Letter XLI
_INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONTINUED._
SECRETION.
Having given you so full an account of the system of digestion in insects, I am now to say something concerning their secretions, and the organs by which they are elaborated. Though no individual amongst them perhaps secretes so many different substances as the warm-blooded animals; yet in general the Class abounds in secretions perhaps as numerous and extraordinary as in the last-mentioned tribes, to some of which a few of them are analogous, while others are altogether peculiar. We know little or nothing of the mode in which the process of secretion in insects is accomplished; in most cases we cannot even discover, except in general, whence the secreted substance originates; and in others, though we are able to trace the vessels that contain it, we are often in the dark as to their structure.--Cuvier, as has been before hinted, from not being able to detect any thing in them like glands, and from their being constantly bathed in the blood or nutritive fluid, conceives that they separate the peculiar substances they contain, by imbibition or infiltration, through the pores of the skin[588]; a circumstance which seems to indicate a certain conformation of the pores both as to size and figure, so as to enable them to admit only one peculiar product.
In treating on this subject, I shall first consider the organs of secretion, and next their products.
I. Organs of Secretion. In general, these are membranous vessels that float in the blood or nutritive fluid, and secrete from it a peculiar substance. They may be denominated according to their products--Silk-secretors, Saliva-secretors, Varnish-secretor, Jelly or Gluten-secretor, Poison-secretor, and Scent-secretors.
i. Silk-Secretors (Sericteria). These organs are most remarkable in the caterpillars of the nocturnal Lepidoptera or moths, especially in that tribe called Bombyces, to which the silkworm belongs: but this faculty is not confined to these insects, but is shared by many other larvæ in different Orders; and in one instance at least, by the imago. In general, the outlet of the silk-secretors is at the mouth; sometimes, however, as in the larva of Myrmeleon and the imago of Hydrophilus, its exit is at the anus. The first is the organ which in the silk-worm provides for us that beautiful substance from which the animal takes its name. There are always two of these vessels, which are long floating tubes, growing slender towards the head of the insect, where they unite to form the spinneret (fusulus) before described[589], which renders the silk. Their lower extremity also is commonly more slender than the middle, and is closed at the end. These organs are usually very much convoluted and twisted[590]. According to Ramdohr[591], they consist of two transparent membranes, between which is found a yellow or transparent jelly. The greater the quantity of silk employed by the caterpillar in the construction of its cocoon, &c., the longer are the silk-secretors. Those of the silkworm are a foot long[592], while those of the larva of the goat-moth are little more than three inches[593].
Other insects spin silk with the posterior extremity of their body. In the great water-beetle (Hydrophilus piceus) the anus is furnished with two spinnerets, with which it spins its egg-pouch[594]; these are in connexion, probably, with the five long and large vessels containing a green fluid, described by Cuvier[595], which surround the base of each branch of the ovaries. The larva of Myrmeleon, which also spins a cocoon with its anus, differs remarkably in this respect from other insects, since its reservoir for the matter of silk is the rectum; this is connected with a horny tube, which the animal can protrude, and thus agglutinate the silk and grains of sand that compose its cocoon[596].
The web of spiders is also a kind of silk remarkable for its lightness and extreme tenuity. It is spun from four anal spinnerets, which never vary in number; two longer organs peculiar to some species have been mistaken for additional ones, but Treviranus affirms that they are merely a kind of anal feeler. Their structure, as far as known, has been before described[597]. The web is secreted in vessels varying in form. In some (Clubiona atrox) they consist of two larger and two smaller ones, at the base of which lie many still more minute[598]. The four larger vessels are wide in the middle, branching at top, and below terminating in a narrow canal leading to the spinnerets[599]. Treviranus thinks the fluid contained in the lower minute vessels different from that furnished by the larger ones--but for what purpose it is employed has not been ascertained.
ii. Saliva-secretors (Sialisteria). These are organs, rendering a fluid to the mouth or stomach, that are found in many insects, especially those that take their food by suction, as the Hemiptera, Lepidoptera, and Diptera, though they are not confined to the perfect insect, being also in some cases visible in the larva. Swammerdam was one of the first that discovered them, and he suspects that they may be salival vessels; though he, as well as Ramdohr, thinks they are the same with the silk vessels of the caterpillar[600]; an opinion which Herold has sufficiently disproved, by showing that at one period of the insect's life they co-exist[601], and Lyonet discovered a very conspicuous pair in the caterpillar of the Cossus, co-existent with the silk-secretors[602]. But the physiologist who has given the fullest account of these organs is Ramdohr:--I shall therefore extract chiefly from him what I have further to communicate with respect to them.
They are variously constructed blind vessels, that are present in almost all insects that take their food by suction, but are mostly wanting in those that masticate it. They have been found, however, in Cryptorhynchus Lapathi, Chrysopa Perla, and Iulus terrestris. The most usual number of the saliva-secretors is two[603]; but sometimes, as in the first of the last-named insects, there is only one[604]; in others (Pentatoma Baccarum) there are three, the exterior one consisting of a pair of reservoirs connecting with the gullet by a single capillary tube[605]; in Pentatoma prasina there appear to be four[606]; in Nepa cinerea, even six--the exterior double pair in this insect, under a powerful lens, is found to consist of spherical vesicles, resembling somewhat a bunch of currants[607]; and in Syrphus arcuatus they are covered with four rows of similar ones[608]. In the flea they consist of two pair of spherical reservoirs, each of which is connected with a short tube, which uniting with that of the other forms a common capillary one connecting with the mouth or gullet[609]; these organs sometimes terminate below in slender vessels;--thus, in Nepa, the inner pair terminates in a single vessel of this description[610], and in Tabanus and Hemerobius apparently in many[611]. It admits of a doubt however, as was lately observed, whether in the Hemiptera, which have usually more than a pair of these organs, some are not rather food-reservoirs as in the Diptera.
The saliva-secretors open either into the instruments of suction themselves (Tabanus, Musca); or into the entrance of the gullet (Pentatoma, &c.); or, lastly, into that of the stomach (Syrphus, Bombylius). Those which lie at the entrance of the stomach consist only of a blind uniform tube[612]; but there is commonly to be distinguished in those that open into the mouth, a reservoir, varying in shape in different species, and terminating in a capillary tube, or tubes, at one or both extremities[613]. In Bugs, two pair of these vessels are often present, one of which opens into the stomach (Reduvius), or gullet (Pentatoma), but the other into the instruments of suction[614]. In the Diptera they open into the stomach when the insect feeds only upon the nectar of flowers (Syrphus), and into the proboscis when it feeds upon both animal and vegetable juices (Tabanus, Musca). The function of the fluid secreted by these organs is to moisten or dilute the food before it is received by the instruments of suction and passed to the stomach[615]. When a common house-fly applies its proboscis to a piece of sugar, it is easy to see that it moistens and dissolves it by some fluid.
iii. Varnish-secretor (Colleterium). In butterflies, moths, and several other insects, one or more vessels called blind vessels open into the oviduct, concerning the use of which, physiologists are not agreed. In the cabbage butterfly there is a pair of ovate ones, or rather a bilobed one, each lobe of which externally terminates in long perplexed convolutions, not easily traced, filled with a yellow fluid, which Reaumur and Herold think is used for varnishing or gumming the eggs, so that they may adhere to the leaves on which they are deposited: it may probably serve likewise for other uses[616]. Another vessel is also to be found in the above butterfly, which enters the oviduct above this, filled with a thick white fluid, the function of which is, probably, to lubricate the passage[617]. A similar organ is found in Phryganea grandis[618].
iv. Jelly-secretor (Corysterium). This is a remarkable organ, related to the preceding, which secretes the jelly of Trichoptera, some Diptera, &c.; this organ in the former, at least in Phryganea grandis, is of an irregular shape, with four horns or processes[619].
Poison-secretor (Ioterium). This organ, which is most conspicuous in the Hymenoptera Order, has not received much notice, except in the case of the Hive-bee and the Scolia: in the former, it is an elliptical membranous vesicle or reservoir, furnished at its lower extremity with a tube which renders to the sting, and at the other by a blind, long, filiform, secretory, vessel, which according to Swammerdam divides into two terminal blind branches[620], though Reaumur could detect but one[621]; in this vessel the poison is secreted and stored up. In Scolia there are two secretory vessels, which enter the reservoir in the middle on each side[622]. In the Scorpion, we learn from Marcel de Serres that the poison-secretor is clothed externally with a horny thickish membrane, containing two yellowish glands, composed of an infinity of spherical glandules, terminating in a canal, enlarged towards its base so as to form a reservoir, and leading to the extremity of the sting[623]. Connected by a slender tube with each mandible in spiders is a vessel with spiral folds, which seems properly to belong to this head--though Treviranus calls it a saliva-vessel[624]--since in the Mygale avicularia and other spiders, the effect of the bite is said to be so venomous as to occasion considerable inflammation, and sometimes death[625].
v. Scent-secretors (Osmateria). Amongst other means with which insects are gifted for the annoyance of their foes and pursuers, are the powerful scents which many of them emit when alarmed and in danger. Concerning the internal organs by which these effluvia are secreted we possess but little information, but more notice has been taken of the external ones by which they are emitted. We may conclude in general, that the secretory organs are membranous sacs or vesicles, perhaps terminating in longer or shorter blind filiform vessels, sometimes secreting a fetid fluid, and at others a fetid gaseous effluvium. The Iulidæ, at least Iulus and Porcellio[626], cover themselves when alarmed, with a fluid of this kind, or emit one, for this faculty is not peculiar to the species noticed by Savi. I observed early in the year, when I handled Iulus terrestris, that it was covered with a slimy secretion, of a powerful scent, which stained my fingers of an orange colour. The spiraculiform pores that mark the sides of the animal are the outlets by which this fluid is emitted, and not spiracles as has been supposed: each of these orifices, as we learn from Savi, terminates internally in a black vesicle, which is the reservoir of the fluid[627]. The most remarkable insect for its powers of annoyance in this way, is one on that account called the bombardier (Brachinus crepitans), which can fire numerous volleys of stinking vapour at its assailants before its ammunition is exhausted[628]. M. Dufour has given a very particular account of the organ that secretes this vapour;--it consists of a double apparatus, one on each side, in the cavity of the abdomen, both formed of two distinct vessels. The first, which is the innermost, presents itself under two different aspects, according as it is contracted or dilated: in the former case it is a whitish, irregularly rounded, soft body, apparently glandular, placed under the last abdominal segments; communicating at one end with the reservoir, and terminating constantly at the other in a very long and slender filament: in the second case, or when it is dilated, it resembles an oblong, membranous, diaphanous sac, filled with air, then occupying the whole length of the abdomen, and appearing free except where it communicates with the reservoir. The second vessel or reservoir is a small, spherical, brown or reddish body, constant in its form, internally hollow, placed under the last dorsal segment, precisely above the rectum, and opening by a small pore into the anus[629]: so that the tail of this little beetle may be regarded as a battery mounted with two pieces of cannon, which our alert bombardier fires alternately without intermission till all his ammunition is expended. The ground-beetles (_Eutrech_in_a_) in general have a pair of these anal scent-secretors, which discharge an acrid and caustic fluid, and sometimes a volatile one[630]. The external organ of the scent-secretors in Gyrinus consists of two minute hairy cylindrical retractile tubes, of a red colour[631]. Numerous insects of other tribes and genera emit scents from their anus, and from various other parts of the body, of which having before given you a very full account[632], I shall proceed to the consideration of the secretions themselves: but first I must observe, that in many cases, as in some of the cottony and powdery Aphides, Chermes, &c., the substance secreted appears to be a transpiration through the pores of the body, a kind of excretion from the superabundance of its fluid contents[633]. In many, however, this secretion transpires through appropriate orifices: thus in Chermes Abietis, which produces those curious galls resembling the cone of a fir[634], the flocoons of seeming cotton that cover it proceed from little oval concavities on its back, four of which are arranged in a transverse line on each dorsal segment of the abdomen: these concavities have minute tubercles probably terminating in a pore[635]. In Aphis Fagi the cottony flocoons are almost an inch long[636].
* * * * *
The secretions of insects may be considered under the following heads--Silk; Saliva; Varnish or Gum; Jelly; Oils; Milk; Honey; Wax; Poisons and Acids; Odorous fluids and Vapours; and Luminous matter.
i. Silk. This valuable product of insects, while in the silk-secretor, assumes in the Lepidoptera the appearance of a viscid gum, but the moment it is exposed to the air it hardens into a silken thread. It is remarkable for the following qualities:--it dries the instant it comes in contact with the air; it is then insoluble not only in water but in the most active solvents, and even heat has no effect upon it to melt or soften it: indeed, without these qualities it would be of no use to us[637]. As soon as it leaves the spinneret it becomes the thread we call silk, which being drawn through two orifices is necessarily double through its whole length. This thread varies considerably in colour and texture, as has been before stated[638], and sometimes resembles cotton or wool rather than silk. In spiders it is of a much softer and more tender texture than that of other spinning insects; and Mr. Murray seems to have proved that it is imbued, in the case of the gossamer, with negative electricity: in the sericterium the fluid that produces it is sometimes white or grey, and at others yellow[639]. A remarkable gnat (Ceroplatus tipuloides), living on an agaric, carpets its station of repose and its paths with something between silk and varnish, which it spins, not in a thread, but in a broad riband[640].
ii. Saliva. Many insects have the power of discharging from their mouth a fluid which seems in some degree analogous to the saliva of larger animals. Thus many, as Lepidoptera, Hemiptera, Diptera, &c., can dilute their food, and render it fitter for deglutition. I have seen a common fly when not employed in eating, emit a globule of fluid as big as a grain of mustard-seed from its proboscis, and retract it again. On a former occasion I observed to you that many predaceous, carnivorous, and some herbivorous beetles, when alarmed emit a drop of coloured acrid fluid from the mouth[641]. That this is not secreted in any of the ordinary salival vessels is evident from Ramdohr's dissections of those beetles[642], who, had there been such an organ, would doubtless have discovered it: but as the stomach of all of them is distinguished by those minute cœca or blind vessels, which he denominates shags (zotten)[643], perhaps these may be the secretors of this fluid, probably analogous to the gastric juice[644]; in which case its primary office would be the digestion of the food. We are not however warranted in considering every fluid effused from the mouth as saliva. The glutinous material with which wasps cement the woody fibres for their paper edifices[645]; that with which some sand-wasps moisten the sand which they scrape away, of which they form the singular tubes that lead to their nests[646]; and that with which the aphidivorous larvæ fix themselves previously to their becoming pupæ[647],--may be a secretion distinct from saliva; possibly intermediate between it and gum or the matter of silk, and secreted by peculiar organs. In the wasp, however, Ramdohr discovered nothing of the kind[648]; and in Syrphus, as before observed, the saliva-secretors are very peculiar in their structure, as if appropriated to the secretion of a peculiar fluid[649]. Something similar has been observed by Reaumur with regard to the larva of Crioceris merdigera, which forms its cocoon with a kind of froth produced from the mouth[650].
iii. Varnish or Gum. The eggs of various insects, when they leave the oviduct, are covered with a kind of varnish or gum by which they adhere to the substances that the young larvæ are to feed upon, or are placed in a proper position for their hatching in an appropriate station. Several instances of this have been already mentioned[651]; I shall therefore not enlarge further upon the subject. With regard to the secretion itself, little has been recorded except its colour, which has been before noticed. Some Lepidoptera also as we learn from Reaumur and Bonnet[652], use a varnish in the construction of their cocoons.
iv. Jelly or Gluten. This secretion is particularly conspicuous in the Trichoptera and some Diptera, serving as a bed or nidus for those eggs that are committed to the water,--upon which I have nothing to add to what has been before said[653]. Under this head also may be noticed the fluid, secreted in peculiar vesicles, that lubricates the oviduct and the passages of the sexual organs[654].
v. Oils. Oily substances are sometimes produced by insects. The common oil-beetle (Meloe Proscarabæus) when touched sends forth a drop of this kind of fluid, of an orange colour, from each joint of its legs[655]: something similar I have observed in Coccinella bipunctata: Ray mentions a locust taken in Spain which emits a yellow oleaginous fluid from between the claws of its fore legs[656]; but the precise nature of these substances has not been ascertained, nor whether they are secreted by peculiar organs.
vi. Milk. A milky fluid is produced by the larva of Chrysomela Populi. Willughby observed a similar effusion from pores in the upper surface of the body of Acilius sulcatus; and other insects emit it from other parts of their body[657].
vii. Honey. It is certain that honey is not an animal secretion; yet the saccharine matter collected from the nectaries of flowers, from which it is derived, seems to undergo some alteration in the stomach; for the consistence of honey is greater than that of any vegetable nectar, and its taste does not vary greatly, while that of the nectar in different plants is probably not the same. Reaumur also has observed, that each honey-cell in a bee-hive is always covered by a cream-like layer of a thicker consistence than the rest, which apparently serves to prevent the more liquid honey, which from time to time is introduced under it, from running out[658]. Now if honey were the unaltered nectar of plants, it is difficult to conceive how this cream could be collected in proper proportions. The last-mentioned naturalist likewise ascertained, that if bees, in a season in which the fields afford a scarcity of food, be supplied with sugar, they will from this substance fill their cells with honey which differs in no respect from the common sort, except that its flavour is a little heightened[659]:--a similar argument may be deduced from the circumstance of the bees imbibing the juices of fruits of various kinds as they are well known to do[660]. It seems therefore evident that the honey collected by bees undergoes some modification in their honey-stomach before it is regurgitated into the cells, and therefore may be regarded in some degree as a peculiar secretion.
Huber says that he has ascertained by a great number of observations that electricity is singularly favourable to the secretion of the substance of which honey is formed by flowers; the bees never collect it in greater abundance, nor is the formation of wax ever more active, than when the wind is in the south, the air humid and warm, and a storm gathering[661].
viii. Wax generally transpires through the pores of the skin of those insects that produce it, either partially or generally, and it is secreted from honey or other saccharine substances taken into the stomach. In the hive-bee, as has been before stated, it is produced partially[662], but in many other insects it is a general transudation of the body. This is particularly the case with a large number of the Homopterous Hemiptera; and those flocoons that look like cotton, and cover the body of several Chermes and Aphides, if closely examined will be found of the nature of wax: this I have particularly noticed with respect to Chermes Fagi, in which the cotton-like flocoons are often so long as to cause the insect to look like a feather, and a leaf covered by them exhibits a very singular appearance, as if clothed with the fine down of a swan[663]. Probably the white powder or threads that appear to transpire through the skin of many other insects is of a waxy nature. In the larva of a beetle described by Reaumur, the flocoons are so arranged as to give the animal some resemblance to a hedgehog, and when rubbed off they are reproduced in twelve hours[664]. Gyllenhal, speaking of Peltis limbata, observes, that when alive it is covered with a white powder resembling mould, which if rubbed off returns again as long as the animal lives[665].
It will not be improper to include under this head what further account I have to give of Lac, which though regarded as a resin, since Cocci sometimes certainly produce wax[666], probably has some analogy with the latter substance. When the females of this Coccus (C. Lacca) have fixed themselves to a part of the branch of the trees on which they feed (Ficus religiosa and indica, Butea frondosa, and Rhamnus Jujuba[667]), a pellucid and glutinous substance begins to exude from the margins of the body, and in the end covers the whole insect with a cell of this substance, which when hardened by exposure to the air becomes lac. So numerous are these insects, and so closely crowded together, that they often entirely cover a branch; and the groups take different shapes, as squares, hexagons, &c., according to the space left round the insect which first began to form its cell. Under these cells the females deposit their eggs, which after a certain period are hatched, and the young ones eat their way out. Though indisputably an animal secretion, many of the properties of lac are not very different from those of the juices of the trees on which the animal feeds, and which therefore would seem to undergo but little alteration.
Wax seems also to form a constituent part of some insects which are not found to secrete it. The yellow substance deposited in vessels containing spiders in alcohol is said to be a true wax, and may be obtained from these animals by gently heating them[668].
ix. Poisons and Acids. The bite as well as the sting of many insects is followed by inflamed tumours, so that the sialisteria of some bugs, Diptera, Aptera and spiders, may be regarded as producing a poisonous fluid; but we know nothing of the real nature of it, nor of that of other venomous insects, except the ant--whose celebrated acid may be considered under the present head,--the bee, the wasp, and the scorpion.
Contrary to the once received doctrine that no acid was to be found in any animal, except as the effect of disease in the alimentary canal, many insects secrete peculiar and powerful ones. I have on a former occasion related an instance in which an acid of this description, secreted in its sialisteria, is employed by a moth to soften its cocoon[669]; and Lister mentions a species of Iulus which produced one resembling that of ants[670]; but this last is the most powerful of all. The fact that blue flowers when thrown into an ant-hill become tinged with red has been long known; but Mr. Fisher of Sheffield, about 1670, seems to have been the first who ascertained that this effect is caused by an acid with which ants abound, and which may be obtained from them by distillation or infusion in water[671]. Margraff and other chemists confirmed this discovery[672]; and concluding that this acid was of a peculiar kind, they gave it the name of the Formic acid. This name, however, is now exploded; the subsequent experiments of Deyeux, Fourcroy and Vauquelin having ascertained that the acid of ants is not of a distinct kind, but a mixture of the Acetic and Malic[673]. These acids are in such considerable quantities, and so concentrated in these animals, that, when a number of Formica rufa are bruised in a mortar, the vapour is so sharp that it is scarcely possible to endure it at a short distance. It also transpires from them, for they leave traces of it on the bodies which they traverse: and hence, according to the experiments of Mr. Coleridge, the vulgar notion that ants cannot pass over a line of chalk is correct; the effervescence produced by the contact of the acid and alkaline being so considerable, as in some degree to burn their legs[674]. The circumstance of much of the food of ants being of a saccharine nature may account for this copious secretion of acid, the use of which is probably to defend themselves and their habitations from the attack and intrusion of their enemies: if a frog be put into a nest of Formica rufa that has been deranged, it will be suffocated in five minutes[675]. That which they ejaculate from their anus when attacked, as formerly stated[676], must be secreted in an ioterium; but their very blood seems of an acid nature. It is very probable, as Dr. Thomson has observed[677], that acids may be obtained from many other insects, and that they are various modifications of the acetic.
From the circumstance that water is absorbed by greasy moths, that crystals of a salt are occasionally found adhering to them, that they change blue litmus paper red,--it has been inferred that their supposed oiliness is in fact an acid or acid salt, having the property of attracting moisture from the air, the infected moths being in fact not greasy, but wet; hence the application of chalk and clay, usually recommended in this case, can have only a temporary and superficial effect. The only effectual remedy, is steeping the body in spirits of wine till all the acid is extracted[678]. This acid is probably the same as Chaussier obtained from silkworms, since called Bombic Acid[679].
The poison of bees and wasps, as to its chemical qualities, is a transparent fluid, at first sweet to the taste, but immediately afterwards hot and acrid like the milky juice of the spurge[680]; soluble in water, but not in alcohol; and separable from the former in the state of white powder, when the latter is added giving a slight red tinge to paper stained with vegetable blue, and when dry and chewed appearing tenacious, gummy and elastic. This last property, as well as solubility in water and not in alcohol, is common also to the poison of the viper, which however differs in being tasteless, and not affecting vegetable blues. From hence Fontana concludes that this fluid is united with an acid, but in a very small proportion, and not with an alkali[681]. The venom of bees is extremely active; a grain in weight, it is conjectured, would kill a pigeon in a few seconds[682]. It is remarkable, however, that while in some constitutions the sting of a single bee or wasp is sufficient sometimes to induce alarming symptoms, in others numerous punctures will produce little or no pain or inflammation. That this fluid, and not the puncture of the sting, is the sole cause of the inflammation that usually follows the wound inflicted by one of these animals, is proved by the facts, that if it be introduced into one made by a needle, the same effect ensues, and that when the whole contents of the poison-bag have been exhausted by the insect's stinging three or four times in succession, its weapon then becomes harmless[683].
The venom of scorpions, though much more potent, probably resembles that of bees, &c., in many of its chemical qualities: it issues from two pores in the sting before described[684], where, when the animal is irritated, it accumulates under the form of two little drops of a whitish colour: spread upon paper this fluid produces a spot like what would be caused by oil or grease, and this part of the paper becomes by desiccation firmer and transparent[685].
x. Odorous fluids and Vapours[686]. The powerful scents which different insects emit are extremely numerous, much more so indeed than the generality of Entomologists have been aware, for there is scarcely a scent odious or agreeable that may not be met with in the insect world. This you will be convinced of, by following a practice which I would recommend to you--that of smelling the insects you take. Some of these scents are peculiar to particular parts or organs, and some are exhaled generally by the whole body; some are emitted by a fluid secretion, and others are gaseous effluvia. On a former occasion I gave you a rather full account of these scents and their organs[687]; I shall relate here only what I there omitted. To begin with sweet odours. Many beetles emit an agreeable scent. The rose-scented Capricorn or musk-beetle (Cerambyx moschatus) has long been noted for the delicious scent of roses which it exhales; this is so powerful as to fill a whole apartment, and the insect retains it long after its death. Captain Hancock also informed me that another species of the same genus, C. sericeus, has in a high degree a scent resembling that of the cedar[688] on which they feed. Though most of the micropterous tribes (Brachyptera) have a fetid smell, yet there are some exceptions to this amongst them. One species (Philonthus suaveolens K. MS.) related to P. micans, which I once took, smelt precisely like a fine high-scented ripe pear; another, Oxytelus morsitans, like the water-lily; a third, O. rugosus, like water-cresses; and lastly, a fourth (P. fuscipes) like saffron[689]: Trichius Eremita, one of the Petalocerous beetles, is stated to have the scent of Russia leather; Geotrupes vernalis, in spite of its stercorarious food, of lavender-water[690]. Mr. Sheppard has observed that Dytiscus marginalis when recently taken smells not unlike liquorice: Bonnet mentions a caterpillar that had the scent of new hay. A little gall-fly (Cynips Quercus Ramuli) has the remarkable odour of Fraxinella: the larva of another species of this genus (C. Rosæ) has an odour which seemed to Reaumur as attractive to cats as that of Nepeta cataria or Teucrium Marum[691]: some Phalangia smell like walnut leaves[692]; and the various species of the genus Prosopis (Melitta * b. K.) have a very agreeable scent of Dracocephalum moldavicum[692].
We next come to fetid odours. These in numerous cases are known to be secreted and emitted by appropriate vessels and organs; they are often exhaled from a fluid secretion, of which, in the letter lately referred to, I gave you almost all the known instances. Savi, in his history of Iulus fœtidissimus, informs us that it emits a yellow fetid fluid from its supposed spiracles, which if applied in sufficient quantity imparts a red colour to the skin, to be removed neither by friction nor washing, but only disappearing by time; when removed from the black vesicles in which it is stored, it shoots into very transparent octahedral crystals[693].
I have before mentioned the coloured fluid which some insects emit when they are disclosed from the pupa, and that it probably exhales some powerful odour which attracts the males[694].
The great Hydrophilus, in its larva state, when first taken into the hand remains without motion; in a minute afterwards it renders itself so flaccid as to appear like a cast skin. Taken by the tail it contracts itself considerably, it then agitates itself briskly, and ejaculates with a slight noise a fetid and blackish fluid[695].
In other cases these odours are produced by gaseous vapours. That of the Bombardiers (Brachinus) is the most celebrated and remarkable. It is whitish, of a powerful and stimulating odour, very like that exhaled by nitrous acid. It is caustic, producing upon the skin the sensation of burning, and forming instantly upon it red spots which soon turn brown, and which, in spite of frequent lotions, remain several days. It turns blue paper red[696]. That amiable, intelligent, and unfortunate traveller Mr. Ritchie,--whose premature death, when attempting to penetrate to the interior of Africa, all lovers of Natural History so deeply lamented, and whose ardour in the pursuit of that science I had an opportunity of witnessing, when, in company with him, Messrs. Savigny, Du Fresne, and W. S. MacLeay in 1817, I visited the forest of Fontainebleau,--in a letter to the last-mentioned gentleman[697], relates that his companion M. Dupont, near Tripoli took a nest consisting of more than a thousand of a species of this genus. "I am making a few experiments," says he, "on the substance which they emit when they crepitate, but do not know whether I can collect enough to arrive at any conclusion. It made Dupont's fingers entirely black when he took them. It is neither alkaline nor acid, and it is soluble in water and in alcohol." From this we may conjecture that it formed crystals.
xi. Phosphorus. On this remarkable secretion I have so fully enlarged on a former occasion[698], that here I shall merely add a few observations which Mr. Murray obligingly communicated to me. He remarks that in a box in which glow-worms were kept--five luminous specks were found secreted by the animal, which seemed to glow and were of a different tinge of light. One put into olive oil at eleven P. M. continued to yield a steady and uninterrupted light until five o'clock the following morning, and then seemed, like the stars, to be only absorbed by superior effulgence. The luminous spherical matter of the glow-worm is evidently enveloped in a sac or capsule perfectly diaphanous, which when ruptured discloses it in a liquid form, of the consistency of cream. M. Macaire, he observes, in the Bibliothèque Universelle, draws the following conclusions from experiments made on the luminous matter of this animal;--that a certain degree of heat is necessary to their voluntary phosphorescence--that it is excited by a degree of heat superior to the first, and inevitably destroyed by a higher--that bodies which coagulate albumen take away the power--that phosphorescence cannot take place but in a gas containing no oxygen--that it is not excited by common electricity, but is so by the Voltaic pile--and lastly, that the matter is chiefly composed of albumen.
xii. Fat. There is one product found in the body of insects most copiously in their larva state, but more or less also in the imago, which may be called their fat. In the former it is a many-lobed mass, occupying the whole of the interior, except the space that is required for the muscles and the internal organs, which it wraps round and protects. It is contained in floating membranes, very numerous, which fill all the interstices, and assume the appearance sometimes of small globules, and sometimes of a thickish mucilage, which easily melts and inflames; in colour it is most commonly white, but sometimes yellow or green. It is imagined to be a kind of epiploon or caul, and is accumulated in the larva as a store of nutriment for the growth and development of the organs of the perfect insect while in the pupa state[699]. The blood in which the different organs float that is not required for their nutriment, is supposed to be expended in the formation of this substance. Marcel de Serres is of opinion that it is secreted from the chyle by passing through the pores of the dorsal vessel, formerly called the heart of insects[700].
Under this head I may mention what little is known with regard to the perspiration of these animals[701]. That a considerable quantity of fluid passes off from them when in the pupa state, is sufficiently proved by the loss of weight which they undergo, and by the experiments of Reaumur, who collected the fluid in closed glass tubes; and that in their perfect state they are constantly passing off perspirable matter by the pores of their skin or crust, is not only rendered probable by the succulent nature of their food and the absence of any urinary discharge, but is proved by what takes place in a swarm of bees. These insects, when crowded together in hot weather in a large mass, become heated to such a degree, and perspire so copiously, that those near the bottom are quite drenched with the moisture it produces, which so relaxes their wings that they are unable to fly[702].
I am, &c.
FOOTNOTES:
[588] Cuv. Anat. Comp. iv. 163--.Return to passage
[589] VOL. III. p. 124--.Return to passage
[590] Malpigh. De Bombyc. t. v. f. 2. Swamm. t. xxxiv. f. 5. Lyonet, t. v. f. 1.Return to passage
[591] Anat. der Ins. 59.Return to passage
[592] Ibid. 60. Malpigh. 20.Return to passage
[593] Lyonet Anat. 111.Return to passage
[594] N. Dict. d'Hist. Nat. xv. 483.Return to passage
[595] Anat. Comp. v. 198.Return to passage
[596] Ramdohr, 60. t. xvii. f. 1. f, g, h, r.Return to passage
[597] VOL. I. p. 403--. Treviran. Arachnid. 42.Return to passage
[598] Treviran. Arachnid. 43. t. iv. f. 42. o. p. 9.Return to passage
[599] Ibid. α, y.Return to passage
[600] Swamm. ii. 21. a. t. xxxvi. f. 1. abcd. Ramdohr 58.Return to passage
[601] Schmet. t. iii. f. 1.Return to passage
[602] Lyonet--. 112. t. v. f. 1. P, Q, R, S.Return to passage
[603] Ramdohr Anat. t. xviii. f. 1. M. f. 5. F.Return to passage
[604] Ibid. t. x. f. 1. m.Return to passage
[605] Ibid. t. xxii. f. 3. M L. Ramdohr regards the double one as a pair; but as they terminate in a single tube, they ought to be reckoned as one.Return to passage
[606] Ibid. f. 4.Return to passage
[607] Ibid. f. 2. K, L, M, N. t. xxiii. f. 6.Return to passage
[608] Ibid. 177. t. xxi. f. 3. F. F.Return to passage
[609] Ibid. f. 2. G, H.Return to passage
[610] Ibid. t. xxii. f. 2. L.Return to passage
[611] Ibid. t. xxi. f. 1. O. t. xvii. f. 6. n.Return to passage
[612] Ramdohr Anat. t. xx. f. 6. D.Return to passage
[613] Ibid. t. xxii. f. 1. K, L. f. 2. I, K, L.Return to passage
[614] Ibid. f. 3, 4, 5.Return to passage
[615] Ibid. 57--.Return to passage
[616] Reaum. ii. 81. Herold Expl. of Plates, x. Malpigh. _De Bombyc._ 37. PLATE XXX. FIG. 12. c.Return to passage
[617] Herold Ibid. x. t. iv. f. 1. p, u, y. Marcel de Serres Mem. du Mus. 1819. 141.Return to passage
[618] Gaede Anat. t. i. f. 3. d.Return to passage
[619] Ibid. 17. t. i. f. 4.Return to passage
[620] Bibl. Nat. t. xix. f. 3. β.Return to passage
[621] Reaum. v. 377. t. xxix. f. 7. s.Return to passage
[622] N. Dict. d'Hist. Nat. xxx. 388.Return to passage
[623] Ibid. 427--.Return to passage
[624] Arachnid. 31. t. ii. f. 21. p. 9.Return to passage
[625] N. Dict. d'Hist. Nat. xxii. 114. 117. comp. VOL. I. p. 127.Return to passage
[626] Ibid. xxviii. 6.Return to passage
[627] Osservazioni, &c. 13--.Return to passage
[628] VOL. II. p. 243--. N. Dict. d'Hist. Nat. iv. 308.Return to passage
[629] Ibid. iv. 309.Return to passage
[630] Ibid. v. 252.Return to passage
[631] De Geer iv. 358. t. xiii. f. 9. m.Return to passage
[632] VOL. II. p. 241--. III. p. 147--.Return to passage
[633] De Geer iii. 41.Return to passage
[634] VOL. I. p. 451, where by mistake it is represented as the work of Aphis Abietis.Return to passage
[635] De Geer iii. 111.Return to passage
[636] Reaum. iii. t. xxvi. f. 4-6.Return to passage
[637] N. Dict. d'Hist. Nat. vi. 305.Return to passage
[638] VOL. III. p. 221.Return to passage
[639] Treviran. Arachnid. 44. In Paraguay a spider is found which makes spherical cocoons of yellow silk, which are spun because of the permanence of the colour. This operation occasions a flow of water from the eyes and nose of the spinners. Azara Voyag. 212. See also Murray in Werner. Trans. 1823. 8--.Return to passage
[640] Reaum. v. 24.Return to passage
[641] VOL. II. p. 244--.Return to passage
[642] Ramdohr Anat. t. ii.-vi.Return to passage
[643] Ibid. 20. See above, p. 107. As some of the Sialisteria render to the stomach (see above, p. 131), there seems no small affinity between these shags and those organs.Return to passage
[644] Cuv. Anat. Comp. iv. 132, 136.Return to passage
[645] Reaum. vi. Pref. xxviii. 177--.Return to passage
[646] Ibid. 253--.Return to passage
[647] Ibid. iii. 375.Return to passage
[648] Anat. t. xii. f. 6.Return to passage
[649] Ibid. xxi. f. 3. I I.Return to passage
[650] Reaum. iii. 230.Return to passage
[651] VOL. III. p. 79--.Return to passage
[652] Reaum. iii. 215. Bonnet ix. 182.Return to passage
[653] VOL. III. p. 68--.Return to passage
[654] Marcel de Serres Mem. du Mus. 1819. 133, 141.Return to passage
[655] De Geer, v. 6.Return to passage
[656] Rai. Hist. Ins. 62.Return to passage
[657] VOL. II. p. 242--. 248. Rai. Hist. Ins. 94, 382.Return to passage
[658] Reaum. v. 448.Return to passage
[659] Ibid. v. 722.Return to passage
[660] VOL. I. p. 196. II. p. 176.Return to passage
[661] Encyclop. Britan. viii. 205. from Journ. de Phys.Return to passage
[662] VOL. II. p. 174.Return to passage
[663] Reaum. iii. 318--. t. xxvi. f. 1-6.Return to passage
[664] Ibid. 396--. t. xxxi. f. 20-29.Return to passage
[665] Insect. Suec. i. 257.Return to passage
[666] VOL. I. p. 327.Return to passage
[667] N. Dict. d'Hist. Nat. xvii. 189.Return to passage
[668] Nicholson's Journ. i. 298--.Return to passage
[669] VOL. III. 281.Return to passage
[670] Philos. Trans. 1670.Return to passage
[671] Philos. Trans. Ibid. Ray's Lett. 74.Return to passage
[672] Amoreux Ins. Venim. 236--.Return to passage
[673] N. Dict. d'Hist. Nat. xii. 94.Return to passage
[674] Southey's Brazil, i. 645.Return to passage
[675] N. Dict. d'Hist. Nat. ubi supr.Return to passage
[676] VOL. II. p. 67.Return to passage
[677] Syst. of Chemist. 533.Return to passage
[678] Germar Mag. der Ent. iii. 445--.Return to passage
[679] Mem. Dijon 1783. ii. 70.Return to passage
[680] Reaum. v. 354.Return to passage
[681] On Poisons, i. 265--.Return to passage
[682] Ibid. 269.Return to passage
[683] Reaum. ubi supr.Return to passage
[684] VOL. I. p. 124. III. p. 716--.Return to passage
[685] N. Dict. d'Hist. Nat. xxx. 427.Return to passage
[686] I use the term odorous, not in the same sense as odoriferous, but to include both sweet and fetid scents.Return to passage
[687] VOL. II. p. 238--. III. p. 147--.Return to passage
[688] A Brazilian wood so called, but differing from the common cedar.Return to passage
[689] Dotharding Insect. Coleopt. Danic.Return to passage
[690] Sturm Deutsch. Fn. i. 27.Return to passage
[691] Reaum. iii. 494.Return to passage
[692] Mon. Ap. Angl. i. 136.Return to reference 1 Return to reference 2
[693] Osservaz. sullo Iulus, &c. 14.Return to passage
[694] VOL. III. p. 297--.Return to passage
[695] N. Dict. d'Hist. Nat. xv. 487.Return to passage
[696] N. Dict. d'Hist. Nat. iv. 308.Return to passage
[697] Dated Tripoli in the West, January 21, 1819.Return to passage
[698] VOL. II. p. 418--.Return to passage
[699] Reaum. i. 145. Lyonet Anat. 106--. N. Dict. d'Hist. Nat. xvi. 224. PLATE XXI. FIG. 5. a.Return to passage
[700] See above, p. 90. note^a.Return to passage
[701] See above, p. 78.Return to passage
[702] Huber i. 273.Return to passage
Letter XLII
_INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONTINUED._
REPRODUCTION.
The reproductive organs of insects in their general denominations and functions correspond with those of the higher classes of animals; but as to number, proportions, and other particular details of their structure, they differ from them very considerably. I shall not now, however, enter at large upon this subject, but confine myself principally to the consideration of those organs in the female which are appropriated to the formation, fecundation, maturation, exclusion and deposition of their eggs, and other circumstances relating to that subject. The organs connected with this function are the Sperm-reservoir; the Oviduct; the Ovaries; and the Ovipositor.
I. The Sperm-reservoir (Spermatheca) is an organ connecting the vagina with the oviduct, which, according to Herold, receives the male sperm as into a reservoir[703], and fecundates the eggs in their transit through that passage. This vessel, which consists of a double tunic, in the cabbage-butterfly terminates the vagina, and is connected with the oviduct by a lateral undulating tube: in shape it is a rather irregular oblong, and is surmounted by a small orbicular vesicle, connected by a short tubular footstalk with the main reservoir[704]. A similar organ was discovered by Malpighi in the imago of the silkworm, who denominates it the uterus; to which indeed it seems analogous, and which he also regards as a reservoir for the sperm for the gradual fecundation of the eggs[705]. But in that fly the organ is of a rather different shape, and the interior vessel terminates in several spherical vesicles[706]. John Hunter by the most decisive experiments, such as covering the eggs of the unimpregnated moth, after exclusion, with the liquor taken from the spermatheca in those which had been impregnated, and rendering them fertile, he demonstrated that this organ was a reservoir for the spermatic fluid, to impregnate the eggs as they were ready for exclusion, and that coition and impregnation were not simultaneous[707]. It is not improbable that in all insects whose eggs are gradually laid, this provision for their gradual fecundation, if carefully sought for, might be detected[708]. Rifferschweils is of opinion, that in these cases the eggs are fertilized in their transit through the oviduct by sperm adhering to the folds of the cloacæ[709]: but this opinion seems less analogous to what takes place in other cases, with regard to the due preparation of the eggs for a safe and effectual transit[710].
II. The Oviduct (Oviductus) is the canal, always separate from the vagina, which receives the eggs from the ovary, transmitting them, often by a peculiar and complex instrument in which it terminates, to their proper station. This canal sometimes opens into the anal passage or cloaca, and at others, as in the cabbage-butterfly[711], is distinct, and lies between the sexual organ and the anus. In the Arachnida there are two oviducts[712].
III. The Ovaries (Ovaria) in insects are the viscera in which the eggs are generated and grow till they arrive at maturity, when they pass through the oviduct, and are extruded or deposited in their appropriate station. They vary considerably in their structure. In all however, except the Iulidæ, in which there is only a single ovary[713], the oviduct at its upper or inner extremity terminates in two branches, usually further subdivided into a number of smaller conical ones, which several ramifications constitute the ovaries, or egg-tubes as they are sometimes called: these tubes generally consist of a single membrane, and are joined to the oviduct by membranous rugose cloacæ[714]: in the Phalangia, however, there are two tunics; the outer one of a cellular substance, and the inner one consisting of spiral fibres like tracheæ--a kind of structure which renders them capable of great extension[715]. Rifferschweils considers the ovaries as formed upon two primary types.--First, flagelliform ovaries, consisting of conical tubes equal in length, and inserted at the same place at the end of the primary branches as in the Lepidoptera, the Bee, &c. Secondly, racemose ovaries, consisting of short conical tubes, so proceeding from the primary branches as to render the ovary racemose or pinnated, as in certain Neuroptera, Coleoptera, and Diptera[716]: but perhaps their structure will be better understood if they are divided into agglomerate ovaries and branching ovaries: in the first the egg-tubes form two bundles, in which the branches are not discernible, as in the Ephemera, the chamæleon-fly, and spiders[717]: and in the second the branches are distinct, as in the Lepidoptera and the majority of insects.
The number of branches varies in different genera and species. In Echinomyia grossa, a large fly, there are only the two primary branches[718]; in the common dung-beetle (Geotrupes stercorarius) these appear divided at their apex into fingers[719]: in Scolia, a Hymenopterous genus, and the butterfly of the nettle, there are three secondary branches on each side[720]: in many other Lepidoptera and the humble-bee there are four[721]; in the common louse there are five[722]; in the rhinoceros-beetle and the cockchafer, six[723]; in the wasp, seven[724]; eight in the cockroach[725]; twelve in the Carabi and the mealworm-beetle[726]; thirty in the large green grasshopper (Acrida viridissima[727]); thirty-two in the cheese-maggot-fly[728]; and in the hive-bee more than a hundred and fifty[729].
The number of eggs also contained in the ovaries varies. In Echinomyia grossa there is only one egg in each, and only two at once in the matrix[730]: in another fly produced by the cheese-maggot there are four[731]; in the louse there are five; in the cockchafer six[732]; in the hive-bee sixteen or seventeen are visible at the same time[733]; and in the silkworm-moth sixty or seventy[734]. Besides the eggs, the tubes contain a pellucid mucus, and at their upper extremity the eggs are lost in a granular mucous mass, in which, however, they may still be discovered with a microscope[735]. With regard to the termination of the ovaries or egg-tubes internally,--in those that have agglomerated ones it is not to be traced, the whole appearing like an oblong obtuse or acute body[736]: but in the branching ones it is more easily traced; at first they converge in most cases to a point; this is seen to advantage in the caterpillar of some butterflies, when near assuming the pupa, in which they are readily discovered, and represent with great truth and elegance the bud of some blossom[737]; but in time they diverge, and sometimes become convoluted[738]; they generally terminate in a slender simple filament, but in the louse in a fork[739]; they are sometimes extremely long, as in the wasp and Lepidoptera[740]; in the hive-bee they appear to be shorter[741].
IV. We are next to consider the Ovipositor, or instrument by which numerous insects are enabled to introduce their eggs into their appropriate situations, and where the new-born larva may immediately meet with its destined food. As this instrument is one of the most striking peculiarities with which the wisdom of the CREATOR has gifted these little animals, and in many cases is extremely curious and wonderful, both in its structure and modes of operation--though on a former occasion I gave you a brief account of several kinds of them[742], I shall now enter more at large into the subject, and describe these often complex machines, as they are exhibited in most of the different Orders of insects.
With regard to the Coleoptera Order, there are doubtless numerous variations in the structure of this organ; but very few have been noticed, and those chiefly belong to insects whose grubs feed on timber. In these it is usually retractile one part within another, like the pieces of a telescope: in Buprestis it consists of three long and sharp laminæ, the two lateral ones forming a sheath to the intermediate one, which probably conveys the eggs[743]: in Elater it is a cylindrical organ, terminating in a pair of conical joints, which seem to form a forceps, and including a tube probably conveying the egg to the forceps, which perhaps introduces it[744]. The Ovipositor of Prionus coriarius differs from that of Callidium violaceum, and many Capricorns before described[745]: it consists merely of a long bivalve piece ending in a kind of forceps, and hollowed above into a channel for the passage of the eggs[746].
In the Orthoptera the instrument of oviposition is more simple; in Locusta consisting merely of four robust three-sided pieces, two above and two below, the former pair at the end curving upwards and the latter downwards[747], these pieces seem calculated when they have entered the earth to enlarge the burrow, and the animal appears able to separate them very widely from each other[748]. The ovipositor of Acrida viridissima, which like that of many Hymenopterous insects forms a kind of appendage or tail to the body, has been described both by De Geer and Latreille as consisting of two valves only[749]; but in reality it consists of six, two upper and four lower, as you may ascertain by means of a pin or the point of a penknife, which will readily separate them. This is confirmed by a figure of Stoll's of a species which seems to connect Conocephalus with Gryllus. In this the ovipositor is considerably longer than the body of the animal, and is composed of six distinct pieces; viz. two external ones stouter than the rest, and within these four others finer than a hair and convolute at the apex[750]. There is a considerable variety in the shape of the ovipositors of the Acridæ and the cognate genera:--thus in A. viridissima this organ is straight, in A. verrucivora bent like a sabre, and in Pterophylla citrifolia and some others, the whole machine is short and boat-shaped; in Scaphura Vigorsii it is also rough with sharp little tubercles[751]. I had an opportunity of observing, with respect to the first of these insects, that in boring, as is the case with the Cicadæ and saw-flies, the motion of the valves was alternately backwards and forwards. It appeared also to me that the two outer pieces of each of the apparent valves were fixed in a groove in the margin of the intermediate one. I saw this clearly with respect to the upper pieces, and it is most probable that the lower are similarly circumstanced. In the cricket tribe (Gryllus) the ovipositor is as long as the abdomen, very slender, terminating in a knob[752]. It is apparently bivalve like that of Acrida, but I believe is resolvable into the same number of pieces.
In the Homopterous Hemiptera there seems to be more than one type on which the ovipositor is constructed. In an insect very common with us, the froth froghopper (Cercopis spumaria), some approach is made to the ovipositors last described, at least the number of pieces is the same--for it has a pair of external valves forming a sheath, which includes three sharp laminæ resembling the blades of a lancet, the middle one of which can be separated into two; this instrument De Geer had reason to think was scored transversely like a file[753]. In the insects of this Order so noted for their song[754] (Cicada), there are only five pieces; namely, two valves forming the sheath, two augers or borers, and an intermediate piece upon which they slide, each being furnished with an internal groove for that purpose, and the middle piece with a ridge to fit; a contrivance of Divine Wisdom, to prevent their dislocation when employed in boring; the augers terminate in a knob which is externally toothed[755]. This structure approaches that of the Hymenoptera, especially the saw-flies. With regard to the Heteropterous section of this Order--as they usually do not introduce their eggs into any substance, they have no call for any remarkable ovipositor, and therefore are not so furnished. A remark which will also apply to the Lepidoptera Order.
In the Libellulina amongst the Neuroptera, an organ of this kind is sometimes discoverable. In Agrion, Reaumur noticed a part which he conjectured to be an ovipositor; it consists of four laminæ or lancets, the interior pair slender, the exterior wider, and all externally serrated[756].
The insects of the Hymenoptera Order have long been celebrated for the organs we are describing, whether used as saws, augers, or darts. I formerly gave you a very general account of the saws,--I shall now give you a very interesting one in detail copied from an admirable little essay of Professor Peck. "This instrument," says he, "is a very curious object; and in order to describe it it will be proper to compare it with the tenon-saw used by cabinet-makers, which being made of a very thin plate of steel, is fitted with a back to prevent its bending. The back is a piece of iron, in which a narrow and deep groove is cut to receive the plate, which is fixed: the saw of the Tenthredo is also furnished with a back, but the groove is in the plate, and receives a prominent ridge of the back, which is not fixed, but permits the saw to slide forward and backward as it is thrown out or retracted. The saw of artificers is single, but that of the Tenthredo is double, and consists of two distinct saws with their backs: the insect in using them, first throws out one, and while it is returning pushes forward the other; and this alternate motion is continued till the incision is effected, when the two saws receding from each other, conduct the egg between them into its place. In the artificial saw the teeth are alternately bent toward the sides, or out of the right line, in order that the fissure or kerf may be made sufficiently wide for the blade to move easily. To answer this purpose in some measure, in that of the Tenthredo the teeth are a little twisted, so as to stand obliquely with respect to the right line, and their point of course projects a little beyond the plane of the blade, without being laterally bent; and all those in each blade thus project a little outwards: but the kerf is more effectually made, and a free range procured for the saws, by small teeth placed on the outer side of each; so that while their vertical effect is that of a saw, their lateral effect is that of a rasp. In the artificial saw the teeth all point outward (towards the end) and are simple; but in the saw of the Tenthredo they point inward, or toward the handle, and their outer edge is beset with smaller teeth which point outwards (towards the end)[757]." Valisnieri, Reaumur, and De Geer describe the groove as being in the back; but in Mr. Peck's insect, if there is no error in his account, it is, as in the Cicadæ, in the saw itself[758]. In the genus Cimbex, belonging to the same tribe, the saw differs in shape, being somewhat sigmoidal or resembling the letter S, while in that of other saw-flies it is cultriform with a concave edge: other minor differences distinguish them, which need not be particularized.
A similar structure, with regard to the organ in question, obtains in the rest of the Hymenoptera, even those that use it as a weapon of offence; but the backs of the saws in them, composed of a single piece, become a sheath for the darts. The valves, however, vary. In most of those with an exerted sting, as Pimpla, they are linear, exerted, and as long as the aculeus itself[759]. In Proctotrupes they appear to be united so as to form a tube for the ovipositor, and are produced by a prolongation of the last abdominal segment. The darts usually run in two grooves of the sheath, and at their apex are retroserrulate[760]. In some cases the sheath itself is serrated[761]. The shanks of the darts are connected with the valves; so that when these open they are pushed out: sometimes on their outer side they have a triangular plate towards the base, which prevents their being pushed out too far[762].
In Sirex and many ichneumons, in which the ovipositor is too long to be withdrawn within the abdomen, it remains always exerted; but in general it is retracted within that part when unemployed. In the gall-fly (Cynips) this instrument is really as long as in Pimpla, &c.; but as it is infinitely more slender, when in repose it is rolled up spirally and concealed within the abdomen. It is the puncture of this minute organ that produces the curious galls formerly described to you[763]. But the most anomalous ovipositor in this Order appears to be that of Chrysis (C. ignita, &c.), which is covered by several demi-tubes or scales enveloping and sliding over each other: when these scales are removed, the true ovipositor appears, which is of a structure similar to that of the rest of the Order, but the valves are long and slender with their summit generally visible without the anus[764].
Though the ovipositor of the majority of Dipterous insects is a tube with retractile joints[765], in the crane-flies this organ is different, and, like that of Acrida above described, consists of what at first sight appear two valves, but each of which is formed of two pieces, the upper ones sharp and longer, and the lower pair blunt. The upper pair forms the auger that bores a hole in the ground, and the lower conducts the eggs into it after it is bored[766].
In the Aptera and Arachnida in general there seems no remarkable instrument of this kind; but Treviranus has described one in spiders for extruding the eggs of a singular construction. It is an oval plate lying between the external genitals and spinning organs, and is composed of a number of small screw-shaped cartilages, connected together in the most wonderful manner. There are few organs, he observes, in the animal kingdom which for their artificial mechanism can be compared with this. Each cartilage inosculates very closely in the adjoining one, and all are besides bound together by a strong skin[767].
The manner in which the eggs of insects are fecundated by the male sperm is one of those mysteries of Nature that are not yet fully elucidated and understood. We can readily conceive that all the eggs may be fertilized by a single intercourse in the case of insects which, like the Ephemeræ and Trichoptera, exclude the whole mass at once; or like many moths and butterflies, in a very short time afterwards; but the subject becomes much more difficult to explain when we advert to the female of the hive-bee, the whole number of whose eggs, deposited in two years, are, as Huber has demonstrated, in like manner fertilized by a single act[768]:--if you bear in mind, however, what I have lately observed with regard to Malpighi's discovery of a sperm-reservoir in insects, you will more readily comprehend how in this case a gradual fecundation may take place. The principal objection to this solution of the difficulty in the case before us, is derived from the very small size of the organ supposed to be destined for this purpose--it being scarcely bigger than the head of a pin[769]: it seems therefore incredible that it should retain any portion of an extraneous fluid at the end of twelve or eighteen months, and still more unlikely that the fluid should in the interval have sufficed for the slightest moistening of not fewer than 30,000 or 40,000 eggs. The only hypothesis that seems at all to square with this fact, is that of Dr. Haighton,--that impregnation is the result not of any actual contact of the sperm with the eggs, but of some unknown sympathetic influence[770], or rather perhaps of some penetrating effluvia or aura seminalis, which, though small in quantity, it may retain the power of emitting for a long period.
Certain female moths, of the species of that family which, from the remarkable cases or sacs the larvæ inhabit, the Germans call sack--träger, before noticed[771], have been supposed to have the faculty of producing fertile eggs without any sexual intercourse; and various observers, after taking great pains, appeared to have satisfactorily proved the fact; so that some doubted whether these insects produced any males at all[772]. The enigma was at length explained by the accurate Von Scheven. At first his experiments were attended with the same result as those of his predecessors; but upon making them more carefully, and separating what he conceived to be the female from the male pupæ, he ascertained not only the existence of a female in the species he examined (Psyche vestita), but that when thus secluded she laid barren eggs; evidently proving that in the contrary instances above alluded to, an unperceived sexual intercourse must have taken place[773]. Though he thus ascertained that these insects do not in this respect deviate from the general rule, he remarked or confirmed several facts in their economy sufficiently anomalous and striking;--as that the female is not only without wings, but with scarcely any feature of a moth, much more closely resembling a caterpillar; and that in ordinary circumstances she never attempts to leave the pupa-case in which she has been disclosed, but that being there impregnated by the male, she there also, apparently after the manner of the female Cocci, deposits her eggs, which hatching produce young larvæ that make their way out of the case, and thus seem to originate without maternal interference[774].
But the most remarkable fact bearing upon this head, though as relating to a viviparous insect it does not strictly belong to it, is the impregnation of the female Aphides, or plant-lice, before alluded to[775]. If you take a young female Aphis at the moment of its birth, and rigorously seclude it from all intercourse with its kind, only providing it with proper food, it will produce a brood of young ones: and not only this; but if one of these be treated in the same way, a similar result will ensue, and so on, at least to the fifth generation!! to which period Bonnet, who first made an accurate series of observations on this almost miraculous fact, successfully carried his experiments, till the approach of winter and the want of proper food forced him to desist[776]; and Lyonet extended it still further[777]. It is now generally admitted as an incontestible fact, that female Aphides have the faculty of giving birth to young ones without having had any intercourse with the other sex. How are we to explain this most extraordinary fact? Are we to suppose with Bonnet that these insects are truly androgynous, as strictly uniting both sexes in one? This supposition, however, is completely overturned by the circumstance, that there are actually male as well as female Aphides, and that these, as was first observed by Lyonet, are united towards the close of the summer in the usual manner[778]. The most likely supposition therefore is, that one conjunction of the sexes suffices for the impregnation of all the females that in a succession of generations spring from that union. It is true that at the first view this supposition appears incredible, contradicting the general laws and course of nature in the production of animals. But the case of the hive-bee, stated above, in which a single intercourse with the male fertilizes all the eggs that are laid for the space of two years, and in the case of a common spider mentioned by Audebert[779], for many years, shows that the sperm preserves its vivifying powers unimpaired for a long period, indeed a longer period than is requisite for the impregnation of all the broods that a female Aphis can produce; and if immediate contact with the fluid be not necessary, who can say that this is impossible? It is, however, one of those mysteries of the CREATOR that human intellect cannot fully penetrate. But this anomaly in nature is not wholly confined to the Aphides; since Jurine has ascertained that the same thing takes place with Daphnia pennata Müll (Monoculus Pulex L.), one of Branchiopod Crustacea[780]. It is worth observing whether the female Aphides in their natural state, I mean those of the summer or viviparous broods, have intercourse with the male. I think I have noticed males amongst them; but they seem to become most numerous in the autumn, preparatory to the impregnation of the oviparous females. The object of this law of the CREATOR is probably the more ready multiplication of the species[781].
* * * * *
As to the period of gestation, most insects begin to lay their eggs soon after fecundation has taken place: but in some Arachnida, as the Scorpion, which seems to be both oviparous and ovo-viviparous, nearly a year intervenes, and the eggs increase to four times the size which they had attained at that period, before they are extruded[782]. The time that is required to lay the whole they are to produce, varies also in insects. In this respect they may be divided into two great classes:--those namely which deposit the whole at once, as Ephemerina, Trichoptera, &c., and those which deposit them in succession, occupying in this operation a longer or shorter period. Many in the first class, as the Trichoptera or caseworm-flies, envelope their eggs in a gelatinous substance[783], which renders their extrusion in a mass more easy. Of the second class, which includes by far the greater proportion of insects, some exclude the whole number in a very short period, others require two or three days or a week, as the cockroach[784]; and others, as the queen-bee, not less than two years. The eggs in the ovaries of the last vary infinitely in size; those that have entered the oviduct have arrived at maturity, while the rest grow gradually smaller as they approach the capillary extremity of the tubes, where they become at length invisible to the highest magnifier[785]. In many insects the eggs seem nearly to have reached their full growth previously to the exclusion of the female from the pupa; and this exclusion and the impregnation and laying of the eggs rapidly succeed each other. One moth (Hypogymna dispar), which is remarkable for the number of eggs she contains, sometimes deposits them, even before they are fecundated, in the pupa-case[786]. But in other cases the sexual union is not so immediate, and some time, longer or shorter, is requisite for the due expansion of the eggs; and the ovaries of the animal swell so much, as often to enlarge the abdomen to an extraordinary bulk: this is seen in a very common beetle (Chrysomela Polygoni) that feeds upon the knot-grass; but in no insect is it so striking as in the female of the white ants, whose wonderful increase of size after impregnation I have related to you on a former occasion[787].
* * * * *
I shall conclude this subject with a few observations upon ovo-viviparous insects; supposed neuters, and hybrids, which, though they do not fall in regularly under any of the foregoing heads, may very well have a place in this letter.
1. It has already been observed that there are a few ovo-viviparous insects[788], the young of which exist in the ovaries at first as eggs, but are hatched within the body of the mother, and come forth in the living form of a larva and sometimes even of a pupa. Of the first description are certain Diptera, the Aphides, and the Scorpion.
Reaumur has described two modes in which the larvæ of the first are arranged in the matrix of the mother. In some they are heaped together without much appearance of order, being placed merely parallel to each other[789]; but in others they are arranged in a kind of riband--the length of the little animals, which are also parallel, forming its thickness--rolled up like the mainspring of a watch[790]. These larvæ in general are not divided into two masses corresponding with the pair of ovaries in other insects, but form only a single one[791]. You must not suppose that these little fetuses lie naked in the womb of the mother; each has its own envelope formed of the finest membrane, which, however, is not entirely divided from that of those adjoining to it, but appears to be one tube, which becomes extremely slender between each individual, so as when drawn out to look like a chain[792]. Reaumur seems to have thought that in these flies the larvæ were never confined in any other case or egg[793]; but De Geer sometimes found eggs in the body of Sarcophaga carnaria, though most generally larvæ, from which he conjectures that it is really ovo-viviparous, the eggs being hatched in the body of the mother[794]. As these flies are all carnivorous, and their office is to remove putrescent flesh, you may see at one glance the object of PROVIDENCE in this law of nature--that no time may be lost, and the animal exercise its function as soon as it is disclosed from the matrix.
The Aphides, so fruitful in singular anomalies, are ovo-viviparous, as I have before hinted[795], at one period of the year, that is during the summer, but strictly oviparous at its close. From the experiments of De Geer, however, upon Aphis Rosæ, it would appear that this faculty is not conferred upon the same individuals, but only upon those of different generations of the same species; all the generations being ovo-viviparous except the last, which is oviparous[796]: nor does it appear, as has been sometimes imagined, that it is common to the whole genus. De Geer observed a species in the fir, which makes curious galls resembling a fir cone (Aphis Abietis), which appeared never to be ovo-viviparous[797].
With regard to scorpions, it does not seem clear that they are always ovo-viviparous: M. Dufour twice found in the midst of the eggs nearly mature, a young scorpion which appeared to him at large in the cavity of the abdomen; it was so large that it was difficult to comprehend how it could possibly be excluded from the animal, without an extraordinary operation[798]. The pupiparous insects (Hippobosca, &c.) have been sufficiently noticed before[799].
2. I have already in several of my former letters stated to you what the modern doctrine of physiologists is with respect to certain individuals, usually forming the most numerous part of the community with insects living in society, that were formerly supposed to be neuters, or as to their sex neither male nor female--that they are in almost every instance a kind of abortive females, fed with a different and less stimulating food than that appropriated to those whose ovaries are to be developed, and in consequence in most instances incapable of conception[800]. Upon these sterile females, you also heard, devolve in general the principal labours of their respective colonies, showing the beneficent design of PROVIDENCE in exempting them from sexual cares and desires, and meriting for them the more appropriate name, now generally used, of workers. The differences in the structure of the female bee and the workers were also then accounted for; and similar reasoning may be had recourse to with regard to those of ants, in which the worker and the female differ still more materially. My reason for introducing this subject here, is to observe to you that I have some grounds for thinking that this system extends further than is usually supposed, and that to each species in some Coleopterous and other genera there are certain individuals intermediate between the male and female; this I seem to have observed more especially in Copris and Onthophagus. For in almost every British species in my cabinet of these genera I possess such an individual, distinguished particularly by having a horn on the head longer than that of the female, but much shorter than that of the male. I once observed a pair of Pentatoma oleracea, a very pretty bug, in coitu, both sexes being ornamented with white spots, and by them stood a third distinguished from them by red ones. I do not, however, build on this circumstance, though singular; but mention it merely that you may keep it in your eye. It would be curious should it turn up, that, to answer some particular end of PROVIDENCE, in some tribes of insects there are two kinds of males, as in the gregarious ones two descriptions of females.
I am, &c.
FOOTNOTES:
[703] Herold Schmetterl. tab. expl. vii.Return to passage
[704] Herold Schmetterl. t. iv. f. 1. x. &c. PLATE XXX. FIG. 12. d.Return to passage
[705] De Bombyc. 36.Return to passage
[706] Ibid. t. xii. f. 1. I. and, f. 2. O. M.Return to passage
[707] Philos. Trans. 1792. 186.Return to passage
[708] Swammerdam, in dissecting the female of Oryctes nasicornis, discovered a blind vessel opening into the vagina, and at the other or inner extremity not terminated by any secretory tube, containing a yellowish matter, that seems analogous to the organ mentioned in the text; and in the hive-bee he found a similar organ covered with air-vessels, which he supposes to be connected with the Colleterium (see above, p. 132.), and which he states to contain a slimy matter. Bibl. Nat. i. 151. b. t. xxx. f. 10. g. 204. b. t. xxix. f. 3. t. Perhaps likewise the organ discovered by M. L. Dufour in Scolia,--which he imagines to belong to the poison-secretor, and which he describes as a sac consisting of a double tunic, the exterior one muscular and the interior membranous, and filled with a blueish-green gelatinous matter (N. Dict. d'Hist. Nat. xxx. 388.)--may be a spermatheca.Return to passage
[709] De Insector. Genital. 17.Return to passage
[710] I allude to those organs above described (p. 132.) for the secretion of matter for varnishing the eggs or lubricating the oviduct. It seems most probable, if the fecundation of the eggs takes place gradually, that upon their passing into the oviduct, a special reservoir should be appropriated to the reception of the male sperm, adapted to maintaining in due activity the vivifying principle, or aura seminalis.Return to passage
[711] Herold Schmett. t. iv. f. 2. m n.Return to passage
[712] Treviran. Arachnid. 36. t. iv. f. 32. aa. Marcel de Serres in Mém. du Mus. 1819. 89.Return to passage
[713] Marcel de Serres, Mém. du Mus. 1819. 115.Return to passage
[714] Rifferschw. De Genital. Ins. 11.Return to passage
[715] Marcel de Serres in Mém. du Mus. 1819. 109. PLATE XXX. FIG. 12. a.Return to passage
[716] Rifferschw. ubi supr. 23--. Swamm. Bibl. Nat. t. xlii. f. 8. a, f, g, h.Return to passage
[717] Ibid. i. 104. t. xv. f. 3. ii. 62. t. xii. f. 8. Treviran. Arachnid. t. iv. f. 32.Return to passage
[718] Reaum. iv. 391.Return to passage
[719] Posselt Anat. der Ins. t. i. f. 28, 29.Return to passage
[720] N. Dict, d'Hist. Nat. xxx. 387--. Swamm. ubi supr. ii. 23. t. xxxv. f. 3.Return to passage
[721] Ibid. i. 203.Return to passage
[722] PLATE XXII. FIG. 2.Return to passage
[723] Swamm. ubi supr. i. 151. Gaede Anat. der Ins. t. ii. f. 3.Return to passage
[724] Swamm. i. 203.Return to passage
[725] Gaede Anat. der Ins. 20. t. i. f. 9.Return to passage
[726] Ibid. 25, 28. t. ii. f. 10.Return to passage
[727] Ibid. 32.Return to passage
[728] Swamm. ii. 74.Return to passage
[729] Ibid. 203. t. xix. f. 3.Return to passage
[730] Reaum. iv. 391--.Return to passage
[731] Swamm. t. xliii. f. 19.Return to passage
[732] Gaede 22.Return to passage
[733] Swamm. Bibl. Nat. i. 203.Return to passage
[734] Ibid.Return to passage
[735] Rifferschw. 11--.Return to passage
[736] Swamm. t. xlii. f. 8. Gaede, t. i. f. 3. cc.Return to passage
[737] Herold Schmett. t. v. f. 10. 12.Return to passage
[738] PLATE XXX. FIG. 12.Return to passage
[739] PLATE XXII. f. 2. b.Return to passage
[740] Swamm. t. xix. f. 4. b.Return to passage
[741] Ibid. f. 3.Return to passage
[742] VOL. I. p. 355--.Return to passage
[743] De Geer iv. 127. t. iv. f. 17.Return to passage
[744] De Geer iv. 143. t. v. f. 15.Return to passage
[745] VOL. I. p. 357.Return to passage
[746] De Geer. v. 62. t. iii. f. 12.Return to passage
[747] PLATE XV. FIG. 18.Return to passage
[748] Stoll Sauterel. t. xxii. b. f. 87, &c.Return to passage
[749] De Geer iii. 418. t. xxi. f. 10, 11. Latr. Gen. Crust. et Ins. iii. 98.Return to passage
[750] Stoll ubi supr. t. xiii. a. f. 51.Return to passage
[751] This insect, which connects Conocephalus, Acrida, &c. with Locusta, is also distinguished by antennæ at first filiform and then setaceous.Return to passage
[752] De Geer iii. t. xxiv. f. 1, 12.Return to passage
[753] Ibid. 176. t. xi. f. 19.Return to passage
[754] VOL. II. p. 397--.Return to passage
[755] Reaum. v. 177--.Return to passage
[756] Ibid. vi. 435. t. xl. f. 6, 7.Return to passage
[757] Natural History of the Slug-worm, 12--. f. 12, 13.Return to passage
[758] Valisn. Esperienz. &c. Musca dé Rosai. Reaum. v. 100--. De Geer ii. 916--. The last writer thought he saw in the back of the saw itself a longitudinal cavity (918), which applied to the groove would form an open canal.Return to passage
[759] PLATE XVI. FIG. 1.Return to passage
[760] Ibid.Return to passage
[761] Reaum. v. 347. t. xlix. f. 10. d, f.Return to passage
[762] See above, VOL. III. 390. a.Return to passage
[763] See above, VOL. I. 448--.Return to passage
[764] De Geer ii. 835. t. xxviii. f. 20, 21. PLATE XV. FIG. 22. This figure was drawn by a friend--the organ seems more exerted than in De Geer's. I cannot make out the little appendage at the end.Return to passage
[765] PLATE XVI. FIG. 2, 3.Return to passage
[766] Reaum. v. 19--. t. iii. f. 3-6.Return to passage
[767] Arachnid. 40.Return to passage
[768] Huber Nouvel. Observ. i. 106.Return to passage
[769] Swamm. Bibl. Nat. t. xix. f. 2.Return to passage
[770] Philos. Trans. 1797. 80.Return to passage
[771] VOL. I. p. 461.Return to passage
[772] Compare Reaum. iii. 153. Pallas Act. Nat. Cur. 1767. iii. 430. Wien. Verzeich. 292.Return to passage
[773] Naturfor St^k. xx. 59--.Return to passage
[774] It does not appear to be clearly decided whether the eggs are extruded from the female, or whether dying immediately after fecundation they are hatched within her body. As the young larvæ certainly are hatched in the pupa (not merely within the exterior case of bits of grass, &c., which includes it) which the body of the insect must fill, it does not seem easy to conceive how she can find room for oviposition; and yet Von Scheven expressly says that one female of Ps. vestita--which being kept from all access to the male actually left the pupa-case and wandered about the glass which contained them--laid unfruitful eggs.Return to passage
[775] VOL. I. p. 32, 175.Return to passage
[776] Bonnet i. 19--.Return to passage
[777] Reaum. vi. 551.Return to passage
[778] Reaum. vi. 552.Return to passage
[779] N. Dict. d'Hist. Nat. ii. 284.Return to passage
[780] N. Dict. d'Hist. Nat. ix. 125. Bonnet and Jurine both found that the female Aphides and Branchiopods that were fertile without the usual intercourse of the sexes were less fruitful than their mother, and those of the last generation less so than the first. Latr. Hist. Nat. des Crust. et Ins. xi. 292.Return to passage
[781] See more on the subject of fecundation, VOL. II. p. 154--. 169--.Return to passage
[782] N. Dict. d'Hist. Nat. xxx. 426.Return to passage
[783] VOL. III. p. 68.Return to passage
[784] De Geer iii. 533.Return to passage
[785] Swamm. i. 203. b. t. xix. f. 3.Return to passage
[786] Reaum. ii. 66.Return to passage
[787] VOL. II. p. 36.Return to passage
[788] VOL. III. p. 64--.Return to passage
[789] PLATE XXII. FIG. 4.Return to passage
[790] Ibid. FIG. 3.Return to passage
[791] Reaum. iv. 414.Return to passage
[792] Ibid. t. xxviii. f. 14, 15.Return to passage
[793] Ibid. 404.Return to passage
[794] De Geer vi. 63--.Return to passage
[795] VOL. I. p. 175.Return to passage
[796] De Geer iii. 70--.Return to passage
[797] Ibid. 128.Return to passage
[798] N. Dict. d'Hist. Nat. xxx. 426--.Return to passage
[799] VOL. III. p. 64--.Return to passage
[800] VOL. II. p. 50, 110--, 118--, 125--, 130--. The neuters of the Termites, however, (p. 33.) seem to be a distinct sex, if I may so speak--and to merit that name.Return to passage
Letter XLIII
_INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONCLUDED._
MOTION.
We have seen upon a former occasion the great variety of movements that insects can perform, and of the external organs with which they perform them[801]: but we are now to consider the internal apparatus, by the immediate action of which they take place--their system of muscles. When we reflect upon the wonderful velocity, their size considered, with which many insects move, and the unparalleled degree of muscular force that many exert[802], we feel no small degree of curiosity to know something of that part of their internal structure that produces these almost incredible effects. I shall in the present letter endeavour in some degree to gratify that curiosity, and give you an account of the muscles of these little animals,--first considering them in general; and then, as far as my information goes, adverting to those in particular that move the different parts and organs of an insect's body.
I. The muscles of insects may be considered in general as to their Origin; Substance and Parts; Shape; Colour; Kinds; Attachment; and Motions.
i. Origin. The origin of the muscular fibre in the higher animals is from the blood, the globules of which, by their coagulation in a series, appear to form it[803]; and in insects it is derived from the same universal source of nutrition and accretion, but not till it has been concreted into the adipose tissue or epiploon before noticed[804]. In the pupa of the cabbage-butterfly, Herold observed that this substance first assumed a fine flocky appearance and a blue-green colour, and that from it so changed were produced tender bundles of muscular fibres, extending in various directions, the epiploon itself decreasing in proportion as they were formed[805].
ii. Substance and Parts. The muscular fibre in vertebrate animals appears to consist of globules arranged in a series, and of no larger diameter than those of the blood,--the mean diameter of which in the human subject, when measured under the microscope by a micrometer, is found to be about 1/5000th part of an inch[806]. When Cuvier published his immortal work in 1805, the powers of any magnifier then constructed were not sufficient to enable this great physiologist to arrive at the simple fibre[807]; but Mr. Bauer, by the use of improved glasses, amongst other discoveries that will immortalize his name, was the first to detect, under the directions of Sir E. Home, the ultimate thread of which the muscular bundles are composed[808]. Chemists distinguish the substance of which we are speaking, by the name of fibrine. By the abundance of azote or nitrogen that enters into its composition, it possesses a character of animalization more marked than any other animal substance; and its elements are so approximated in the blood, that the slightest stagnation causes them to coagulate: and the muscles are without doubt, in the living subject, the only organs that can separate this matter from the mass of blood, and appropriate it to themselves[809]. The primary bundles of muscles are formed of the simple fibres, and the secondary are the result of an aggregation of the primary. The smaller bundles are not always exactly parallel to each other, but must in many cases diverge more or less, to produce those variations in shape observable in the muscles themselves: there are intervals therefore between the bundles, which in some animals are filled by a cellular substance[810]. Probably much of this statement will apply in most instances to the muscles of insects, but we may conclude that the globules that form them are infinitely smaller[811]. Lyonet has given some interesting observations with regard to those of the caterpillar of the Cossus: he describes them as of a soft transparent substance, capable of great extension, covered and filled by silver tubes of the bronchiæ, penetrated by the nerves, and containing oily particles. Each muscle was enveloped in membrane, and was composed of many parallel bands, consisting of bundles of fibres enveloped likewise in separate membranes. The fibres themselves, (but it is doubtful whether he arrived at the ultimate term of muscular fibre,) in a favourable light and under a good magnifier, appeared to be twisted spirally[812]. In spiders the muscles seemed to him to consist of two substances, the one soft and the other hard, the last forming a kind of stiff twisted filament[813]. A muscle thus composed of different bundles of fibres may be stated as to its parts, in insects, to consist of base, middle, and apex: the base is that part by which they are fixed to any given point of the internal surface of the crust, or of one of its processes, which serves as their fulcrum; the apex is that part by which they are fixed, either mediately or immediately, to the organ to be moved; and the middle is the remainder of the muscle. We usually discover in them no inflation of the middle corresponding with the belly of the muscles in vertebrate animals; they occasionally, however, terminate in a tendon, as those of the thighs and legs; but these tendons are of a different nature from the fibrous ones of warm-blooded animals; for they are hard, elastic, and without apparent fibres: the fleshy ones of the muscle envelope them, and are inserted in their surface[814].
iii. Shape. The muscles of insects are usually linear, with parallel sides; some are cylindrical, as those of the wings of the Libellulina[815]; and others, as those that move the legs in the caterpillar of the Cossus, are triangular[816]. In the suctorious mandibles of the grub of a common water-beetle[817] they are penniform, or shaped like a feather; and some in the Cossus are forked[818]. Under this head I may also observe, that the muscles are sometimes extremely slender threads, crossing each other, and often curiously interwoven in various directions, so as to resemble lace or fine gauze, as may be seen in the alimentary canal of some caterpillars[819]; sometimes also they surround part of this organ, like a series of minute rings[820].
iv. Colour. The most usual colour of the muscles of insects is white: those for flight however, according to Chabrier, differ from the rest, by being of a deeper and reddish colour[821]; and I have observed likewise that those in the head of the stag-beetle, when dried at least, are red, and look something like the flesh of warm-blooded animals.
v. Kinds and Denomination. In general, muscles may be regarded as divided into primary and secondary--the primary being the muscles by which the principal movements of any organs are effected, and the secondary their auxiliaries which are the cause of subordinate movements[822]. Every muscle almost has its antagonist, the action of which is in an opposite direction; so that when it is equal, the organ to which they are attached remains without motion; but when that of one preponderates, a movement in proportion takes place[823]. The principal antagonist muscles that may be found in insects are the following. 1. Levator muscles that raise an organ, and Depressors that depress it. 2. Flexors that bend an organ, and Extensors that unbend or extend it. 3. Abductors that draw an organ back, and Adductors that draw it forwards. 4. Constrictors that contract an opening, and Laxators that relax it. 5. Supinators that turn the underside of an organ upwards, and Pronators that return it to its natural situation. Some of these muscles in insects, like some of their articulations and their spinal chord[824], seem to exercise a double function,--thus the levators and depressors of the wings are constrictors and laxators of the trunk[825]. At first it may seem that insects, not having the power of turning up the hand, cannot have the Supinator and Pronator muscles; but some muscle of this kind must be in the Gryllotalpa, and in those that have a versatile head[826].
v. Attachment and Insertion. The attachment and insertion of the muscles in insects in general is to the interior of the crust, or to some of its internal processes as a fulcrum, and to the organ to be moved. In some cases, however, the muscles act upon the organ by the intervention of other bodies. Thus, those that move the wings are often attached to little bones, as Chabrier calls them[827], which are connected with the base of the wings by ligaments. In the Dynastidæ and other Lamellicorns, and the Libellulina, &c., a remarkable provision is made for giving a vast increment of force to the muscles of the wings, by means of caps or cupules surmounted by a tendon, which receive their extremity; the tendon terminating in a fine point attached to the wing, and thus more muscles are brought to bear upon it[828]. Chabrier seems to think that, in some cases, the back that intervenes between each pair of wings is the medium by which the muscles act upon it[829].
vi. Motions. Irritability is the universal distinction of the muscular fibre,--when put in action by the will or involuntarily, it causes it to contract or become shorter; and the intermediate agents of the will and other causes are the nerves, which, as galvanic experiments seem in some degree to prove, are the conductors of an invisible fluid or power which immediately causes that action. If a nerve is divided, the muscles to which it renders obey it no longer, evidently proving that the nerves cause muscular irritability[830]. How this contraction is immediately effected,--whether the fibre, as some suppose, undergoes any crispation, or becomes zigzag[831], or whether there is any sudden change in their chemical composition that rapidly and strongly augments their cohesion, as Cuvier hints[832], cannot be clearly ascertained, unless a Bauer could submit the living fibre to his glasses. All that we know certainly on the subject is, that muscles alternately contract and relax at the bidding of the will or involuntarily, and so occasion all the movements of animal bodies.
II. Having considered the muscles of insects in general, I must next make a few observations, as far as my means of information will enable me, upon those that move their different parts and organs--at least the principal ones; since to descend to minutiæ would be an endless and unprofitable labour. As larvæ, except those whose metamorphosis is semicomplete[833], differ widely in their system of muscles from perfect insects, I shall begin my observations with them.
We owe by far the most accurate and detailed account of the muscles of larvæ to the illustrious Lyonet, who, with incredible labour and patience without example, dissected the caterpillar of the Cossus, and has described every air-vessel, every nerve, and every muscle that could be detected by the microscope. Cuvier also has given a description of the muscles not only of caterpillars, but of the larvæ of the Lamellicorn beetles, the Hydrophili, and the Capricorn beetles[834]. From these sources are derived what I have now to lay before you. If you look at one of Lyonet's plates[835], the layers of longitudinal muscles look like so many parallel ribands, others run in an oblique, and others again in a transverse direction[836]. He divides them into dorsal, ventral, and lateral muscles[837], terms which sufficiently explain themselves. Of the longitudinal muscles there are four principal rows[838], the others are more numerous. The principal object of these muscles, which are flexors and extensors, is to shorten or lengthen the body, or to act on any particular segment as the circumstances of the animal may require. I shall not here notice the muscles of the head and legs, as they are not remarkably different from those of perfect insects. The prolegs are moved by two muscles--the anterior one covering in part the posterior--of a remarkable structure: one of their points of attachment is by many branches or tails to the sole of the foot, and by several heads to the skin of the animal; so that they can draw the proleg within the body or push it out, and perform other necessary movements[839].
I shall now call your attention to the muscles of the perfect insect, as they move the head and its organs; the Trunk; the Abdomen; and the Viscera.
i. The Head. This part in insects moves upwards, downwards, inwards, to right and left, is pushed forth or drawn in, is often capable in part of a rotatory movement, and is sometimes versatile, turning as it were upon a pivot. All these movements are of course produced by an appropriate apparatus of muscles, which have their attachment in the anterior part of the trunk, mostly in the manitrunk, while their insertion is in the posterior part of the head, in the margin of the occipital cavity. To enumerate and describe them all would be tedious and uninteresting--I shall only mention some of the principal ones. The levators of the head are usually a pair of muscles situated in the manitrunk, to the upper side of which they are attached, and perhaps in Coleoptera and some others to the phragma, which probably Cuvier means by the anterior part of the scutellum[840]; they are inserted in the posterior margin of the upper part of the head, in Coleoptera in a pair of notches (Myoglyphides[841]), or a single one[842]. In Cordylia Palmarum these muscles as they approach the head, to judge from the dead animal, divide into two branches or a fork: thus, as the muscle-notches are wide in this insect, the muscle acts upon each extremity of the sinus--these branches appear to be tendinous[843]. The depressors of the head are the antagonist muscles to the above, and have their attachment to the antepectus and its antefurca[844]. A circumstance distinguishes these muscles in many Coleoptera, that seems hitherto to have been overlooked. If you take the common dung-beetle (Geotrupes stercorarius), and carefully extract the head with its muscles from the trunk, you will see on each side of the depressors a subovate corneous scale, of a pitch colour[845], which is attached only to the muscle, and designed to strengthen it: if you then examine the anterior cavity of the manitrunk, you will perceive on each side, just within the lower margin, a minute triangular scale, of a similar substance; these ligaments, like the pax-wax, or ligamenta nuchæ, in mammalia, though in a lower situation, are doubtless intended to sustain the action of the muscles.
With regard to the moveable organs of the head--the antennæ, maxillæ, palpi, tongue, mandibulæ, &c., have each their appropriate apparatus of muscles: but I shall only notice those of the last, the mandibulæ. These are principally abductors and adductors to open and shut them: from the work that the jaws of some insects have to do, you may conjecture that they must be furnished with powerful muscles. In caterpillars and other larvæ, in which state the action of the mandibles is most in requisition, the muscles are what Cuvier calls penniform[846], and are attached on each side to a tendinous lamina or cartilage. In the grub of Dytiscus the power and magnitude of the adductor muscle is wonderful[847]. In the Orthoptera this structure of the mandibular muscles takes place also in the imago[848]; but in the Coleoptera, at least in the stag-beetle and some others that I have examined, these muscles in this state have no cartilage or tendon. Their attachment is always to the parietes of the head, of the cavity of which the adductors, in some cases, occupy a considerable portion[849]. As to their insertion--these last, in some Orthoptera, enter more or less the interior of the mandible[850]; but commonly they are inserted at or near the interior angle of the mandibular basal cavity, and the abductors at the exterior.
ii. The Trunk. We have little information with regard to the muscles of the parts of the trunk itself, by which, in some insects, the manitrunk is enabled to move independently of the alitrunk: it is more probable that the levators have in part at least their attachment to the anterior surface of the prophragm[851], than that the levators of the head should be there fixed, as Cuvier seems to think; since both the phragma and the ligament that appears in many cases to close the cavity of the manitrunk round the viscera[852], would prevent all communication between those muscles and any part connected with the scutellum: probably the depressors have their attachment partly on the anterior face of the medifurca[853]. These points, however, must be left to future investigators.
With regard to the organs of the trunk, we have more certain and satisfactory information;--the muscles of the legs having been described by Lyonet and Cuvier, and those of the wings most particularly by Chabrier. In caterpillars, the muscles are situated in the interior of the articulations that form the legs: they consist of several bundles appropriated to each, which have their attachment in the parietes of the preceding joint, near the margin, and are inserted in the margin of that they move[854]. Lyonet counted twenty-one muscles in the leg of the caterpillar of the Cossus; but eight of these were appropriated to the claw, or rather formed a pair of semipenniform muscles, having their insertion at the inner angle of its base[855]. In perfect insects, according to Cuvier, each joint of the legs is furnished with a pair of antagonist muscles--a flexor and extensor, the former being the lower, and the latter the upper muscle; and this pair has its insertion in the joint it moves, and its attachment usually in the preceding one: but those of the coxæ--which are rotators, causing it to turn backwards or forwards--and the extensor of the thigh, have their attachment in the parietes of the trunk, and to the endosternum; one of the rotators of the anterior coxa, and the extensor of the anterior thigh to the antefurca; of the intermediate pairs to the medifurca, and of the posterior to the postfurca[856]. Every joint of the tarsus has also its flexor and extensor. In the ground- and water-beetles (_Eutrech_in_a_ and _Eunech_in_a_), &c., whose posterior coxæ are immoveable, the thigh includes two pair of antagonist muscles[857]. In extracting the posterior leg of Necrophorus Vespillo I observed more than a single pair of muscles that had their attachment in the coxa; and probably many other variations in this respect exist.
Little was known with respect to the most interesting part of the muscular apparatus of insects, that by which such wonderfully rapid and varied motions are imparted to their organs of flight, till Chabrier undertook to elucidate it; which he has done in a manner that will confer a lasting honour upon his name, as one of the most able successors to Swammerdam and Lyonet in their peculiar department. He has given a most admirable account of the internal anatomy of the trunk of insects in general, as far as it relates to their flight; particularly of that of the cockchafer (Melolontha vulgaris), of one of the Libellulina (Æshna grandis), and of a bumble-bee (Bombus); and I believe he has thus illustrated insects of some of the other Orders, but his memoirs on these I have not had an opportunity of consulting. What I have to say on this subject, therefore, will be principally derived from what he has communicated with respect to the above insects.
A considerable difference in the volume of the muscles of the wings takes place in insects according to the force of their flight. Where it is rapid and powerful, the alitrunk is nearly filled by them, and the alimentary canal is much attenuated; but in those whose flight is feeble, they occupy less space, and the alimentary canal is proportionally enlarged[858]. In the Lepidoptera, Hymenoptera and Diptera, the principal muscles of both wings have their attachment in the anterior portion of the alitrunk[859]; in the Coleoptera, in the posterior[860]; and in the Libellulina, those of the anterior wings are confined to the anterior portion, and those of the posterior pair to the posterior[861]. The muscles for flight in general differ from others by their mass, length, and colour; the bundles of fibres are very distinct, strong, and parallel; their direction is uniform, according to the motion they are to produce; their fibres are either attached to the solid parts to be moved, or to cupules, but they never terminate in a tendon; the muscles are perfectly independent of each other, and the wings can be moved by them separately[862]. As to their denomination and kind--the principal ones are the levators and depressors, which with respect to the trunk, as was before observed, are constrictors and laxators. The levator muscles form several distinct bundles in Coleoptera, Lepidoptera, &c.; in the Diptera there are three[863]; in the Libellulina they seem to be single, are all environed with a blackish pellicle, with numerous aërial vesicles, symmetrically arranged, filling the interstices[864]. The most common number is a levator to each wing; there are often, however, as in the cockchafer and the dragon-fly, two depressors[865]: but in the Hemiptera, Lepidoptera, and saw-flies (Serrifera) amongst the Hymenoptera, the secondary wings have distinct levators, but not depressors[866]; the other insects of that Order have only a pair of each[867]. The other wing-muscles are of a secondary description, and auxiliary to the above. Their office is to extend and close the wings: so that though the denomination of extensor will suit the former, that of flexor is not so proper for their antagonists; their office being not so much to bend, as to bring back the wing to its station of repose. The folding of certain wings, as those of Coleoptera, Dermaptera, the Vespidæ, &c., seems more the function of the abdomen than of the wing-muscles; this you may easily see, as I have often done, if you attend to any Staphylinus, when after alighting from flight it proceeds to fold up its wings under the elytra. Perhaps the term retractor might not be inapplicable to the muscles in question. Both these and the extensors are usually small slender muscles, but sometimes numerous[868]. They are larger in the Coleoptera, Lepidoptera, and saw-flies[869]. The muscles that open and shut the elytra of Coleoptera, and probably of Heteropterous Hemiptera, and which also aid their movements during flight, are very slender[870]. With regard to the attachment and insertion of the wing-muscles, it is according to two very distinct types, one of which appertains to insects in general, and the other is peculiar to the Libellulina. In insects in general, the principal muscles for flight have not their insertion in the wings, but act upon their bases by the intervention of small long pieces. The depressors occupy the middle and upper region of the alitrunk, and are inserted anteriorly and posteriorly upon the concave surfaces of two transverse horny semi-partitions, adapted by their elasticity to dilate the trunk--and thus acting the part of both diaphragm and ribs[871]: but in the Libellulina, as in birds, these muscles are placed on each side of the point of support of the humerus[872]; the depressors being attached immediately to the wings without it, and the levators within it, with this sole difference, that they are connected to the internal extremity of the base of the wing by the intervention of a cupule terminating in a tendon; all are disposed perpendicularly to the arms of the levers on which they act, and all incline more or less outwards, the one to dilate, and the other to contract the trunk[873]. It may be observed in general, that in insects formed upon the first type, the great action of these muscles is the dilatation and contraction of the alitrunk, the main tendency of which is to depress and raise the wings[874]. I shall add here a few words upon the attachment of the wing-muscles in the different Orders: but first I must request you to read what I have said on the partitions and chambers of the alitrunk in a former letter[875]. In most insects of the first type, the depressors are longitudinal dorsal muscles that have their posterior point of attachment in the metaphragm (costale Chabr.); but the anterior varies:--in those that have elytra, tegmina, or hemelytra, the muscles for them seem to be contained in the chamber, varying in size, that lies between the prophragm and mesophragm; and the anterior point of attachment of their depressor muscles is the mesophragm: they are also attached in some to the metathorax or back of the posterior portion of the alitrunk[876]. The levator muscles in Coleoptera, at least in the cockchafer, by a long tendon have their posterior attachment in the lower part of the posterior coxæ[877], their anterior attachment to the solid parts to be moved. In the Cockchafer and the Dynastidæ, but not in Geotrupes, on each side of the cavity of the metathorax under the base of the wing is a large and small cupule, which from their lateral situation one would think must receive the levator muscles--apparently unnoticed by M. Chabrier; but as there is a pair of these cupules on each side, there must have been also a pair of muscles attached to them, which does not agree with his statement[878]. In the Hymenoptera and Diptera the anterior attachment of the depressors is to the back of the alitrunk and to the prophragm, and the levators to the breast, and the sides of the back of the trunk[879]. In the Libellulina the depressors and levators that terminate, by a tendon surmounting a cupule, in the base of the wings, have their posterior attachment in the breast. These cylindrical muscles with their cupule and tendon look like so many syringes[880].
Having thus described to you the powerful muscular apparatus by which, either mediately or immediately, the wings of insects are moved, it will not be out of place if I add a few words upon their flight itself. The great object in this is to generate a centrifugal force which may counteract the weight of the body. Its wings are the external organs by which the insect as it were takes hold of the air when they fall, and is impelled by it when they rise; its head makes way for it; its abdomen, as a rudder, steers it; and by alternately increasing and diminishing in volume, and rising and falling, enables it to win an easy way through the fluctuations of the atmospheric sea. The trunk by its elasticity admits the internal action of antagonist muscles, which by turns compress and dilate it; an action promoting the elevation and depression of the wings, and keeping up the elasticity of the internal air, which is thus now rarified and now condensed: in the former state flowing like a tide, accompanied by the blood, into the nervures of the wings[881], and thus increasing their tension and centrifugal force;--in the latter ebbing and receding to the trunk, thus relaxing the one and diminishing the other. The spiracles by which the air enters or is expelled, open and shut at the animal's pleasure[882]; and besides, many insects are furnished, as we have seen[883], with numerous vesicles or reservoirs, which can give out a supply of internal air when wanted: and thus they can vary their aërial motions, diminish or increase the counteracting centrifugal force; rise and fall, and move onwards and in different directions, as their occasions demand.
iii. The Abdomen is perhaps capable of the greatest variety of motions of the three primary sections of the body. Even when the insect is reposing, a constant dilatation and contraction usually takes place in it[884]; and from its annular structure, its parts capable of separate motion are numerous:--it expands and contracts; it rises and falls; it bends in various directions; and its segments can often be lengthened or retracted. Besides all this, its spiracles open and shut, and its reproductive and other anal organs have their appropriate motions. In numerous Coleoptera, however, and some Hemiptera, the upper-side of the abdomen is almost the only part that is moveable, especially near the trunk; the under-side, having its first segments soldered together, is only capable of motion near the tail[885]. The muscles that produce the various motions of this part must be entitled to all the denominations stated above[886]. I have on a former occasion explained to you how, in insects that have a petiolate abdomen, that part is elevated and depressed[887]. In those with a sessile one the base is attached to the metaphragm by strong ligaments[888], and the muscles that move the first piece act from one segment to another. The partial movements of the segments of this part, where they have place, are produced by muscular fibres which extend from the whole anterior margin of one to the whole posterior one of that which precedes it. If those, for example, of the back contract, the abdomen becoming shorter above, bends upwards; and if those of the sides or belly, it bends sideways or downwards[889]: this is a beautiful as well as simple contrivance.
The alternate rush of air from the abdomen into the alitrunk, and from the atmosphere into the abdomen, is attended by the constriction or expansion of that part as it rises or falls in flight[890], which seems to require the action of constrictor and laxator muscles.
iv. The Viscera. Having before had occasion sufficiently to notice the muscles by which the systole and diastole of the dorsal vessel of insects is maintained[891], I shall now only mention those that are woven round their alimentary canal, by which the peristaltic motion of that organ, causing its contractions and the propulsion of its contents, takes place. One would at first think that a view of the intestines of any animal could under no circumstances afford any very pleasing spectacle to the eye of any but a scientific spectator; but any lady who is fond of going to Disons to be tempted with an exhibition of fine lace, would experience an unexpected gratification could she be brought to examine those of a caterpillar under a microscope: with wonder and delight she would survey the innumerable muscular threads that in various directions envelope the gullet, stomach, and lower intestines of one of these little animals; some running longitudinally, others transversely, others crossing each other obliquely, so as to form a pattern of rhomboids or squares; others again, surrounding the intestine like so many rings, and almost all exhibiting the appearance of being woven, and resembling fine lace,--one pattern ornamenting one organ; another, a second; and another, a third. This will suffice to give some idea of this part of the muscular structure of these little animals[892].
Lyonet counted the muscles contained in the body of the caterpillar of the Cossus. In the head he found 228; in the body, 1647; and enveloping the intestines, no less than 2186; which, after deducting 20 that are common to the gullet and the head, gives a total of 4061[893]. In the human subject only 529 have been counted[894]: so that this minute animal has 3532 muscles more than the Lord of the creation!
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The muscles of the Arachnida seem less numerous than those of insects. In the Scorpionidea they appear to be robust, formed of simple straight fibres, of a whitish gray colour: a muscular web, rather strong, clothes the parietes, but rarely adheres to them, of the abdomen, and envelopes the viscera, with the exception of the lungs, and probably of the heart. The dorsal part of this web gives birth to seven pairs of filiform muscles, which traverse the liver, and are attached to a muscular riband which, passing above the lungs, runs the whole length of the ventral parietes. These muscles when exposed to view resemble extended cords. The abdominal segment preceding the tail is filled with a powerful muscular mass which moves that organ[895]. Treviranus discovered two longitudinal muscles in Scorpio europæus, running from the breast to the tail, which above and below each gill were connected by another running transversely across the heart, thus forming a quadrangular area in which the gills are situate[896]. The heart appears to be moved by muscles not very dissimilar to those of the Cossus[897], as is likewise that of the Araneidea; in Clubiona atrox the wider part of this organ is muscular, and incloses a considerable cavity[898]. In this tribe the muscles of the abdomen, the skin of which is soft and unfit to act as a lever to them, are attached to a cartilage, and thus their action is better sustained[899].
Having thus laid before you all of importance that I can collect with regard to the apparatus of muscles discoverable in insects, I shall next say something upon a few other points connected with that subject. When I enlarged upon their motions, I related a few instances of the extraordinary power of that apparatus[900] in leaping ones; but this power is not confined to that circumstance. The flea, not more remarkable for its compressed form, enabling it to glide between the hairs of animals, and its elastic coat of mail, by which it can resist the ordinary pressure of the fingers, than for its muscular strength, has attracted notice on this account from ancient times. Mouffet relates that an ingenious English mechanic, named Mark, made a golden chain of the length of a finger, with a lock and key, which was dragged by a flea;--he had heard of another that was harnessed to a golden chariot, which it drew with the greatest ease[901]. Another English workman made an ivory coach with six horses, a coachman on the seat with a dog between his legs, a postillion, four persons in the coach, and four lacqueys behind--which also was dragged by a single flea. At such a spectacle one would hardly know which most to admire, the strength and agility of the insect, or the patience of the workman. Latreille mentions a flea of a moderate size dragging a silver cannon on wheels, that was twenty-four times its own weight, which being charged with powder, was fired without the flea appearing alarmed[902]. Many caterpillars are accustomed to extend their bodies from a twig, supported merely by the four hind feet, in one fixed attitude, either in an oblique, horizontal, or vertical direction, either upwards or downwards, and that for hours together. We may conceive what prodigious muscular force must be exerted upon this occasion, by reflecting that the most expert rope-dancer, though endued with the power of grasping with his feet like a bird with its claws, could not maintain himself in a horizontal position even for an instant. Bradley asserts that he has seen a stag-beetle carry a wand half a yard long and half an inch thick, and fly with it several yards[903]. Some insects have the faculty of resisting pressure in a wonderful degree. If you take a common dung-chafer (Geotrupes) in your hand and press it with all your strength, you will find with what wonderful force it resists you; and that you can scarcely overcome the counteraction, and retain the insect in your hand: was it not for this quality, the grub of the gad-fly must be crushed probably in passing through the anal sphincter of the horse[904]. But that of Eristalis tenax affords a more surprising instance of this power of counteraction:--an inhabitant of muddy pools, it has occasionally been taken up with the water used in paper-making, and strange to say, according to Linné, has resisted without injury the immense pressure given to the surrounding pulp[905]; like leather-coat Jack mentioned by Mr. Bell[906], who, from a similar force of muscle, could suffer carriages to drive over him without receiving any injury. Almost as remarkable is the state of extreme relaxation into which the muscles of some larvæ fall, when their animation is suspended; and the revived tension to which a subsequent resumption of the vital powers restores them. Bonnet having suspended the animation of the caterpillar of Sphinx Ligustri by keeping it submerged, squeezed it between his fingers, until it had wholly lost its cylindrical form and was as flat and supple as the empty finger of a glove; yet in less than an hour the very same caterpillar became as firm, as compact, as cylindrical, and in short, as well, as though it had never been submitted to treatment so rough[907].
It is fortunate that animals of a large size, as has been well remarked, especially noxious ones, have not been endowed with a muscular power proportionable to that of insects. A cockchafer, respect being had to their size, would be six times stronger than a horse; and if the elephant, as Linné has observed, was strong in proportion to the stag-beetle, it would be able to pull up rocks by the root, and to level mountains[908]. Were the lion and the tiger as strong and as swift for their magnitude as the Cicindela and the Carabus, nothing could have escaped them by precaution, or withstood them by strength. Could the viper and the rattlesnake move with a rapidity and force equivalent to that of the Iulus and Scolopendra, who could have avoided their venemous bite? But the CREATOR in these little creatures has manifested his Almighty POWER, in showing what he could have done had he so willed; and his GOODNESS in not creating the higher animals endued with powers and velocity upon the same scale with that of insects, which would probably have caused the early desolation of the world that he has made. From this instance we may conjecture, that after the resurrection, our bodies by a change in the structure and composition of their muscular fibre--for we know that their locomotive powers and organs, as far as the muscle is concerned, will then be of a very different nature[909]--may become fitted for motions and a potent agency of which we have now no conception.
This wonderful strength of insects is doubtless the result of something peculiar in the structure and arrangement of their muscles, and principally their extraordinary power of contraction, excited by the extent of their respiration: for animals that respire but little, as the fœtus in the womb and the pullet in the egg, have very little contractile muscular power[910]. To get some idea from facts of this extraordinary contractile power in insects,--extract the sting of a bee or a wasp, with its muscles, which appear to be attached to powerful cartilaginous plates[911], and you will find it continue for a long time to dart forth its spicula, almost as powerfully as when moved by the will of the animal. A still more extraordinary instance of irritability is exhibited by the antlia, or instrument of suction of the butterfly. If this organ, which the insect can roll up spirally like a watchspring or extend in a straight direction, be cut off as soon as the animal is disclosed from the chrysalis, it will continue to roll up and unroll itself as if still attached to its head: and if after having apparently ceased to move for three or four hours it be merely touched, it will again begin to move and resume the same action. This surprising irritability and contractility of muscle doubtless depends upon the peculiar structure of the antlia, which is composed of an infinite number of horny rings, acted upon by muscles, more numerous probably than those which move the trunk of the elephant. The motion only ceases when the muscles become dry and rigid.
* * * * *
I have already, under another head[912], considered the annual sleep, or winter state of torpidity of insects, during which an intermission for the most part of muscular motion and action takes place. I shall now make a few observations with respect to their diurnal sleep, which may very properly have its place in the present letter. That insects, usually so incessantly busy and moving in every direction, require their intervals of repose, seems to call for no proof. We see some that appear only in the day, and others only in the night, others again only at certain hours; which leads to the conclusion, that when they withdraw from action and observation, it is to devote themselves to rest and sleep. The cockchafer flies only in the evening; but if you chance to meet with it roosting in a tree in the earlier part of the day, you will find it perfectly still and motionless, with its antennæ folded and applied to the breast:--we cannot indeed say that its eyes are shut; for as insects have no eyelids, that sign of sleep can never be found in them. Again, if a Lepidopterist goes into the wood to capture moths in the day-time, he finds them often perched on the lichens that cover the north side of the trunk of a tree, with their wings and antennæ folded, and themselves without motion, and insensible of his approach and their own danger. Thus it was that I captured that rare insect the lobster-moth (Stauropus Fagi) in the New Forest. Some, however, have asserted that the caterpillar of the silkworm, except when they moult, never intermits feeding day or night, and consequently does not sleep: but the accuracy of this statement, both from analogy and observation, admits of great doubt. Malpighi informs us that these caterpillars for an hour and more, twice a day, remain immoveable with their heads bent down as though asleep, and even if disturbed, resume again the same inactive posture[913]; and other larvæ in great numbers certainly seem to have regular intermissions from eating of considerable duration: those called Geometers, for hours together remain motionless projected from a twig, to which they adhere by their posterior prolegs alone; and the processionary caterpillars make only nightly sorties from their nests, passing the day in inaction and repose[914]. Bees have been often seen by Huber, when apparently wearied with exertion, even in the middle of the day, to insert the half of their bodies into an empty cell, and remain there, as if taking a nap, without motion for half an hour or longer[915]; and at night they regularly muster in a state of sleep-like silence. Mr. Brightwell once observed an individual living specimen of Haltica concinna, which appeared to remain motionless on the same spot of a wall for three successive days.
Before concluding these remarks on the Internal Anatomy and Physiology of Insects, I shall explain to you, as you will probably feel inclined occasionally to pursue the subject, the best mode of dissecting them.--By far the most useful dissecting instruments for this purpose are very fine-pointed and sharp scissors, as these will enable you to divide the integument and separate other parts with much less risk of injuring their delicate structure than any knife. These scissors are what Swammerdam chiefly used; and he had some so extremely small and fine, that he was necessitated to employ a lens when he sharpened them. If to these be added a sharp and fine-pointed knife or two, some needles fixed in handles, also fine-pointed--(you will find them more convenient than any other instrument for detaching minute parts and fibres,) a pair of fine and accurately adjusted pliers, and an assortment of camel's-hair brushes,--you will be nearly set up as an Entomological dissector. You will still, however, require a small dissecting table, with a projecting and moveable arm for lenses of various descriptions, so as to admit both the hands to be employed upon the subject under examination; and for this purpose probably no contrivance can be better adapted than that of Lyonet, of which the figure in Adams On the Microscope will convey a better idea than any description[916].
Previously to dissecting any insect, it must be killed by plunging it into boiling water, which is recommended by Lyonet, or spirits of wine or of turpentine; and it is often useful to let larvæ remain a few days in the latter, by which means the vessels become firmer and stronger. The parts of pupæ become much more distinct if they are boiled for a few minutes: and the same mode may be adopted in the examination of spiders.
The most convenient mode of proceeding, which was that also of Lyonet, is to dissect the insect in water, or, to avoid putridity, in diluted spirits,--if small, upon a concave glass, to which it should be fastened by means of a little melted wax; if larger, in the bottom of a common chip box, surrounded with a border of wax to retain the fluid. The integuments of the insect, being carefully divided longitudinally with scissors, should if flexible be turned back, and fixed by small pins stuck in by a fine pair of pliers, while the skin at the same time is stretched by another. After making such observations as present themselves without further dissection, the viscera must be cautiously extracted, washing away the fat which surrounds them with spirits of turpentine, in which it is soluble, applied by camel's-hair pencils. After separation they may conveniently be examined by putting them into water, and gently shaking them so as to cause the parts to unfold. If endowed with the patience of Swammerdam, you may even arrive at injecting these minute parts with wax or coloured fluids, conveyed by delicate glass tubes having one end as fine as a hair, which he also employed to fill the viscera with air; and afterwards drying them in the shade, and anointing them with oil of spike in which a little resin had been dissolved, he succeeded in preserving them. If it is not convenient to finish the dissection of an insect at once, it should be covered with spirits of wine. Swammerdam found a mixture of spirits and distilled vinegar very useful for keeping caterpillars previously to dissecting them, as it consolidated the parts[917].
* * * * *
And now having brought to a close my long wanderings in this ample and intricate field, and having threaded, as well as my slender powers and limited knowledge enabled me, the infinite turnings and convolutions of this Dædalean labyrinth--the Anatomy and Physiology of insects,--will you not own that the volume of wonders I have laid before you proves irrefragably that, though these minims of nature apparently rank so low in the scale of being, yet in their structure, instead of being, as might be expected, more simple, they are infinitely more complex and highly wrought than those animals that are placed the nearest to ourselves? the CREATOR in the latter doing every thing by a beautiful simplicity; while in the former, the more to magnify his power and skill, because they afford no apparent space for it, by a wonderfully curious and intricate multiplicity: and whether we study the one or the other, we shall in both trace the footsteps of that adorable LOVE which has shown attention to the comfort and well-being of the lowest insect, as well as of the highest of his creatures.
I am, &c.
FOOTNOTES:
[801] VOL. II. LETTER XXII. VOL. III. LETTERS XXXIV.-XXXVI.Return to passage
[802] VOL. II. p. 280, 295--, 306, 310--. &c.Return to passage
[803] Philos. Trans. 1818. 174. t. viii. f. 4-6.Return to passage
[804] See above, p. 150--.Return to passage
[805] Schmetterl. 105.Return to passage
[806] Philos. Trans. 1819. 172, 174, 187.Return to passage
[807] Anat. Comp. i. 90.Return to passage
[808] Philos. Trans. 1819. 175.Return to passage
[809] Cuv. ubi supr. 90--.Return to passage
[810] Cuv. Ibid. i. 89--.Return to passage
[811] See above, p. 85.Return to passage
[812] Lyonet Anat. t. iv. f. 3.Return to passage
[813] Ibid. 93--.Return to passage
[814] Cuv. Anat. Comp. i. 134.Return to passage
[815] Chabrier Sur le Vol des Ins. c. i. 445.Return to passage
[816] PLATE XXI. FIG. 6. a.Return to passage
[817] De Geer iv. t. xv. f. 11. m n, o p.Return to passage
[818] Lyonet Anat. 93.Return to passage
[819] Lyonet Anat. t. xiii. f. 1, 2.Return to passage
[820] Ramdohr Anat. t. v. f. 1. e. f. 3.Return to passage
[821] Chabr. ubi supr. 440--.Return to passage
[822] Ibid. 442, &c.Return to passage
[823] N. Dict. d'Hist. Nat. xxii. 80.Return to passage
[824] VOL. III. p. 663, 670. See above p. 21.Return to passage
[825] Chabrier Sur le Vol des Ins. c. i. 446.Return to passage
[826] VOL. III. p. 411.Return to passage
[827] Ubi supr. 437, 439.Return to passage
[828] PLATE XXII. FIG. 11, 12. c. Chabrier ubi supr. c. iii. t. xi. viii. f. 9. S. D. i, k. c. i. 440--.Return to passage
[829] PLATE XXII. FIG. 11, 12. c. Chabrier Sur le Vol des Ins. c. iii. t. xi. viii. f. 9. S. D. i, k. c. i. 440--.Return to passage
[830] Cuv. Anat. Comp. i. 94--.Return to passage
[831] N. Dict. d'Hist. Nat. xxii. 80.Return to passage
[832] Ubi supr. 101--.Return to passage
[833] VOL. I. p. 67.Return to passage
[834] Anat. Comp. i. 432--.Return to passage
[835] Anat. t. vii. f. 2. left hand.Return to passage
[836] Ibid. right hand.Return to passage
[837] Ibid. 115--.Return to passage
[838] Cuv. ubi supr.Return to passage
[839] VOL. III. p. 135--.Return to passage
[840] Anat. Comp. i. 447.Return to passage
[841] VOL. III. p. 366. PLATE XXVII. FIG. 1, 4. n´.Return to passage
[842] Ibid. FIG. 3. n´.Return to passage
[843] PLATE XXVII. FIG. 1. a.Return to passage
[844] VOL. III. p. 367--, 541, 584. PLATE XXII. FIG. 7. Cuv. ubi supr. 448.Return to passage
[845] PLATE XXVII. FIG. 5. a.Return to passage
[846] Anat. Comp. i. 136.Return to passage
[847] De Geer iv. t. xv. f. 11. o, p.Return to passage
[848] Marcel de Serres Comparaison, &c. 3--.Return to passage
[849] Ibid. 4.Return to passage
[850] Ibid. 5.Return to passage
[851] PLATE XXII. FIG. 11. h´.Return to passage
[852] VOL. III. p. 579.Return to passage
[853] PLATE XXII. FIG. 6. VOL. III. p. 585--.Return to passage
[854] Cuv. Anat. Comp. i. 436. PLATE XXI. FIG. 6.Return to passage
[855] Ibid. a, b. Lyonet Anat. 37.Return to passage
[856] Cuv. ubi supr. 458--. VOL. III. p. 368, 378, 382.Return to passage
[857] Cuv. ubi supr. 459.Return to passage
[858] Chabr. Sur le Vol des Ins. c. i. 441.Return to passage
[859] Chabr. Sur le Vol des Ins. c. i. 415.Return to passage
[860] Ibid.Return to passage
[861] Ibid. c. iii. 344. t. viii. f. 8, 9.Return to passage
[862] Ibid. c. i. 440.Return to passage
[863] Ibid. 444.Return to passage
[864] Ibid. 445. c. iii. 359.Return to passage
[865] Ibid. c. ii. 332. c. iii. 359.Return to passage
[866] Ibid. c. i. 445.Return to passage
[867] Ibid. c. iv. 78.Return to passage
[868] Chabr. Sur le Vol des Ins. c. i. 415, 442. c. iv. 80.Return to passage
[869] Ibid. c. i. 442.Return to passage
[870] Ibid. 439--.Return to passage
[871] Chabrier Analyse, 28. The latter part of this passage is copied from a MS. note of the author's in my copy.--W. K.Return to passage
[872] Chabrier Analyse, Ibid. Sur le Vol des Ins. c. i. 445. VOL. III. p. 617.Return to passage
[873] Analyse ubi supr.Return to passage
[874] Sur le Vol des Ins. c. i. 448, c. ii. 336.Return to passage
[875] VOL. III. p. 579.--Return to passage
[876] Chabr. Ibid. c. i. 443. ii. 316, 332.Return to passage
[877] Chabr. Sur le Vol des Ins. c. ii. 333.Return to passage
[878] Ibid. 332. PLATE XXII. FIG. 11, 12. c. A cupuliform process is also observable at the side of the metaphragm. Ibid. FIG. 10. a.Return to passage
[879] Chabr. Ibid. c. iv. t. xi.-4. f. 14.Return to passage
[880] Ibid. c. i. 445. xi.-8. f. 8, 9.Return to passage
[881] Chabr. Sur le Vol des Ins. c. ii. 336. note 1. VOL. III. p. 292--.Return to passage
[882] Chabr. Ibid. c. i. 447.Return to passage
[883] See above, p. 66--.Return to passage
[884] See above, p. 73--.Return to passage
[885] Chabrier Sur le Vol des Ins. c. i. Addend. 298.Return to passage
[886] See above, p. 178--.Return to passage
[887] VOL. III. p. 700--.Return to passage
[888] Chabr. ubi supr. c. i. 422.Return to passage
[889] Cuv. Anat. Comp. i. 451.Return to passage
[890] Chabr. Analyse 25. Sur le Vol des Ins. c. i. 423, 452. Addend. 301.Return to passage
[891] See above, p. 83.Return to passage
[892] Lyonet Anat. t. xiii. f. 1, 2.Return to passage
[893] Lyonet Anat. t. xiii. 188--, 584.Return to passage
[894] Ibid. 189.Return to passage
[895] N. Dict. d'Hist. Nat. xxx. 421.Return to passage
[896] Arachnid. 9. t. i. f. 7. r.Return to passage
[897] Ibid. o.Return to passage
[898] Ibid. 10.Return to passage
[899] Arachnid. 45. t. iii. f. 31. m, n, q, r, t.Return to passage
[900] VOL. II. p. 309--.Return to passage
[901] Mouffet Theatr. 275.Return to passage
[902] N. Dict. d'Hist. Nat. xxviii. 249.Return to passage
[903] Phil. Acc. of Works of Nat. 144.Return to passage
[904] Clark in Linn. Trans. iii. 309.Return to passage
[905] Fn. Suec. 1799.Return to passage
[906] Anatomy of Expression in Painting, 170.Return to passage
[907] Bonnet Œuvr. ii. 124.Return to passage
[908] N. Dict. d'Hist. Nat. xxii. 81.Return to passage
[909] 1 Cor. xv. 50--.Return to passage
[910] N. Dict. d'Hist. Nat. ubi. supr.Return to passage
[911] Swamm. Bibl. Nat. t. xviii. f. 2. l, m, n, o. Reaum. v. t. xxix. f. 7. m, n, o, p, q.Return to passage
[912] VOL. II. LETTER XXVI.Return to passage
[913] De Bombyc. 5.Return to passage
[914] Reaum. ii. 185--.Return to passage
[915] VOL. II. p. 186.Return to passage
[916] t. vi. f. 3.Return to passage
[917] These directions for dissecting are chiefly taken from Swammerdam, Life xiv.-- and Lyonet Anat. 7--.Return to passage
Letter XLIV
DISEASES OF INSECTS.
Having laid before you what observations I thought might sufficiently explain all the principal features of the Anatomy of insects both external and internal, you will next expect to be informed whether, like the higher animals, they are subject to have the admirable order observable in their frame interrupted by Disease; and you will perhaps imagine, from the multiplicity of their organs and vessels, that they must be peculiarly exposed to derangements of the vital and other functions. That they have their diseases is certain; but, except in the case of their appropriate parasitic assailants, which is a part of their economy, it does not appear that their maladies are more numerous and frequent than those of other animals. The same ALMIGHTY POWER which endowed them with so complex a structure, generally upholds them in health during their destined career, until they have fulfilled the purpose of their creation, when they die and return again to their dust[918].
But perhaps I may seem to you as making too great a parade about these little insignificant creatures if I assign a separate letter to the consideration of their diseases: but when you recollect that Aristotle has a chapter on this subject[919], and that the learned Willdenow has devoted a distinct portion of his excellent introductory work on Botany to the diseases of Plants[920],--you will perhaps be of a different mind: indeed, some facts I shall have to communicate are so remarkable and interesting, that I am sure, when you have read this letter, you will not think the subject one that deserves to be slighted.
Insect diseases may, I think, be divided into two great classes; those resulting, namely, from some accidental external injury or internal derangement, and those produced by parasitic assailants.
I. Under the first head we may begin with wounds, fractures, mutilations, and other extraneous causes of disease. To these--insects are peculiarly subject; and though they are not, like the Crustacea and Arachnida[921] and some other invertebrate animals, endowed with the power of reproducing a mutilated limb, yet their wounds appear to heal very rapidly, and at the time they are inflicted to produce little pain[922]. But if those important members, their antennæ, are mutilated, insects seem to suffer a kind of derangement; the great organ of their communication with each other, and in various respects with the external world, being removed, all their instincts at once fail them. I formerly related how the amputation of these affects the queen-bee[923]. A similar result, as Huber tells us[924], follows, when the same experiment is repeated on the workers or drones: they immediately become unable to take any further part in the labours of the hive; they can no longer guide themselves except in the light; if they petition one of their fellow-citizens for honey, they are unable to direct their tongue to its mouth to receive it; they remain near the entrance of the hive, and when the light is intercepted they rush out of it to return no more.
Insects occasionally are subject to tumours or a preternatural enlargement of their parts and organs. The antennæ of bees sometimes swell at their extremity so as to resemble the bud of a flower ready to open, becoming at the same time very yellow, as does the fore part of the head[925]. I once saw a specimen of a Hydrobius--agreeing with H. fuscipes in every other respect even to the most minute punctum--which had a large tumour on each side of the prothorax, evidently accidental, occasioned probably by the stoppage of the pores by which the superfluous moisture and air escape when it undergoes its last change. The converse of this I have observed to take place sometimes in the same part of Geotrupes foveatus, the ordinary lateral foveæ becoming very considerably enlarged;--this was the case with the specimen from which Mr. Marsham made his description of that insect. The species is, however, very distinct in other respects, and may always be known by its small size. It happens now and then also, that these tumours represent blisters. I saw one once on one elytrum of a beetle and not on the other. Those of Serropalpus (as Mr. MacLeay, on the authority of M. Clairville, informs me) are particularly subject to this disease. But, of all the organs, the wings are most exposed to derangements of this kind. De Geer, in a specimen of Pieris Cratægi just excluded from the chrysalis, observed that one of these was distended by a considerable quantity of extravasated green fluid--two or three large drops following an incision. This disease appeared to arise from the lower membrane not adhering to the upper; so that the nervures--which are rather longitudinal channels, being open below, than tubes--were not closed to confine the fluid to its proper course. The malady, which might be called a dropsy of the wing, carried off the insect the day after its exclusion[926]. Reaumur observed that the wings of some flies were affected by an air-dropsy, as he calls it, which appeared to arise from the air escaping from its natural channels, and thus separating, the two membranes that form the wing, and filling the cavity produced by their separation[927].
Sometimes also monstrosities are to be met with in these animals, or variations from a symmetrical structure in organs that are pairs. I have a beetle in which the terminal joint of one of the maxillary palpi is short, ovate, and acute; and that of the other, long, semiovate, and rather obtuse. A specimen of Blaps mortisaga in my cabinet, taken by Mr. Denny, besides the terminal mucro of the elytra, has a long diverging lateral one. Goeze had the larva of a Semblis brought to him in which one of the two fore-legs, though perfect in all its parts, was only half the length of the other[928]; which he regarded as a reproduction, but it seems rather a malformation. Müller mentions a most extraordinary fact of one of the Noctuidæ, which when disclosed from the pupa retained the head of the larva[929]. One of the most remarkable instances of this kind that have fallen under my own observation, may be seen in a specimen of Chrysomela hæmoptera in the cabinet of our friend Curtis; in which one of the thighs produces a double tibia, but only one of these is furnished with a tarsus.
The diseases of insects which arise from some internal cause are not very numerous. The first that I shall mention is a kind of vertigo. "Ants have also their maladies," says M. P. Huber: "I have noticed one extremely singular; the individuals attacked by it lose their power of guiding themselves in a straight line, they can walk only by turning round in a circle of small diameter and always in the same direction. A virgin female shut up in one of my glasses was seized on a sudden with this distemper; she described a circle of an inch in diameter, and made about a thousand turns in an hour, or not quite seventeen in a minute. She continued constantly turning round for seven days, and when I visited her in the night I found her still in motion. I gave her honey--and I think that she ate some of it." He observed that some workers were attacked by a similar disease: one of these, however, had the power of walking from time to time in a straight line; when placed upon its head it continued its gyrations[930]. Similar motions of a little moth, mentioned on a former occasion[931], may perhaps have been produced by the same cause. Bees are also subject to vertigo, which has been attributed to their eating poisonous honey[932]--but may not this disease in all these cases arise from some derangement of the nervous system? One of the ants which was so affected had lost one of its antennæ; but as this was not the case with the others, no great stress is to be laid upon the circumstance. Huber does not inform us whether those attacked by this disease recovered or not.
I have observed more than once, that the flesh-fly and some others of the same tribe are subject in particular seasons to a kind of convulsions. When thus attacked, they kick and struggle, and seem unable to fly. Sometimes they lie upon their backs without motion, but if a finger be placed near them their convulsive motions are renewed. When thrown into the air, instead of flying, they fall to the ground. Had this distemper occurred earlier or later in the year I should have attributed it to the benumbing effects of cold; but as my observations were made one year (1816) in May, and in another (1811) in the latter end of June, this could scarcely be the case. In the year last mentioned I observed that many flies died under its influence. In wet seasons this tribe is subject to another disease, which proves fatal to many of them, and indeed to other Diptera. A white crust appears to be formed upon the abdomen both above and below, of a granular appearance, much resembling fine moist sugar. On the back of that part this crust does not cover the margins of the segments, which gives it the appearance of white bands; so that deceived by it, I have often at first flattered myself that I had met with some new species. The under-side of the abdomen is wholly covered by it, divided in the middle into two longitudinal masses, the anal segment being bare. De Geer has noticed this or a similar disease, which, when flies are attacked by it, causes the abdomen to swell so as even to burst, and the segments become dislocated. Upon opening the abdomen it is found filled with a white unctuous substance, which often accumulates (as above described) on its external surface[933]. Dr. Host says that in this disease when the animal is dead, the wings, which were before incumbent, become extended, and its almost invisible pubescence grows into long hairs[934]. De Geer seems to think that these flies are thus affected in consequence of having eaten some poisonous food[935]; but I rather suspect, as I have observed it become prevalent chiefly in wet seasons, that it arises from a superabundance of the nutritive fluid, or of the fat, so that it seems to be a kind of plethora. I once observed a fly fixed to a pane of glass, round which was a semicircle of what appeared to be merely vapour, whose radius was nearly three-fourths of an inch. Taking it for an aqueous fluid that had transpired from the dead animal, I paid no further attention to it at that time: but observing from day to day that the moisture did not evaporate, after two or three months had elapsed, I had the curiosity to examine it more closely, and, upon scraping some of it off with a penknife, I found it was a white substance of a fatty nature. In this case, then, the fat must have exploded on all sides with considerable violence from half the body or the abdomen. Probably this was a more intense degree of plethora. When I examined this appearance the fly had fallen off, and I could not find it.
Mr. Sheppard once brought me a panicle of grass, the glumes of which were rough with hairs, or small bristles, to which several specimens of a fly related to Xylota pipiens adhered by their proboscis. At first I thought that having been entrapped by the bristles, and unable to extricate themselves, they had perished from want of food; but since when touched they readily dropped from the glumes, some other cause, perhaps disease, probably occasioned this singular suspension of themselves.
The maladies to which bees and silkworms are subject are more interesting to us than those of flies, on account of their utility as cultivated insects. One of the worst distempers which attacks the first of these animals is a kind of looseness or dysentery: this happens early in the year, when they are fed with too much honey without any portion of bee-bread[936], and sometimes destroys whole hives. Their excrements, instead of a yellowish red, then become black, and the odour they emit is insupportable; the bees no longer observe their usual neatness, inducing them to leave the hive when they void their excrements, but they defile it, their cells, and each other. Several remedies have been prescribed for this disease. To prevent it, a syrup made by an equal mixture of good wine and honey is recommended; and as a cure, to place in the hive combs containing cells filled with bee-bread[937]. But one of the worst maladies to which these useful animals are subject, is that called by Schirach Faux Couvain. It originates with the larvæ; and is caused either by their being fed with unwholesome food, or when the queen, as sometimes happens, lays her eggs so that the head of the grub is not in a proper position for emerging from the cell when the period for its disclosure is arrived:--the consequence is, that in both cases it dies and becomes putrid, which sometimes produces a real pestilence in a hive. The remedy for this evil is to cut away the infected combs, and to make the bees undergo a fast of two days[938]. The hive should be cleaned and fumigated, by burning under it aromatic plants.
The cultivators of the silkworm in France have given names to several diseases to which that animal is subject. One is called La Rouge, and is supposed to be occasioned either by too great heat, or by too sudden a transition from cold to heat. It takes place when the caterpillar is first hatched; which lives perhaps, but in a very sickly state, till it should spin its cocoon and assume the pupa, when it expires. Another degree of the same disease is called Les Harpions or Passis. A second distemper of this animal is Des Vaches, Le Gras or La Saune: this is a mortal disease, supposed to be of a putrid nature, and produced by mephitic air; it shows itself after the second moult, but rarely after the subsequent ones. When a caterpillar is first attacked, changing the air may prove a remedy; but when the disease has made progress, it is best to burn or bury them, since if the poultry pick them up they might be poisoned by them. A third disease of silkworms is called Les Morts Blancs, or Tripes, which is also occasioned by impure air, when the leaves the animal feeds upon are heaped so as to produce fermentation. The caterpillars attacked by it die suddenly, and preserve after their death the semblance of life and health. Too great heat, whether artificial or natural, occasions La Touffe, a fourth, which, when the heat continues long, destroys all those that are arrived at their last stage of existence in their larva state. Black points scattered over different parts of the body, or livid and blackish spots in the vicinity of the spiracles, followed by a yellowish or reddish tint, are symptoms of a fifth malady, called La Muscardine. After this the animal soon dies, and becomes mouldy, but does not stink. This disease is not contagious, and is thought to be caused by a moist heat, attended by pernicious exhalations. La Luzette, Luisette, or Clairène, is another malady, which shows itself most commonly after the fourth moult. It seems to arise from some original defect in the egg. The caterpillars attacked by it may be known by their clear red and afterwards dirty white colour; their body becomes transparent, and the matter of silk exudes in drops from their spinnerets; consequently, though as voracious as the rest, they are never able to construct a cocoon, and should be destroyed. Les Dragées is the name given to cocoons which include a larva that never becomes a pupa. The cause of this disorder has not been ascertained, and whole broods are sometimes subject to it, which, as in the last, seems to imply some defect in the eggs. But as the caterpillar spins its cocoon, and the silk is as good as usual, it is a malady of no great importance. Lastly, sometimes the mulberry leaves have a gummy rather acrid secretion, which purges the silkworms; their excrement is no longer solid; they become weak and languid; and if the secretion is abundant, their transpiration is impeded, and at the time of moulting they are become so feeble as to be unable to cast their skin[939].
In the case of many caterpillars of Lepidoptera that died, Bonnet found by dissection that the disease was remotely occasioned by a diarrhea, which taking place immediately before they became pupæ, prevented the inner membrane of their intestines from being rejected, as it would have been if no extraordinary cause had prevented it, attached to the hard excrement. He found this membrane converted into a jelly occupying great part of the stomach, which he conjectured was the proximate cause of their death[940].
To conclude this head--spiders are reputed to be subject to the stone: I do not say Calculus in Vesica; but we are informed by Lesser that Dr. John Franck having shut up fourteen spiders in a glass with some valerian root, one of them voided an ash-coloured calculus with small black dots[941].
II. I now come to that class of diseases which appears to prevail almost universally amongst insects--I mean those resulting from the attack of parasitic enemies. Thus millions and millions annually perish before they have arrived at their perfect state. Diseases of this kind proceed either from vegetable or animal parasites. I shall begin with the first, which will not occupy us long.
i. As insects pass often no small portion of their life in a state of torpidity, in which they remain chiefly without motion, it will not seem wonderful, should any partial moisture accidentally accumulate upon them, that it affords a seed plot for certain minute fungi to come up and grow in. Persoon observes with regard to his genus Isaria, that one species grows upon the larvæ of insects (I. truncata), and another upon pupæ (I. crassa[942]):--as he does not say upon dead larvæ and pupæ, as upon a former occasion[943], perhaps in these cases these plants may constitute an insect disease; but I lay no stress upon it, and only mention the circumstance here as connected with the history of these animals. Mr. Dickson has described a Sphæria under the name of entomorhiza that grows upon dead larvæ; it has a slender long stipes and spherical granulated head: on the pupa of a species of Cicada in my cabinet, another kind of Sphæria, with a twisted thickish stipes and oblong head, springs up in the space between the eyes. I observed something similar but longer, in the grub of some large beetle in M. Du Fresne's museum at Paris; and I have a memorandum of having noticed something of the kind on the rostrum of a Calandra. Bees and humble-bees have been sometimes thought to have some species of mucor or other Fungilli occasionally growing upon them; but Mr. Brown is of opinion that stamina which they have filched from flowers have been mistaken for these Fungilli, since he has detected those of Orchideæ in some of this tribe, and upon a beetle shown to him by Mr. MacLeay, one which he knew to be the stamen of an Aristolochia. I once observed a bunch of what I mistook for a singular mucor that adorned the vertex of a humble-bee, between the antennæ, which doubtless were of the same description; and I even saw one upon its wing. Upon a former occasion I mentioned a parallel circumstance with respect to a species of Xylocopa[944].
ii. The animal parasites that infest insects are either themselves insects; or worms.
1. Their insect infesters, as far as we know at present, are confined to the Orders Strepsiptera, Hymenoptera, Diptera, and Aptera: they attack them sometimes in their egg state, most frequently when they are larvæ, occasionally when pupæ, and very rarely in their perfect state. Upon many of these I have formerly enlarged[945], and I shall now add such further circumstances as I then omitted. The Strepsiptera Order, as at present known, consists only of two genera, Stylops and Xenos; the first being appropriated to the imago of Andrena, a kind of bee, and the latter to that of the wasps. Their eggs appear to be deposited in the abdomen of these insects in which they feed, till having attained their full growth they perforate the membrane that connects its segments; and at the proper time their pupa-case bursts, they emerge, and take their flight. Sometimes four or five infest a single bee. Whether the latter dies upon their quitting it I have not been able to ascertain, but from their flying, when the little parasite is very near leaving them, with their usual activity, it should seem that this disease is not mortal; but it probably prevents their breeding: I do not recollect observing the exuviæ of one in a male bee[946].
The great body of insect parasites, however, belong to the Hymenoptera Order, and chiefly to the Linnean genus Ichneumon. The insects of this order have been denominated Principes, because of the wonderful instincts of ants, wasps, bees, and other gregarious tribes that belong to it; and they merit a name of honour not less for the benefits that they confer upon mankind, by keeping within their proper limits the various insect-destroyers of the produce of the globe. It deserves notice that when these latter increase to a degree to occasion alarm, their parasites are observed to increase in a much greater, so as to prevent the great majority of them from breeding[947]. Though these benefactors of the human race constitute numerous genera, at present not well ascertained, I shall speak of most of them under the common name of Ichneumon.
The appearance of these little four-winged flies puzzled much the earlier naturalists:--that a caterpillar usually turning to a moth or butterfly should give birth to myriads of flies, was one of those deep mysteries of nature which they knew not how to fathom[948]: even the penetrating genius of our great Ray, though he ultimately ascertained the real fact[949], was at one time here quite at fault; for he seems at first to have thought, when from any defect or weakness nature could not bring a caterpillar to a butterfly, in order that her aim might not be entirely defeated, that she stopped short, and formed them into more imperfect animals[950].
Before I detail more particularly the proceedings of Ichneumons, I shall make a few general remarks upon them. The structure of the instrument by which they are enabled to deposit their eggs in their appropriate station has been before sufficiently described[951]; it is long or short according to the situation and circumstances of the larva which receives them: if this lives in the open air, and the access to it is easy, it is usually short and retracted within the body; but if it lies concealed in deep holes or cavities, or shuns all approach, it is often very long. Thus in Pimpla Manifestator, which commits its eggs to the grub of a wild bee inhabiting the bottom of deep holes bored in posts and rails, the ovipositor is nearly an inch and half in length, and in some extra-European species three inches. How the egg is propelled so as to pass in safety from the oviduct, along this extended and very slender instrument to the grub for which it is destined, has not been certainly ascertained: but from an observation of Reaumur's[952] it should seem that it is aided in its passage by some fluid ejected at the same time with it, or is so lubricated as to slide easily without being displaced. The flies we are speaking of, by some authors are called Muscæ vibrantes, because when searching for the destined nidus of their eggs their antennæ vibrate incessantly, and it is by the use of these wonderful organs that they discover it wherever it lurks. Bergman observed that Fœnus Jaculator searches for the latent grub of certain bees and other Hymenoptera with its antennæ[953]: and from Mr. Marsham we learn that Pimpla Manifestator, before it inserts its ovipositor in the nest of the grub of Chelostoma maxillosa, explores it first with one antenna and then with the other, plunging them all the while intensely quivering up to the very root[954]. With respect to their size, Ichneumons vary greatly; some being so extremely minute as to be invisible to the naked eye, unless moving upon glass; while others, as to their length, emulate the giants amongst insects. The former, unless appropriated to the eggs themselves, usually commit many eggs to a single larva, while the latter are directed by their instinct to introduce into them only one. Some of the former description are endowed with the faculty of leaping[955]. The food of Ichneumons, and indeed of other internal parasites, is chiefly the epiploon or fat of the larva, but they never touch any vital organ; so that it continues to feed, and probably more voraciously, grow, cast its skin, and often it changes to a chrysalis, although at the same time inhabited by an army of these little devourers.
Ichneumons, as far as has been at present ascertained, are parasitic upon other insects chiefly in their three first states, a solitary instance only having been observed of their inhabiting an imago; but from their first exclusion as eggs from the ovary till their assumption of that state they give them no rest. I shall therefore first treat of those that infest the eggs; next those appropriated to larvæ; and lastly those that devour pupæ.
Vallisnieri appears to have been the first naturalist who discovered that Ichneumons were appropriated to the eggs of other insects. He observed one proceed from those of the emperor-moth (Saturnia Spini): finding two holes in each egg, one larger than the other, he conjectured that one was made when it entered, and the other when it emerged. In this case the egg of the Ichneumon must be fixed on the outside of the egg it was to feed upon; though some appear to pierce it with their ovipositor, and consequently introduce their egg within: for he says afterwards; "I have seen with my own eyes a certain kind of wild flies deposit their eggs upon other eggs, and bore and pierce others with an aculeus--by which they have introduced the egg[956]." Count Zinanni, a correspondent of Reaumur's, saw an Ichneumon pierce the eggs with her ovipositor repeatedly; which in about fifteen days were filled with the pupa, and in six more produced the imago[957]. I. Ovulorum L. is the only known species of egg-devourers; but most likely there are many, varying in size, according to the size of the egg they inhabit. Probably I. Atomus L., and I. Punctum Shaw, are of this description[958]. It is wonderful what a number these little flies destroy:--out of a mass of more than sixty eggs which was brought to De Geer, not one had escaped the Ichneumon[959]. But the most extraordinary thing is, that even these little creatures we are told are destroyed by another still more minute[960].
Though the animals we are speaking of usually destroy only a single egg, yet some appear not so to confine themselves. Geoffrey informs us that the larva of one of the Ichneumons whose females are without wings (Cryptus) devours the eggs of the nests of spiders, and from its size--it is nearly a quarter of an inch long--it must require several of them to bring it to maturity[961]. One of those also which destroys the gnat infesting the wheat (I. inserens) appears to devour them in their egg state, and could not be brought to perfection by the food that a single one would furnish[962].
The Ichneumons that are parasitic upon larvæ are the most numerous of all. Some of them are deposited by the parent fly on the outside of their prey, and others introduced into its interior. Ophion luteum is one of the former tribe; it plants its eggs in the skin of the caterpillar of the puss-moth (Cerura Vinula). Each egg is furnished with a footstalk terminating in a bulb[963], which is so deeply and firmly fixed that it is impossible to extract it without detaching a portion of the animal with it, and even when the caterpillar changes its skin it is not displaced. After it is hatched, the grub, while feeding, keeps its posterior extremity in the egg-shell, to which it adheres so pertinaciously, that it is scarcely possible to disengage it without crushing it. It fixes itself by its mandibles to the skin of the caterpillar, and keeps constantly sucking the contents of its body till it dies: sometimes nine or ten of these larvæ inhabit a single caterpillar[964]. Reaumur has given an account of other external Ichneumons. Upon one caterpillar that he examined, they were so numerous as to render the poor animal quite a spectacle, and they underwent their metamorphosis attached to it[965]. One species of this description avenges the cause of insects upon their most pitiless foes, the all-devouring spider--for in the midst of her toils and lines of circumvallation it makes her its prey. De Geer, meeting one day with a young spider of a common kind, observed with surprise, engaged in sucking it, a small white grub, which was firmly attached to the abdomen near the trunk. Putting it by in a glass, after some days he examined it again; when he observed that it had spun the outline of a vertical web, had stretched threads from the top to the bottom of the glass and from one side to the other, and had also spun the radii that meet in the centre, and this was all;--but what was remarkable, the larva that had fed upon it was suspended in the centre of this web, where it was engaged in spinning its own cocoon, while the spider, exhausted by this last effort, had fallen dead to the bottom of the glass. It cannot be asserted positively that this suspension of the larva of the Ichneumon in the centre of the web always takes place; but if it does, as seems most probable, it shows that this little parasite is endowed with an instinct which causes it so to act upon the spider as may induce it to spin a web so nicely timed as to be sufficiently complete at the period of its death and of the change of the Ichneumon, for the latter to cast it down and assume its station[966].
But the great bulk of the parasitic Hymenopterous devourers of larvæ have their assigned station within the body. As Entomologists in breeding insects have paid their principal attention to Lepidoptera, it necessarily follows that their Ichneumon infesters must be most generally known; but doubtless the larvæ of the other Orders are not wholly liberated from this scourge: they also require to be kept within due limits, and have their appropriate parasites. Some, however, in most of them have been detected: of which I shall now proceed to state to you the most interesting examples, beginning with the Coleoptera.
Alysia Manducator[967], remarkable for having mandibulæ that do not close, and toothed at the end, usually attends masses of dung, both of man and cattle, probably for the purpose of depositing its eggs in some of the Coleopterous larvæ that inhabit it. Mr. Stephens, one of the most accurate observers as well as one of the best Entomologists of the present day, informs me that he once captured three specimens of Timarcha tenebricosa, from each of which forty or fifty minute Ichneumons emerged. An insect also of this Order, that is a great benefactor to mankind, as a destroyer of the plant-lice,--I mean the lady-bird (Coccinella), in its larva state is itself subject to the attack, as we learn from De Geer, of one of these small parasites[968]. He detected them also in that of two species of weevils: and in the pupa of some large grub of a beetle inhabiting the wood of the elm, perhaps that of the stag-beetle, he found the pupa of one of those Ichneumons that have an exserted ovipositor[969]. Doubtless, did we know their history, we should find that numberless species have their internal assailants belonging to this tribe.
Orthopterous larvæ seem not to have been yet announced as affording a pabulum to these animals: but the late Dr. Arnold, whose tact for observation with regard to the manners and economy of insects has rendered his loss irreparable, discovered that the remarkable parasitic genus Evania was appropriated to the all-devouring Blatta. Whether it attacked it in its egg or larva state I have not been informed. This little benefactor is here extremely rare, at least in the country; perhaps in towns, where the cock-roach abounds, it may be more common.
The observations of naturalists have chiefly been confined to the Hemipterous genus Aphis; but these early attracted their notice. Leeuwenhoek has given a particular and entertaining account of the proceedings of I. Aphidum. As soon as the little flies approached their prey, they bent their abdomen, which is rather long, between their legs, so that the anus projected beyond the head; then with their ovipositor they pierced the body of the Aphis, at the same time carefully avoiding all contact with it in every other part: whenever they succeeded in their attempt, a tremulous motion of the abdomen succeeded. Only a single egg is committed to one Aphis: when hatched, the latter becomes very smooth and appears swelled; it is, however, full of life, and moves when touched. Those that are thus pricked separate themselves from their sound companions, and take their station on the underside of a leaf. After some days the inclosed grub pierces the belly of the Aphis, and attaches the margin of the orifice to the leaf by silken threads; upon this it dies, becomes white, and resembles a brilliant bead or pearl[970]. De Geer observed also an Ichneumon on the Coccus of the elm, I. Coccorum[971].
Amongst the Neuropterous tribes likewise, probably the Ichneumonidæ commit their usual ravages; but their exploits, as far as I recollect, have met with no historian. I have a small species related to Chelonus, which a memorandum made when I took it tells me was obtained from Æshna viatica; yet I do not remember ever tracing that species to its final change, so that I must have taken this Ichneumon from the perfect insect. It suffices, however, to prove that this tribe is also exposed to the attack of these parasites. Where larvæ and pupæ are aquatic, it seems probable, if any attack is made upon them, that it must take place after they have quitted the water.
In the Hymenoptera Order itself, almost every genus has been ascertained to have its Ichneumon parasites. Not even the fortified habitations of the gall-flies (Cynips) can escape them, almost every species becoming their prey; a circumstance which puzzled not a little some of the older naturalists, when they at one time saw a fly not remarkable for its colours or brilliancy emerge from the curious moss-like Bedeguar of the wild rose, and at another were struck by the appearance of one of those splendid minims of nature which almost dazzle the sight of the beholder[972]. Immunity, however, from this pest seems to have been granted to the gregarious Hymenoptera; at least none has yet been discovered to attack the ant, the wasp, the humble-bee, or the hive-bee; in which last, had there been one appropriated to it, it could never have escaped the notice of the Reaumurs and the Hubers. The solitary bees, however, as we have seen above[973], do not escape; and Epipona spinipes, a solitary wasp which feeds its own young with a number of green caterpillars[974], is itself, when a larva, though concealed in a deep burrow, the prey of the grub of an Ichneumon, which by means of a long ovipositor introduces its egg into its body[975]. Even these parasites, whose universal office it is in their first state to prey upon insects, are themselves subject to the same malady. Ichneumonidan devourers are kept in check by other Ichneumonidan devourers. These in some cases are so numerous as to destroy the tithe of the kinds they attack[976]. Thus an ever-watchful PROVIDENCE prevents these parasites from becoming so numerous as to annihilate in any place the species necessary for the maintenance of the general economy and proportion of animal and vegetable productions. Amongst the assailants of the Hymenoptera, none seem to have a more laborious task assigned them than those that pierce the various galls in which the larvæ of the Cynips tribe are inclosed. To look at an oak-apple, we should think it a work of difficulty, requiring much sagacity and address, for one of our little flies to discover the several chambers lurking in its womb, and to direct their ovipositor to each of them. Its CREATOR, however, has enabled it instinctively to discover this, and furnished it with an appropriate elongated instrument, which will open a way to the deep and hidden cells in which the grubs reside, penetrate their bodies, and to each commit an egg. When it prepares to perforate the gall, the Ichneumon begins by depressing this organ, that it may extricate it from its sheath; it next elevates its body as high as possible, and bending the instrument till it becomes perpendicular to the body and to the gall, so as to touch the latter with its point, it then gradually plunges it in, till it is quite buried[977]. A very remarkable Hymenopterous parasite (Leucospis), which when unemployed turns its ovipositor over the back of its abdomen, so that its end points to its head, is said to deposit its eggs in the nest of the mason-bee, most probably in the larva: but the curious observations that are stated to have been made by M. Amédée Lepelletier upon its history have not yet been given to the public[978].
Dipterous insects, likewise, do not escape from these pests of their Class: but few observations, however, have been recorded as to the species assailed by them. We learn from De Geer, that a gnat (Cecidomyia Juniperi), which forms galls upon the juniper is devoured by an external Ichneumon[979]; that which injures the wheat in the ear, whose ravages I formerly mentioned to you[980], affords food to three of these parasites,--one I lately mentioned as probably devouring its eggs; another pierces the glumes of the floret, where its destined prey is concealed; and the third enters it. I once placed a number of the larvæ of the gnat upon a sheet of paper, at no great distance from each other, and then set down one of these last Ichneumons in the midst of them. She began immediately to pace about, vibrating her antennæ very briskly: a larva was soon discovered, upon which she fixed herself, the motion of her antennæ increasing intensely; then bending her abdomen obliquely under her breast, she inserted her ovipositor, and while the egg was depositing these organs became perfectly motionless. The larva when pricked gave a violent wriggle. This operation was repeated with all that had not already received an egg, for only one is committed to each larva. I have often seen it mount one that was already pricked, but it soon discovered its mistake, and quitted it untouched[981]. The Hessian Fly also (Cecidomyia Destructor) related to the preceding, whose alarming ravages I formerly described to you[982], has a peculiar parasite attached to it, which keeps it in check. The only other Dipterous insects that I have seen mentioned as affording pabulum to an Ichneumon, are--one of the aphidivorous flies mentioned by De Geer, who does not note the species, to the larva of which the Ichneumon commits only a single egg, producing a grub that intirely devours its interior[983];--and two described by Scopoli, one, the larva of a fly frequenting hemp; and the other, which feeds on a Boletus, that of a gnat[984].
The Lepidoptera, however, is the Order over the larvæ of which the Ichneumons reign with undisputed sway; attacking all indiscriminately, from the minute one that forms its labyrinth within the thickness of a leaf, to the giant caterpillar of the hawk-moth. The most useful of all, however, the silkworm, appears at least with us, exempted from this scourge. De Geer, out of fifteen larvæ that were mining between the two cuticles of a rose-leaf, belonging to the first tribe here alluded to, found that fourteen were destroyed by one of these parasites, only one coming forth to display itself in all its brilliancy and miniature magnificence[985]. One of the most useful to us is that which destroys the clothes-moth, which the same writer also traced[986]. Another, equally serviceable, takes up its abode in the caterpillar that ravages our cabbages and brocoli (Pontia Brassicæ) which perish by hundreds from its attacks. As this falls frequently under our notice, it will not be uninteresting to give a fuller account of it. Reaumur has traced and related its whole history. One of these little flies that he observed, was so intent upon the business in which she was engaged, that she suffered him to watch her motions under a lens, without being discomposed. She pursued nearly the same plan of proceeding with that of the Ichneumon of the wheat-gnat just described; except that she repeated her operations frequently on the same caterpillar in different parts, alternately plunging in and extracting her ovipositor. She seemed to prefer the spot where the segments of the body are united, particularly where the eighth meets the ninth, and the ninth the tenth. When the fly had completed its work and quitted the caterpillar, Reaumur gave it food, and it did not seem less lively and vigorous than others of its kind; in less than a fortnight it assumed the pupa; and in four days the whole of its interior being devoured, it died: but its parasites, perhaps not finding a sufficient supply of nutriment in it, never came to perfection[987]. Sometimes, however, these little grubs arrive at maturity before the caterpillar has become a chrysalis, when they pierce the skin and begin to emerge. First appears a little white tubercle, which gradually elevates itself in a direction perpendicular to the body; while this is doing, a second appears in another place; and so on, till fifteen or sixteen are seen on each side, giving the caterpillar a very grotesque appearance. By the alternate contraction and relaxation of their bodies the grubs effect their complete liberation, which takes place with respect to the whole in less than half an hour. When entirely disengaged, they place themselves close to the sides of the caterpillar: even before this they begin spinning, and draw unequal threads in different directions, of which they form a cottony bed, which serves as the base of the separate cocoon of each individual, which they next construct of a beautiful silk thread of a lovely yellow, which, if it could be unwound and in sufficient quantity, would yield a silk unrivalled in lustre and fineness[988].
De Geer has recorded a very singular fact which deserves your notice. An Ichneumon, appropriated to one of the Tortrices, had deposited its eggs in two of their caterpillars; each produced a considerable number; but those that emerged from one were all females, and those from the other, males[989]. He observed a similar fact take place with Misocampus Puparum[990]. One might conjecture from this circumstance, that as in the queen-bee[991], so in these Ichneumons, the eggs producing the two sexes were arranged separately in the ovaries. Reaumur has related, that in one instance three or four males were produced to one female; and in another four or five females to one male[992].
But though the great majority of insects are subject to this Scolechiasis[993] in their larva state, yet sometimes they are not attacked by the Ichneumon till they have become pupæ. Of this kind is one just mentioned (M. Puparum), which commits its eggs to the chrysalis of the butterfly of the nettle, Vanessa Urticæ: the moment this caterpillar quits its skin to assume that state, while it is yet soft they pierce it and confide to it their eggs[994]. De Geer and others have supposed that this same Ichneumon attacks the Cocci and Coccinellæ[995]; but this probably is an erroneous supposition. Cryptus Compunctor also attacks the pupæ of butterflies[996].
If we consider the great purpose of PROVIDENCE in giving being to this tribe of destroyers--the keeping of insects within their proper limits,--we may readily conceive that this purpose is more effectually answered by destroying them in their preparatory than in their ultimate state, since at that time the laying of their eggs and a future progeny could not so effectually be prevented;--this will account for there being few or no Ichneumons appropriated to them in their latter state.
The next tribe of insect parasites are to be found in the Diptera Order. The species that has been particularly noticed as such is the Tachina Larvarum; its larva is polyphagous, laying its eggs upon the bodies of caterpillars of different kinds. Sometimes a pair is placed on the first segment, sometimes on the head itself, and sometimes near the anus. These eggs are very hard, convex, of an oval figure, polished and shining like a mirror. They are fixed so firmly that if you attempt to remove them with a penknife the skin comes off with them. When hatched, they enter the body and feed on the interior, and, undergoing their metamorphosis within it, do not emerge till they enter their perfect state. The caterpillar thus attacked lives long enough to spin its cocoon, when it dies[997]. Sometimes, however, these animals quit their prey sooner. Reaumur saw a grub of one of the Muscidæ come out of a caterpillar, and then become a pupa, which was so large that he wondered how it could have been contained in the animal it had quitted[998].
We have now done with those parasites that produce in insects the disease I have called Scolechiasis[999]: the rest, which belong to the Aptera Order, will afford us examples both of Phthiriasis and Acariasis[1000].
I begin with the first. Mr. Sheppard once brought me a specimen of a bird-louse (Nirmus) which he took upon a butterfly (Vanessa Io): and should such a capture be more than once repeated, it would afford a certain instance of the first of these diseases amongst insects;--but most probably the specimen in question had dropped from some bird upon the butterfly. The only remaining animal belonging to the apterous hexapods that is parasitic on insects, is by many supposed to be the larva of a giant-beetle (Meloe Proscarabæus). I have before alluded to this animal[1001], and shall now resume the subject. Gœdart, Frisch, and De Geer, observed that it deposited in the earth one or two considerable masses, containing an infinite number of very minute orange-coloured eggs adhering to each other, which in about a month were hatched, and produced a number of small hexapods distinguished by two pairs of anal setæ and a proleg, by means of which they could move readily upon glass, as I have myself seen: these little animals precisely corresponded with one found by the latter author upon Eristalis intricarius; and when that fly was placed amongst them, they immediately attached themselves to it, so as to leave no doubt of their identity[1002]. A congenerous species had been detected upon wild bees, and described by Linné under the name of Pediculus Apis. De Geer is so thoroughly to be depended upon for his veracity and accuracy of observation, that we cannot suppose there is any incorrectness in his statement. If the mass of eggs be, as he represents it, of the size of a hazel-nut, it must have been the product of a very large insect: in confirmation of this opinion it may be further observed, that the larva of the kindred genus Cantharis agrees with it in having anal setæ, though it appears to differ in having only two conspicuous segments in the trunk[1003]. Those which infest wild bees make their first appearance upon acrid plants, which the Meloe likewise feeds upon; from whence with wonderful agility they leap upon the Andrenæ, &c. that visit these flowers. Strong, however, as all these facts appear, still we cannot help exclaiming with the illustrious Swede last named, Who could ever have imagined that the larva of this great beetle would be found upon the body of flies,--and we may add, or bees? Who could ever imagine that it would feed like a bird-louse and resemble it so closely? that in the insertion of its palpi it should exhibit a character exclusively belonging to that tribe[1004]? Another circumstance seems to indicate that these hexapods at the time that they take their station in bees or flies are perfect insects--they do not vary in size, at least not materially. Where, we may also ask, if they are to become large beetles, where do they take their principal growth? It cannot be as parasites on the little bees or flies that they are usually found upon; they must soon desert them, and like their kindred blister-beetles, as is most probable, have recourse to vegetable food. What an anomaly in rerum natura! It is much to be wished that some skilful insect-anatomist would carefully dissect the Meloe; or perhaps by digging round the roots of the ranunculuses and other acrid plants the larva of that beetle might be discovered in a later stage of growth, and so this mystery be cleared up. I should observe here, that Scopoli has described three parasites as Pediculi; viz. P. rostratus, coccineus, and Cerambycinus; the first of which Fabricius has adopted under the name of P. Gryllotalpæ, but which are all evidently hexapod Acarina[1005].
Acariasis seems a disease almost as universal amongst insects as Scolechiasis; with this difference however, that Acari most commonly take their station upon them in their perfect state. You have doubtless often observed the common dung-beetles (Geotrupes) covered on the underside of their body with small mites (Gamasus Coleoptratorum) which look as if they were engaged in suction--they are often so numerous that no part is uncovered; they also attack other beetles[1006], and are sometimes found on humble-bees. They are easily disturbed, run with great swiftness, and may often be seen in hotbeds and fermenting dung prowling in search of the stercorarious beetles. But the most remarkable insect of this kind is the Uropoda vegetans: it derives its nutriment from the insects it assails not by its mouth, but by means of a long anal pedicle by which it is attached to them. De Geer found these in such numbers upon a species of Leptura, that its whole body was almost covered with them; they hung from the legs and antennæ in bunches, and gave the animal a most hideous and disgusting appearance. Under this load of vermin it could scarcely walk or move, and all its efforts to get rid of them were in vain: many were attached to its body and to each other by their anal pedicles, but others had cast them off and were walking about. When put into a glass with earth, they began to abandon their prey, so that in a few days it was quite freed from its plagues. He found that these parasites lived long in alcohol[1007].
If you inquire--How are these mites originally fixed by their pedicles? it seems most probable, that as the Hemerobii, when they lay their eggs, know how to place them upon a kind of footstalk, so the parent Uropoda has the same power; and this pedicle appears to act the part of an umbilical chord, conveying nutriment to the fœtus not from a placenta, but from the body of the insect to which it is attached; till having thus attained a certain maturity of growth and structure, it disengages itself and becomes locomotive. Many eggs of the aquatic Acarina (Hydrachna, &c.) are also furnished with a short pedicle by which they are fixed to Dytisci and other water insects. De Geer found some of this description on the underside of the water-scorpion, so thickly set as to leave no void space: they were oval, of a very bright red, and of different sizes on different individuals; whence it was evident that they grow when thus fixed: when hatched or released--for perhaps they may be regarded as fœtuses in their amnios rather than eggs--they cease to be parasitical. Let us admire on this occasion, (piously observes this great Entomologist,) the different and infinitely varied means by which the AUTHOR of Nature has endowed animals, particularly insects, for their propagation and preservation: for it is a most extraordinary sight to see eggs grow, and pump as it were their nutriment from the body of another living animal[1008]. As these mites are fixed to the crust as well as its inosculations, they must have some means of forcing their nutriment through its pores.
Another insect, remarkable for its resemblance in some respects to the scorpion--called in this country the book-crab (Chelifer cancroides), from its being sometimes found in books--occasionally is parasitic upon flies, especially the common blue-bottle-fly (Musca vomitoria). They adhere to it very pertinaciously under the wings; and if you attempt to disturb them, they run backwards, forwards, or sideways, with equal facility.
Spiders also are infested by mites. Mr. Briggs once found a very small Theridion, to the thorax of which were attached four oblong bright scarlet mites, each of which was as large as the thorax itself. He afterwards met with another spider still smaller, attacked by two of these swoln parasites, one of which appeared to him nearly equal to the spider in size. This mite was probably either Leptus Phalangii, or Astoma parasiticum.
2. We now come to a perfectly distinct tribe of insect parasites, which belong to that section or order of intestinal worms which Rudolph has denominated Entozoa nematoidea, and Lamarck Vers rigidules[1009]. To this tribe belong the Gordius of Linné and the Filaria of modern zoologists, which from the experiments and observations of De Geer, Dr. Matthey, &c. appear to have been too hastily separated, being really congenerous, and living indifferently in water and in the intestines of insects and other animals[1010]. To this genus belong the guinea-worm (Gordius medinensis[1011]), the Furia infernalis, and several others that are found in various vertebrate animals. These little worms have been discovered in insects of almost every Order; and their attack generally produces the death of the animal, though they appear not to devour those parts that are essential to life[1012]. I once took a specimen of Pœcilus azureus, and upon immersing it in boiling water I was surprised to see what at first I mistook for an intestine, thrust itself forth; but upon a nearer inspection, to my great surprise I found it was one of these worms, thicker than a horse-hair and of a brown colour. Mr. W. S. MacLeay also once found one in Abax Striola. It still remains in my specimen, making it appear as if it had a long tail. De Geer long ago found these worms in grasshoppers[1012]; but Dr. Matthey has given the fullest account of one which infested Acrida viridissima. A friend of his noticing one of these insects which had not strength enough to leap and could scarcely even walk, being struck with the circumstance, caught the animal, upon which its hind legs were immediately detached from it. His surprise was greatly increased when he saw issue from its body a cylindrical worm about two feet and a half in length. Upon being called, Dr. M. soon recognised it for a Gordius or Filaria; and on his putting it into water, it moved in it with great velocity, twisting its long and slender body in all directions. Upon opening the body of the grasshopper, nothing appeared within it but the intestine shrunk up to a thread. A few days after, another was brought, which appeared in full vigour, but its abdomen was enormously distended, and from it another worm was extracted, which remained without motion rolled in a spiral direction: intending to preserve this in spirits of wine--as it had become flat he first immersed it in water, that it might recover if possible its cylindrical form. Upon immersion a movement took place in the animal, and it gradually recovered its plumpness; but it still remained without motion, as if dead, for nearly five days, when another living specimen being brought and placed with it, as soon as water was poured on them, the seemingly dead one began to show by a slight oscillation in its extremities that life was not extinct in it. Fresh water being poured upon it, at the end of the day it had recovered all its strength and agility. He afterwards often repeated the same experiment with a similar result[1013]. From this account it appears that the Gordius or Filaria has a property resembling that of the Vibrio Tritici, so well described and so beautifully figured by M. Bauer[1014], of apparently dying and being resuscitated by immersion in water. How long it can retain this property remains to be ascertained.
De Geer states that he had seen them of the length of two feet[1015]; but they vary considerably in this respect. In ants, in which Gould detected them, he states their length to be not more than half an inch[1016]. In caterpillars, which they sometimes infest, they are longer; in that of Notodonta Ziczac, De Geer found one three inches and a half long[1017]; and Rösel three, of six inches, in that of Deilephila Euphorbiæ[1018]; and in Phalangium cornutum, according to Latreille, they extend to more than seven inches[1019]. In the larva of a Phryganea L. the author first named found one which was more than a foot long, corresponding exactly with the Gordius aquaticus of Linné; being forked at one extremity, brown above, gray below, and black at each end[1020]. These animals appear to die as soon as they leave the body[1021] they have preyed upon; except this happens in water, when their activity has no repose. In this element they give their bodies every possible inflexion, often tying themselves in knots in various places, interlacing and twisting themselves in a hundred different ways; so that when confined in the body of an insect, from their extreme suppleness and power of contortion they find sufficient space wherein to pack their often enormous length[1022]. Linné makes one of their habitats clay; and Mr. W. S. MacLeay finds them very common at Putney in clay at the bottom of pools.
Dr. Matthey asks--How does the Gordius get into Acrida viridissima[1023]? And De Geer--Why do they die after having quitted a caterpillar? and where do they perpetuate their species[1024]? These questions, without further observations, cannot easily be answered. However, it may be supposed that carnivorous insects, such as Harpali, &c. may swallow them when found apparently dead in clay, where the water has been evaporated, or when they have been ejected by other insects; and they may revive in their bodies, as Dr. Matthey found them to do in water. It is not difficult to conjecture that the larvæ of Phryganeæ may meet with them when young in the water, and sometimes unluckily swallow them with their food. Why they become as dead when they emerge from their prey we cannot at present conjecture; but no doubt to answer some wise purpose;--in rainy seasons they probably revive and get into little hollows full of rain-water. Upon De Geer's last question--How they perpetuate their species--at present I can offer no conjecture.
I am, &c.
FOOTNOTES:
[918] Ps. civ. 29.Return to passage
[919] Hist. Animal. l. viii. c. 27.Return to passage
[920] The Principles of Botany and of Vegetable Physiology, § 310-353.Return to passage
[921] Dr. Leach, from a communication of Sir Joseph Banks, has given a very interesting history of a spider which, having lost five of its legs, from a web-weaver had become a hunter; these legs it afterwards reproduced, though shorter than the others. Linn. Trans. xi. 393. Comp. N. Dict. d'Hist. Nat. ii. 282.Return to passage
[922] VOL. I. p. 55--.Return to passage
[923] VOL. II. p. 166--.Return to passage
[924] Huber Abeilles ii. 409.Return to passage
[925] N. Dict. d'Hist. Nat. i. 42.Return to passage
[926] De Geer i. 72--.Return to passage
[927] Reaum. iv. 342.Return to passage
[928] Naturf. xii. 224. t. v. f. 8.Return to passage
[929] Naturf. xvi. t. iv. f. 1-3.Return to passage
[930] Huber Fourmis, 174. note 1.Return to passage
[931] VOL. II. p. 365.Return to passage
[932] N. Dict. d'Hist. Nat. i. 42.Return to passage
[933] De Geer vi. 75. Latr. Hist. Nat. xiv. 371.Return to passage
[934] Jacquin Collectan. iii. t. xxiii. f. 7.Return to passage
[935] De Geer ubi supr.Return to passage
[936] Dr. Bevan asserts (The Honey-bee, 197) "that we have no evidence that pollen constitutes any part of the food of adult bees." Had he consulted Reaumur (v. 418) he would have found that this great man examined the proceedings of a bee with a magnifying glass, and distinctly saw her devour very deliberately the masses of pollen on her hind legs. He says also (Ibid. 419.) that if the stomach and intestines are opened, they will be found filled with that substance.Return to passage
[937] Schirach Hist. &c. 54. Reaum. v. 713. N. Dict. d'Hist. Nat. i. 42.Return to passage
[938] N. Dict. d'Hist. Nat. i. 42. Schirach Hist. 56.Return to passage
[939] Latr. Hist. Nat. xiv. 163--. N. Dict. d'Hist. Nat. iv. 134--.Return to passage
[940] Œuvr. ii. 48--.Return to passage
[941] Lesser L. ii. 121.Return to passage
[942] Synops. Meth. Fung. 687. g. 63. n. 1, 2.Return to passage
[943] Ibid. 4. g. 1. n. 4.Return to passage
[944] VOL. III. p. 335--.Return to passage
[945] VOL. I. p. 267--.Return to passage
[946] Mon. Ap. Angl. ii. 111. Linn. Trans. xi. 90--.Return to passage
[947] Reaum. ii. 439.Return to passage
[948] Ibid. 415. Mouffet 57.Return to passage
[949] Hist. Ins. Præf. xv.Return to passage
[950] Cat. Cant. 137.Return to passage
[951] See above, p. 162--.Return to passage
[952] Reaum. vi. 306.Return to passage
[953] Fn. Suec. 1626.Return to passage
[954] Linn. Trans. iii. 26.Return to passage
[955] De Geer i. 608. Linné has made a mistake with regard to the Ichneumon here alluded to, in calling De Geer's saltatorious Ichneumon I. Muscarum, and referring for it to t. xxxii. f. 19, 20 of that author; whereas the Ichneumon that preys upon the aphidivorous flies does not jump, and is figured by De Geer 605. t. xxxiv. f. 26-29. The jumping one feeds on the larva of a Coccinella.Return to passage
[956] Vallisnieri Lettere, &c. 80.Return to passage
[957] Reaum. vi. 296--.Return to passage
[958] Linné evidently has described another species under I. Ovulorum, in Fn. Suec. 1644.Return to passage
[959] De Geer i. 593--.Return to passage
[960] N. Dict. d'Hist. Nat. vi. 10.Return to passage
[961] Geoffr. Hist. Ins. Par. ii. 361.Return to passage
[962] Linn. Trans. v. 102--.Return to passage
[963] PLATE XX. FIG. 22. a.Return to passage
[964] De Geer ii. 850--.Return to passage
[965] Reaum. ii. 444--.Return to passage
[966] De Geer ii. 863--.Return to passage
[967] Panzer Fn. Germ. Init. lxxii. 4.Return to passage
[968] De Geer, i. 583--. ii. 822--. 907.Return to passage
[969] Reaum. vi. 312.Return to passage
[970] Leeuwenh. Epist. Oct. 6, 1700. De Geer ii. 869.Return to passage
[971] Ibid. i. 604.Return to passage
[972] Rai. Hist. Ins. 259--.Return to passage
[973] See above, p. 217; and VOL. I. p. 356.Return to passage
[974] Ibid. 348.Return to passage
[975] Reaum. vi. 303--.Return to passage
[976] Reaum. ii. 454--.Return to passage
[977] De Geer ii. 879--.Return to passage
[978] N. Dict. d'Hist. Nat. xvii. 513.Return to passage
[979] De Geer vi. 411--.Return to passage
[980] VOL. I. p. 172.Return to passage
[981] Linn. Trans. iv. 236.Return to passage
[982] VOL. I. p. 50--. 170--. Since that Volume of the present Edition was printed, Say's account of the Hessian Fly has been met with, where he distinguishes it by the above name. (Journal of the Acad. of Nat. Sciences of Philadelphia 1817.) The Ichneumon he calls Ceraphron Destructor.Return to passage
[983] De Geer, i. 605. This, as before observed, is not the I. Muscarum of Linné; but it ought to have that name, and the other instead to be named, I. Coccinellæ.Return to passage
[984] Ent. Carn. 760, 761.Return to passage
[985] De Geer i. 587.Return to passage
[986] Ibid. ii. 876.Return to passage
[987] Reaum. ii. 417--.Return to passage
[988] Reaum. ii. 419--.Return to passage
[989] De Geer i. 583--.Return to passage
[990] Ibid. ii. 884.Return to passage
[991] See above, p. 164.Return to passage
[992] Reaum. vi. 312.Return to passage
[993] VOL. I. p. 99.Return to passage
[994] De Geer ubi supr.Return to passage
[995] Ibid. 883.Return to passage
[996] Linn. Fn. Suec. 1609.Return to passage
[997] Reaum. ii. 443. De Geer i. 196--, 550--. vi. 24.Return to passage
[998] Reaum. ii. 440--.Return to passage
[999] VOL. I. p. 99.Return to passage
[1000] Ibid. 84, 97.
[1001] VOL. I. p. 163. note^c. VOL. III. p. 162. note^b.
[1002] De Geer v. 8--.
[1003] Naturf. xxiii. t. i. f. 8.
[1004] N. Dict. d'Hist. Nat. xx. 110--.
[1005] Ent. Carn. 1052-4.
[1006] Hister particularly.
[1007] De Geer vii. 126--.
[1008] De Geer vii. 144--.
[1009] Lamarck Anim. sans Vert. iii. 196.
[1010] De Geer ii. 554--. Pictet Bibliotheq. Univers. num. ult.
[1011] The existence of this animal has been satisfactorily ascertained by M. de Blainville, who had a specimen, extracted from a human body, sent him by M. Girard, a surgeon of Guadaloupe.
[1012] De Geer ii. 555.
[1013] Matthey ubi supr.
[1014] Philos. Trans. 1823. 8. t. i. ii.
[1015] De Geer ii. 556.
[1016] Gould Ants, 63.
[1017] De Geer i. 551.
[1018] Rösel I. iii. 20.
[1019] Latr. Fourmis, 373.
[1020] De Geer ii. ubi supr. t. xiv. f. 12-14.
[1021] Ibid. i. 553.
[1022] Ibid. ii. 556. t. xiv. f. 12, 13.
[1023] Ubi supra.
[1024] De Geer i. 553.