, and that a separate nerve renders to every separate lens in a compound eye[91]; the optic nerve in Dytiscus and Carabus is pyramidal, with the base of the pyramid at the eye and the summit at the brain[92]; in Eristalis tenax it is very large, cylindrical, and of a diameter equal to the length of the last-mentioned part, upon the side of which it is supported; it terminates in a very large bulb corresponding to the eye[93]: in Scolopendra morsitans the optic nerves divide into four branches long before they arrive at the eyes, and in this insect the nerves which render to the antennæ are so thick as to appear portions of the brain, which they equal in diameter[94]. Swammerdam discovered in the grub of the rhinoceros-beetle and in the caterpillar of the silk-worm, a pair of nerves which he regarded as analogous to the recurrent nerves in the human subject, and therefore he distinguishes them by the same name[95]: they issue from the lower surface of the brain, or that which rests on the œsophagus, and at first go towards the mouth, but afterwards turn back, and uniting form a small ganglion; this produces a single nerve, which passing below the brain follows the œsophagus to the stomach, where it swells into another ganglion, from which issue some small nerves that render to the stomach, and one more considerable which accompanies the intestinal canal, producing at intervals lateral filaments which lose themselves in the tunics of that tube[96]. Lyonet afterwards discovered these nerves in the caterpillar of the goat-moth[97], and Cuvier in other insects[98].

The other nerves which issue from the brain exhibit no remarkable features. Those which originate in the spinal marrow are mostly derived from the ganglions, and are sometimes interwoven with the muscles, as the woof with the warp in a piece of cloth[99]; those from the three or four first commonly rendering to the muscles of the legs, wings, and other parts of the trunk, and those from the remainder to the abdomen. After their origin they often divide and subdivide, and terminate in numerous ramifications that connect every part of the body with the sensorium commune. A pair of nerves is the most usual number that proceeds from each side of a ganglion[100]; but this is by no means constant, since in the louse, the hive-bee, and several other insects, only a single nerve thus proceeds[101]; and in the larva of Ephemeræ, while two pairs issue from the six first ganglions, only a single one is emitted by the five last[102]. In the spinal marrow of the rhinoceros-beetle, both larva and imago, the nerves consist of simple filaments which diverge like rays in all directions[103]: the same circumstance distinguishes the cheese-maggot, only some of the nerves appear to branch at the end[104]: in the louse, the last ganglion sends forth posteriorly three pairs of nerves which render to the abdomen[105]. Sometimes, though rarely, nerves originate in the internodes of the spinal marrow. Cuvier indeed has asserted that in invertebrate animals all the nerves spring from the ganglions, and never immediately from the spinal marrow; but Swammerdam, in describing those of the silk-worm, mentions and figures four pairs as proceeding from the four anterior internodes, excluding the first[106]; and at the same time he gives it as his opinion, that all the nerves in insects really originate from the marrow itself, and not from the ganglions, which he asserts are of a different substance, and are inclosed in the marrow for the sake of giving it greater firmness[107]. In this opinion, however, he seems singular[108]. Those remarkable nerves described by Lyonet under the name of spinal bridle (bride épinière) also take their origin, not from the ganglions, but from a bifurcation of the spinal marrow. Of these, in the caterpillar of the goat-moth there are ten, the first issuing from the bifurcation of the internode between the fourth and fifth ganglions, and the remainder from the succeeding ones. After approaching the succeeding ganglion, these nerves form a pair of branches that diverge nearly at right angles from the bridle, and producing several lesser branches, lose themselves in the sides of the animal[109]. Besides the nerves above mentioned, two generally issue from the posterior part of the last ganglion, diverging in opposite and oblique directions: some of these render to the parts of generation; and in the silk-worm, and probably other species, the innermost pair is perforated for the passage of the vasa deferentia[110].

After duly considering this general outline of the nervous system of insects, the question will continually occur to you,--is then what you have called the brain the sensorium commune of these animals, in the same manner as it is in those with warm blood? To this query a negative must be returned. In the latter, the brain is the common centre to which, by means of the nerves and spinal marrow, all the sensations of the animal are conveyed, and in which all its perceptions terminate. The nerves and spinal marrow are merely the roads by which the sensations travel; and if their communication with the brain, by any means be cut off at the neck, the whole trunk of the animal becomes paralytic, evidently proving that the organ by which it feels is the brain. This, however, is so far from being the case in insects, that in them, if the head be cut off, the remainder of the body will continue to give proofs of life and sensation longer than the head: both portions will live after the separation, sometimes for a considerable period; but the largest will survive the longest, and will move, walk, and occasionally even fly, at first almost as actively without the head, as when united to it. Lyonet informs us, that he has seen motion in the body of a wasp three days after it had been separated from the head; and that a caterpillar even walked some days after that operation; and when touched, the headless animal made the same movements as when intire[111]. Dr. Shaw has observed--an observation confirmed in Unzer's Kleine Schriften,--that if Geophilus electricus be cut in two, the halves will live and appear vigorous even for a fortnight afterwards; and what is more remarkable, that the tail part always survives the head two or three days[112]. The sensorium commune of insects, therefore, does not, as in the warm-blooded animals, reside in the brain alone, but in the spinal marrow also. It was on this account probably that Linné denied the existence of a brain in insects, regarding it merely as the first ganglion of the spine.

Cuvier and other modern physiologists, from the ganglionic structure of this organ, are of opinion that it is not the analogue of the cerebro-spinal system of vertebrate animals, but rather of their great sympathetic nerves. Indeed, considering solely the external structure of the nervous system of insects, a great resemblance strikes us between it and these nerves; for besides its general ganglionic structure, there is also in them an upper ganglion in the neck, seemingly corresponding with what we have named the brain of insects, from which the nervous chord dips to the lower part of the neck, where it forms a second ganglion, which appears to correspond with what we have considered as their second ganglion[113]. We may observe, however, that at least in one respect there is even an external resemblance between the brain of insects and that of vertebrate animals:--it most commonly consists, as has been stated, like them, of two lobes, often very distinct; a circumstance which not unfrequently distinguishes the other ganglions[114], and is not borrowed from the ganglions of the great sympathetics. With respect to the internal structure of the ganglions and spinal marrow of insects, we know little to build any theory upon, except that the internal substance of the former is filled with air-vessels; at least so Lyonet, as has been already observed, found in the goat-moth, while only the tunics of the latter are covered by them. Taking the above resemblance to the brain of vertebrates into consideration, there appears ground for thinking that the nervous system of insects, like some of their articulations[115], is of a mixed kind, combining in it both the cerebro-spinal and the ganglionic systems; and this will appear further if we consider its functions.

That learned and acute physiologist Dr. Virey, assuming as an hypothesis, that the structure of the system in question is simply ganglionic, and merely analogous to the sympathetic system of vertebrate animals, has built a theory upon the assumption, which appears evidently contradicted by facts. Because, as he conceives after Cuvier, insects are not gifted with a real brain and spinal marrow, he would make it a necessary consequence that they have no degree of intellect, no memory, judgement or free will; but are guided in every respect by instinct and spontaneous impulses,--that they are incapable of instruction, and can superadd no acquired habits to those which are instinctive and inbred[116]. This consequence would certainly necessarily follow, was their nervous system perfectly analogous to the sympathetic of warm-blooded animals. But when we come to take into consideration the functions that in insects this system confessedly discharges, we are led to doubt very strongly the correctness of the assumption. Now in these animals the system in question not only renders to the nutritive and reproductive organs, which is the principal function of the great sympathetic nerves in the vertebrates; but by the common organs maintains a connexion with the external world, and acquires ideas of things without, which in them is a function of the cerebral system: from the same centre also issue those powers which at the bidding of the will put the limbs in action, which also belongs to the cerebral system. That insects have memory, and consequently a real brain, has been before largely proved, as also that they have that degree of intellect and judgement which enables them to profit by the notices furnished by their senses[117]. What can be the use of eyes,--of the senses of hearing, smelling, feeling, &c. if they are not instructed by them what to choose and what to avoid? And if they are thus instructed--they must have sufficient intellect to apprehend it, and a portion of free will to enable them to act according to it. With regard to the assertion that they are incapable of instruction, or of acquiring new habits; few or no experiments have been tried with the express purpose of ascertaining this point: but some well-authenticated facts are related, from which it seems to result that insects may be taught some things, and acquire habits not instinctive. They could scarcely be brought from their wild state, and domesticated, as bees have been so universally, and both ants and wasps occasionally[118], without some departure from the habits of their wild state; and the fact of the corsair-bees, that acquire predatory habits before described[119], shows this more evidently: but one of the most remarkable stories to our purpose upon record, is that of M. Pelisson, who, when he was confined in the Bastile, tamed a spider, and taught it to come for food at the sound of an instrument. A manufacturer also in Paris, fed 800 spiders in an apartment, which became so tame that whenever he entered it, which he usually did bringing a dish filled with flies but not always, they immediately came down to him to receive their food[120].

All these circumstances having their due consideration and weight, it seems, I think, most probable, that as insects have their communication with the external world by means of certain organs in connexion with their nervous system, and appear to have some degree of intellect, memory, and free will, all of which in the higher animals are functions of a cerebral system, and at the same time in other respects manifest those which are peculiar to the sympathetic system,--it is most probable, I say, as was above hinted, that in their system both are united.

I must bespeak your attention to a circumstance connected with the subject of this letter, which merits particular consideration: I mean the gradual change that takes place in the nervous system when insects undergo their metamorphoses; so that, except in the Orthoptera, Hemiptera, and Neuroptera Orders, in which no change is undergone, the number of ganglions of the spinal chord is less in the imago than in the larva. There seems an exception indeed to this rule in the case of the rhinoceros-beetle, in the larva of which there is only one ganglion, while in the imago there are four[121]. But as this one ganglion occupies the whole spinal marrow, it is really of greater extent than the four of the imago; so that even in this case there is a concentration of the cerebral pulp. In some cases, as in Dytiscus marginalis, and Hydrophilus piceus[122], the imago has only one ganglion less than the larva, but more generally it loses four or five. Dr. Herold has traced the gradual changes that take place in the spinal marrow of the common cabbage-butterfly (Pontia Brassicæ), from the time that it has attained its full size to its assumption of the imago. Of these I shall now give you some account.

In the full-grown caterpillar, besides the brain there are eleven ganglions, the chords of the four first internodes being double, and the rest single: from each ganglion proceed two pairs of nerves, one from each side. In this the lobes of the brain form an angle with each other[123]. In two days the double chords mutually recede, so as to diminish the interval between the ganglions, and the single ones have become curved: thus the length of the spinal marrow is shortened about a fourth, and the fourth and fifth ganglions have made an approach to each other[124]. On the eighth day, when the insect has assumed the pupa but remains still in the skin of the caterpillar, the flexure of the internodes is much increased; the first ganglion is now united to the brain, and the fourth and fifth have joined each other, though they are still distinct; the spinal marrow has now lost considerably more than a third of its length[125]. On the fourteenth day, the internodes, except the double ones, have become nearly straight again; the fourth and fifth ganglions have coalesced so as to form one, and the sixth and seventh have each lost their pairs of nerves[126]. Shortly after this, these last ganglions have nearly disappeared, and the chords of the three first internodes have again approached each other[127]. The next change exhibited is the absorption of the first ganglion by the brain, the union of the chords of the first internode, which is now straight, the approximation of the second and third ganglions, and the enlargement of the one formed by the union of the fourth and fifth, at the expense perhaps of the sixth and seventh, which have now intirely disappeared, and in their place is a very long internode. These united ganglions retain the pairs of nerves they had when separate[128]. Just before the assumption of the imago, the direction of the lobes of the brain becomes horizontal, the second and third ganglions unite, and the internode between the third and fourth is shortened[129]. Lastly, when the animal is become a butterfly, the second and third ganglions have coalesced, and are joined to that formed by the union of the fourth and fifth; a short isthmus or rather constriction, with an orifice, being their only separation: each of these united ganglions send forth laterally four pairs of nerves[130]. In his figure, Dr. Herold has not represented the orifice for the passage of the gullet, but doubtless one exists, which for an animal that imbibes only fluid food is probably very minute. In Hypogymna dispar, we learn from Cuvier, this orifice is of that description, and of a triangular shape[131].

It can admit of no reasonable doubt that one of the principal intentions of these changes is to accommodate the nervous system to the altered functions of the animal in its new stage of existence, in which the antennæ, eyes, and other organs of the senses, as well as the limbs and muscles moving them, and the sexual organs, being very different from those of the larva, and if not wholly new, yet expanded from minute germs to their full size, may well demand corresponding changes in the structure of the nervous system by which they are acted upon.

But are these changes also concerned, as Dr. Virey conjectures, in producing that remarkable alteration which usually takes place between the instincts of the larva and imago? In order to answer this question, it will be requisite first to quote the ingenious illustration with which this able physiologist elucidates his ideas on this point. "The more readily," he observes, "to comprehend the action of instinct, let us compare the insect to one of those hand-organs in which a revolving cylinder presents different tunes noted at its surface, and pressing the keys of the pipes of the organ, gives birth to all the tones of a song: if the tune is to be changed, the cylinder must be pulled out or pushed in one or more notches, to present other notes to the keys. In the same manner let us suppose that nature has impressed or engraved certain determinations or notes of action, fixed in a determinate series in the nervous system and the ganglions of the caterpillar, by which alone she lives, she will act according to a certain sequence of operations; and, so to speak, she will sing the air engraven within her. When she undergoes her metamorphosis into a butterfly, her nervous system being, if I may so express myself, pulled out a notch, like the cylinder, will present the notes of another tune, another series of instinctive operations; and the animal will even find itself as perfectly instructed and as capable of employing its new organs, as it was to use the old ones. The relations will be the same; it will always be the play of the instrument[132]."

This illustration is doubtless at the first glance very striking and plausible: but a closer examination will, I think, show, that, as in so many other instances in metaphysical reasoning, when fanciful analogies are substituted for a rigid adherence to stubborn facts, it is satisfactory only on a superficial view, and will not stand the test of investigation; and as this is a question intimately connected with what I have advanced on the subject of instinct in a former letter, I must be permitted to go somewhat into detail in considering it.

To prove his position, Dr. Virey ought at least to be able to show that, whenever a change takes place in the instincts of insects in their different states of larva and imago, a corresponding change takes place in the external structure of the nervous chord. But what are the facts? In three whole orders, viz. Orthoptera Hemiptera, and Neuroptera, as mentioned above[133], the structure of the nervous chord is not changed; and yet we know that many tribes of these orders acquire instincts in their imago state altogether different from those which directed them in their state of larvæ. A perfect Locust, for instance, acquires the new instincts of using its wings; of undertaking those distant migrations of which so many remarkable instances were laid before you in a former letter[134]; and, if a female, of depositing its eggs in an appropriate situation. But if such striking changes in the instinct of these tribes can be effected without any perceptible alteration in the structure of the nervous chord, it is contrary to the received rules of philosophical induction to refer to this alteration the changes in the instincts of other tribes where it is found. Is it not far more probable that this alteration has in fact no connexion with the changes of instinct, but is solely concerned with those remarkable changes in the organs of sense and motion, which occur in the larva and imago states of the orders in which it is observed? In a common caterpillar, the form of the body, the legs, the eyes, and other organs of the senses, all strikingly differ from those of the imago; whereas, with the exception of the acquisition of new wings, a perfect locust differs little from its larva: so that we may reasonably expect a corresponding change, such as we find it, in the structure of the nervous chord of the lepidopterous insect, not called for in that of the neuropterous species, in which accordingly it does not take place.

This reasoning, in opposition to Dr. Virey's theory, that the changes of instinct depend on the altered structure of the nervous system, becomes greatly strengthened when we advert to the higher classes of animals, which surely in any investigation of the nature of instinct ought to be closely kept in view; for the faculty, though often less perfect in them than in insects, is still of the same kind, and may consequently be expected to follow the same general laws. In a young swallow, for example, all its instincts are not developed at once any more than in an insect. The instinct which leads it to migrate does not appear for some months after its birth, and that of building a nest still later. But we have not the slightest ground for believing that these new instincts are preceded by any change in the structure of the great sympathetic nerve, or of any other portion of the nervous system: and the same may be said as to the sexual instincts developed in quadrupeds some years subsequent to their birth. If, then, these remarkable changes in the instinct of the higher classes of animals can take place independently of any visible change in the nerves, what substantial reason can be assigned why they may not also in the class of insects?

On the whole, I think you will agree with me, that there is nothing in Dr. Virey's hypothesis which should lead me to alter the opinion I have already so strongly expressed in a former letter[135], as to the insufficiency of the mechanical theories of instinct hitherto promulgated, adequately to explain all the phenomena; and unless they do this they are evidently of small value. Such theories as I have there adverted to may often seem to be supported by a few insulated facts, but with others, far more numerous, they are utterly at variance; and, to omit many other instances, I am strongly inclined to doubt the possibility of satisfactorily explaining the variety of instincts exercised by a bee[136], or the extraordinary development of new ones in particular circumstances only[137], on any merely mechanical grounds.

And after all, even suppose it could be demonstratively shown that every instinct is as clearly dependent on secondary causes, as I have formerly admitted that some doubtless seem to be, yet what would this teach us as to the essential nature of instinct? We have advanced indeed a step; but still, as I have before observed in referring to the theories of Brown and Tucker, we have only placed the world upon the tortoise, and instinct, as to its essence, which is what we want to detect, is as mysterious as ever: just as, though we can clearly prove that the mind is acted upon by the senses, yet this throws no light upon the essential nature of the mind, which we are forced to admit is inscrutable, as if to teach us humility, and prevent our vainly fancying, that though allowed to discover some of the arcana of nature, we shall ever be able to penetrate into her inmost sanctuaries.

That Dr. Virey should regard instinct in insects as purely mechanical was the natural consequence of his denying them any portion of intellect; but his opinion cannot I think be consistently assented to, if it be the fact, as I have just shown[138], that they are not wholly devoid of the intellectual principle. Whatever is merely mechanical, must, under similar circumstances, always act precisely in the same way. An automaton once constructed, whilst its machinery remains in order, will invariably perform the same actions; and Des Cartes, when he had constructed his celebrated female automaton, imagined that he had irrefragably proved his principle, that brutes are mere machines. But if, instead of losing himself in the wilds of metaphysical speculation, he had soberly attended to facts, he would have seen that the instinct of animals can be modified and counteracted by their intellect, and consequently cannot be regarded as simply mechanical. Though the instinctive impulse of an empty stomach powerfully impel a dog to gratify his appetite, yet, if he be well tutored, the fear of correction will make him abstain from the most tempting dainties: and in like manner a bee will quit the nectary of a flower, however amply replenished with sweets, if alarmed by any interruption. The ants on which Buonaparte amused himself with experiments at St. Helena, though they stormed his sugar-basin when defended by a fosse of water, controlled their instinct and desisted when it was surrounded with vinegar[139]: and in the remarkable instance communicated to Dr. Leach by Sir Joseph Banks, the instinct of a crippled spider so completely changed, that from a sedentary web-weaver it became a hunter[140]. There is evidently, therefore, no analogy between actions strictly mechanical and instincts, which, though they may often seem to be excited by mechanical causes, are liable to be restrained or modified by the connexion of the instinctive and intellectual faculties[141]; and while we are ignorant how this connexion takes place, it is obviously impossible to reason logically on the subject.

In thus denying that any existing mechanical theory of instinct is satisfactory, I by no means intend to assert that instinct is purely intellectual. I have already given you my opinion[142], that it is not the effect of any immediate agency of the Deity; nor am I prepared to assent to the doctrine of a writer, who has in some respects written ably on the subject in question, who says, that "the Divine Energy does in reality act not immediately, but mediately, or through the medium of moral and intellectual influences upon the nature or consciousness of the creature, in the production of the various, and in many instances truly wonderful, actions which they perform[143]." The same objection applies to this as to so many other metaphysical theories, that it is not adequately supported by facts; and all theories not so supported are injurious to science in proportion as their plausibility is greater, by leading the student to relax in that observation of nature and attentive study of the instincts of animals, on which alone sound hypothesis on this subject can be ultimately founded.

I shall conclude these remarks on the nature of instinct with a few observations as to the circumstances in which insects may be supposed to be guided by this faculty, and those in which intellect seems to direct them. The bee, when it takes its flight to a field where flowers abound, is governed by intellect in the use of its senses; for these are given to it as guides: and when it arrives there, they direct it to the flowers, and enable it to ascertain which contains the treasures it is in search of; but having made this discovery, its instinct teaches it to imbibe the nectar and load its hind legs with pollen.--Again: its senses, aided by memory, enable it to retrace its way to the hive, where instinct once more impels it in its various operations. So that when we ascribe a certain degree of intellect to these animals, we do not place them upon a par with man; since all the most wonderful parts of their economy, and those manipulations that exceed all our powers, we admit not to be the contrivance of the animals themselves, but the necessary results of faculties implanted in their constitution at the first creation by their MAKER. I may further repeat, that the mere fact of being endowed with the external organs of sense, proves a certain degree of intellect in insects. For if in all their actions they were directed merely by their instinct, they might do as well without sight, hearing, smell, touch, &c. but having these senses and their organs, it seems to me a necessary consequence, that they must have a sufficient degree of intellect, memory, and judgement, to enable them advantageously to employ them.

There is this difference between intellect in man, and the rest of the animal creation. Their intellect teaches them to follow the lead of their senses, and make such use of the external world as their appetites or instincts incline them to,--and this is their wisdom; while the intellect of man, being associated with an immortal principle, and being in connexion with a world above that which his senses reveal to him, can, by aid derived from heaven, control those senses, and bring under his instinctive appetites, so as to render them obedient to the το ἡγεμονικον, or governing power of his nature: AND THIS IS HIS WISDOM.

I am, &c.

FOOTNOTES:

[1] Το Ἡγεμονικον.Return to passage

[2] See Hooper's Medical Dictionary, under Nervous Fluid, and Mr. Sandwith's useful Introduction to Anatomy and Physiology, 83.Return to passage

[3] N. Dict. d'Hist. Nat. xvi. 305--.Return to passage

[4] Cuv. Anat. Comp. ii. 362. Compare MacLeay Hor. Entomolog. 215--.Return to passage

[5] N. Dict. d'Hist. Nat. ubi. supr.Return to passage

[6] Cuv. Anat. Comp. ii. 360. MacLeay Hor. Ent. 201.Return to passage

[7] N. Dict. d'Hist. Nat. xvi. 306. MacLeay Hor. Ent. 200--.Return to passage

[8] Ibid. 307. The great sympathetic nerves in fishes are said to have no ganglions. Cuv. ubi. supr. 297.Return to passage

[9] They are called trisplanchnic because they render to the three cavities of the viscera:--viz. the thoracic, the abdominal and the pelvic. N. Dict. d'Hist. Nat. xxii. 524. 527.Return to passage

[10] In Hemiplegia, &c.Return to passage

[11] N. Dict. d'Hist. Nat. xvi. 307.Return to passage

[12] Thus in the Molluscæ there must be a great difference in this respect, since in some of these the brain or cerebral ganglion has been cut off with the head, and another reproduced. Ibid. xvi. 306. Comp. v. 391.Return to passage

[13] VOL. III. p. 29.Return to passage

[14] Comp. PLATE XXX. FIG. 1. and 6. and Carus. Introd. to Comp. Anat. i. 64.Return to passage

[15] Lyonet Anatom. 100.Return to passage

[16] Ibid. 101.Return to passage

[17] Lyonet Anatom. 100. In man and the vertebrate animals, the medullary pulp is every where homogeneous; under the microscope it appears to consist of a number of minute conglomerated globules. M. Vauquelin has analysed it, and found it to contain, of water 80 parts; of albumen in a state of demicoagulation 7·0; of phosphorus 1·50; of osmazone 1·12; of a white and transparent oily matter 4·53; of a similar red do. 0·75; of a little sulphur and some salts 5·15. N. Dict. d'Hist. Nat. xxii. 531--.Return to passage

[18] Anat. 99.Return to passage

[19] Malpigh. de Bombyc. 20. Swamm. Bibl. Nat. i. 224. a.Return to passage

[20] Anat. Comp. ii. 348.Return to passage

[21] Lyonet Anat. 100. t. iv. f. 6. Sandwith Introd. 59--.Return to passage

[22] PLATE XXI. FIG. 1. 7. 8. a.Return to passage

[23] N. Dict. d'Hist. Nat. xxii. 527.Return to passage

[24] Ibid. v. 591.Return to passage

[25] Cuv. Anat. Comp. ii. 318. Swamm. Bibl. Nat. t. xxix. f. 7. Herold Schmetterl. t. ii. f. 1-10. a.Return to passage

[26] Cuv. Ibid. 322. 337.Return to passage

[27] Cuv. Anat. Comp. 324.Return to passage

[28] Arachnid. t. i. f. 13. m.m.Return to passage

[29] Cuv. ubi supr. 343. 346. Treviranus Arachnid. t. v. f. 45. a. PLATE XXI. FIG. 8. a.Return to passage

[30] Ibid. FIG. 1. b.b.Return to passage

[31] Cuv. ubi supr. 337.Return to passage

[32] PLATE XXI. FIG. 8. Swamm. Bibl. Nat. i. 36. b.Return to passage

[33] Arachnid. t. v. f. 45.Return to passage

[34] Swamm. ubi supr. t. xliii. f. 7.Return to passage

[35] Ibid. 112. a.Return to passage

[36] Cuv. Anat. Comp. ii. 337. 343--.Return to passage

[37] Ibid. 336.Return to passage

[38] Herold Schmetterl. t. ii. f. 1.Return to passage

[39] Lyonet Anat. 98.Return to passage

[40] Cuv. ubi supr. 342. Gaede N. Act. Acad. Cæs. XL. ii. 323. Cuv. Ibid. 351.Return to passage

[41] Cuv. ubi. supr. 348.Return to passage

[42] Treviranus Arachnid. t. v. f. 45.Return to passage

[43] PLATE XXI. FIG. 7. 8. Swamm. Bibl. Nat. t. xliii. f. 7.Return to passage

[44] PLATE XXI. FIG. 7. 8. c.Return to passage

[45] Lyonet Anat. 100.Return to passage

[46] N. Dict. d'Hist. Nat. xxii. 522--.Return to passage

[47] Lyonet ubi supr. t. ix. f. 1-4.Return to passage

[48] Cuv. Anat. Comp. ii. 339. 343.Return to passage

[49] PLATE XXI. FIG. 7.Return to passage

[50] Swamm. ubi supr. t. xl. f. 5. Cuvier (ii. 332.) accuses Swammerdam of representing the spinal marrow in this grub as producing nerves only on one side; whereas he expressly states (ii. 50. b.) that a considerable number spring on each side from the eleven ganglions, but that to avoid confusion he had omitted some.Return to passage

[51] Cuv. ubi supr. 325.Return to passage

[52] Swamm. Bibl. Nat. t. xv. f. 6.Return to passage

[53] Treviran. Arachnid. t. l. f. 13. 1-4.Return to passage

[54] Swamm. ubi supr. t. xxii. f. 7.Return to passage

[55] Treviran. ubi supr. t. v. f. 45.Return to passage

[56] PLATE XXI. FIG. 7.Return to passage

[57] Cuv. Anat. Comp. ii. 346.Return to passage

[58] PLATE XXI. FIG. 8.Return to passage

[59] Cuv. ubi supr. 337.Return to passage

[60] Ibid. 335--.Return to passage

[61] Cuv. ubi supr. 348.Return to passage

[62] Ibid. 320--.Return to passage

[63] Ibid. 340--.Return to passage

[64] Ibid. 338--.Return to passage

[65] Gaede ubi supr.Return to passage

[66] Cuv. ubi supr. 323--. 327--. Mr. Bauer (Phil. Trans. 1824. t. ii. f. 1.) has figured only seven, excluding the brain, in that of the silk-worm, and Malpighi (De Bombyc. t. vi. f. 2.) ten,--Swammerdam (Bibl. Nat. t. xxviii. f. 3.) however has twelve.Return to passage

[67] Ibid. 326.Return to passage

[68] Ibid. 352.Return to passage

[69] Ibid. 343--.Return to passage

[70] Ibid. 345.Return to passage

[71] Ibid. 325--.Return to passage

[72] Ibid. 351.Return to passage

[73] Cuv. ubi supr. 339.Return to passage

[74] Ibid. 335--.Return to passage

[75] Lyonet Anat. 190.Return to passage

[76] Cuv. ubi supr. ii. 340. Malpigh. de Bombyc. t. vi. f. 2.Return to passage

[77] Cuv. Ibid. 348.Return to passage

[78] Swamm. Bibl. Nat. t. xlviii. f. 7.Return to passage

[79] Cuv. Ibid. 319.Return to passage

[80] N. Dict. d'Hist. Nat. xxx. 420.Return to passage

[81] Treviran, Arachnid. t. v. f. 45. m.Return to passage

[82] PLATE XXI. FIG. 1. 7. 8. d.Return to passage

[83] Lyonet ubi supr. t. x. f. 5. 6.Return to passage

[84] Ibid. 192.Return to passage

[85] Cuv. ubi supr. 323. 335.Return to passage

[86] Ibid. ii. 339.Return to passage

[87] Ibid. 342.Return to passage

[88] Swamm. Bibl. Nat. t. xxii. f. 6. m.m.Return to passage

[89] Cuv. ubi supr. 350.Return to passage

[90] Ibid. 335.Return to passage

[91] VOL. III. p. 495--. Lyonet. Anat. 581.Return to passage

[92] Cuv. ubi supr. 337.Return to passage

[93] Cuv. ubi supr. 351.Return to passage

[94] Ibid. 352.Return to passage

[95] Cuvier (Ibid. 319.) seems not to have been aware that Swammerdam was the first discoverer of these nerves, since he attributes their name to Lyonet.Return to passage

[96] Bibl. Nat. i. 138. b. t. xxviii. f. 2. a, b, c. f. 3. g.Return to passage

[97] Ubi supr. 578.Return to passage

[98] Ubi supr. 320. 339, &c.Return to passage

[99] Cuv. ubi supr. 349.Return to passage

[100] Lyonet Anat. t. ix. x.Return to passage

[101] PLATE XXI. FIG. 8. Swamm. Bibl. Nat. t. xxii. f. 6.Return to passage

[102] Ibid. t. xv. f. 6.Return to passage

[103] PLATE XXI. FIG. 7.Return to passage

[104] Swamm. ubi supr. t. xliii. f. 7. h, h.Return to passage

[105] PLATE XXI. FIG. 8.Return to passage

[106] In Mr. Bauer's figure (Philos. Trans. 1824. t. ii. f. 1.) no less than eighteen pairs of nerves are represented as issuing from the internodes; but it should seem as if in the specimen from which his figure was taken, several of the ganglions, perhaps from some injury received in the dissection, had become obliterated, while their nerves remained: yet still, even making allowance for these, many pairs will appear to take their origin from the spinal chord.Return to passage

[107] Comp. Cuv. Anat. Comp. ii. 102-123.; with Swamm. Expl. of PLATES XXXII. t. xxviii. f. 3. k.Return to passage

[108] Malpighi seems, however, to agree with him. De Bombyc. t. vi. f. 1.Return to passage

[109] Lyonet ubi supr. 201. t. ix. f. 1, 2. n. 1, 2. &c.Return to passage

[110] Swamm. ubi supr. 1. 139. a. t. xxviii. f. 3. s, s.Return to passage

[111] In Lesser Insecto-theol. ii. 84. note *.Return to passage

[112] Linn. Trans. ii. 8. Aristotle had observed this vitality of insects, and that that of the myriapods is greatest. Hist. Animal. l. iv. c. 7. De Respiratione, c. 3. Reptiles have also this faculty. N. Dict. d'Hist. Nat. xxix. 161.Return to passage

[113] Cuv. Anat. Comp. ii. 283--. These are named "the upper and lower cervical ganglions."Return to passage

[114] Lyonet Anat. t. ix. x. PLATE XXI. FIG. 1. a. b.Return to passage

[115] VOL. III. p. 663. 670.Return to passage

[116] N. Dict. d'Hist. Nat. ii. 47--. v. 592. xvi. 308--.Return to passage

[117] VOL. II. p. 519--. 507--.Return to passage

[118] Huber Fourmis, 260--. Reaum. vi. 172--.Return to passage

[119] VOL. II. p. 204.Return to passage

[120] N. Dict. d'Hist. Nat. ii. 279--.Return to passage

[121] Cuv. Anat. Comp. ii. 319. 337.Return to passage

[122] Ibid. ii. 322, 323--; 338. 339--.Return to passage

[123] PLATE XXX. FIG. 1.Return to passage

[124] Ibid. FIG. 2.Return to passage

[125] PLATE XXX. FIG. 3.Return to passage

[126] Herold Schmett. t. ii. f. 6.Return to passage

[127] Ibid. t. ii. f. 7.Return to passage

[128] PLATE XXX. FIG. 4.Return to passage

[129] Ibid. FIG. 5.Return to passage

[130] Ibid. FIG. 6.Return to passage

[131] Anat. Comp. ii. 348.Return to passage

[132] N. Dict. d'Hist. Nat. xvi. 313. Comp. i. 420.Return to passage

[133] See above, p. 23.Return to passage

[134] VOL. I. p. 217--.Return to passage

[135] VOL. II. p. 461.Return to passage

[136] VOL. II. p. 493.Return to passage

[137] Ibid. p. 503.Return to passage

[138] See above, p. 21.Return to passage

[139] Antommarchi's Last Days of Napoleon.Return to passage

[140] Linn. Trans. xi. 393.Return to passage

[141] VOL. II. p. 509.Return to passage

[142] VOL. II. p. 463, 5.Return to passage

[143] Zoological Journal, n^o. i. 5.Return to passage

Letter XXXVIII

_INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS CONTINUED._

RESPIRATION.

"Life and flame have this in common," says Cuvier, "that neither the one nor the other can subsist without air; all living beings, from man to the most minute vegetable, perish when they are utterly deprived of that fluid[144]." The ancients, however, not perceiving insects to be furnished with any thing resembling lungs, took it for granted that they did not breathe; though Pliny seems to hesitate on the subject[145]. But the microscopic and anatomical observations of Malpighi, Swammerdam and Lyonet, and the experiments of more modern physiologists, have incontestably proved that insects are provided with respiratory organs, and that the respiration of air is as necessary to them as to other animals. They can exist indeed for a time in irrespirable air; and immersion in hydrogen or carbonic acid gases is not, as I have often ascertained, so instantly fatal to them as it would be to vertebrate animals; but like them, they speedily perish in air altogether deprived of its oxygen, or placed in situations to which all access to this essential element is excluded. Their respiration too of atmospheric air produces the same change in it with that of the vertebrate animals, the oxygen disappearing, and carbonic acid gas being produced in its place. Boyle had long since ascertained, that when bees, flies, and other insects were placed under an exhausted receiver, they often perished[146]: and the same effect was even observed by the ancients to ensue, when their bodies were by any means covered with oil or grease, which necessarily closed the orifices of their respiratory organs[147].

But for the first series of experiments ascertaining the necessity of a supply of air to insects, and their conversion of it into carbonic acid, we are indebted to the illustrious Scheele[148]; and his experiments have been repeated and confirmed by Spallanzani, Vauquelin, and other chemists. The former found, that when caterpillars and maggots were confined in vessels containing only about eleven cubic inches of atmospheric air, though furnished with sufficient food, they soon died, and sooner when the space was more confined[149]. He ascertained too, that a larva weighing only a few grains consumed, in a given time, as much oxygen as an amphibious animal a thousand times as voluminous[150]. A male grasshopper (Acrida viridissima) in six cubic inches of oxygen lived but eighteen hours, and the female placed in eight cubic inches of atmospheric air, only thirty-six hours. The usual tests in both instances detected the conversion of the oxygen present into carbonic acid[151]. Precisely the same result was obtained by Sorg and Ellis, who, having placed a number of flies in nine cubic inches of atmospheric air, found them all dead by the third day, the oxygen intirely vanished, and a quantity of carbonic acid nearly equal in bulk produced[152].

It is ascertained too, that insects like other animals require in the process of respiration not merely oxygen, but such a mixture of it with nitrogen or azote as composes atmospheric air: for Vauquelin found that a grasshopper placed in six cubic inches of oxygen lived only half as long (eighteen hours) as another placed in eight inches of atmospheric air; its breathing was much more laborious, and it died when not more than one-twentieth of the oxygen had been converted into carbonic acid[153]. That a large quantity of oxygen penetrates all parts of insects, is evident also from the acid prevalent in the fluids of most of them, as likewise from the wonderful power of their muscles. That azote is also received, seems probable from the ammonia which has been extracted from the fluids of many, and from the rapid putrescence of these animals[154].

The mode, however, in which the respiration of insects is carried on, differs greatly from that which obtains in the higher animals. They have no lungs, no organs confined to a particular part of the body, by means of which the whole of the blood is regularly exposed to the action of the inspired air. They do not breathe through the mouth, but through numerous orifices called spiracles, and the respiratory vessels connected with these are conducted to every part of the body. In some indeed, that we have included under the denomination of insects, as the Arachnida, an approach is made to the branchial respiration of fishes.

The respiratory apparatus of insects may be considered under two principal heads:--viz. the orifices or spiracles, and other external organs by which the air is alternately received and expelled; and the internal ones, by which it is distributed. Each of these is well worthy of your attention.

I. The external respiratory organs of insects may be divided into three kinds. Spiracles; Respiratory plates; and branchiform and other pneumatic appendages.

i. Spiracles[155] (Spiracula), or breathing pores, are small orifices in the trunk or abdomen of insects, opening into the tracheæ, by which the air enters the body, or is expelled from it[156]. They may be considered principally as to their composition and substance; shape; colour; magnitude; situation; and number.

1. Composition and substance. Perhaps you may not be aware that the structure of these minute apertures is not so simple as at the first view it may seem; but when you recollect that by them the insect breathes, you will suspect that provision may be made for their opening and shutting. A spiracle therefore, speaking analogically, may be regarded in numerous cases as a mouth closed by lips. In caterpillars and many other insects, the substance of the crust where it surrounds the spiracle, is elevated so as to form a ring round it. The lips, properly speaking, are formed of a single cartilaginous piece or platform, with a central longitudinal cleft or opening, when closed often extending the whole length of the piece[157]; but in some appearing always open and circular: of the former description are those covered by the elytra in the common cockchafer; and of the latter, those that are not so covered: in some, as in the antepectoral pair of the mole-cricket, there appear to be no lips, the orifice being merely closed with hairs[158]. Though the aperture is usually in the middle of the platform, in the female of Dytiscus marginalis, it is nearer the posterior side, the anterior or upper lip being the longest. In the majority, the mouth or cleft is nearly as long as the spiracle; yet in the puss-moth (Cerura Vinula) it is shorter[159]. Some spiracles, however, are unilabiate, or have only one lip. This is the case with Gonyleptes and perhaps others[160]. The lips are usually horizontal, but sometimes they dip so as to make the spiracle appear open.

With regard to the substance of these organs, it is more or less cartilaginous, and probably elastic; the surface frequently appears to be corrugate or plaited; this is very distinctly seen in the stag-beetle and the cockchafer: in the last insect, under a powerful magnifier, we are told that the lips appear to consist of parallel cartilaginous processes, separated by a cellular web[161]. In some species of Copris the corrugations form a perplexed labyrinth; in the caterpillar of the puss-moth the plaits are so narrow as to look like rays[162]; and in some Dynastidæ the lips approach to a lamellated structure. Again, in Hydrophilus caraboides the upper lip, and in Dytiscus circumflexus, both lips seem formed of elegant plumes[163]: a similar ornament distinguishes the inner edge of the lips in the caterpillar of the great goat-moth (Cossus ligniperda) and others[164]. In the grub of the rhinoceros-beetle (Oryctes nasicornis) the margin of the lower or inner lip is decorated by pinnated rays, which enter the cellular membrane that covers the upper lip[165]: in this larva, and that likewise of the cockchafer, the two lips are formed of different substances; in the last the upper or outer one consists of a perforated cellular membrane, through which the air can pass, while the lower or inner one is a cartilaginous valve that closes the orifice[166]: in the former this valve is surmounted by a boss[167]. In the pupa of Smerinthus Populi, a hawk-moth not uncommon, and of some dragon-flies (Libellula depressa), the margin of the two lips is crenated, probably with notches which alternate, that the mouth of the spiracle may shut more accurately[168]. The substance is unusually thick in the spinose caterpillars of butterflies; and in the pupa of one, Uria Proteus, it is villose.

Under the present head I may observe, that in some cases, as in the puss-moth, and the larva of the common water-beetle (Dytiscus marginalis), the spiracles are closed by a semifluid substance, which however, according to Sprengel, is permeable to the air[169]. The animal, where these organs are furnished with lips, has doubtless, by means of a muscular apparatus, the power of opening and shutting them: this is done, we are told, by elevating and depressing, or rather by contracting and relaxing them. Sorg counted in one case (Oryctes nasicornis) twenty, and in another (Acrida viridissima) fifty, of these motions to take place in little more than two minutes[170]: but the quickness and force of this motion is not always uniform; for the same physiologist observed, that in Carabus auratus, when feeding or moving its body rapidly, the contraction of the spiracles took place at very short intervals; but when it was fasting, and its motions were slow, the intervals were longer[171]: it is probable also, that the temperature may accelerate or retard the motion. In the summer I examined a specimen of Phyllopertha horticola, that had indeed been somewhat injured, with this view: the pulses of the abdomen, which alternately rose and fell, were at about the rate of the pulse of a man in health, sixty in a minute, and the spiracles appeared to me to keep pace with this motion: later in the year, when the temperature was lower, as I was walking, I took a specimen of some grasshopper (Locusta). Upon viewing it under a lens, I observed one of the convex pectoral spiracles open and shut, and the interval between two breathings appeared nearly half a minute.

2. With regard to their shape, spiracles vary considerably. In general we may observe that the abdominal ones are usually flat, while those of the trunk are often convex[172]. Sometimes they are very narrow and nearly linear, as in many pupæ of Lepidoptera, and those in the metathorax of the sand-wasps (Ammophila) and affinities; at others they are wider and nearly elliptical, as in Lucanus and many Lamellicorn beetles: again, in Copris they are circular; in Cordylia Palmarum ovate; in Dytiscus oblong[173]; in Goerius olens lunulate; in Gonyleptes nearly of the shape of a horse-shoe[174]; and probably many other forms might be traced, if a thorough investigation with this view were undertaken.

3. The colour of spiracles will not detain us long. In the caterpillars of Lepidoptera this is often so contrasted with that of the rest of the body, as to produce a striking and pleasing effect. Thus when the body is of a dark colour, they are usually of a pale one[175]; or if the body is pale, they are dark[176], or surrounded with a dark ring[177]. This contrast is often rendered more striking by their position with regard to the partial colours that often ornament caterpillars: in those whose sides are decorated by a longitudinal stripe, the spiracles are often planted in it[178]; or just above it[179]; or between two[180]: in some hawkmoths the intermediate ones are set in white or pale spots, which gives great life to the appearance of the animal. In general, in perfect insects the most prevalent colour is buff, or reddish-yellow. In the larva of the great water-beetle these organs resemble the iris of the eye, being circular with concentric rings alternately pale and dark[181].

4. The size of spiracles varies considerably. Those in the larva last mentioned are so minute as to be scarcely visible except under a lens, while those behind the fore-legs in the mole-cricket are a full line in length, and those in the pleura of Acrocinus accentifer, a Brazilian Capricorn beetle, are more than twice as long. In the same species they are often found of different sizes;--thus the anal pairs in the water-beetle lately alluded to, I mean in the perfect insect, are much larger than the rest[182], probably that the animal may imbibe a larger quantity of air when it rises to the surface of the water, where it suspends itself by the tail. In those Lamellicorn beetles in which the terminal part of the abdomen is not protected by the elytra, the covered spiracles are the largest.

5. Under the next head, the situation of spiracles, I shall not only consider the part of the body in which they are situated, but likewise their position in the crust; to which last, as it will not detain us long, I shall first call your attention. Their position in this respect is most commonly oblique: but in the abdomen of the above water-beetle they are transverse, and in a larva I possess, probably of an Elater, they are longitudinal. In spinose caterpillars these organs are generally planted between two spines, one being above and the other below. The lateral line of the body most commonly marks their situation; but in many cases they become ventral, and in others dorsal. The most important circumstance, however, connected with the present head is their appropriation to particular segments or parts of the body, for, like the ganglions of the spinal marrow, they are distributed to almost every segment. Let us take a summary view of their arrangement in this respect.

No insect has any spiracle in the head; but in caterpillars and many other larvæ there is a pair in the first segment of the trunk. This is also to be found in the other states, but is not easily detected in the pupæ of Lepidoptera: in the Coleoptera order, in the grub of the Lamellicorn beetles, it is extremely conspicuous, and planted in the side of the first segment[183]; in other Coleopterous grubs it is not so readily found, but probably its station is somewhere behind the base of the arms, where it is very visible in that of the Staphylinidæ. In the imago of insects of this order, this antepectoral spiracle has been overlooked, and indeed is not soon discovered: to see it clearly, the manitrunk should be separated from the alitrunk; and then if you examine the lower side of the cavity, you will see a pair of, usually, large spiracles planted just above the arms, in the ligament that unites these two parts of the trunk to each other: in the common rove-beetle, however, (Goerius olens)you may easily see it without dissection[184]. In the Orthoptera it is situated behind the arms, as in Gryllotalpa: or between them and the prothorax, as in Blatta: in the Hemiptera and Neuroptera probably the situation is not very different. In the Lepidoptera this pair of spiracles is planted just before the base of the upper or primary wings[185]: a similar situation, I suspect, is appropriated to it in the Trichoptera, but covered by a tubercle or scale. Something similar has been noticed by M. Chabrier, in the same situation and circumstances, in the collar of Hymenoptera[186]. In numerous Diptera this breathing pore is planted on each side between the collar and the dorsolum above the arms[187], and in Hippobosca in the collar itself[188].

In Lepidopterous, Coleopterous, and some other larvæ, the two segments of the body corresponding with the alitrunk in the perfect insect, are without spiracles, neither have they in this state, though pneumatic organs have been discovered[189], any real ones in that part: but not so the remaining orders, all of which have these organs in that section of the trunk. To begin with the Orthoptera:--in Blatta there seems to be a long narrow one behind the intermediate leg; in the Gryllotalpa there is one in the posterior part of the pleura; and in Locusta, above both the intermediate and hind legs[190]. It is probable, that in general those that have no spiracles in the manitrunk have four in the alitrunk, which seems the natural number belonging to the trunk. In many of the Heteropterous Hemiptera in the parapleura there is an open spiracle without lips[191], to which, as in that beautiful bug Scutellera Stockeri, a channel sometimes leads. The space in which this spiracle is planted in other genera of bugs (Pentatoma &c.) is covered with a kind of membranous skin, often much corrugated[192]. In the aquatic insects of this section, and many terrestrial ones, as Reduvius, &c. this spiracle is obsolete. There is another circumstance, possibly connected with their respiration, relating to many of the bugs, which may be mentioned here. If you examine Pentatoma rufipes, a very common one, you will find between the scapula and parapleura a long orifice or chink; this upon a closer inspection, under a good magnifier, you will see completely filled with minute stiff hairs or bristles, which fringe the posterior margin of the scapula[193]. In a Brazilian species of Lygæus (sexmaculatus K. M. S.) with incrassated posterior thighs, these hairs are replaced by lamellæ which have the aspect of gills. A red, vertical, convex spiracle, with its orifice towards the head, and terminating posteriorly in a kind of conical sac, is situated towards the hinder part of the pleura in the giant water-scorpion (Belostoma grandis[194]); this seems analogous to one lately mentioned in the mole cricket. In the other section of this Order it is not easy to decipher the parts of the under side of the alitrunk. In Fulgora, Cicada, and many others of its genera, there appears to be more than one opening into the chest; but whether they are of a pneumatic nature or not, can only be ascertained by an inspection of the living animal. There is a very visible spiracle over each of the four last legs of the Libellulina[195], but in the remainder of the Neuroptera Order they have eluded my search. In the Hymenoptera and Diptera they are nearly in the same situation, being placed behind the wings on each side of the metathorax; in the latter Order with the poiser near them on the inner side[196]: in this also, the spiracles of the trunk are without lips, except in the larvæ, but are often merely an orifice, sometimes fringed with hairs; this is particularly conspicuous in Syrphus, in which these orifices are very large, and in some species closed by an elegant double fringe of white hairs. This is doubtless to prevent the entrance of any particles of dust or the like.

We are next to consider the situation of the spiracles of the abdomen: these which are supposed to be appropriated exclusively to inspiration, are usually more numerous than those of the trunk, by which it is probable that expiration is performed, and have principally attracted the notice of Entomologists: they are either dorsal, lateral, or ventral. In Dytiscus, Copris, &c. amongst the beetles, all the spiracles are dorsal; in the larvæ of Coleoptera and Lepidoptera they are lateral; and in the Heteropterous Hemiptera they are usually ventral: in Dynastes they are commonly found of all three descriptions;--the three first being dorsal, the two next lateral, and the last pair ventral[197]. In some instances, as in Perga Kirbii, and probably other Hymenoptera, these organs are planted in that portion of the dorsal segment which turns under, as was observed in a former letter[198], and becomes ventral. Generally there is a pair of spiracles to each segment, and in those insects that have a hypochondriack joint[199] there is often a spiracle in it. The last segment of the abdomen is always without these orifices, as is the basal one in Velia, Ranatra, and some other bugs. A singular anomaly distinguishes the Libellulina: they appear to have no abdominal spiracles[200], yet I have seen the abdomen of Libellula depressa when reposing, contract and dilate alternately, from whence it follows that this part is concerned in respiration. Sprengel says that the larvæ in this tribe have seven or nine on each side[201], and Reaumur speaks of them as discoverable in the pupa[202]. I have carefully examined the pupa-skin of most of the genera of Libellulina, under a powerful magnifier, but have not succeeded in discovering any thing like these organs in the abdomen. The Ephemera and probably the other Neuroptera have abdominal spiracles[203]. M. Latreille observed one on each side of the base of the scale on the footstalk of the abdomen in ants[204]. Generally the abdominal spiracles may be described as planted in the crust of the insect; but in many cases their station is in the membranous folds, which I have therefore named the pulmonarium, that sometimes separate the dorsal from the ventral segments: these folds allow of a considerable distention of the abdomen, which is probably necessary when all the air-vessels are full. In a gravid Ichneumon I once saw it enlarged to more than twice its natural size by means of this membrane, through which the eggs were distinctly visible.--Before I bid adieu to this subject, I must say a few words upon the situation of the organs in question in the myriapods. In Iulus, in each segment is a pair of orifices which have usually been regarded as spiracles, but M. Savi found that these orifices opened into vesicles containing a fetid fluid, and upon a very close examination he discovered the real spiracles above the base of the legs, in connexion with tracheæ[205]. In some of the larger species of Scolopendræ large open spiracles in the same situation are extremely visible[206]. Cermatia presents a singular anomaly:--a single series of spiracles of the usual form, each planted in a cleft of the posterior margin of the dorsal scuta, runs along the back of the animal[207]: unless we may suppose that, like the seeming spiracles of Iulus just mentioned, these are merely orifices by which it covers itself with some secretion.

6. A few words upon the number of spiracles.--If you examine the common dog-tick (Ixodes Ricinus), you will find only one of these organs on each side of the abdomen[208]; the Libellulina, as we have seen, have only four, all in the trunk; in the Dynastidæ, Melolontha, and the larva of Dytiscus, there are fourteen; sixteen in the Copridæ; eighteen in Dytiscus, and probably the majority of Coleoptera, both larva and imago, and Lepidoptera; and a pair to each segment except the last, in the Myriapods.

ii. Respiratory plates (Respiratoria). The nearest approach to spiracles is made by those remarkable plates that are found in such larvæ of Diptera, as in that state inhabit substances that might impede or altogether stop the entrance or exit of the air by the ordinary spiracles, such as dead or living flesh, dung, or the like. The CREATOR therefore, as he has seen it good for wise reasons[209] to commission certain insects to feed on unclean food, has fitted them for the offices that devolve upon them, and has placed their orifices for breathing in plates at each extremity of the body. There are usually two of these plates at the head, and two at the tail. In the grub of the common flesh-fly (Sarcophaga carnaria), at the junction of the first segment of the body with the second, two of these plates are planted, which are concave and circular, with a denticulated margin; in the cavity near the lower side is a round spiracle. These plates the animal can withdraw within the body, so as to prevent this spiracle from being stopped up by any greasy substance[210]. The posterior extremity of this grub is truncated, and has a large and deep cavity surrounded by several fleshy prominences: at the bottom of this are two oval brown plates, in each of which are three oval spiracles, placed obliquely: by the contraction of the fleshy prominences, this cavity also can be closed at the will of the animal[211]. In some cases, several stiff rays or spines replace the prominences[212]. In Echinomyia grossa and others the anal plates appear not to be perforated, being surmounted only by a central boss[213]; but this, most probably, as in the case of Œstrus Ovis[214], is a valve that closes the respiratory orifices. In the gad-fly of the ox (Œ. Bovis) there are no plates at the anterior extremity of the body; but those planted in the other end are very remarkable, and demand particular attention. Each is separated by a curved line into two unequal portions; the smallest of which is contiguous to the convex belly, and the largest to the concave back of the animal. This last is distinguished by two hard, brown, kidney-shaped pieces, a little elevated with the concave sides turned towards each other: in this sinus is a single, small, white spot, which appears to be a spiracle: in the smallest portion are eight minute circular orifices, arranged in a line[215]. As the only communication which this grub has with the atmosphere is at its anal extremity, it has no occasion for respiratory organs at the other. The gad-fly of the horse (Gasterophilus Equi, &c.) which has no communication at all with the external air, breathing that which is received into the stomach, has these plates at both ends of the body.

iii. Respiratory Appendages[216]. These may be divided into two kinds; those by which the animal has immediate communication with the atmosphere, and those by which it extracts air from water.

1. To begin with the first. These are often found in insects which, during their two first states, live in the water. No better example, nor one more easy to be examined, of this structure, can be selected, than the gnat (Culex). You must have occasionally observed in tubs of rain-water, numerous little wriggling worm-like animals, which frequently ascend to the surface; there remain a while, and then bending their head under the body rapidly sink to the bottom again. These are the larvæ of some species of the genus just named; and if you take one out of the water and examine it, you will perceive that it is furnished near the end of its body with a singular organ, which varies in length according to the species, and forms an angle with the last segment but one[217]. The mouth of this organ is tunnel-shaped, and terminates in five points like a star; and by this it is usually suspended at the surface of the water, and preserves its communication with the atmosphere: in its interior is a tube which is connected with the tracheæ, and terminates in several openings, visible under a microscope, at the mouth of the organ. The points or rays of the mouth when the animal is disposed to sink in the water, are used to close it, and cut off its communication with the atmosphere. When the animal is immersed, a globule of air remains attached to the end of the tube, so that it is in fact of less specific gravity than that element, and it is not without some effort that it descends to the bottom; but when it wishes to rise again, it has only to unclose the tube, and it rises without an effort to the surface, and remains suspended for any length of time. Its anal extremity is clothed with bunches of hairs, which are furnished with some repellent material which prevents their becoming wet[218]: it is this repellent quality that probably causes a dimple or depression of the surface, which if you look narrowly you will discover round the mouth of the tube[219].

When the gnat undergoes its first change and assumes the pupa, instead of a single respiratory appendage it is furnished with a pair, each in shape resembling a cornucopia, and, what is remarkable, placed near the opposite extremity of the body, for they proceed from the upper side of the trunk[220]. By these tubular horns, which Reaumur compares to asses' ears[221], they respire, and are suspended at the surface.

Other respiratory tubes or horns are more complex. The rat-tailed grub of a fly (Helophilus pendulus), like the gnat, breathes by a tube: but as if the CREATOR willed to show those whose delight it is to investigate his works, by how many varying processes he can accomplish the same end, this respiratory organ is of a construction totally different from that we have been considering. It is not fixed to the side of the tail, but is a continuation of the tail itself, and is composed of two tubes, the inner one, like the tube of a telescope, being retractile within the other[222]. The extremity, which is very slender, and through which the air finds admission by a pair of spiracles, terminates in five diverging hairs or rays, which probably maintain it in equilibrio at its station at the surface[223]. As these larvæ seek their food amongst the mud at the bottom of shallow pools, in which they are constantly employed, they require an apparatus capable of being lengthened or shortened, to suit the depth of the water, that they may maintain their necessary communication with the atmosphere; and for this purpose a single tube would not have been sufficient: therefore PROVIDENCE has furnished them with two, and both are extremely elastic, consisting of annular fibres, so as to admit their being stretched to an extraordinary length. Reaumur found that these animals could extend their tails to near twelve times their own length. The mechanism by which the terminal piece is pushed forth or retracted, is very curious, though extremely simple. Two large parallel tracheæ, the direction of which is from the head[224] of the grub to its tail, occupy a considerable portion of its interior: near the origin of the tail, where they are very ample, they suddenly grow very small, so as to form a pair of very slender tubes, but so long that, in order to find room in a very contracted space, they form numerous zigzag folds attached to the terminal tube; when this issues from the outer tube they consequently begin to unfold, and when it is intirely disengaged, they are become quite straight and parallel to each other. Reaumur has figured them as being united at the base of the inner tube[225]; most probably, however, they do not here stop short, but, as in other instances, proceed to the end, and terminate in the two spiracles mentioned above: he conjectures that when the animal has occasion to push forth its respiratory apparatus, it injects into these vessels part of the air contained in the body of the tracheæ, which of course would cause them to unfold and push forth the tube[226]. When this insect assumes the pupa, instead of its anal respiratory organ it has four respiratory horns in the trunk near the head[227].

The larva of the chamæleon-fly (Stratyomis Chamæleon) is furnished with a respiratory organ of a still different and more elegant structure, exhibiting some resemblance to the tentacula of what are called sea anemones. In this larva the last joint of the body is extremely long, and terminates in an orifice to receive the air, which is surrounded by a circle of about thirty diverging rays, consisting of beautifully feathered hairs or plumes[228]. This apparatus serves the same purpose with that above described of the larva of the gnat. The feathery hairs are so prepared as to repel the water, and thus to suspend the animal by its tail at the surface, and preserve a constant access of air. When it has occasion to sink, it turns these hairs in and shuts the orifice, carrying down with it an air-bubble that shines like quicksilver, and which Swammerdam conjectures enables it again to become buoyant when it wants to breathe[229].

In the red aquatic larva of a small gnat (Chironomus plumosus) there are two anal respiratory subcylindrical horns, with the orifice fringed with hairs[230]; and in another gnat Reaumur discovered four[231]. The larva of Tanypus maculatus, whose remarkable legs I formerly noticed[232], exhibits in the interior of its trunk two long, oval, opaque bodies, which De Geer conjectures may be air-reservoirs; these, when the animal assumes the pupa, according to every appearance become external, and are placed on the back, precisely where the respiratory horns of aquatic pupæ are usually situated,--they appear to terminate in a transparent point[233]. The pupa of a Tipula observed by Reaumur, instead of two has only one of these respiratory organs, in the form of a very fine hair proceeding from the anterior end of the trunk, and considerably longer than the animal itself[234].

It is observable that aquatic insects that come to the surface of the water for air, receive it at the anus, often carrying it down with them as a brilliant bubble of quicksilver. This is generally done by means of spiracles in perfect insects, but in the water-scorpion tribe in that state respiration is by means of a long hollow tube, consisting of two concavo-convex pieces which apply exactly to each other. This is found in both sexes, and therefore cannot be an ovipositor, as some have thought[235].

These respiratory organs, however, are not invariably confined to aquatic larvæ and pupæ, for those of some aphidivorous flies have anal ones, and the pupa of Dolichopus nobilitatus, or a fly nearly related to it, which is terrestrial, has likewise a pair of long sigmoidal ones on the back of the trunk[236]. The pupa also of the rat-tailed larva just noticed as having four horns, resides under the earth, the insect being only aquatic in its grub state.

2. I am next to consider those respiratory appendages by which aquatic insects, since they do not come to the surface for that purpose, appear to extract air for respiration from the water; so that they may be looked upon in some degree as analogous to the gills of fishes: there is, however, this difference between them--in fishes, the blood is conveyed in minute ramifications of the arteries to the surface of the branchial laminæ, through the membranes of which they abstract the air combined with the water; but as insects have no circulation, the process in them must be different, and their branchiform appendages may be regarded as presenting some analogy rather than any affinity to those of fishes. The first approach to this structure is exhibited by the pupa of a gnat lately mentioned (Chironomus plumosus); for on each side of the trunk this animal has a pencil consisting of five hairs elegantly feathered, which, when they diverge, form a beautiful star; its anus also is furnished with a fan-shaped pencil of diverging hairs[237].

On most of the abdominal segments of the larvæ and pupæ of the Trichoptera are a number of white membranous floating threads, arranged in bundles, four on each segment, two above and two below, and traversed longitudinally by several air-vessels or bronchiæ, which run in a serpentine direction, growing more slender as they approach the extremity, and in some places sending forth very fine ramifications,--these are their respiratory organs[238]. The caterpillar also of a little aquatic moth (Hydrocampa stratiotata) at first sight appears to be covered on each side with hairs, but which examined under a microscope are found to be branching flattish filaments, each furnished with tubes from the tracheæ. These caterpillars have also the semblance of spiracles, but apparently found in the usual situation[239]. The larva of a little beetle often mentioned in my letters (Gyrinus Natator), is furnished on each side of every abdominal segment with a long, hairy, slender, acute, conical process, of the substance of the segment, through each of which an air-tube meanders; the last segment but one has four of these processes, longer than the rest[240].

Laminose or foliaceous respiratory appendages distinguish the sides of the abdomen of the larvæ and pupæ of the Ephemeræ, whose history you found so interesting[241]. In them these organs wear much the appearance of gills. In the different species they vary both in their number and structure. With regard to their number, some have only six pair of them, while others have seven. In their structure the variations are more numerous, and sometimes present to the admiring physiologist very beautiful forms[242]. They usually consist of two branches, but occasionally are single, with one part folding over the other, as in one figured by Reaumur, which precisely resembles the leaf of some plant, the air-vessels or bronchiæ in connexion with the tracheæ branching and traversing it in all directions, like the veins of leaves[243]. The double ones differ in form. In the larva and pupa of Ephemera vulgata there are six of these double false gills on each side of the abdomen, the three last segments being without them; each branch consists of a long fusiform piece, rather tumid and terminating in a point, which is fringed on each side with a number of flattish filaments, blunt at the end. An air-vessel from the trachea enters the gill at its base; is first divided into two larger branches, each of which enters a branch of the false gill. These branches send forth on each side numerous lesser ramifications, one of which enters each of the filaments[244]. In another species (E. vespertina) each false gill presents the appearance of a pair of ovate leaves with a long acumen, and the air-vessels represent the midrib of the leaf, with veins branching from it on each side[245]; and, to name no more, in E. fusco-grisea, one branch represents the leaf of a Begonia, the sides not being symmetrical, with its veins, while the other consists only of numerous branching filaments[246]. In other aquatic larvæ, as in that of the common May-fly (Sialis lutaria), these appendages consist of several joints[247].

By the above apparatus these aquatic animals are enabled to separate the air from the water, as the fish by their gills; but how this separation is made has not been precisely explained. The false gills in many species are kept in continual and intense agitation. When they move briskly to one side, Reaumur conjectures they may receive the air, and when they return back they may emit it[248]. This brisk motion probably disengages it from the water. In many species, when in repose, they are laid upon the back of the animal[249], but in others they are not[250].

The larvæ of the Agrionidæ appear to respire like those of the Ephemeræ, &c. by means of long foliaceous laminæ or false gills filled with air-vessels; but instead of being ventral, they proceed from the anus. They are three in number, one dorsal and two lateral, perpendicular to the horizon, of a lanceolate shape, beautifully veined, with a longitudinal middle nervure, from which others diverge towards the margin, which are probably bronchiæ. They are used by the animal, which swims like a fish, as fins, but it does not appear to imbibe the water like the other Libellulinæ, nor to propel itself by ejecting it,--a circumstance which furnishes an additional argument for the more received opinion, that this action in them is for the purpose of respiration as much as for motion[251].

The larvæ and pupæ of the Libellulinæ, receive the water and air that they respire by a large anal aperture, which is closed at the will of the animal by five hard, moveable, triangular, concavo-convex pieces, all very acute and fringed with hairs. These pieces are placed so that there is one above, which is the largest of all; one on each side, which are the smallest, and two below; when these are closed they form together a conical point[252]. Sometimes only three of these pieces are conspicuous[253]: three other cartilaginous pieces, resembling the valves of a bivalve shell, close the passage within the pointed pieces[254]. At this orifice the water is received; and when, by an internal process to be described afterwards, it has parted with its oxygen, is again expelled.

Under this head I shall mention a fact which may be connected with respiration of the insects concerned. In dissecting a moth related to Catocala Pronuba, but I do not recollect the particular species,--at the base of the abdomen of the male I discovered two bunches of long fawn-coloured parallel hairs, planted each in an oval plate, plane above, but below convex and fleshy; while the plates remained attached to the insect, they appeared to have a distinct pulsation. The hairs, which are about half an inch long, diverge a little, and form a tuft not very unlike a shaving-brush[255]. I have not since met with this species, but I have preserved the brush and scale. Somewhere in Bonnet's works, but I do not recollect where, I have since found mention of a similar fact in another moth.

II. Having considered the external respiratory organs of insects, by which the air is received, we are next to consider the internal ones, by which it is distributed. These are gills; tracheæ and bronchiæ; and sacs or pouches[256].

i. Gills (Branchiæ[257]). Having lately described what may be denominated false gills, or branchiform appendages, I shall now call your attention to what may be denominated true ones, which are peculiar to the Arachnida Class: but what is remarkable, the animals that breathe by them are very rarely inhabitants of the water, so that their functions cannot be perfectly analogous to those of fishes.

In the Scorpion, on each side of the four first ventral segments a spiracle may be discovered, which has no lip as in other insects, but is merely a circular orifice. These orifices do not lead to tracheæ or vesicles, but to true gills, which are situated below a muscular web which clothes the internal surface of the crust. Each gill consists of many semicircular very thin plates, of a dead milky white, which are connected together at the dorsal end like the leaves of a book. There appear to be more than twenty of these leaves, which when strongly magnified look transparent and destitute of any vessels. Each gill is fastened at the back to the spiracle[258]. In the spiders also, gills are discoverable, but differently circumstanced. On the under side of the abdomen, near the base, is a transverse depression, on each side of which is a longitudinal opening leading to a cavity, which is covered from above by a cartilaginous plate. In this cavity is situated a true gill, which is white, triangular, and covered with a fine skin; the leaves of this gill are far more numerous and much finer and softer than those of the gills of the scorpion. On account of their softness they have often the appearance of a slimy skin; but their laminated structure shows itself very clearly in old specimens, and in such as have been immersed in boiling water[259].

ii. Tracheæ and Bronchiæ[260]. Parallel with each side of the body of most insects and extending its whole length, run two cylindrical tubes[261], which communicate with the spiracles[262], and from which issue, at points opposite to those organs, other tubes which ramify ad infinitum, and are distributed to every part of the body[263]. The first of these tubes are called the tracheæ and the latter the bronchiæ. This structure appears, however, not to be universal: it is to be found in caterpillars and many Dipterous larvæ; but in that of the rhinoceros-beetle and other Lamellicorns, the bronchiæ branch directly from the spiracle, the bottom or interior mouth of which is lined by a membrane from which they proceed[263]: something similar has been observed to take place in many insects in other states, as the common cockchafer[264]; in the pupa of Smerinthus Populi[265]; in the Cicadæ[266]; in the Locust tribe[267]; and many others. In the Cossus, or larva of the great goat-moth, the trachea commences with the first spiracle, and finishes a little beyond the last, after which it diminishes considerably in diameter, and terminates in several branches or bronchiæ, which proceed to the anal extremity of the body[268]. The bronchiæ which originate from the tracheæ in the vicinity of each spiracle, may be considered as consisting in general of three packets;--dorsal ones, which are distributed to the back and sides of the animal; visceral ones, which enter the cavity of the body, and are lost amongst the viscera and the caul; and ventral ones, which dipping from the tracheæ overrun the lower part of the sides and belly[269].

The tracheæ and bronchiæ consist of three tunics[270]: the first or external one is a thickish membrane, strengthened by a vast number of fibres or vessels, which form round it a number of irregular circles; the second is a membrane more thin and transparent, without a vascular covering[271]; the third is formed of a cartilaginous thread running in a spiral direction, which may be easily unwound[272]. This structure gives a great elasticity to these organs, so that they are capable of considerable tension, after which they return to their usual length[273]. The Bronchiæ are cylindrical or slightly conical, insensibly diminishing in size as they leave the trunk, in which they originate. In larvæ, after losing their spiral fibre, they appear to terminate in membrane, but in perfect insects they pass into vesicles[274]. In the Cossus the trachea is flattened, and in every segment, except the first and two last, is bound by a fleshy cord four or five times as thick as its threads. Where this occurs, there is a slight constriction,--probably here is a sphincter, by the contraction of which Lyonet supposes the trachea may be shut when it is necessary to stop the passage of the air, and direct it to any particular point[275]. The structure here described is admirably adapted for the purpose it is intended to serve; for had these vessels been composed of membrane, they could not possibly have been prevented from collapsing; but by the intervention of a spiral cartilaginous thread this accident is effectually guarded against, and the necessary tension of the tubes provided for. However violent the contortions of the insect, however small the diameter of these vessels, they are sure to remain constantly open, and pervious to the air. And by this circumstance they may be always distinguished from the other organs of the animal, and likewise by their pearly or silvery hue, for from being constantly filled with air, these tubes, when viewed under a powerful microscope in a recently dissected insect, present a most beautiful and brilliant appearance, resembling a branching tree of highly polished silver or pearl:--though sometimes they are blue, or of a lead colour, and sometimes assume a tint of gold. In the dead insect the larger tubes soon turn brown, but the finer ones preserve their lustre several weeks[276]. The ramifications of the tracheal tree may be seen without dissection through the transparent skin of the common louse[277] and most of the thin skinned larvæ.

You will not expect to view in this way the minuter ramifications of the bronchiæ, when I have mentioned their number and incredible smallness. Nothing but the scalpel of a Lyonet and the most powerful lenses are adequate to trace the extremities of these vessels; and even with every help, they at last become so inconceivably slender as to elude the most piercing sight. That illustrious anatomist found that the two tracheæ of the larva of the Cossus gave birth to 236 bronchial tubes, and that these ramify into no less than 1336 smaller tubes, to which, if 232, the number of the detached bronchiæ, be added, the whole will amount to 1804 branches[278]. Surprising as this number may appear, it is not greater than we may readily conceive to be necessary for communicating with so many different parts. For, like the arterial and venous trees, which convey and return the blood to and from every part of the body in vertebrate animals, the bronchiæ are not only carried along the intestines and spinal marrow, each ganglion of which they penetrate and fill, but they are distributed also to the skin and every organ of the body, entering and traversing the legs and wings, the eyes, antennæ, and palpi, and accompanying the most minute nerves through their whole course[279]. How essential to the existence of the animal must the element be that is thus anxiously conveyed by a thousand channels, so exquisitely formed, to every minute part and portion of it! Upon considering this wonderful apparatus we may well exclaim, This hath GOD wrought, and this is the work of his hands.

Though in general there is only a pair of tracheæ, yet in some larvæ a larger number have been discovered. In those of the Libellulinæ there are six. According to M. Cuvier, Reaumur, who mentions only four, overlooked the two lateral ones that are connected with the spiracles[280]. The reason of this and other parts of their internal structure I shall explain under the next head. In the grub of the gad-flies of the horse (Gasterophili,) Mr. B. Clark discovered eight longitudinal tracheæ,--six arranged in a circle and two minute ones, which appeared to him to terminate in a pair of external nipples (spiracles) in the neck of the animal[281]. This is a singular anomaly, as the other Œstridæ have only a pair of tracheæ[282].

iii. Respiratory Sacs or Pouches. Besides their tracheæ and bronchiæ, many insects are furnished with reservoirs for the air, under the form of sacs, pouches, or vesicles. These are commonly formed by the bronchial tubes being dilated at intervals, especially in the abdomen, into oblong inflated vesicles; from which other bronchial tubes diverge, and again at intervals expand into smaller vesicles, so as to exhibit no unapt resemblance--as Swammerdam has observed with respect to those of the rhinoceros-beetle--to a specimen of Fucus vesiculosus. Cuvier compares them in the Lamellicorn beetles in general to a tree very thickly laden with leaves[283]; and Chabrier observes that they particularly occur in the intestinal canal[284]. This structure of the pulmonary organs may be seen also in the common hive-bee, and other Hymenoptera; but the vesicles are less numerous, and those at the base of the abdomen much larger than the rest[285]. These vesicles, by a very rough dissection, may be distinctly seen in the abdomen of the cockchafer, which appears to be almost filled with them. Not being composed of cartilaginous rings like the air-tubes, but of mere membrane, if a pin pierces one, the air that inflates it escapes, and it collapses. In the larva of a little gnat (Corethra culiciformis) the tracheæ appear to proceed from a pair of oblong vesicles of considerable size[286] in the trunk, and towards the anus they form two other smaller ones[287],--upon piercing the former, De Geer observed a considerable quantity of air to make its escape[288]. Another species, probably of the same genus, described by Reaumur, exhibits something similar[289].

But one of the most remarkable structures, in this respect, is to be seen in the larva and pupa of the dragon-flies (Libellulina). I have before noticed the number of their tracheæ, but I shall here describe their whole internal respiratory apparatus. I must observe that Reaumur, Cuvier, and most modern writers on the physiological department of Entomology, have affirmed that they respire the water, and that they receive it for that purpose at their anal extremity: but M. Sprengel, from having observed in the larvæ abdominal spiracles, is unwilling to admit this as a fact[290]; and De Geer also seems to hesitate upon it, especially as he discovered that the animal seemed to absorb the water to aid it in its motions[291]. But when we consider that it is by the action of a pneumatic apparatus that the absorption and expulsion of the water takes place, and that the animal when it has been taken out of that element, upon being restored to it, immediately has eager recourse to this action[292], we shall feel inclined rather to adopt the opinion of those great physiologists Reaumur, Lyonet, and Cuvier, and admit that it absorbs water for the purpose of respiration. I shall now explain how this takes place. The pieces both internal and external that close the anal orifice have been before described; the others employed in the admission and expulsion of the water are evidently respiratory organs. When this orifice is opened, the parts that are above it are drawn back in an opposite direction, so that the five last segments of the abdomen become entirely empty, and form a chamber to receive the water that enters by it. When the water is to be expelled, the whole mass of air-vessels which had receded towards the trunk, is pushed forwards, and forms a piston that again expels the water in a jet. It consists of an infinite number of bronchiæ, entangled with each other, which proceed from the middle and posterior end of the tracheæ. M. Cuvier in the interior of the rectum of the larva discovered twelve longitudinal rows of little black spots, in pairs, which exhibited the resemblance of six pinnated leaves. These are minute conical tubes, of the spiral structure of tracheæ, which decompose the water, and absorb the air contained in it. He also discovered that each of these tubes gave birth to another outside the rectum, which connected itself with one of the six great longitudinal tracheæ; two of which are of enormous size, and appear to serve as reservoirs, since they furnish air by transverse branches to two other tubes; they have each a recurrent branch, which follows the course of the intestinal canal, and furnishes it with an infinity of bronchiæ[293]. These tracheæ are found in the perfect insect. The principal ones in some send forth many branches, terminating in vesicles, which in shape resemble the seed-vessels of some species of Thlaspi, while others appear to form a file of oblong ones[294]. Near each of their spiracles also is a vesicle which appears to be a reservoir[295].

But this kind of structure is not confined to insects strictly aquatic. Even such species of terrestrial ones as live upon aquatic plants, and are, consequently, necessarily or accidentally often a considerable time under water, are furnished with some apparatus by means of which they can exist in this element for a considerable period. For example, most of the Weevils (Rhyncophora) die in a short time if immersed in water; yet the species of the genera Tanysphyrus, Bagous, and Ceutorhynchus which feed on aquatic plants, can exist for days under water, as I have ascertained by experiment. C. leucogaster and another of the same tribe, swims like a Hydrophilus, and will live a long time in a bottle filled with water and corked tight. Other insects also, that are not at all aquatic, have pneumatic pouches. A striated or channeled vesicle I have found under the lateral angles of the collar in the humble-bee, where Chabrier supposes the vocal spiracles are situate; and also at the mouth of the spiracles of the metathorax in Vespa, &c.[296] In Sphinx Ligustri the bronchiæ terminate in oblong vesiculoso-cellular bodies, almost like lungs[297]; in Smerinthus Tiliæ these are preceded by a simple vesicle bound with spiral fibres[298]. M. Chabrier thinks that these air-bladders of insects, amongst other functions, give more fixity and force to the muscles for flight[299].

Many physiologists have seen an analogy between the spiral vessels of plants and the tracheæ of insects; and some of great name, as Comparetti, Decandolle, and Kieser, have thought that in some instances they terminated in the oscula or cortical pores: but Sprengel contends that they are not accurate in this opinion[300]. In fact, the principal analogy seems to be in the spiral structure of both these vessels.

* * * * *

Having considered the different organs of respiration both external and internal, I shall make a few further observations upon this function. We know little more respecting the mode in which insects respire, except that they breathe out the air by the same kind of organs by which they receive it,--namely, the spiracles, or their representatives. This has been satisfactorily proved by Bonnet, who showed that the experiments by which Reaumur thought it established that insects inspire by their spiracles, but expire through the mouth, anus, or pores of the skin, are founded on an erroneous assumption. This physiologist, having observed on the surface of submerged insects numerous bubbles of air, concluded that they had passed through the above orifices[301]: but Bonnet found by various experiments carefully conducted, that this appearance was caused by air which adhered to the skin and its hairs, and that when the access of this was precluded by carefully moistening the skin with water previously to immersion, this accumulation of air-bubbles on its surface did not take place[302]. And in a variety of instances he observed large ones issue from all the spiracles, especially the anterior ones. These bubbles sometimes were alternately emitted and absorbed without quitting the spiracle[303], and at others were darted with force to the surface of the water, where they appeared to burst with noise[304]. This author is of opinion that the first and last pair of these organs are of most importance to respiration[305]. Reaumur subsequently owned that Bonnet's arguments had shaken his opinion[306]; and some observations of his own, with respect to the respiration of the bot of the ox, go to prove that expiration and inspiration are not by the same spiracles; for he found that the air in this animal was expired by the eight little lower orifices before mentioned[307], from which he clearly saw the air-bubbles issue--the upper one he conjectures receives the air[308]. As the only communication that this grub has with the atmosphere is by its posterior extremity, it follows, reasoning from analogy, that the anterior respiratory plates of Dipterous larvæ, which may be regarded as representing the spiracles of the trunk in insects in general, are destined for the escape of the air, after it has parted with its oxygen, received by the anal ones[309]. So that there seems very good ground for M. Chabrier's opinion that inspiration is ordinarily by the abdominal spiracles, and expiration by those of the trunk of insects[310]. He seems to have been led to the adoption of this opinion, not so much by experiments similar to that of Reaumur just stated, but by observing that in many instances these two sets of spiracles differ from each other, the latter having a convex and the former a concave mouth or bed[311]. In some cases, however,--for instance during flight,--he supposes the spiracles of the trunk may receive as well as emit the air[312]: he likewise is of opinion, and it seems not improbable, that by means of these openings in the trunk, from the rush of the superfluous air through them, insects produce those sounds for which they are remarkable,--as the humming of bees and flies. In the former he thinks the sound is produced by the pneumatic apparatus covered by the ends of the collar; while in the latter he attributes it to the spiracles in the metathorax behind the wings attended by a poiser[313]. I incline, however, to M. Dufour's opinion[314],--that the vocal spiracles in the Hymenoptera, as well as in the Diptera, are those behind the wings. Perhaps both theories may be right; for if you take any common humble-bee, you will find that, in the hand, it produces one kind of sound when its wings are motionless, and another more complex and intense when they vibrate. In numerous instances, however, there is no very striking external difference between the spiracles of the trunk and those of the abdomen: this observation applies more particularly to the caterpillars of Lepidoptera; but whether these receive the air by those of the abdomen, and return it by those of the trunk, has not yet been ascertained; and indeed, too little is at present known upon the subject, and too few facts have been collected, to admit of dogmatizing.

The external signs of respiration in insects are not universally to be discovered. The alternate contraction and expansion of the abdomen is, however, very visible in some beetles, bees, the larger dragon-flies, and grasshoppers. In one of the latter, Acrida viridissima, Vauquelin observed that the inspirations were from fifty to fifty-five times in a minute in atmospheric air, and from sixty to sixty-five when in oxygen gas[315]. But M. Chabrier has given the most satisfactory account of these signs: The abdomen, says he, is the principal organ of inspiration; it can dilate and contract, lengthen and shorten, elevate and depress itself. In flight, in elevating its extremity at the same time with the wings, it contracts itself, pushes the air into the trunk, and diminishes the weight of the body by the centrifugal ascending force[316]. In the majority of insects perhaps the dilatation of the abdomen takes place by the recession of the segments from each other by means of the elastic ligaments that connect them; in others, as the Dynastidæ, Galeodes, &c. by the longitudinal folded membrane that unites the dorsal and ventral segments--in the Libellulina by similar ventral folds; and in Cimbex by membranous pieces in the first dorsal segment, which De Geer observed was elevated and depressed at the will of the animal[317].

Air is as essential to insects in their pupa as in their larva or perfect states. Lyonet, however, Musschenbroek, Martinet, and some other physiologists, have doubted whether quiescent pupæ breathed[318]; but Reaumur and De Geer seem to have proved that they do[319]: and if thrown into water, the same proof of respiration, by the emission and retraction of a bubble of air takes place, as in the larvæ; and De Geer found that if one be transferred under water from one spiracle to another, it will be absorbed by it[320]. Indeed, unless these pupæ had breathed, where would have been the necessity for the spiracles with which all are furnished? It is remarkable, however, that all these spiracles do not seem of equal importance in this respect. Reaumur found that if the posterior spiracles only were closed with oil, the insect suffered no injury; but that if the anterior ones were similarly treated, it infallibly died[321]. The respiration however of pupæ seems more perfect in those that have recently assumed that state, than in those that are more advanced towards the imago; in which at first, from Reaumur's experiments[322], it appears that the posterior spiracles were stopped; and in others still older, from Musschenbroek's[323], even the anterior ones. Those quiescent pupæ that during that state remain submerged, respire air. De Geer has given an interesting record of this, in the case of Hydrocampa stratiotata. This insect spins a double cocoon, the outer one thin, and the inner one of a close texture. In the pupa there are three pair of conspicuous spiracles on the second, third, and fourth segments of the abdomen, which are placed on cylindrical tubes, and they appear to have no other air-vessels. The respiratory gills of the larva having vanished, like some others of the same genus, they know how to surround themselves with an atmosphere of air in the midst of the water, so that the interior of their inner cocoon is impervious to the latter element--how they renew the air has not been ascertained. Though they respire air, water is equally necessary, for the animal died when kept out of water[324].

The great majority of insects respire in much the same manner in all their states, particularly as to their external organs; for when the larva breathes by the lateral spiracles, the pupa and imago usually do the same. The converse of this, however, by no means holds; for it not unfrequently happens that the two latter breathe by means of lateral spiracles, though they received the air in their larva state by an apparatus altogether different. Thus the larvæ of many Diptera breathe by an anal tube, while the pupa and imago follow the general system. Sometimes a tribe of insects breathe by an apparatus quite different in all their states, as we have seen to be the case with the common gnat[325], which has an anal respiratory tube in its first state, thoracic respiratory horns in its second, and the ordinary lateral spiracles in its third.

Changes also take place in their internal organs. In the larvæ the respiratory apparatus, especially the tracheal tubes, is often much larger and more ramified than in the imago; and as the former is the principal feeding state, there seems good ground for Mr. B. Clark's opinion--that the respiration is intimately connected with the conversion of the food[326]. In the imago, there appears to be more provision for storing up the air in vesicular reservoirs, than in the larva. Wonderful is the mode in which some of the changes in the internal structure, which these variations indicate, must necessarily take place. They are, however, probably not more singular than those which less obviously occur in the air-vessels of all insects in their great change out of the larva into the pupa state. But having before enlarged on this subject, I need not repeat my observations[327].

The access of air is as necessary to insects even in their egg state[328], and in many cases its presence seems provided for with equal care, by means as beautiful as those Sir H. Davy and Sir E. Home have shown to occur in the oxygenation of the eggs and fœtuses of vertebrate animals[329]. It is only necessary to view the admirable net-work of air-vessels which Swammerdam discovered spread over the surface of the eggs of the hive-bee while in the ovaries[330],--a provision which, from analogy, we may conclude obtains generally; from the importance which nature has attached to the oxygenation of the germ while in the matrix. And judging from analogy, we may infer that the access of this element is as carefully secured after the egg is laid, as before. The eggs of most insects being of a porous texture, often attached to the leaves of plants, and some of them embedded in the very substance of a leaf or twig[331], are in a situation for the abundant absorption of oxygen: and the pouch of silk in which the eggs of spiders and Hydrophili are deposited, may probably, from Count Rumford's experiments, be of utility in the same point of view. In the case of the Trichoptera and other insects[332] whose eggs are dropped into the water enveloped in a mass of jelly, this substance perhaps serves for aërating the included embryo, in the same way with the jelly surrounding the eggs of the frog, dog-fish, &c. It would be desirable to ascertain whether the former jelly be of the same nature as the experiments of Mr. Brande have shown the latter to be[333]. It is not improbable that the singular rays that terminate the eggs of Nepa[334] may in some way be connected with the aëration of the egg.

To what I have before remarked with regard to the vital heat of insects[335], I may under this head very properly add a few further observations. I there stated, that the temperature of these animals is usually that of the medium they inhabit, but that bees, and perhaps other gregarious ones, furnish an exception to this rule[336]. A confirmation of this remark is afforded by Inch, a German writer, who, upon putting a thermometer into a bee-hive in winter, found it stand 27° higher than in the open air; in an anthill, he found it 6° or 7° higher; in a vessel containing many blister-beetles (Cantharis vesicatoria,) 4° or 5° higher. A thermometer, standing in the air at 14° R., put into a glass vessel with Acrida viridissima, in nine minutes rose to 17°, and a similar result was observed with respect to other insects[337]. Dr. Martine says that caterpillars have but two degrees of heat above that of the air they live in[338]. Coleopterous insects are said to move slowly and with difficulty when the thermometer sinks to 36°, to become torpid at 34°, and to lose muscular irritability at a lower degree[339]. I have before observed that some insects will bear to be frozen into an icicle, and yet survive[340]: they share this power with reptiles, fishes, and amphibia. But, however small the excess of it in some insects above that of the medium they inhabit, it proves that they possess the power of generating heat. Whether, like the warm-blooded animals, they generally possess that of resisting heat by perspiration, &c. is not so clear. Yet the heat to which some can bear to be exposed, basking at noon, as Dr. Clarke informs us[341], on rocky and sandy places, exposed to the full action of the sun, appears sufficient, if not resisted by some principle of counteraction, to roast them to a cinder. That bees perspire is well known, but probably not singly.

When the respiration of insects is suspended by immersion in any fluid, it is often resumed, even when it has been long and they are apparently dead, if they be brought into contact with the atmosphere. Reaumur found this to be the case with bees[342]; and Swammerdam tells us that the maggot of the cheese-fly (Tyrophaga Casei) lived six or seven days in rain-water[343]: he found it so difficult to kill the larva of Stratyomis Chamæleon, which he first immersed twenty-four hours in spirits of wine, and then put them several days in water, without killing them,--that he lost his patience, and dissected them alive. He tried to drown them also in vinegar, in which they held out more than two days[344].

That the suspended animation and subsequent death of most terrestrial insects when thrown into water is caused by the want of air, is evident from this,--that the same effect ensues if the spiracles be covered with any oily or fatty matter. In this case too, their vital powers soon become suspended: they revive, if the suffocating matter be soon removed; and if this be not done, infallibly perish. This fact was known to the ancients, for Pliny observes that bees die if dipped in oil or honey[345]. One exception to this law has been before mentioned[346]: a similar contrivance secures the cheese-maggot from having its respiration interrupted by its moist and greasy food; the grub also of Sarcophaga carnaria, and of other Muscidæ probably, has its posterior spiracles placed in a plate at the bottom of a kind of fleshy pouch, which has the shape of a hollow, truncated, and reversed cone. This pouch the grub can close whenever it pleases, so as to cover its spiracles[347]. And numerous other larvæ, both of Diptera and Coleoptera that devour unclean and oily food, have doubtless some protection of this kind for their spiracles and respiratory plates.

I am, &c.

FOOTNOTES:

[144] Anat. Compar. iv. 296.Return to passage

[145] Plin. Hist. Nat. l. xi. c. 3. Even Aristotle seems to have given into the common opinion. De Respirat. c. 3, 9. &c.Return to passage

[146] Philos. Trans. v. 2011. Works, 4to. i. 79, 112.Return to passage

[147] Aristot. Hist. Animal. l. viii. c. 27.Return to passage

[148] On Air and Fire, 148, 155.Return to passage

[149] Tracts, 208.Return to passage

[150] Mem. on Respirat. 75.Return to passage

[151] Ann. de Chimie, xii. 273.Return to passage

[152] F. L. A. Sorg, Respirat. Insect. et Verm. Ellis, Inquiry into Chang. prod. on Atmosph. Air by Respirat. &c. 69.Return to passage

[153] Ann. de Chimie, xii. 273.Return to passage

[154] Sprengel, Commentar. &c. 27--.Return to passage

[155] PLATE XXIII. FIG. 2. and PLATES VIII. IX. XVI. XXIX. c´,h´´, m´´, A´´, D´´.Return to passage

[156] Moldenhawers (Anat. der Pflanz. 314--.) affirms that the spiracles of most insects are quite closed: but Sprengel (Commentar. § 8.) has satisfactorily refuted that opinion.Return to passage

[157] PLATE XXIII. FIG. 2.Return to passage

[158] Sprengel, Commentar. § 7.Return to passage

[159] Ibid. t. iii. f. 30.Return to passage

[160] PLATE XXIX. FIG. 23.Return to passage

[161] Ibid. 8.Return to passage

[162] Sprengel, 7. t. iii. f. 30.Return to passage

[163] Ibid. t. ii. f. 22. t. iii. f. 29.Return to passage

[164] PLATE XXIX. FIG. 29.Return to passage

[165] Ibid. FIG. 16. Sprengel, Ibid. 9. t. 1. f. 4-6.Return to passage

[166] Ibid. 9. t. i. f. 9.Return to passage

[167] PLATE XXIX. FIG. 16. a.Return to passage

[168] Sprengel, Ibid. t. iii. f. 27.Return to passage

[169] Sprengel, Commentar. 7--.Return to passage

[170] Sprengel, from whom I have borrowed this quotation, expresses the time by "scripulo horæ." This word is of uncertain meaning, being scarcely ever applied to time; but as it means the twenty-fourth part of an ounce, Faber conjectures it may mean the same portion of an hour.Return to passage

[171] Sorg, Disquisit. circa respirat. insect. 27, 46, 66. Sprengel ubi supr. 11--.Return to passage

[172] Chabrier sur le Vol des Ins. c. l. 454.Return to passage

[173] PLATE XXIX. FIG. 28. A´´.Return to passage

[174] Ibid. FIG. 23.Return to passage

[175] Sepp. I. iv. t. ii. f. 3.Return to passage

[176] Ibid. t. xiv. f. 3.Return to passage

[177] Ibid. t. v. f. 6, 7.Return to passage

[178] Ibid. t. i. f. 7, 8.Return to passage

[179] Ibid. t. x. f. 6, 7.Return to passage

[180] Ibid. v. t. i. f. 3.Return to passage

[181] Sphinx Labruscæ Merian Surinam. 34.Return to passage

[182] PLATE XXIX. FIG. 28. A´´.Return to passage

[183] Swammerd. Bibl. Nat. t. xxvii. f. 5. Compare Sturm Deutsch. Fu. i. t. v. f. r.Return to passage

[184] PLATE XXIX. FIG. 12. c´.Return to passage

[185] De Geer, i. 81. t. v. f. 10. f.Return to passage

[186] Sur le Vol des Ins. c. i. 459.Return to passage

[187] Reaum. iv. 246. t. xix. f. 8. s.Return to passage

[188] In this tribe, which I forgot to remark before, (see VOL. III. p. 549--.) there seems both prothorax and collar.Return to passage

[189] VOL. III. p. 550, 559. &c.Return to passage

[190] PLATE VIII. FIG. 14. h´´.Return to passage

[191] PLATE XXIX. FIG. 14, 15. m´´.Return to passage

[192] Ibid. FIG. 15. a.Return to passage

[193] Ibid. FIG. 14, 15. b.Return to passage

[194] Ibid. FIG. 25. k´´.Return to passage

[195] Chabrier sur le Vol des Ins. c. iii. t. vi. f. 4. Sa, Sp.Return to passage

[196] PLATE IX. FIG. 21. m´´.Return to passage

[197] PLATE VIII. FIG. 9.Return to passage

[198] VOL. III. p. 705--.Return to passage

[199] VOL. III. p. 708.Return to passage

[200] Sprengel, Comment. 3.Return to passage

[201] Ibid.Return to passage

[202] vi. 398.Return to passage

[203] De Geer, ii. 635.Return to passage

[204] Fourmis, 22.Return to passage

[205] Osservaz. &c. sullo Iulus fœtid. 14--.Return to passage

[206] They are particularly visible in an undescribed East Indian species, (S. alternata K. M. S.) with scuta alternately black and yellow.Return to passage

[207] PLATE XXIX. FIG. 20. A´´.Return to passage

[208] De Geer, vii. t. vi. f. 3.Return to passage

[209] VOL. I. p. 254--.Return to passage

[210] De Geer vi. 67. t. iii. f. 10. ss. 14. Mr. W. S. MacLeay (Philos. Mag. N. Ser. No. 9. 178.) says that in this grub the longitudinal trunks of the Tracheæ send off at equal distances lateral branches just as if there were spiracula to correspond with them. This is evidently a preparatory step to the formation of those that ultimately appear in the perfect insect.Return to passage

[211] De Geer 66. t. iii. f. 13.Return to passage

[212] PLATE XIX. FIG. 11. a.Return to passage

[213] Reaum. iv. 375--. t. xxvi. f. 7, 8.Return to passage

[214] Ibid. 555. t. xxxv. f. 10. ss.Return to passage

[215] Ibid. 519--. t. xxxvii. f. 3, 4.Return to passage

[216] PLATES XVI. FIG. 9. a b. XIX. FIG. 9, 10, 12, 13. a. XXIX. FIG. 3-7.Return to passage

[217] PLATE XIX. FIG. 9. a.Return to passage

[218] PLATE XIX. FIG. 9. b.Return to passage

[219] Compare Swamm. Bibl. Nat. i. 154. t. xxxi. f. 5. Reaum. iv. 601--. t. xliii. De Geer vi. 317--. t. xvii. f. 2-8.Return to passage

[220] Swamm. Ibid. t. xxxi. f. 7, 8.Return to passage

[221] Reaum. iv. 607.Return to passage

[222] PLATE XIX. FIG. 12. a.Return to passage

[223] Reaum. iv. t. xxxii. f. 2. e.Return to passage

[224] Mr. W. S. MacLeay (Philos. Mag. N. Ser. n. 9. 179.) asserts that what Reaumur (iv. 487. t. xxx. f. 6. ll) calls the first pair of legs of this grub, are the usual palmated stigmata which occur on the humerus of the larvæ of Muscidæ. It does not appear whether he has himself examined this grub, but Reaumur (443) states that it has seven pairs of legs all armed with claws. If this is correct, it is not properly a palmated organ.Return to passage

[225] Reaum. iv. t. xxx. f. 10.Return to passage

[226] Reaum. iv. t. xxx. f. 447--.Return to passage

[227] Ibid. 456. t. xxxi. f. 1-7.Return to passage

[228] PLATE XIX. FIG. 13. a.Return to passage

[229] Bibl. Nat. ii. 44.Return to passage

[230] PLATE XIX. FIG. 10. a.Return to passage

[231] Reaum. v. t. iv. f. 6. s, u.Return to passage

[232] VOL. II. p. 275--.Return to passage

[233] De Geer vi. 395--. t. xxiv. f. 16. 18. d.Return to passage

[234] v. t. vi. f. 1, 2.Return to passage

[235] De Geer iii. 367. t. xviii. f. 1, 2, 9.Return to passage

[236] Ibid. vi. 36. 194--. t. ii. f. 2, 3. s.Return to passage

[237] PLATE XVI. FIG. 9. a. b.Return to passage

[238] De Geer ii. 539--. t. xi. f. 12, 16, &c.Return to passage

[239] De Geer i. 526--. t. xxxvii. f. 2-6.Return to passage

[240] Ibid. iv. 362--. t. xiii. f. 16-19.Return to passage

[241] VOL. I. p. 282--. II. 365--.Return to passage

[242] See Reaum. vi. t. xlii.--xlvi. and PLATE XXIX. FIG. 3-5.Return to passage

[243] Reaum. Ibid. t. xlv. f. 2.Return to passage

[244] PLATE XXIX. FIG. 5. De Geer ii. 624--.Return to passage

[245] Ibid. FIG. 4. De Geer Ibid. 647--.Return to passage

[246] Ibid. FIG. 3. De Geer Ibid. 653--.Return to passage

[247] Ibid. FIG. 6. De Geer Ibid. 727--.Return to passage

[248] Reaum. vi. 465.Return to passage

[249] Ibid. t. xlii. f. 4, 5. De Geer ii. 623.Return to passage

[250] Ibid. 648. t. xvii. f. 11, 12.Return to passage

[251] VOL. III. p. 154. De Geer ii. 697--. t. xxi. f. 4, 5, 12.Return to passage

[252] De Geer Ibid. 666--. t. xix. f. 6.Return to passage

[253] Reaum. vi. 393. t. xxxvi. f. 8, 9. t. t.Return to passage

[254] Reaum. vi. 395. t. xxxvi. f. 8-9. c. c.Return to passage

[255] PLATE XXIX. FIG. 21.Return to passage

[256] Marcel de Serres (Mem. du Mus. 1819. 137, &c.) calls the tubular tracheæ that receive the air, arterial tracheæ, and the vesicular ones which act as reservoirs, pulmonary tracheæ.Return to passage

[257] PLATE XXIX. FIG. 1. 2.Return to passage

[258] Treviranus Arachnid. 7--. t. l. f. 1. r. f. 10. Comp. N. Dict. d'Hist. Nat. xxx. 419. Latreille calls these gills Pneumo-branches.Return to passage

[259] Treviranus Ibid. 24. PLATE XXIX. FIG. 1.Return to passage

[260] PLATE XXI. FIG. 3. a b.Return to passage

[261] Ibid. a.Return to passage

[262] Ibid. b.Return to passage

[263] Sprengel Commentar. t. i. f. 1.Return to reference 1 Return to reference 2

[264] Ibid. f. 10.Return to passage

[265] Ibid. t. ii. f. 15.Return to passage

[266] Malpigh. De Bombyc. t. iii. f. 3.Return to passage

[267] Ibid. t. iv. f. 1.Return to passage

[268] Lyonet Anat. 101.Return to passage

[269] Lyonet Anat. 101.Return to passage

[270] Sprengel (ubi. supr. 16.) says that he never found more than two; but as Lyonet affirms that he has very often separated them (102), his accuracy cannot be questioned.Return to passage

[271] Lyonet Anat. 103.Return to passage

[272] Ibid. Cuv. Anat. Comp. iv. 438. This author says that the intermediate tunic is the spiral thread (437).Return to passage

[273] Lyonet 102.Return to passage

[274] Ibid. 104. Sprengel Commentar. 17.Return to passage

[275] Lyonet 104. Sprengel Commentar. 17.Return to passage

[276] Lyonet 102. Malpigh. De Bombyc. 12. Reaum. i. 130.Return to passage

[277] Swamm. Bibl. Nat. t. ii. f. 7.Return to passage

[278] Lyonet 411.Return to passage

[279] Professor Kidd (Philos. Trans. 1825. 235.) conjectures that the tracheæ, as well as air-vessels, may possibly be blood-vessels; but this hypothesis is inconsistent with the fact recently discovered by Dr. Carus, of a circulation, by other means, in larvæ. See Carus Introd. to Comp. Anat. &c. ii. 400.Return to passage

[280] N. Dict. d'Hist. Nat. xvii. 541. Reaum. vi. 397. PLATE XXIX. FIG. 8. shows three of them at a.Return to passage

[281] Essay on the Bots, &c. 23. t. i. f. 7, 32, &c.Return to passage

[282] Ibid. 49. Valisnieri i. 101. t. vi. f. 4. &c.Return to passage

[283] Bibl. Nat. i. 149. a. t. xxix. f. a. Cuv. Anat. Comp. iv. 439. Malpigh. De Bombyc. t. iii. f. 2.Return to passage

[284] Sur le Vol des Ins. c. ii. 336. note 1.Return to passage

[285] Swamm. Bibl. Nat. t. xvii. f. 9. Cuvier Ibid. 440.Return to passage

[286] PLATE XXIX. FIG. 10. a.Return to passage

[287] Ibid. b.Return to passage

[288] De Geer vi. 374.Return to passage

[289] Reaum. v. 40. t. vi. f. 4, 7.Return to passage

[290] Sprengel Comment. 4.Return to passage

[291] De Geer ii. 667, 675.Return to passage

[292] Reaum. vi. 394--.Return to passage

[293] Reaum. vi. 394--. Cuv. Anat. Comp. iv. 440--. N. Dict. d'Hist. Nat. xvii. 540--.Return to passage

[294] PLATE XXIX. FIG. 9. a, b. Reaum. vi. 418--. 450.Return to passage

[295] Cuv. Anat. Comp. iv. 441.Return to passage

[296] VOL. III. p. 583.Return to passage

[297] Sprengel Comment. 17. t. iii. f. 24.Return to passage

[298] Ibid. t. i. f. 11.Return to passage

[299] Sur le Vol des Ins. c. ii. 336. note 1.Return to passage

[300] Sprengel Comment. 13--. These oscula or pores in the straw of Triticum hybernum, as figured by Mr. Bauer's admirable pencil, (Sir J. Banks On the Blight, &c. t. ii. f. 3.) exactly resemble the spiracles of insects.Return to passage

[301] Reaum. i. 136.Return to passage

[302] Bonnet Œuvr. iii. 39--.Return to passage

[303] Ibid. 43.Return to passage

[304] Ibid. 50.Return to passage

[305] Ibid. 69.Return to passage

[306] De Geer ii. 117.Return to passage

[307] See above, p. 50.Return to passage

[308] Reaum. iv. 520.Return to passage

[309] Mr. B. Clark thinks that he has discovered spiracles in this larva in the usual situation, (Essay on the Bots, &c. 48. t. ii. f. 3.) but they are probably analogous to the spiraculiform tubercles of Œ. Ovis. Reaum. iv. 566. t. xxxv. 17-19. t. Vallisnieri (Esperienz. &c. 136) notices them.Return to passage

[310] Sur le Vol des Ins. c. i. 423.Return to passage

[311] Ibid. 454. and c. iv. 66. note 1.Return to passage

[312] Ibid. c. i. 453.Return to passage

[313] Ibid. 459, 456.Return to passage

[314] Ibid. 459.Return to passage

[315] Annal. de Chim. xii.Return to passage

[316] Sur le Vol des Ins. c. i. 423, 454. c. iii. 344. c. iv. 66.Return to passage

[317] De Geer ii. 946--.Return to passage

[318] Lesser, L. i. 124. note *. Lyonet Anatom. pref. xii. De Geer ii. 132.Return to passage

[319] Reaum. i. 399--. De Geer i. 37--.Return to passage

[320] Ibid. 40.Return to passage

[321] Reaum. i. 400.Return to passage

[322] Ibid.Return to passage

[323] De Geer ii. 129.Return to passage

[324] De Geer i. 531--. t. xxxvii. f. 13. s. Compare Reaum. ii. 396--.Return to passage

[325] See above, p. 51--.Return to passage

[326] In Linn. Trans. iii. 302.Return to passage

[327] VOL. III. p. 195--.Return to passage

[328] Spallanzani found that the eggs of insects placed under the exhausted receiver of an air-pump, or in any small closed vessels, did not hatch, though every other condition for their development was present. Opusc. de. Phys. i. 141.Return to passage

[329] Philos. Trans. 1820. 213.Return to passage

[330] Bibl. Nat. i. 204. b. t. xix. f. 5.Return to passage

[331] VOL. I. p. 446--. III. p. 76.Return to passage

[332] Ibid. 68--.Return to passage

[333] Philos. Trans. 1820. 218.Return to passage

[334] VOL. III. p. 94.Return to passage

[335] VOL. II. p. 228--.Return to passage

[336] Ibid. p. 211.Return to passage

[337] Inch, c. iv. Ideen zu Einer Zoochemie, 68--.Return to passage

[338] On Thermom. 141.Return to passage

[339] Carlisle in Philos. Trans. 1805. 25.Return to passage

[340] VOL. II. p. 229.Return to passage

[341] Travels ii. 482.Return to passage

[342] Reaum. v. 540.Return to passage

[343] Swamm. Bibl. Nat. ii. 65. a.Return to passage

[344] Ibid. 48. a.Return to passage

[345] Hist. Nat. l. xi. c. 19.Return to passage

[346] Swamm. Bibl. Nat. ii. 64. a.Return to passage

[347] Reaum. iv. 428. t. xxix. f. 2. c, s.Return to passage

Letter XXXIX

_INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONTINUED._

CIRCULATION.

We learn from the highest authority, that the blood is the life of the animal[348]: every object of creation, therefore, that is gifted with animal life, we may conclude, in some sense, has blood, which in this large sense may be defined--The fluid that visits and nourishes every part of a living body[349]. But the GREAT AUTHOR of nature has varied the machinery by which this nutritive fluid is formed and distributed, gradually proceeding from the most simple to the most complex structure; in which he seems to have seen it fit to invert the process observable in the systems of sensation and respiration, where the ascent is from the most complex, to the most simple structure. In the lowest members of the animal creation, the blood seems the portion they imbibe of the fluid medium in which they reside, which when chylified, distributes new molecules to all parts of their frame[350]. In others, as in insects, it is formed by the chyle that transpires through the intestinal canal into the general cavity of the body, where it receives oxygen from the air-vessels, and is fitted for nutrition[351]. In these animals it is accompanied by a long dorsal vessel, the first step towards a heart, which alternately contracts and dilates with an irregular systole and diastole, but appears to have no vascular system connected with it, though in their preparatory states it has an extra-vascular circulation which ceases in the perfect insect. Again: in others, as the Tubicoles, Annelida, &c., a real circulation has been discovered; that is to say, a system of veins and arteries, but unaccompanied by a muscular heart[352]. In the Arachnida and Branchiopod Crustacea the long dorsal vessel is also found; but in these it is connected with an arterial and venous system, which receives, distributes, and returns the blood[353]. It has therefore now become a true heart, and there is a regular circulation; and in the Decapod Crustacea the dorsal vessel is contracted into an oval form, and placed nearly in the centre of the trunk[354]. In the great majority of invertebrate animals the blood is white, but in the Annelida, to which Class the common dew-worm belongs, a curious anomaly takes place--for it is red[355]. Thus a gradual ascent is made to the circulating system of the vertebrate and red-blooded animals. In all, however, the blood is the principal instrument of nutrition and accretion; and is on that account properly so denominated, though not connected with a circulating system.

Having given you this general outline of the means by which the blood is distributed in the different Classes of animals, I shall now confine myself to the case of insects and Arachnida, beginning with the former.

I. If you examine attentively the back of any smooth caterpillar with a transparent skin, you will perceive in that part an evident pulsation, as though a fluid were pushed at regular intervals towards the head, along a narrow tube which seems to run the whole length of the body. Accurate dissections have proved that this appearance is real, that there is actually present in the back of most insects, placed immediately under the skin and furnished with numerous air-vessels, a longitudinal vessel[356] originating in the head near the mouth[357], running parallel with the alimentary canal nearly to the anus, containing a fluid which is propelled in regular pulsations of from 20 to 100 per minute, more or less as the weather is colder or warmer[358], causing a sensible alternate systole and diastole from the anal extremity towards the head. In the Cossus these pulses were observed by Lyonet to begin in the eleventh segment, from which they passed from segment to segment, till they arrived at the fourth, where they terminated[359]. This vessel is what Malpighi, who first discovered it, termed a heart, or rather series of hearts[360]; but which Reaumur, who injected it, regarded as a simple artery without striking contractions[361]: but to steer clear of any hypothesis, I shall merely call it the dorsal vessel (Pseudo-cardia). When carefully taken out of the body it is found to be a membranous tube, appearing to be closed at each end[362], in many larvæ of equal diameter every where, but in perfect insects usually widest at the anal extremity[363], and attenuated into a very slender filament towards the head. In some insects, however, as in the larva of the chamæleon-fly (Stratyomis Chamæleon), it is attenuated at both ends, and in the Ephemera is alternately constricted and dilated as Malpighi describes that of the silkworm[364], a dilated portion belonging to each segment[365]. In the Cossus, and probably others, after the third segment, it is furnished with nine pair, the three posterior pair being the largest, of triangular transverse bundles of muscular fibres, which Lyonet denominates its wings[366], the action of which produces its systole and diastole, and their propagation from the tail towards the head[367]. Under the last pair of these wings it is strengthened by a large number of circular muscular fibres[368]. I have stated it as appearing to be closed at each extremity, because Cuvier and most writers have so regarded it, and probably it is so closed in the perfect insect; but from Lyonet's words it should seem that, in the larva of the Cossus, he considered it as open and expanded at its anterior end[369]. He seems also to suspect, that, by means of what he calls the frontal ganglions, a fluid is derived from the dorsal vessel to the spinal marrow. He likewise describes a large nerve as passing through it and becoming recurrent[370]. Carus, as we shall soon see, has also proved that this tube is not closed in larvæ.

The fluid which this vessel contains is very abundant; in the animal it appears colourless and transparent like water, but when collected in drops it becomes more or less yellow, and even orange[371]. Examined under the microscope it appears filled with a prodigious number of transparent globules, of incredible minuteness[372]. When mixed with water, which it does readily, its globules lose all their transparency, and coagulate into small clammy masses. After evaporation it becomes hard, and cracks like gum, as blood does also. This gummy substance is so abundant, that the fluid contained in the dorsal vessel of the caterpillar of the Cossus yields a mass of it of the size of a grey pea[373].

From the situation of this dorsal vessel, which is precisely the same with that of the heart in Arachnida and the Branchiopod Crustacea, and from the systole and diastole which keep its fluid contents in constant motion, who can wonder that the physiologists who first discovered it, reasoning analogically, maintained that it was a true heart? But modern comparative anatomists, and those of the highest name, from the absence of a vascular system for a circulation, have contended that it is not a true heart, but an organ appropriated to other purposes: a third hypothesis, and intermediate between these two, has very recently been promulgated, that the organ in question, namely, is a real heart, and in the preparatory states of insects, the centre of a real circulation, which, in the imago state, ceases with the full development of the wings; but that this circulation is extravascular, or without peculiar vessels analogous to veins and arteries.

I shall now enlarge a little upon each of these hypotheses, beginning with the first or original one.

No one will deny that the argument from analogy is strongly in favour of this: I need not therefore dwell upon it, but proceed to others. Swammerdam, to whose exactness in observing, and scrupulous accuracy, every reader of his immortal work will bear testimony, expressly asserts that he has seen vessels issuing from the dorsal vessel in the silkworm, and even succeeded in injecting them with a coloured fluid[374]. Now it seems extremely improbable that so practised and expert an anatomist should have been deceived, especially upon a point which would naturally excite his most earnest and undivided attention. Without this recorded experiment, perhaps, it might be thought, though this was very unlikely, that he had mistaken bronchiæ for veins and arteries: but how could they have been injected from the supposed heart? Another great physiologist, Reaumur, in the caterpillar of the saw-fly of the rose (Hylotoma Rosæ) observed, besides the dorsal vessel, a ventral one of similar form, in which also was a pulsation, but slower than that of the other. This he supposes may be the principal trunk of the veins[375]. Bonnet thought he discovered a similar vessel in a large caterpillar, but with all his attention could perceive no motion in it[376]. Reaumur also fancied he perceived in the grub of Musca vomitoria, in which he in vain looked for the dorsal vessel, a fleshy part which exhibited alternate pulsations; and when with a pair of scissors he made a lateral incision in the insect, amongst other parts that came out, there was one that had movements of contraction and dilatation for several minutes,--this experiment was repeated with the same result upon several grubs[377]. De Geer, whose love of truth and accuracy no one will call in question, saw the appearance of blood-vessels in the leg of the larva of a Phryganea L. (as Lyonet did in those of a flea[378]); and in the transparent thigh of Ornithomia avicularia he discovered a pulse like that of an artery[379]. Baker, whose only object was to record what he saw, speaks of the current of the blood being remarkably visible in the legs of some small bugs[380]: what he meant by that term is uncertain, but they could not be spiders, which he had just distinguished. This author has likewise seen a green fluid passing through the vessels of the wings of grasshoppers[381]; and M. Chabrier is of opinion that insects possess the power of propelling a fluid into the nervures of their wings and withdrawing it at pleasure, as they are elevated or depressed[382]; but this last fact may be independent of a circulation.

But though these arguments, which I have stated in their full force, appear strong, and at first sight conclusive, those which may be urged for the more modern opinion--that no circulation exists in insects, properly so called,--appear to have still greater weight. Lyonet, whose piercing eye and skilful hand traced the course of so many hundred nerves and bronchiæ long after they became invisible to the unassisted eye, and which were a thousand times smaller than the principal blood-vessels, opening into so large an organ as the supposed heart of insects, might be expected to be, could never discover any thing like them. His most painful researches, and repeated attempts to inject them with coloured liquors, were unable to detect the most minute opening in the dorsal vessel, or the slightest trace of any artery or vein proceeding from or communicating with it[383]. And Cuvier, whose unrivalled skill in Comparative Anatomy peculiarly qualified him for the investigation, repeated these inquiries, and tried all the known modes of injection, with equal want of success; and is thus led to the conclusion, that insects have no circulation, that their dorsal vessel is no heart, and therefore ought not to be called by that name: that it is rather a secretory vessel, like many others of that kind in those animals. As to the nature of the fluid that it secretes, and its use, he thinks it impossible, from our present information on the subject, to form any satisfactory conclusion[384]. Marcel de Serres informs us--which further seems to prove that it can be no real heart--that this vessel may be totally removed without causing the immediate death of the insect[385]. This opinion receives additional confirmation from the mode in which respiration is performed in insects. In those animals that have a circulation, this takes place by means of lungs or gills;--thus we find, even in the Crustacea and Arachnida so nearly related to insects, that the organs of this function are true gills; whereas in insects, though in some of their states their respiratory tubes are branchiform, yet they are not gills, and the respiration is by tubes and spiracles. And these tubes, as you have seen, are so numerous and so infinitely ramified and dispersed, as to occupy the place of arteries and veins, and to imitate their distribution,--and thus to oxygenate what may be deemed the real analogue of the blood, which bathes every internal part of the body of an insect. Those animals likewise that have a circulation are furnished with a liver, as is the case with the Arachnida and even many aggregate animals that have a heart; but in insects there are only hepatic ducts. M. Cuvier has also proved that the conglomerate glands, which exist in all animals that have a heart and blood-vessels, do not exist in insects, in which they are replaced by long slender secretory tubes, which without being united float in the interior of the body: from this circumstance, he is led to conclude that their nutrition is by imbibition or immediate absorption, as in the Polypi and other zoophytes, the chyle transpiring through the alimentary canal, and running uniformly to all parts of the body[386].

These arguments appear so satisfactory, that Physiologists in general seem to have been convinced by them that no circulation, at any time, takes place in insects, and that their supposed heart is merely a secretory vessel, though of what kind they were at a loss to conjecture[387]. But, convincing as they seem, they appear to have been founded in error, and on the idea that a circulation, as well as a heart, necessarily implies a vascular system consisting of veins and arteries; for by the recent discoveries of M. Carus, it has been satisfactorily proved that insects in their preparatory states, have an extravascular circulation, the arterial and venose currents not being confined by parietes. The observations upon which M. Carus' hypothesis is founded, were made in the Autumn of 1826; and an abstract of their results presented to the Union of German Naturalists and Physicians, which then held its meeting at Dresden, many of the members of which, as MM. Oken, Husche, Heyne, Purkinje, Otto, Weber, and Müller, had ocular proofs of the reality of the phenomena.

His first observations were made on the larva of Agrion Puella, which swims by means of three vertical laminæ attached to the tail; which, when the wings first appear as rudiments, begin to be exsiccated and are finally detached. Each of these laminæ, in its natural vertical position, presents an inferior abdominal and a superior dorsal edge, has two tracheæ running along its centre with ramifying bronchiæ, and consists of granular substance contained between two strata of the external integuments. A current of blood-globules enters each lamina somewhat nearer to its abdominal than to its dorsal edge, and running through the greater part of its length suddenly turns and bends its course back towards the body, somewhat nearer to the dorsal than to the abdominal margin of the lamina. The channel thus formed in the midst of the granular substance is perfectly transparent, except where it is occupied by the blood-globules, or crossed by the bronchiæ. The parietes of the channel are not strictly defined, nor formed by any thing like the coats of a vessel, the blood circulating through the granular Parenchyma; a circumstance however which is not peculiar to this case, but also occurs generally in the first states of the circulation, as it presents itself for instance in the embryo of Fishes, and in the figura venosa of the incubated egg[394]. The blood-globules are elongated like a grain of wheat, considerably larger than those of the human blood, and float in a fluid which is invisible because of its transparency, but the existence of which is proved by the variations in the position of the globules in the current, sometimes following its direction, at others crossing it transversely, or more or less obliquely.

When the animal is vigorous, the current is uninterrupted, although its velocity is accelerated at regular intervals; and that not only in the excurrent (arterial), but also in the recurrent (venous) part of its course through the lamina. When the animal becomes exhausted, or the laminæ exsiccated, the circulation is interrupted, and in the same manner, as under the same circumstances, in the larvæ of frogs and lizards; the disturbance displaying itself not merely by a cessation of the process, but also by retrograde movements of the currents, or by oscillatory motions of the blood-globules.

In proportion as the wings are developed, the circulation in the laminæ diminishes, and ultimately ceases, preparatory to the detachment of the laminæ themselves. At the same time, however, it presents itself under a new form in the wings. In these the excurrent or arterial stream takes its course along the inner margin of the wing, and the recurrent or venous returning along the outer; whilst, occasionally, other transverse currents take their course through the net-work of the wing from its inner to its outer margin. As the wings are further developed, the circulation in them, like that in the caudal laminæ, gradually becomes weaker and ultimately ceases[395].

The next observations were made on the transparent larva of a neuropterous insect (probably a Semblis or Sialis), in which the pulsations of the dorsal vessel were distinctly seen at its posterior extremity, from which they were propagated towards the anterior; these two divisions of that vessel appearing to bear to each other the relation of a heart and aorta. There were no traces of other vessels, though regular and rapid currents of blood-globules, exterior to the tracheæ, proceeded from the head towards the posterior extremity of the body, where each of these currents entered the heart, which again propelled its contents with accelerated velocity through the anterior part of the dorsal vessel towards the head. The lateral currents also were accelerated upon each contraction of the heart, proving that they must communicate with the dorsal vessel at the anterior part of the body, though the opacity of the head rendered it impossible to ascertain the mode of anastomosis. An excurrent and returning current were also traced to each of the legs[396]. But the phenomena of the circulation was most distinctly visible in the larva of Ephemera vulgata, even more distinctly than it is possible to trace it in the larvæ of frogs and newts. In this animal the circulation, with the help of the microscope, is at once visible in the three last segments of the body; and with a little attention is discoverable not only in the three terminal caudulæ, and in the upper joints of the legs, but also in the head, and particularly the roots of the antennæ. In the posterior part of the body there are on each side two currents of blood, not bounded by parietes, situate on each side of the intestinal canal, the inner one being the most considerable. The external one communicates with the internal by several intermediate branches; from this probably the streams are detached, which in the form of loops are seen at the upper joints of the legs, though it is not possible precisely to ascertain this, nor even whether these lateral currents continue distinct in the thorax, which probably they do. At the ninth abdominal segment these currents which flow posteriorly from the head, change their direction, and are inflected so as to enter the pulsating heart, from which the current again flows towards the head. Before they enter the heart they give off three streams, one for each of the three caudulæ. The currents in these caudulæ present the phenomena of the circulation with peculiar distinctness, and are particularly remarkable from the circumstance, that the excurrent and recurrent streams, though closely approximated without any visible separation, flow without disturbing each other. The excurrent stream is accelerated in correspondence with the pulsations of the heart; the recurrent on the contrary being always somewhat more sluggish, and the first to stagnate and cease when the strength of the animal is impaired. In the anterior part of the head currents can be discovered, forming loops like those of the legs, at the roots of the antennæ; each current proceeding from the cranial surface, and in returning taking its course towards the region of the larynx[397].

M. Carus has likewise observed currents of blood in the larvæ of water-beetles (Hydrophilus and Dytiscus)[398]; but at present he appears to have detected it in no terrestrial larva. Whether this is occasioned by their opacity, or it exists only in the ovum, as he seems to suspect[399], must be left for determination to future observers; it is scarcely probable, however, that the larvæ of Dytisci and Hydrophili should differ from other Coleoptera in their circulation.

The endeavours of M. Carus to discover any proofs of a circulation in insects in their last state, except in the wings at their first development, were without success[400]. He observes that the fact of the currents of fluids in larvæ not being defined by vascular parietes, enables us to comprehend the rapidity and facility with which the traces of the circulation are lost in the perfect insect. On the other hand, that the existence of a circulation at one period, and its cessation at another, elucidate many circumstances connected with the physiology of these animals: for instance, the contrast between the rapid growth and transformations of the larvæ, and the stationary existence of the imago, &c. Lastly he remarks, that the phenomena of this circulation do not throw any light on the obscure subject of the mode of nutrition in perfect insects; which therefore must still be supposed to be effected according to the idea of Cuvier, without the intervention of vessels[401].

Whatever be the functions of the dorsal vessel, this seems the most proper place to state to you what further is known respecting it. Its construction is nearly alike in insects in all their states, except that in the imago it is shorter and narrower. Reaumur has affirmed, and before him Malpighi made a similar observation, that in chrysalises newly disclosed from the larva, and yet transparent, the motion of the included fluid is the reverse of what it has been in that state, it being propelled from the head to the tail, which he found to be the case also in the imago[402]. If this be true, and there is no reason to doubt his accuracy, when they are more advanced, it resumes its old course, as Lyonet observed, from the tail to the head[403]. But probably it is not always uniformly in the same direction, since Malpighi states that a very slight cause will change its course, and that the pulsations differ in quickness in different portions of the heart[404]. If its course were really always the same, and in one direction, without any reflux, it would seem to follow that the fluid must be absorbed at one end, and, if there was no outlet, transpire at the other, which would be a kind of circulation. In Syrphus Pyrastri and other aphidivorous flies, this dorsal vessel, instead of the usual form which it had in the larva, assumes a very peculiar appearance. If, taking one of these flies by the head and wings and holding it up to the light, you survey under a lens the base of the lower part of its abdomen, you will see through its transparent skin, which exactly forms such a window as physicians have sometimes wished for in order to view the interior of their patients, a flask-shaped vessel having its long end directed towards the trunk, in which there is a manifest pulsation and transmission of some fluid. This vessel extends in length from the junction of the trunk with the abdomen to about the termination of the second segment. The included fluid does not run in the dorsal vessel in a regular course, but is propelled at intervals by drops, as if from a syringe, first from the wide end towards the trunk, and then in the contrary direction, forming a very interesting and agreeable spectacle. One circumstance led Reaumur to conjecture that the neck of this vessel, which he at first regarded as simple, is in fact composed of two or more approximated tubes, and that the blood is conveyed forward by the outward ones, and backward by the intermediate one[405]: he even thinks that he saw a kind of secondary heart, at the extremity next the trunk, for the purpose of causing the reflux. This illustrious author observed the above remarkable structure not only in the Syrphi, but in many of their affinities, and thinks that it is also widely diffused amongst the Muscidæ[406].

I must now say something upon what I conceive to be the real blood of insects; for I think no one will object to that name being given to their nutritive fluid, especially in the larva, though it does not circulate by means of a vascular system. The chyle that is produced in the intestines of animals from the food, is that fluid substance from which their blood is formed: in insects it is not absorbed by the lacteals, but transpires through the pores of the intestinal canal into the general cavity of the body, where, being exposed to the influence of the oxygen in the air-vessels, it becomes, though retaining its colour, a different fluid from what it was before, and analogous to blood in its use and office[407]; only that in these animals, as Cuvier has observed, at least in their perfect state, the blood, for want of a circulating system, not being able to seek the air, the air goes to seek the blood[408]. The dispersion of this fluid appears to be universal, so that all the parts and organs contain it in a greater or less degree[409]. In many insects, if you break only an antenna or a leg, a drop of fluid flows out at the wound. In larvæ, the fluid which bathes[410], or visits, all the internal parts and organs is not only sufficient for their nutriment, but a large quantity of seemingly superfluous blood remains that is not wanted for this purpose. This is expended in the production of the caul or epiploon (Corps graisseux Reaum.), which laps over and defends all the viscera of the animal, and goes principally to the formation of the imago[411]. I have said that Cuvier conceives nutrition in insects to take place by imbibition or immediate absorption; that is, I suppose, the different parts and organs thus constantly bathed in the blood, imbibe from it the particles necessary for their constant accretion. M. Chabrier seems to think that it is the compression and dilatation of the trunk that duly distributes the nutritive fluid[412]; Lyonet compares the nutrition of insects by their fibres from this fluid, when formed into the corps graisseux, to that of plants that draw their support by their roots from the earth[413]. Much obscurity, however, at present rests upon this subject--much for future investigation to explore; but in all the works of the MOST HIGH there is always something inscrutable, something beyond the reach of our senses and faculties, which teaches us humbly to adore his infinite perfections.

II. The circulation of the Arachnida is next to be considered; and the term applied to these becomes strictly proper. Two great tribes, in our view of the subject, constitute this Class,--the spiders (Araneidea) and scorpions (Scorpionidea): I shall give you some account of the circulating vessels of each.--In spiders, the heart in general is a long dorsal vessel as in insects, but supposed to be confined to the abdomen, growing slenderer towards each extremity, particularly the anal. In some also, as in Aranea domestica, like that of insects, it has lateral muscular appendages; but in others, as in Clubiona atrox, it is without them[414]. It exhibits a pair of vessels that appear to connect with the gills, by which the oxygenation of the blood takes place, and a number of others that ramify minutely and are lost in the analogue of the epiploon, supposed to be their liver[415]. Whether these last are to be regarded merely as veins, has not been ascertained; they seem rather to convey the blood outwards, than to return it back to the heart: but this question must be left for future investigation. I may observe, however, that though the heart of the spider has been traced only in the abdomen, it may probably extend into the trunk.

The heart of the scorpion has been examined both by Treviranus and Marcel de Serres; but I shall principally confine myself to the description of the latter, as the most clear and intelligible. The heart, then, of these animals is elongated, almost cylindrical, but attenuated at each end; it is extended from the head to the extremity of the tail, and appears to have four pairs of lateral muscles. On each side are four pairs of principal vessels, which go to the pulmonary pouches, and there ramify. These may be assimilated to veins. Besides these, there are four other vessels that cross them, forming with them an acute angle, and which, with four branches of smaller size, receive the blood from the pulmonary pouches, and distribute it to the different parts of the body,--these are the arteries. Before it enters the tail, the heart throws out two vascular branches which do not go to the gills, but distributing the blood to different parts, ought to be considered as arteries[416]. Treviranus mentions bunches of reticulated vessels, concerning the use and origin of which he seems uncertain[417]; but as they approach the gills they are probably the branching extremities of what M. de Serres considers as the veins.

I am, &c.

FOOTNOTES:

[348] Genes. ix. 4.Return to passage

[349] N. Dict. d'Hist. Nat. xxx. 130.Return to passage

[350] Cuv. Anat. Comp. iv. 167.Return to passage

[351] Herold Schmetterl. 25. note *. VOL. III. p. 53.Return to passage

[352] N. Dict. d'Hist. Nat. vii. 313. Cuv. Anat. Comp. iv. 411.Return to passage

[353] Ibid. 419, 407.Return to passage

[354] Ibid.Return to passage

[355] Ibid. 410.Return to passage

[356] PLATE XXII. FIG. 15.Return to passage

[357] Lyonet Anat. 105.Return to passage

[358] Ibid. 425.Return to passage

[359] Ibid. 105--.Return to passage

[360] De Bombyc. 15--.Return to passage

[361] Reaum. i. 160--.Return to passage

[362] Cuv. Anat. Comp. iv. 418.Return to passage

[363] Marcel de Serres Mem. du Mus. 1819. 69.Return to passage

[364] Swamm. Bibl. Nat. t. xl. f. 4. t. xv. f. 4.Return to passage

[365] De Bombyc. t. iii. f. 4.Return to passage

[366] Ubi supr. 414.Return to passage

[367] Ibid. 425--.Return to passage

[368] Ibid. 419.Return to passage

[369] Ibid. 412.Return to passage

[370] Lyonet Anat. 413.Return to passage

[371] Lyonet Ibid. 426. Cuv. Anat. Comp. iv. 419.Return to passage

[372] Lyonet says (426), "au-delà de trois millions de fois plus petits qu'un grain de sable"!!Return to passage

[373] Ibid.Return to passage

[374] His words are--"In silkworms I have clearly seen various small vessels spring from and approaching to the heart, which I have even filled with a coloured liquid. But whether they were veins or arteries I cannot yet affirm." i. 112. a. 176. a. According to Cuvier (Anat. Comp. iv. 418), but I cannot find the passage, Swammerdam also mentions having seen a red fluid issue from small vessels in grasshoppers.Return to passage

[375] Reaum. v. 103.Return to passage

[376] Bonnet ii. 309. Perhaps in both cases the alimentary canal was the organ seen.Return to passage

[377] Reaum. iv. 171--.Return to passage

[378] Lesser L. ii. 84. note.Return to passage

[379] De Geer ii. 505--. vi. 287.Return to passage

[380] On the Microscope. i. 130.Return to passage

[381] Ibid.Return to passage

[382] Sur le Vol des Ins. 325--.Return to passage

[383] Lyonet Anat. 427--.Return to passage

[384] Cuv. Anat. Comp. iv. 418--.Return to passage

[385] Mem. du Mus. 1819. 71.Return to passage

[386] N. Dict. d'Hist. Nat. xvi. 208.Return to passage

[387] Marcel de Serres, in his Observations on the Dorsal Vessel of Insects[388], endeavours to prove that the principal use of that vessel is the more perfect animalization of the chyle that, transuding through the pores of the intestinal canal, is imbibed by it. In insects, he observes, that undergo metamorphoses, in which the growth or development of parts is often very rapid, it is requisite that a considerable portion of the chyle should be in reserve for this purpose. On this account it is that the Epiploon or adipose tissue is so abundant in larvæ to what it is in the perfect insect. That the importance also of this part to insects is proved by the circumstance, that all their interior parts communicate by fibrils with this tissue, and that probably their various organs derive the nutriment from it by their means. He then asks by which of the viscera is the fat elaborated, or by what means does the chyle which transudes from the intestinal canal pass to the state of fat? Facts seem to indicate, says he, that the function of the dorsal vessel is to pump up the chyle, and to cause it then to transude through the meshes of the adipose tissue, where it finishes by elaborating that mass of fat so abundant in larvæ and certain perfect insects, which are thus enabled to sustain the effects of a long fast. So that this vessel is only a secretory organ, analogous to so many others that exist in insects; but the secretion which it has to produce is the most important of all, since the support of the vital powers depends upon it: it is, in effect, that vessel which completes the function of animalization, and which itself prepares the nutritive fluid[389]. He observes, amongst other reasons he brings to support his theory, that the colour of the fluid which it contains is always analogous to that of the adipose tissue that surrounds it, and that the colour of that tissue never changes without that of the fluid undergoing a corresponding alteration,--that when, as in many perfect insects, the quantity of fat diminishes, the dorsal vessel also diminishes in size, and that the same reagents which coagulate the fat, coagulate equally the fluid in the dorsal vessel, which seems to indicate an identity between them[390].Return to passage

But there are circumstances that militate against this hypothesis. The analysis which Lyonet has given of the fluid contained in the dorsal vessel of the Cossus[391], seems to prove that it is more analogous to gum or varnish. He saw indeed a few globules, which appeared ten times as big as the others, which swam upon the water, but which he did not regard as component parts of the fluid, but as little drops of grease extravasated by dissection. The fluid of the vessel itself easily mixed with water, and appeared to sink in it to the bottom[392]. This proves that it is not of a fatty or oleaginous nature. But the strongest objection is stated by M. Carus, who judiciously observes[393], That it is contradictory to suppose that a canal should absorb or exude fluids by its parietes in a different form. Further experiments however seem necessary to ascertain the nature of the fluid and its object.

[388] Mem. du Mus. 1819.

[389] Ibid. 68--.

[390] Ibid. 69--.

[391] See above, p. 85.Return to passage

[392] Lyonet Anat. 426--.Return to passage

[393] Introd. to Comp. Anat. ii. 277. Engl. Trans.Return to passage

[394] This seems some confirmation of Dr. Virey's opinion, that insects in their first states are still a kind of fœtus. See above, VOL. III. p. 61--.Return to passage

[395] Introd. to Comp. Anat. ii. 393--. Engl. Trans.Return to passage

[396] Introd. to Comp. Anat. ii. 395--. Engl. Trans.Return to passage

[397] Introd. to Comp. Anat. ii. 396--. Engl. Trans.Return to passage

[398] Ibid. 398.Return to passage

[399] Ibid. 399.Return to passage

[400] Ibid. 398.Return to passage

[401] Introd. to Comp. Anat. ii. 399--. Engl. Trans.Return to passage

[402] Reaum. i. 409, 643--. Malpigh. De Bombyc. 38.Return to passage

[403] Lesser L. ii. 87 note *.Return to passage

[404] Ubi supra.Return to passage

[405] Reaumur iv. 264.Return to passage

[406] Ibid. 260--.Return to passage

[407] Herold Schmetterl. 24.Return to passage

[408] Anat. Comp. iv. 165.Return to passage

[409] Marcel de Serres (p. 67.) speaks of this fluid as being, after it has transuded through the intestinal canal, a fluid in repose, which seems to indicate that it is perfectly stagnant; but when we consider that it is not only incessantly entering the body and making its way to every part, but is also, by means of the various secretory organs, constantly converted into new products, and so going out again in many cases, it will appear evident that it cannot be considered as a stagnant fluid, since there must be a constant though probably slow motion towards the points of absorption or imbibition.Return to passage

[410] Dr. Kidd (Philos. Trans. 1825. 236.) did not find the abdominal viscera of the mole-cricket thus circumstanced, nor more lubricated than the intestines of the higher animals.Return to passage

[411] Cuv. Anat. Comp. iv. 158. Herold Schmetterl. 28.Return to passage

[412] Sur le Vol des Ins. c. iv. 88. note 1.Return to passage

[413] Anat. 428.Return to passage

[414] Treviranus Arachnid. 28. t. iii. f. 28, 29.Return to passage

[415] Ibid. 29. t. iii. f. 30, 31.Return to passage

[416] N. Dict. d'Hist. Nat. xxx. 420. Comp. Treviran. Arachnid. 10--.Return to passage

[417] Ibid. 9--.Return to passage

Letter XL

_INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONTINUED._

DIGESTION.

"The immense Class of insects," says the immortal Cuvier, "in the structure of its alimentary canal exhibits as many variations as those of all the vertebrate animals together: there are not only the differences that strike us in going from family to family and from species to species; but one and the same individual has often a canal quite different, according as we examine it in its larva or imago state; and all these variations have relations very exact, often easily estimable, with the temporary or constant mode of life of the animals in which it is observable. Thus the voracious larvæ of the Scarabæi and butterflies have intestines ten times as large as the winged and sober insects--if I may use such an expression--to which they give birth[418]."

In the natural families of these creatures, the same analogy takes place with respect to this part that is observable in the rest of the Animal Kingdom; the length and complication of the intestines are here, as in the other Classes, often an index of a less substantial kind of nutriment; while their shortness and slenderness indicate that the insect lives by prey[419].

In considering therefore the parts connected with the digestive functions of the insect world, it will not be amiss to have reference to their food, and their mode of taking it; but first it will be proper to state and define the parts of this important organ.

In general the alimentary canal[420] is composed of the same essential tunicks as that of the vertebrate animals, consisting of an interior epidermis, a papillary and cellular tunick, and an exterior muscular one[421]. The first is usually tender, smooth, and transparent; but not always discoverable, perhaps on account of its tender substance[422]. Ramdohr does not notice the papillary and cellular tunicks; they are probably synonymous with what he denominates--the flocky layer (Die flockige lage), and which he describes, when highly magnified, as appearing to consist of very minute globules or dark points, and as being of a cellular structure[423]. The exterior tunick is thicker and stronger than the interior, and composed of muscular fibres, running either longitudinally, or transversely so as to form rings round the canal. This tunick mostly begins at the mouth, and goes to the anus, changing its conformation in different parts of the above intestine. Sometimes however it originates only at the beginning of the stomach[424]. With respect to its general disposition, that canal--in its relative length, in the size of its different parts, in the number and form of its dilatations, and particularly of its stomachs and its cœcums, and in the folds of its interior--exhibits variations altogether analogous to those of vertebrate animals, and which produce similar effects[425]. As to its parts, it may be considered as consisting of two larger portions, between which the biliary or hepatic vessels form the point of separation. In the first, the most universal parts are the gullet and the stomach; and in the second, the small intestine and the large intestine[426].

1. The gullet (Œsophagus[427]) is that portion of the intestinal canal which, receiving the food from the pharynx, or immediately from the mouth, conveys it to the stomach. Though it often ends just behind the head[428], it is usually continued through the trunk, and sometimes even extends into the middle of the abdomen[429]; it therefore seldom much exceeds in length half the body. It is constantly long when the head is connected with the trunk by a narrow canal--as in the Hymenoptera, Neuroptera, Lepidoptera, &c.; but is frequently short when these parts are more intimately united[430]. It often ends in a kind of sac analogous to the crop of birds. Under this head I must mention a part discovered by Ramdohr, which he calls the food-bag (Speisesack), as he thinks, peculiar to Diptera[431]. From the mouth in these proceeds a narrow tube into the abdomen, where it expands into a blind sac having no connexion with the stomach; so that the fluid food, as blood, &c. stored in it, must be regurgitated into the mouth before it can pass into that organ[432]. Thus these animals, besides their stomach, have a reservoir in which to store up their food; the product therefore of a single meal will require several days to digest it.

2. The stomach (Ventriculus[433]) is that part of the intestinal canal immediately above the bile-vessels, which receives the food from the gullet for digestion, and transmits it when digested to the lower intestines[434]. By its admixture with the gastric juice, the food acquires in the stomach a quite different colour from what it had in the gullet. In herbivorous insects it contains no acid, but, like the gastric juice of herbivorous quadrupeds, is of an alkaline nature[435]. The chyle is forced through this organ, probably in part by the pressure of the muscular fibres during the peristaltic motion; and being pressed through the inner skin, is first collected in the intermediate cellular part, and ultimately forced through the outer skin[436]. At its posterior end it terminates in the pylorus, a fleshy ring or sphincter formed of annular muscular fibres[437]. The stomach often consists of two or more successive divisions, which are separated from each other, and are often of an entirely different conformation and shape[438]. In the Orthoptera, Predaceous Coleoptera, and several other insects, an organ of this kind precedes the ordinary stomach, which from its structure Cuvier denominates a second stomach or gizzard[439]; Posselt improperly calls it Cardia[440]; and by Ramdohr it is named the plaited-stomach (Falten-magen[441]). It is a short fleshy part consisting of two skins, placed above the opening of the stomach, and perhaps rather belongs to the gullet. The inner skin is formed into longitudinal folds, and sometimes armed with horns, teeth, or bristles. Its cavity is very small and compressed, so as to admit only small masses of food, and yet present them to a wide surface for the action of the teeth or bristles;--in this stomach therefore, as in the gizzard of birds, to which it seems clearly analogous[442], the food is more effectually comminuted and rendered fit for digestion. The muscles, by which its action upon the food is supported, in some species amount to many thousands[443]. Rudiments of a gizzard are sometimes found concealed in the gullet of many insects[444]. The idea of Swammerdam, Cuvier, &c. that grasshoppers and other insects that have this kind of stomach, chew the cud[445], Ramdohr affirms is entirely erroneous[446]. Besides its divisions, the stomach has other appendages that require notice. In most Orthoptera, a pair or more of blind intestines or cœca may be found at the point of union of the gizzard with the stomach[447], which have been regarded as forming a third stomach: they also begin the stomach in the louse[448]; they form a coronet round the apex of that organ, in the grub of the cockchafer[449]; and in that of the rose-beetle, there is one at the apex, one in the middle, and a third at the base[450]. Besides these appendages, which are formed of the skin of the stomach, there are others that are not so. In the Predaceous and some other beetles, the whole external surface of this organ is covered with small blind appendages opening into the space between its two skins, which cause it to resemble a shaggy cloth; these Ramdohr calls shags (zotte[451]), and Cuvier, hairs[452] (villi). These appendages the latter author seems to regard as organs that secrete the gastric juice and render it to the stomach[453]; but the former thinks their use uncertain[454].

3. The small intestines (Intestina parva) are the portion of intestines next the stomach, and consist often of three distinct canals;--the first is supposed to be analogous to the duodenum; it is found only in the Coleopterous genera Silpha L. and Lampyris L., and is distinguished from the succeeding intestine by being perfectly smooth[455]. Next follows the thin intestine (Dünndarm), which in the above insects is wrinkled; it most commonly immediately follows the stomach. Sometimes it is wholly wanting, as in Agrion, the Hemiptera[456], &c. Ramdohr conjectures that it is not solely destined for conveying the excrement, but that probably some juices are separated in it from the food especially for the nutrition of the gall-vessels, as their principal convolutions are mostly near this intestine[457]; which perhaps may in some cases be regarded as analogous to the jejunum in vertebrate animals. The third pair of the small intestines, which perhaps represents the ileum, Ramdohr distinguishes by the name of club-shaped (Keulförmigen Darm[458]). It may generally be regarded as only a continuation of the former thickened at the end so as to resemble a club reversed. It is however sometimes separated from the thin intestine, as in Cerambyx moschatus[459].

4. The large intestines (Intestina magna) consist sometimes of two portions. The thick intestine (Dicken-darm), which may be regarded as a kind of cœcum, is found only in the larvæ of the Lamellicorn beetles, but never in the perfect insect. In shape it is oval and folded; whence it is thicker than the rest of the intestinal canal, and is constantly filled with excrement[460]. The second portion of these intestines is the rectum (Mastdarm), which terminates in the anal passage. This part is scarcely ever wanting, except when the insect evacuates no excrement, which is the case with the grubs of bees, wasps, and the antlion (Myrmeleon). In the imago of Telephorus, at least in T. fuscus, it is also obsolete[461]: in most cases, however, it is very distinct from the preceding intestine. Sometimes it consists of only one tunick composed of muscular fibres[462]. When the gullet is wide, the rectum is usually so likewise; but when it follows a club-shaped or thick intestine, it is narrow[463]. It generally may be termed short[464]. When wide, it often contains a great quantity of excrement, as the gullet does of undigested food; but when narrow, the excrement seldom remains long in it. This intestine also in a few cases has a lateral enlargement or cœcum (Blind-darm), being a continuation of the same skin; but perhaps this enlargement is really analogous to what Ramdohr calls the thick intestine, though in these cases he regards it as an appendage of the rectum[465].

I must now call your attention to the bile-vessels of insects. These, by Malpighi[466] and the earlier physiologists, who regarded them as a kind of lacteals, were denominated varicose vessels: but Cuvier--and his opinion after some hesitation has been adopted by Ramdohr--considers them as vessels for the secretion of bile, and as analogous to the liver of animals that have a circulation[467]. As the want of blood-vessels prevents insects from having any gland, the bile is produced with them, as all their other secretions, by slender vessels that float in their nutritive fluid, and from thence secrete the elements proper to form that important product, which usually tinges them with its own yellow hue; though in the Lamellicorns and Capricorns they are of an opaque white, and in the Dytisci of a deep brown colour[468]. Their bitter taste further proves that they contain the bile[469]. They are long, slender, filiform, tortuous or convoluted, and mostly simple vessels; sometimes gradually smaller toward the base[470], at others towards the apex[471]. In some, screw-shaped[472]: in one larva, with hemispherical elevations[473]: in the cockchafer, part of them are fringed on each side with an infinity of short, blind, minute, setiform tubes, while the rest are naked[474]; they are composed of a single, thin, transparent membrane, according to Ramdohr[475]; but Cuvier thinks their texture is spongy[476]. They appear to contain a number of small, irregular, dark granules, which float in a peculiar fluid, with which, however, they are not always filled throughout, nor are they constantly permeable from one end to the other. Thus in the meal-worm beetle (Tenebrio Molitor), the common trunk by which they are attached to the intestinal canal is composed of gelatinous granules[477]. The place of their insertion is generally a little below the pylorus, but in the common cockroach they are inserted into the stomach just above that part[478]. Usually each vessel opens singly into the intestinal canal, which the whole number surround at an equal distance from each other[479]. Sometimes, however, they are connected with it by a common tube in which they all unite, as in the asparagus-beetle (Lema Asparagi[480]), and the mole-cricket (Gryllotalpa vulgaris[481]); in the house-fly (Musca domestica), and other Muscidæ, each pair unites so as to form a single branch on each side of the canal previously to their insertion[482]; in the field-cricket (Gryllus campestris) they are all inserted in one spot[483]; and when numerous, they are generally attached singly though irregularly[484]. These vessels at their base do not open into the cavity of the intestinal canal, but merely into the space between its outer and inner tunicks, the last being constantly imperforated[485].

With regard to their apex, the bile-vessels are sometimes fixed singly or connectedly to the intestine merely by a few muscular fibres; for they do not enter it, their ends having no orifice. This structure is mostly to be met with in the Coleoptera[486]. In caterpillars, the tops of these vessels perforate the outer skin of the rectum, and proceeding in dense convolutions to the anus, become at last so fine that their terminations cannot be discovered[487]. In other cases, the extremities of a pair of these vessels unite so as to form a double one: this may be seen in those of Philonthus politus[488], and probably other rove-beetles: and lastly, in others the bile-vessels are free, hanging down by the intestinal canal, without being attached to it or to each other. This structure is constantly found in the Orthoptera and Hymenoptera Orders, &c.[489].

With regard to their number, the bile-vessels vary from two to upwards of one hundred and fifty, yet so that their whole amount is constantly the product of the number two,--at least as far as they have been counted: and even when those on one side are not alike, a similar variation takes place in the other, as may be seen in Galleruca Vitellinæ, where on each side are two long ones and one shorter[490]; the most usual numbers are, four--six--or many, that is, more than twenty--

Two bile-vessels are found in the larva of Cetonia aurata[491].

Four most Coleoptera, Diptera, and Hemiptera[492].

Six Lepidoptera, some Coleoptera[493], &c.

Eight Myrmeleon, Hemerobius[494].

Fourteen Formica rufa[495].

Twenty larva of Clavellaria Amerinæ[496].

Many Libellulina, Orthoptera, and Hymenoptera[497].

The bile-vessels vary considerably in length: in many cases where they are free they are short[498]; they are often very long, and perhaps those that are fixed may be generally stated as the longest. In the Lamellicorn beetles they are remarkable for their great length[499].

Having given you this general account of the intestinal canal and its parts and appendages, I shall now state some of the peculiarities that in this respect distinguish particular tribes and families.

The Coleoptera alone, exhibit as many variations in the structure of the alimentary tube as all the other Orders of insects together:--to particularize these would occupy too large a portion of this letter, I shall therefore only notice a few of the most remarkable. In general they may be stated as having universally a stomach, a small intestine and rectum, and not more than three pairs of fixed or united bile-vessels. In the Predaceous beetles, the gullet mostly widens at the base into a considerable crop, followed by a gizzard, a shaggy stomach, and two pairs of united bile-vessels. The whole alimentary canal in these, is never less than double, and sometimes treble the length of the body[500]. In the carnivorous beetles, at least the Staphylinidæ and Silphidæ, there is little or no crop, and the gizzard is hidden: in the former, the whole length of the intestinal canal is not twice, while in the latter it is more than four times that of the body[501]. In these also the intermediate portion of the large intestine is singularly annulated[502]. In the Petalocera the stomach is usually longer than all the rest of the intestines together, and often convoluted: in the cockchafer the whole intestinal canal is nearly five times the length of the body, four parts of which is occupied by the stomach[503]. In the grub the canal scarcely exceeds the length of the animal[504]. In Lampyris the stomach exhibits a remarkable appearance, having on each side a series of spherical folds or vesicles[505]. Have these any thing to do with the secretion of its phosphoric matter? Tenebrio has a gizzard armed internally with calluses, and a shaggy stomach, and Blaps does not differ materially; their entire canal is more than twice the length of the body[506]. In the vesicatory beetles (Cantharis, Meloe, &c.) there is no gizzard, and the canal is less than twice the length of the body[507]. Little is known with regard to the alimentary canal of the beetles distinguished by a rostrum (Rhyncophora). In the only two that appear to have been examined, Rhynchites Betuleti and Cryptorhynchus Lapathi, that canal is moderately long, the stomach partially shaggy, and the small intestine inversely claviform; but in other respects they differ materially[508]. In the former there is no crop or gizzard, the stomach is fringed on each side, except at its upper extremity, with a series of small cœca or shags, and there are three pairs of bile-vessels[509]; while in the latter the gullet is dilated into a crop which includes a gizzard in which the skill of a DIVINE artist is singularly conspicuous:--though so minute as scarcely to exceed a large pin's head in size, it is stated to be armed internally with more than 400 pairs of teeth, moved by an infinitely greater number of muscles[510]. A transverse section of this gizzard represents two concentric stars, with nine rays each[511]: the object of this structure is, the comminution of the timber which this beetle has to perforate and probably devour[512]. The stomach is very slender, but dilates in the middle into a spherical vesicle[513], and there are only two pairs of bile-vessels[514]. In the Capricorn beetles, the part we are considering varies much: in general we may observe that it is more than double the length of the body, that the stomach is long and slender, and usually naked, that the gullet terminates in a crop without a distinct gizzard, and that there are three pairs of bile-vessels[515]. In the Herbivorous beetles (Chrysomela, Cassida, &c.) the canal is more than double the length of the body, and in some much longer[516], the stomach is long, and commonly naked; but in Chrysomela violacea it is covered with hemispherical prominences[517], and in Chrysomela Populi it is shaggy[518]; in the insect last named and Galleruca Vitellinæ the rectum consists of two pieces[519]. In this tribe the intestines of the larva resemble those of the perfect insect[520].

In the Orthoptera the alimentary canal, which continues the same in every state, is short, or only moderately long; the gullet has one or two lateral pouches or crops[521], and terminates in a gizzard of curious construction, with singular folds and teeth[522]; then follows a short stomach, usually with a pair or more of cœca at its upper extremity[523]; the lower intestines are not distinct, and the bile-vessels numerous, short and free[524].

In the Neuroptera, many of the genera are distinguished by the remarkable length of the gullet, and by the lower intestines forming one short piece[525]. In the Libellulina the bile-vessels are numerous, short, and free, as in the Orthoptera[526]. In Hemerobius and Myrmeleon there is a gizzard[527], and just above it a cœcum, in the former very remarkable, is connected with the gullet[528].

The Hymenoptera appear all to be distinguished by a long slender gullet, terminating in a dilated crop forming the honey-bag; their stomach is variable, their small intestine slender, and the rectum dilated;--their bile-vessels, like those of the two preceding Orders, are numerous, short, and free[529]. In the ants and ichneumons there is an approach to a gizzard[530]. In the wasp and humble-bee the stomach is very long, with muscular rings surrounding it[531]. In this Order the larvæ at first have no lower intestines and void no excrement[532], but as they approach to the pupa state one begins to appear[533].

The next insects whose alimentary canal we are to consider, are those which, taking their food by suction, have no occasion for masticating organs: this may in part be predicated of the preceding Order, in which most of the tribes in their perfect state imbibe fluid food, and use the ordinary organs of mastication principally in operations connected with their economy; and their crop, in which the honey in many is stored up for regurgitation, may be regarded in some degree as analogous to the food-bag of the Diptera and other suctorious insects.

The two sections of the Hemiptera Order differ widely in the canal we are considering, and I shall therefore give a separate account of each. In the Heteropterous section, appended to the gullet by a long convoluted capillary tube, besides the usual saliva-reservoirs there is often a double vessel, which Ramdohr regards as discharging the same function, but which in many respects seems rather analogous to the food-reservoir of the Diptera[534]. As I have had no opportunity of examining this vessel, I shall content myself with stating this idea, and describe the vessel more fully hereafter. The gullet, in these, usually terminates in an ample crop consisting of many folds[535], followed by a long, slender, cylindrical tube, dilated at its base into a spherical tumour; these two may be said to form the first stomach: to this succeeds a second[536], which Ramdohr denominates the bug-stomach (Wanzen-magen), which varies in its figure, and in Pentatoma consists of four demi-tubes, so as to form a quadrangular canal[537]. In the Homopterous section of this Order Ramdohr seems to have examined but few; Chermes however and Aphis exhibit one remarkable feature; they have no bile-vessels, at least he could discover no trace of these organs[538]. Their intestinal canal is very simple, their stomach very long, widest above, and somewhat convoluted, with a very slender gullet[539]. In Cercopis spumaria the structure is more complex, and extremely singular. It has two or rather three stomachs; the two first of a horny substance, and the last a slender somewhat convoluted membranous tube, which becoming reversed, is attached by what should be deemed its lower extremity to the first stomach, from the other side of which emerge the lower intestines, terminating in a thick pear-shaped rectum. At the same point of the first stomach the four bile-vessels are attached, they grow gradually thicker for about a third of their length, when they become twisted like a cord, and taper towards the rectum, to which also they are attached[540]. From this structure it should seem that the food has to pass twice through the first stomach, before the process of digestion is complete, and it is rejected at the anus.

The next suctorious Order is the Lepidoptera: in these the gullet is long and slender, surrounded at the beginning with a loose transparent skin, and at the base furnished with a pair of lateral sacs, forming the honey-stomach, and probably analogous to the food-reservoirs of the Diptera, which when blown up are of an oval form; the stomach, as in the bugs, consists of two portions, the first being the longest[541]. There are three free bile-vessels on each side, proceeding from a single branch[542]. It will not be uninteresting here to abstract from Herold the progressive changes which take place in the intestinal canal in this Order, during the transition of the animal from the larva to the imago state. In the larva, the gullet, the small intestine, and the rectum, are short and thick[543], there are a pair of silk reservoirs (sericteria), as well as vessels for the secretion of saliva (sialisteria): if you examine it two days after its first change, you will find the gullet and the small intestine much lengthened and become very slender; the stomach contracted both in length and size; the rectum also changed, and the silk vessels contracted[544]. These in a pupa eight days old have wholly disappeared; the gullet is become still longer, its base is dilated into a crop or food-reservoir; the stomach is still more contracted, and instead of a cylinder represents a spindle; the small intestine also is lengthened[545]: at a still more advanced period, when it is near appearing under its last form, the gullet and small intestine are still more drawn out; and the honey-bag, though very minute, has become a lateral appendage of the gullet[546]; and lastly, in the butterfly it appears as a large vesicle[547]; the small intestine is grown very long[548]; and the rectum has changed its form and acquired a cœcum[549]. When we consider the adaptation of all these changes of form, the loss of old organs and the acquisition of new ones, to the new functions and mode of life of the animal, we see evidently the all-powerful hand of that ALMIGHTY BEING who created the universe, upholding by his providence, and the law that he has given to every creature, the system that he at first brought into existence.

We now come to the Diptera. These have a very slender gullet, to which is attached on one side a long filiform tube, terminating in the food-reservoir, which in some instances is simple[550], but most generally consists of two or more vessels[551], collapsing when empty, but varying in shape and size when inflated with food: the mouth of the stomach in many cases is dilated into a kind of ring[552]; sometimes there is on each side a blind appendage or cœcum opening into it, in Bombylius covered with shags, which though not connected with the mouth by a tube, Ramdohr regards as saliva-reservoirs[553]; in Musca vomitoria the beginning of this organ below the mouth is covered with hemispherical prominences, and in Tipula it is dilated and marked with transverse folds. There are usually two pairs of bile-vessels; in the Muscidæ pedunculate and free[554]; in Tipula, Bombylius, and Leptis, sessile and united[555]; and in Tabanus sessile and fixed[556]. It is remarkable that in some of this Order--the reverse of what usually happens--the alimentary canal appears to be much longer in the larva than it is in the imago; in Musca vomitoria, its length in the former is two inches and a quarter, while in the latter it is only one inch and one third[557]. A singular organ distinguishes the imago of this species, the use of which appears not to be discovered. It succeeds the rectum, and has on each side two short club-shaped appendages, open at the end, which receive tracheæ, and terminate in a short piece that opens into the anus[558].

In Hippobosca and its affinities the canal in question differs from that of other Diptera, in having no food-reservoir; in other respects it resembles it[559].

From the above statement it appears that the principal character which distinguishes those that take their food by suction, from those that masticate it, is the faculty with which they are furnished by means of an ample crop, honey-stomach, or food-reservoir, of regurgitating the food they may have stored up. Another distinction still more striking, which will appear more evidently hereafter, is to be seen in the saliva-secretors with which the suctorious tribes are furnished, to be found in very few masticators, by which they are enabled to render the juices more fluid and fit for suction.

The only insect amongst the Aptera whose alimentary canal I shall notice, is the common harvest-man (Phalangium Opilio): in this, though the stomach and lower intestine are remarkably simple, yet their cœcal appendages are numerous and singular: the former, which has no distinct gullet, is pear-shaped[560]; and the latter, tapering downwards, and truncated at the end[561]; connected with it above are no less than twenty-three cœca or blind appendages, of various forms and dimensions; the last pair but one of which is very remarkable, being bent like a bow, and furnished externally with four short clavate processes[562]. It is probable that some of these organs are analogous to the bile-vessels of other insects.

* * * * *

When the CREATOR in his wisdom fixed the limits of the various tribes of animals, he united them all into one harmonious system by means of certain intermediate forms, exhibiting characters taken some from those that were to precede, and others from those that were to follow them, and this not only in their external structure, but likewise in their internal organization; so that we are not to wonder if in the same individual we meet with organs that belong to two distinct tribes, or if, remaining nearly the same in their prima facie appearance, they begin to exercise new functions. An instance of this we have seen in the dorsal vessel of insects, which in the Arachnida, though not materially different in situation or general form, by the addition of a small apparatus of arteries and veins becomes the centre and fountain of a regular system of circulation[563]. From the circumstances here alluded to, physiologists have been led to entertain very different sentiments with regard to the structure of the alimentary organs of the Class we are now to enter upon, the Arachnida: what some regard as a real liver, others look upon as an epiploon or caul; and what the last denominate bile-vessels are by some of the former considered as appropriated to the secretion of chyle[564]. Yet both these opinions have some foundation in nature. When, in the Arachnida, we discover a lobular substance consisting of granules filling the whole cavity of the body and wrapped round the intestines, every one will see in it no small analogy to the epiploon which in insects performs the same function: but when, upon a further examination, we detect certain vessels communicating with this substance and the intestinal canal[565], the idea that these may be hepatic ducts, and this substance analogous to the liver, immediately strikes us as not improbable. Again: when we discover pairs of other capillary and tortuous vessels connecting with the intestinal canal either at the pylorus[566] or below it[567], which in appearance strikingly resemble the bile-vessels which we so constantly find in insects, we seem warranted in concluding that they are of the same nature and use: but when a nearer inspection enables us to detect the hepatic ducts just mentioned in the scorpion, and we find that these capillary vessels in the spider are in a very different situation from those in insects which we suppose them to represent, it occurs to us as not unlikely, that their function may be different.

Let us now consider how the intestinal canal is circumstanced in the two sections into which the Class Arachnida is divided; the Scorpionidea, and Araneidea. In the Scorpions, this organ proceeds from the mouth to the anus without any flexure or convolution, so that its length is scarcely equal to that of the body[568]; it is slender, and its diameter, with the exception of an irregular dilatation here and there, is nearly the same in its whole extent; the gullet is short; the stomach long, and nearly cylindrical; the duodenum shorter and thicker than the stomach, from which, as well as from the rectum, it is separated by a valve; the latter is cylindrical, and opens at the anus above the insertion of the vesicle that secretes the poison[569]. With regard to the biliary system and its organs: The liver is of a pulpy granular consistence and of a brownish colour, fills the whole cavity of the trunk and abdomen, and serves as a bed for the other intestines. It is divided longitudinally into two portions, by the channel in which the heart reposes--its anterior part is formed into many irregular lobes, by the sinuosities of the trunk; at the other extremity, it terminates in two acute ends, which enter the first joint of the tail; its surface presents a reticular appearance, the result of the approximation of polygonous lobuli; its interior is a tissue of infinitely minute glands: in Scorpio occitanus there are about forty pyramidal lobuli detached from each other, the summits of which, by their union, form bunches that have their excretory canals, varying in number in different species, which convey the bile to the alimentary tube; in the above insect there are six pairs, three in the trunk and three in the abdomen, and in S. Europæus a smaller number[570]; these vessels run transversely from the liver, or aggregation of conglomerate glands, to the intestinal canal[571]; the bunches consist of an infinite number of spherical glands, generally filled with a brown thick fluid[572]: besides the transverse vessels, from the base of the stomach there issue two pairs of very slender tortuous ones, seemingly analogous to the common bile-vessels; one pair of which runs upwards, one on each side that organ towards the mouth, forming here and there some ramifications which enter the liver; and the other runs nearly transversely to it[573]. As the fluid contained in these vessels is different from that contained in the glands of the liver, M. Marcel de Serres supposes they may be chyliferous[574].

In the Araneidea also the alimentary canal is nearly straight, and scarcely exceeds the length of the body: the gullet is rather thick and cylindrical[575]; the stomach is distinguished anteriorly by two pairs of sacs, the upper pair being much the largest and nearly triangular, the lower linear[576]; from these sacs a narrow tube runs towards the rectum, but which is so entangled with the liver, muscles, &c., as not to be easily made out[577]; the rectum is rather tumid, and has a lateral cœcum[578]. The disposition of the liver or conglomerate glands is stated to be similar to that of the scorpion[579]; it is usually white, but in some species it is yellowish, or reddish, and its lower surface has sometimes regular excavations[580]; no transverse hepatic ducts connecting it with the alimentary canal, as in the scorpion, appear to have been at present discovered: two pairs of capillary free vessels are attached to the base of the rectum on one side, which, except in their situation, seem analogous to the bile-vessels of insects[581].

From the above detailed account of the alimentary canal of the animals whose internal anatomy we are considering, it appears that M. Cuvier's observation--that the length and complication of the intestines indicate a less substantial kind of nutriment--does not hold universally: thus, in Necrophorus and Silpha, carnivorous insects, the intestinal canal in its length and convolutions exceeds those of most herbivorous ones, and in Cassida viridis and some others of the latter tribe are not longer than those of the predaceous beetles. In herbivorous larvæ also, in general, the length of the alimentary canal does not exceed that of the body, but in those of some flesh-flies (Musca vomitoria) it very greatly exceeds it[582]. So true is the observation--that there is no general rule without exceptions.

In this letter it may not be out of place to say a few words upon the excrements of insects; which, strange as the observation may seem, but it is no less true than strange, are sometimes pleasing to the eye, from their symmetry, and to the taste, from their sweetness. In those that masticate their food they are solid, and in those that take it by suction, fluid or semi-fluid. In the caterpillars of Lepidoptera they are of the former description, and every grain wears some resemblance to an insect's egg: as the passage in many of these consists of six fleshy parts separated by channels, so the excrement represents six little prisms separated by six channels[583]. The Aphides all secrete a fluid excrement as sweet as honey, of which the ants are so fond[584], which is ejected not only at the anal passage, but, in many, by two little siphonets also above it[585]. A semi-fluid excrement is produced by some species of Chermes, as that which inhabits the Box, which often comes from the animal in long convoluted strings resembling vermicelli. Reaumur says its taste is agreeable, much more so than that of manna[586]. Under this head should be included the abundant spume with which the larva of Cercopis spumaria envelopes itself[587].

I am, &c.

FOOTNOTES:

[418] Anat. Comp. iv. 129.Return to passage

[419] Cuv. Anat. Comp. iv. 129.Return to passage

[420] PLATE XXI. FIG. 3. c, d, e, is the intestinal canal of the larva of the Cossus.Return to passage

[421] Cuv. Ibid. 112.Return to passage

[422] Ramdohr Anat. der Ins. 6.Return to passage

[423] Ibid. 25.Return to passage

[424] Ibid. 6.Return to passage

[425] Cuv. ubi supr. 113.Return to passage

[426] Comp. Ramdohr Anat. 7.Return to passage

[427] PLATE XXI. FIG. 3. c.Return to passage

[428] Tenebrio Ramdohr, ubi supr. 9. t. iv. f. 1.Return to passage

[429] Agrion. Ibid. t. xv. f. 4. a, b.Return to passage

[430] Ibid.Return to passage

[431] Many other insects that live by suction have something similar, as the honey-bag of butterflies, PLATE XXX. FIG. 10, 11. a. Ramdohr t. xviii. f. 2. with t. xix. f. 1-3. and xxi. 1, 3, &c.Return to passage

[432] Ramdohr Anat. 11--.Return to passage

[433] PLATE XXI. FIG. 3. d.Return to passage

[434] Ramdohr Ibid. 28--.Return to passage

[435] Herold (Schmetterl. 24) says that Ramdohr is mistaken here, and denies the existence of this juice in insects; but as Ramdohr's researches were so widely extended, he is most likely to be right.Return to passage

[436] Ramdohr Ibid. 29.Return to passage

[437] Ibid. 31.Return to passage

[438] Ibid. 28.Return to passage

[439] Anat. Comp. iv. 135. Comp. Dr. Kidd in Philos. Trans. 1825. 223. t. xv. f. 6, 7.Return to passage

[440] Ramdohr Anat. 15.Return to passage

[441] Ibid. 15.Return to passage

[442] Ibid. 18.Return to passage

[443] Ibid.Return to passage

[444] Ibid.Return to passage

[445] Swamm. Bibl. Nat. i. 94. b. Cuv. Anat. Comp. iv. 134.Return to passage

[446] Ubi supr. 18.Return to passage

[447] Ibid. t. i. f. 1. e. 5. c. 9. g, h.Return to passage

[448] Ibid. t. xxv. f. 4. bb.Return to passage

[449] Ramdohr Anat. t. viii. f. 3. cc.Return to passage

[450] Ibid. t. vii. f. 2.Return to passage

[451] Ibid. 20.Return to passage

[452] Anat. Comp. iv. 132.Return to passage

[453] Ibid. and 136.Return to passage

[454] Ubi supr. 30.Return to passage

[455] Ibid. 31. t. iv. f. 2. c. t. v. f. 1. d. f. 4. D.Return to passage

[456] Ibid. 32.Return to passage

[457] Ibid. 34.Return to passage

[458] Ramdohr Anat. 35.Return to passage

[459] Ibid. t. xxiv. f. 1. F.Return to passage

[460] Ibid. 36. t. vii. f. 2. kk. t. viii. f. 3. g, hh.Return to passage

[461] Ibid. t. xii. f. 1. t. xvii. f. 1. t. vii. f. 5.Return to passage

[462] Ibid. 37.Return to passage

[463] Ibid. 38.Return to passage

[464] Ibid.Return to passage

[465] Ramdohr Anat. 40.Return to passage

[466] De Bombyc. 18--.Return to passage

[467] Anat. Comp. iv. 153.Return to passage

[468] Ibid.Return to passage

[469] Ibid.Return to passage

[470] Ramdohr 43. Cicindela campestris, t. iii. f. 1. K.Return to passage

[471] Phryganea grandis, Ibid. t. xvi. f. 2.Return to passage

[472] Notonecta glauca, Ibid. t. xxiii. f. 5.Return to passage

[473] Of Musca vomitoria, Ibid. t. xix. f. 5.Return to passage

[474] Ibid. t. viii. f. 1. H. and G. f. 2.Return to passage

[475] Ibid. 50.Return to passage

[476] Ibid.Return to passage

[477] Ibid.Return to passage

[478] Ibid. 44. t. i. f. 9.Return to passage

[479] Ibid.Return to passage

[480] Ibid. t. vi. f. 5. H.Return to passage

[481] Kidd in Philos. Trans. 1825. t. xv. f. 6.Return to passage

[482] Ibid. t. xix. f. 1. N, N, O, f. 2. P, P, O.Return to passage

[483] Ibid. t. 1. f. 1. kkk.Return to passage

[484] Ramdohr, t. xiii. f. 1-3.Return to passage

[485] Ibid. 44.Return to passage

[486] Ibid. 45.Return to passage

[487] Ibid. 45. PLATE XXI. FIG. 3. f. f.Return to passage

[488] Rhamdohr, Ibid. t. iii. f. 6. E.Return to passage

[489] Ibid. t. i. f. 1. 5. 9. t. xiv. f. 1-3.Return to passage

[490] Ibid. 46. t. vi. f. 3.Return to passage

[491] Ramdohr, t. vii. f. 2.Return to passage

[492] Ibid. t. ii. iii. &c. t. xx. f. 1, 2. 6. t. xxii. f. 1-5. &c.Return to passage

[493] Ibid. t. xviii. f. 1. 5. t. iv. f. 1. See also t. vi. f. 1. 3.Return to passage

[494] Ibid. Anat. t. xvii. f. 1, 2. 6.Return to passage

[495] Ibid. t. xiv. f. 3.Return to passage

[496] Ibid. t. xiii. f. 4.Return to passage

[497] Ibid. t. xv. f. 3, 4. t. 1. f. 1. 5. 9. t. xii. f. 4, 5, 6, &c.Return to passage

[498] Ibid. t. xi. f. 4. t. xii. f. 4-6. t. xiii. f. 2-4, &c.Return to passage

[499] Ibid. t. vii. f. 1. t. viii. f. 1, &c.Return to passage

[500] Ramdohr Anat. t. ii. iii. xxv.Return to passage

[501] Ibid. t. iii. f. 6. t. iv. f. 2. t. v. f. 1.Return to passage

[502] Ibid. f. l. e. f. 3.Return to passage

[503] Ibid. 122.Return to passage

[504] Ibid. 123.Return to passage

[505] Ibid. t. v. f. 4. B.Return to passage

[506] Ibid. 94.Return to passage

[507] Ibid. 96--.Return to passage

[508] Ramdohr t. x. f. 1. 8.Return to passage

[509] Ibid. f. 8. b. c.Return to passage

[510] Ibid. 98. t. x. f. 2-4. From Ramdohr's figure, compared with the size of the insect, it appears that the gizzard could scarcely have been of greater diameter.Return to passage

[511] Ibid. f. 2.Return to passage

[512] See W. Curtis in Linn. Trans. i. 88.Return to passage

[513] Ramdohr t. x. f. 1. d.Return to passage

[514] Ibid. l l.Return to passage

[515] Ibid. t. ix. f. 1, 2. t. xi. f. 3. t. xxiv. f. 1, 2.Return to passage

[516] Ramdohr 103.Return to passage

[517] Ibid. 104. t. vi. f. 4. D.Return to passage

[518] Ibid. f. 2. B.Return to passage

[519] Ibid. t. vi. f. 3. E.Return to passage

[520] Ibid. 101.Return to passage

[521] Ibid. t. i. f. 1. 5. 9.Return to passage

[522] Ibid. f. 2, 3, 4. 7, 8. 12.Return to passage

[523] Ibid. f. 1. e, f. 5. c. f. 9. g h.Return to passage

[524] Ibid. f. 1. 9. k.Return to passage

[525] Ibid. t. xv. f. 3, 4. t. xvii. f. 2. 6.Return to passage

[526] Ibid. t. xv. f. 3, 4, f.Return to passage

[527] Ibid. t. xvii. f. 2. c. f. 6. d.Return to passage

[528] Ibid. f. 2. b. f. 6. c.Return to passage

[529] Ramdohr t. xii. f. 6. H. t. xiii. f. 1. f.Return to passage

[530] Ibid. t. xiv. f. 2, 3, C.Return to passage

[531] Ibid. t. xii. f. 6. D. t. xiii. f. 1. b.Return to passage

[532] Ibid. 133. t. xii. f. 1-3.Return to passage

[533] Ibid. f. 4.Return to passage

[534] Comp. Ramdohr t. xxii. f. 3. M. FIG. 4. 3. with t. xxi. f. 1. I.Return to passage

[535] Ramdohr t. xxii. f. 1. c. f. 3, 4. B--.Return to passage

[536] Ibid. f. 1. D E. f. 3. C D.Return to passage

[537] Ibid. t. xxii. f. 1. D, E. f. 3. C, D. f. 4. C.Return to passage

[538] Ibid. 198.Return to passage

[539] Ibid. t. xxvi. f. 2. 4.Return to passage

[540] Ibid. t. xxxiii. f. 3.Return to passage

[541] Ramdohr t. xviii. f. 1. F, G.Return to passage

[542] Ibid. L, K.Return to passage

[543] PLATE XXX. FIG. 7.Return to passage

[544] Ibid. FIG. 8.Return to passage

[545] Ibid. FIG. 9.Return to passage

[546] PLATE XXX. FIG. 10.Return to passage

[547] Ibid. FIG. 11. a.Return to passage

[548] Ibid. e.Return to passage

[549] Ibid. d.Return to passage

[550] Ramdohr, Ibid. t. xx. f. 1. E. f. 6. C.Return to passage

[551] Ibid. t. xix. f. 2. C. f. 3. CCD. t. xx. f. 2. E.Return to passage

[552] Ibid. t. xix. f. 2. D.Return to passage

[553] Ibid. t. xx. f. 2. FF. f. 6. DD. 184. 180.--Return to passage

[554] Ibid. t. xix. f. 1. ON. f. 2. OP. f. 3. F. t. xxviii. f. 1, 2. p. q.Return to passage

[555] Ramdohr, Ibid. t. xx. f. 1. G. f. 2, 3. L.Return to passage

[556] Ibid. t. xxi. f. 1. D.Return to passage

[557] Ibid. 172.Return to passage

[558] Ibid. t. xix. f. 2. K L. This organ seems analogous to that with four retractile fleshy horns, observed by Reaumur and De Geer in other species of Muscidæ. Reaum. iv. t. xxviii. f. 13. a, s. De Geer vi. t. iii. f. 18. c, d.Return to passage

[559] Ramdohr t. xxi. f. 6.Return to passage

[560] Ramdohr t. xxix. f. 1 *. A.Return to passage

[561] Ibid. and f. 3. B, D.Return to passage

[562] Ibid. f. 2, 3. 5. &c.Return to passage

[563] See above, p. 99--.Return to passage

[564] Treviranus and Ramdohr are of the former opinion; and Meckel, Cuvier, Marcel de Serres, and Leon du Four, of the latter.Return to passage

[565] Treviran Arachnid. t. 1. f. 6. v.Return to passage

[566] Ibid. n.Return to passage

[567] Ibid. t. ii. f. 24. β.Return to passage

[568] Treviran Arachnid. f. 6. B B.Return to passage

[569] N. Dict. d'Hist. Nat. xxx. 423--. Comp. Treviranus, Arachnid. t. i. f. 6.Return to passage

[570] Treviranus, Ibid. v.Return to passage

[571] N. Dict. d'Hist. Nat. xxx. 421--. Comp. Treviran. Ibid.Return to passage

[572] N. Dict. d'Hist. Nat. Ibid.Return to passage

[573] Treviran. Ibid. t. i. f. 6. i i, c c.Return to passage

[574] N. Dict. d'Hist. Nat. Ibid.Return to passage

[575] Treviran. Ibid. t. ii. f. 24. a.Return to passage

[576] Ibid. v, b.Return to passage

[577] Ibid. c, d, f.Return to passage

[578] Ibid. g, n.Return to passage

[579] N. Dict. d'Hist. Nat. Ibid.Return to passage

[580] Treviran. Ibid. 28.Return to passage

[581] Ibid. t. ii. f. 24. β.Return to passage

[582] Ramdohr, t. xix. f. 1.Return to passage

[583] Reaum. i. 143. t. v. f. 9.Return to passage

[584] VOL. II. p. 88--.Return to passage

[585] De Geer iii. 26.Return to passage

[586] Reaum. iii. 357. t. xxix. f. 6-10.Return to passage

[587] VOL. II. p. 225.Return to passage