PLATE I.PLATE I.7—MATURE INSECTS.Fig. 1, Cricket; 2, Earwig; 3,Aphis; 4,Scolytus; 5,Anthrax; 6,Balaninus; 7,Cynips; 8, Ant; 9, Wasp.
PLATE I.7—MATURE INSECTS.Fig. 1, Cricket; 2, Earwig; 3,Aphis; 4,Scolytus; 5,Anthrax; 6,Balaninus; 7,Cynips; 8, Ant; 9, Wasp.
PLATE II.PLATE II.—LARVÆ OF THE INSECTS REPRESENTED ON PLATE I.Fig. 1, Larva of Cricket; 2, Larva ofAphis; 3, Larva of Earwig; 4, Larva ofScolytus(Beetle); 5, Larva ofAnthrax(Fly); 6, Larva ofBalaninus(Nut Weevil); 7, Larva ofCynips; 8, Larva of Ant; 9, Larva of Wasp.
PLATE II.—LARVÆ OF THE INSECTS REPRESENTED ON PLATE I.
Fig. 1, Larva of Cricket; 2, Larva ofAphis; 3, Larva of Earwig; 4, Larva ofScolytus(Beetle); 5, Larva ofAnthrax(Fly); 6, Larva ofBalaninus(Nut Weevil); 7, Larva ofCynips; 8, Larva of Ant; 9, Larva of Wasp.
In the Cynipidæ (PlateI., Fig. 7) the female is provided with an organ corresponding to the saw of the sawfly, but resembling a needle. With this she stings or punctures the surface of leaves, buds, stalks, or even roots of various plants. In the wound thus produced she lays one or more eggs. The effects of this proceeding, and particularly of the irritating fluid which she injects into the wound, is to produce a tumour or gall, within which the egg hatches, and on which the larva, a thick fleshy grub (PlateII., Fig. 7), feeds. In some species each gall contains a single larva; in others, several live together.
The oak supports several kinds of gallflies: one produces the well-known oak-apple, one a small swelling on the leaf resembling a currant, another a gall somewhat like an acorn, another attacks the root; the species making the bullet-like galls, which are now so common, has only existed for a few years in this country; the beautiful little spangles so common in autumn on the under side of oak leaves are the work of another species, theCynus longipennis. One curious point about this group is, that in some of the commonest species the females alone are known, no one yet having ever succeeded in finding a male.
Another great family of the Hymenoptera is that of the ichneumons; the females lay their eggs either in or on other insects, within the bodies of which the larvæ live. These larvæ are thick, fleshy, legless grubs, and feed on the fatty tissues of their hosts, but do not attack the vital organs. When full-grown, the grubs eat their way through the skin ofthe insect, and turn into chrysalides. Almost every kind of insect is subject to the attacks of these little creatures, which are no doubt useful in preventing the too great multiplication of insects, and especially of caterpillars. Some species are so minute that they actually lay their eggs within those of other insects Figs. (15, 16). These parasites assume very curious forms in their larval state.
But of all the Hymenoptera, the group containing the ant, the bee, and the wasp is the most interesting. This is especially the case with the social species, though the solitary ones also are extremely remarkable. The solitary bee or wasp, for instance, forms a cell generally in the ground, places in it a sufficient amount of food, lays an egg, and closes the cell. In the case of bees, the food consists of honey; in that of wasps, the larva requires animal food, and the mother therefore places a certain number of insects in the cell, each species having its own special prey, some selecting small caterpillars, some beetles, some spiders.Cerceris bupresticida, as its name denotes, attacks beetles belonging to the genusBuprestis. Now if theCerceriswere to kill the beetle before placing it in the cell, it would decay, and the young larva, when hatched, would find only a mass of corruption. On the other hand, if the beetle were buried uninjured, in its struggles to escape it would be almost certain to destroy the egg. The wasp has, however, the instinct of stinging its prey in the centre of the nervous system, thus depriving it of motion, and let us hope of suffering, but not of life; consequently, when the young larva leavesthe egg, it finds ready a sufficient store of wholesome food.
Other wasps are social, and, like the bees and ants, dwell together in communities. They live for one season, dying in autumn, except some of the females, which hibernate, awake in the spring, and form new colonies. These, however, do not, under ordinary circumstances, live through a second winter. One specimen which I kept tame through last spring and summer, lived until the end of February, but then died. The larvæ of wasps (PlateII., Fig. 9) are fat, fleshy, legless grubs. When full-grown they spin for themselves a silken covering, within which they turn into chrysalides. The oval bodies which are so numerous in ants' nests, and which are generally called ants' eggs, are really not eggs but cocoons. Ants are very fond of the honey-dew which is formed by the Aphides, and have been seen to tap the Aphides with their antennæ, as if to induce them to emit some of the sweet secretion. There is a species ofAphiswhich lives on the roots of grass, and some ants collect these into their nests, keeping them, in fact, just as we do cows. Moreover they collect the eggs in the autumn and tend them through the winter (when they are of no use) with the same care as their own, so as to have a supply of young Aphides in the spring. This is one of the most remarkable facts I know in the whole history of animal life. One species of red ant does no work for itself, but makes slaves of a black kind, which then do everything for their masters. The slave makers will not even put food into their own mouths, but would starve in the midst of plenty, if theyhad not a slave to feed them. I found, however, that I could keep them in life and health for months if I gave them a slave for an hour or two in a week to clean and feed them.
PLATE III.PLATE III.--MATURE INSECTS.Fig. 1,Chloëon;2,Meloë(after Shuckard); 3,Calepteryx; 4,Sitaris(after Shuckard); 5,Campodea(after Gervais); 6,Acilius; 7,Termes; 8,Stylops(female); 9,Thrips.
PLATE III.--MATURE INSECTS.
Fig. 1,Chloëon;2,Meloë(after Shuckard); 3,Calepteryx; 4,Sitaris(after Shuckard); 5,Campodea(after Gervais); 6,Acilius; 7,Termes; 8,Stylops(female); 9,Thrips.
PLATE IV.PLATE IV. YOUNG FORMS OF THE INSECTS REPRESENTED ONPLATE III.--Fig. 1, Larva ofChloëon;2, Larva ofMeloë(after Chapuis and Candèze); 3, Larva ofCalepteryx(after Léon Dufour); 4, Larva ofSitaris; 5, Larva ofCampodea; 6, Larva ofAcilius; 7, Larva ofTermes(after Blanchard); 8, Larva ofStylops; 9, Larva ofThrips.
PLATE IV. YOUNG FORMS OF THE INSECTS REPRESENTED ONPLATE III.--Fig. 1, Larva ofChloëon;2, Larva ofMeloë(after Chapuis and Candèze); 3, Larva ofCalepteryx(after Léon Dufour); 4, Larva ofSitaris; 5, Larva ofCampodea; 6, Larva ofAcilius; 7, Larva ofTermes(after Blanchard); 8, Larva ofStylops; 9, Larva ofThrips.
Ants also keep a variety of beetles and other insects in their nests. That they have some reason for this seems clear, because they readily attack any unwelcome intruder; but what that reason is, we do not yet know. If these insects are to be regarded as the domestic animals of the ants, then we must admit that the ants possess more domestic animals than we do.
Some indeed of these beetles produce a secretion which is licked by the ants like the honeydew; there are others, however, which have not yet been shown to be of any use to the ants, and yet are rarely, if‘ ever, found, excepting in ants' nests.
M. Lespès, who regards these insects as true domestic animals, has recorded8some interesting observations on the relations between one of them (Claviger Duvalii) and the ants (Lasius niger) with which it lives. This species ofClavigeris never met with except in ants' nests, though on the other hand there are many communities ofLasiuswhich possess none of these beetles; and M. Lespès found that when he placedClavigersin a nest of ants which had none of their own, the beetles were immediately killed and eaten, the ants themselves being on the other hand kindly received by other communities of the same species. He concludes from these observations that some communities of ants are more advanced in civilization than others; the suggestion isno doubt ingenious, and the fact curiously resembles the experience of navigators who have endeavoured to introduce domestic animals among barbarous tribes; but M. Lepès has not yet, so far as I am aware, published the details of his observations, without which it is impossible to form a decided opinion. I have sometimes wondered whether the ants have any feeling of reverence for these beetles; but the whole subject is as yet very obscure, and would well repay careful study.
The order Strepsiptera are a small, but very remarkable group of insects, parasitic on bees and wasps. The larva (Pl.IV., Fig. 8) is minute, six-legged, and very active; it passes through its transformations within the body of the bee or wasp. The male and female are very dissimilar. The males are minute, very active, short-lived, and excitable, with one pair of large membranous wings. The females (Pl.III., Fig. 8), on the contrary, are almost motionless, and shaped very much like a bottle; they never quit the body of the bee, but only thrust out the top of the bottle between the abdominal rings of the bee.
In the order Coleoptera, the larvæ differ very much in form. The majority are elongated, active, hexapod, and more or less depressed; but those of theWeevils(Pl.II., Fig. 6), ofScolytus(Pl.II., Fig. 4), &c., which are vegetable feeders, and live surrounded by their food,—as, for instance, in grain, nuts, &c.,—are apod, white, fleshy grubs, not unlike those of bees and ants. The larvæ of the Longicorns, which live inside trees, are long, soft, and fleshy, with six short legs. The Geodephaga, corresponding with the Linnæan generaCicindelaandCarabus, have six-legged, slender, carnivorous larvæ; those ofCicindela, which waylay their prey, being less active than the hunting larvæ of the Carabidæ. The Hydradephaga, or water-beetles (Dyticidæ and Gyrinidæ), have long and narrow larvæ (Pl.IV., Fig. 6), with strong sickle-shaped jaws, short antennæ, four palpi, and six small eyes on each side of the head; they are very voracious. The larvæ of the Staphylinidæ are by no means unlike the perfect insect, and are found in similar situations; their jaws are powerful, and their legs moderately strong. The larvæ of the Lamellicorn beetles Figs. (1-6)—cockchafers, stag-beetles, &c.—feed on vegetable substances or on dead animal matter. They are long, soft, fleshy grubs, with the abdomen somewhat curved, and generally lie on their side. The larvæ of the Elateridæ, known as wireworms, are long and slender, with short legs. That of the glowworm (Lampyridæ) is not unlike the apterous female. The male glowworm, on the contrary, is very different. It has long, thin, brown wing-cases, and often flies into rooms at night, attracted by the light, which it probably mistakes for that of its mate.
The metamorphoses of the Cantharidæ are very remarkable, and will be described subsequently. The larvæ are active and hexapod. The Phytophaga (Crioceris,Galeruca,Haltica,Chrysomela, &c.) are vegetable feeders, both as larvæ and in the perfect state. The larvæ are furnished with legs, and are not unlike the caterpillars of certain Lepidoptera.
The larva ofCoccinella(the Ladybird) is somewhat depressed, of an elongated ovate form, with asmall head, and moderately strong legs. It feeds on Aphides.
Thus, then, we see that there are among the Coleoptera many different forms of larvæ. Macleay considered that there were five principal types.
1. Carnivorous hexapod larvæ, with an elongated, more or less flattened body, six eyes on each side of the head, and sharp falciform mandibles (Carabus,Dyticus, &c.).
2. Herbivorous hexapod larvæ, with fleshy, cylindrical bodies, somewhat curved, so that they lie on their side.
3. Apod grub-like larvæ, with scarcely the rudiments of antennæ (Curculio).
4. Hexapod antenniferous larvæ, with a subovate body, the second segment being somewhat larger than the others (Chrysomela,Coccinella).
5. Hexapod antenniferous larvæ, of oblong form, somewhat resembling the former, but with caudal appendages (Meloë,Sitaris).
The pupa of the Coleoptera is quiescent, and “the parts of the future beetle are plainly perceivable, being encased in distinct sheaths; the head is applied against the breast; the antennæ lie along the sides of the thorax; the elytra and wings are short and folded at the sides of the body, meeting on the under side of the abdomen; the two anterior pairs of legs are entirely exposed, but the hind pair are covered by wing-cases, the extremity of the thigh only appearing beyond the sides of the body.”9
In the next three orders—namely, the Orthoptera (grasshoppers, locusts, crickets, walking-stick insects,cockroaches, &c.), Euplexoptera (earwigs), and Thysanoptera, a small group of insects well known to gardeners under the name ofThrips(Pl.I.andII., Figs.1 and 2)—the larvæ when they quit the egg already much resemble the mature form, differing, in fact, principally in the absence of wings, which are more or less gradually acquired, as the insect increases in size. They are active throughout life. Those specimens which have rudimentary wings are, however, usually called pupæ.
The Neuroptera present, perhaps, more differences in the character of their metamorphoses than any other order of insects. Their larvæ are generally active, hexapod little creatures, and do not vary from one another in appearance so much, for instance, as those of the Coleoptera, but their pupæ differ essentially; some groups, namely, the Psocidæ, Termitidæ, Libellulidæ, Ephemeridæ, and Perlidæ, remaining active throughout life, like the Orthoptera; while a second division, including the Myrmeleonidæ, Hemerobiidæ, Sialidæ, Panorpidæ, Raphidiidæ, and Mantispidæ, have quiescent pupæ, which, however, in some cases, acquire more or less power of locomotion shortly before they assume the mature state; thus that ofRaphidia, though motionless at first, at length acquires strength enough to walk, even while still enclosed in the pupa skin, which is very thin.10
One of the most remarkable families belonging to this order is that of the Termites, or white ants. They abound in the tropics, where they are a perfect pest, and a serious impediment to human development. Their colonies are extremely numerous, andthey attack woodwork and furniture of all kinds, generally working from within, so that their presence is often unsuspected, until it is suddenly found that they have completely eaten away the interior of some post or table, leaving nothing but a thin outer shell. Their nests, which are made of earth, are sometimes ten or twelve feet high, and strong enough to bear a man. One species,Termes lucifugus, is found in the South of France, where it has been carefully studied by Latreille. He found in these communities five kinds of individuals—(1) males; (2) females, which grow to a very large size, their bodies being distended with eggs, of which they sometimes lay as many as 80,000 in a day; (3) a form described by some observers as Pupæ, but by others as neuters. These differ very much from the others, having a long, soft body without wings, but with an immense head, and very large, strong jaws. These individuals act as soldiers, doing apparently no work, but keeping watch over the nest and attacking intruders with great boldness. (4) Apterous, eyeless individuals, somewhat resembling the winged ones, but with a larger and more rounded head; these constitute the greater part of the community, and, like the workers of ants and bees, perform all the labour, building the nest and collecting food. (5) Latreille mentions another kind of individual which he regards as the pupa, and which resembles the workers, but has four white tubercles on the back, where the wings afterwards make their appearance. There is still, however, much difference of opinion among entomologists, with reference to the true nature of these different classes of individuals. M.Lespès, who has recently studied the same species, describes a second kind of male and a second kind of female, and the subject, indeed, is one which offers a most promising field for future study.
Another interesting family of Neuroptera is that of the Ephemeræ, or Mayflies (Pl.III., Fig. 1), so well known to fishermen. The larvæ (Pl.IV., Fig. 1) are semi-transparent, active, six-legged little creatures, which live in water; having at first no gills, they respire through the general surface of the body. They grow rapidly and change their skin every few days. After one or two moults they acquire seven pairs of branchiæ, or gills, which are generally in the form of leaves, one pair to the segment. When the larvæ are about half grown, the posterior angles of the two posterior thoracic segments begin to elongate. These elongations become more and more marked with every change of skin. One morning, in the month of June, some years ago, I observed a full-grown larva, which had a glistening appearance, owing to the presence of a film of air under the skin. I put it under the microscope, and, having added a drop of water with a pipette, looked through the glass. To my astonishment, the insect was gone, and an empty skin only remained. I then caught a second specimen, in a similar condition, and put it under the microscope, hoping to see it come out. Nor was I disappointed. Very few moments had elapsed, when I had the satisfaction of seeing the thorax open along the middle of the back; the two sides turned over; the insect literally walked out of itself, unfolded its wings, and in an instant flew up to the window. Several times since, I have had the pleasure ofwitnessing this marvellous change, and it is really wonderful how rapidly it takes place: from the moment when the skin first cracks, not ten seconds are over before the insect has flown away.
Another family of Neuroptera, the Dragon-flies, or Horse-stingers, as they are sometimes called, from a mistaken idea that they sting severely enough to hurt a horse, though in fact they are quite harmless, also spend their early days in the water. The larvæ are brown, sluggish, ugly creatures, with six legs. They feed on small water-animals, for which they wait very patiently, either at the bottom of the water, or on some aquatic plant. The lower jaws are attached to a long folding rod; and when any unwary little creature approaches too near the larva, this apparatus is shot out with such velocity that the prey which comes within its reach seldom escapes. In their perfect condition, also, Dragon-flies feed on other insects, and may often be seen hawking round ponds. The so-called Ant-lions in many respects resemble the Dragon-flies, but the habits of the larvæ are very dissimilar. They do not live in the water, but prefer dry places, where they bury themselves in the loose sand, and seize with their long jaws any small insect which may pass. The true Ant-lion makes itself a round, shallow pit in loose ground or sand, and buries itself at the bottom. Any inattentive little insect which steps over the edge of this pit immediately falls to the bottom, and is instantaneously seized by the Ant-lion. Should the insect escape, and attempt to climb up the side of the pit, the Ant-lion is said to throw sand at it, knocking it down again.
One other family of Neuroptera which I mustmention, is the Hemerobiidæ. The perfect insect is a beautiful, lace-winged, very delicate, green creature, something like a tender Dragon-fly, and with bright, green, touching eyes. The female deposits her eggs on leaves, not directly on the plant itself, but attached to it by a long white slender footstalk. The larva has six legs and powerful jaws, and makes itself very useful in destroying the Hop-fly.
The insects forming the order Trichoptera are well known in their larval condition, under the name of caddis worms. These larvæ are not altogether unlike caterpillars in form, but they live in water—which is the case with very few lepidopterous larvæ—and form for themselves cylindrical cases or tubes, built up of sand, little stones, bits of stick, leaves, or even shells. They generally feed on vegetable substances, but will also attack minute freshwater animals. When full grown, the larva fastens its case to a stone, the stem of a plant, or some other fixed substance, and closes the two ends with an open grating of silken threads, so as to admit the free access of water, while excluding enemies. It then turns into a pupa which bears some resemblance to the perfect insect, “except that the antennæ, palpi, wings, and legs are shorter, enclosed in separate sheaths, and arranged upon the breast.” The pupa remains quiet in the tube until nearly ready to emerge, when it comes to the surface, and in some cases creeps out of the water. It is not therefore so completely motionless as the pupæ of Lepidoptera.
The Diptera, or Flies, comprise insects with two wings only, the hinder pair being represented by minute club-shaped organs called “haltères.” Fliesquit the egg generally in the form of fat, fleshy, legless grubs. They feed principally on decaying animal or vegetable matter, and are no doubt useful as scavengers. Other species, as the gadflies, deposit their eggs on the bodies of animals, within which the grubs feed, when hatched. The mouth is generally furnished with two hooks which serve instead of jaws. The pupæ of Diptera are of two kinds. In the true flies, the outer skin of the full-grown larva is not shed, but contracts and hardens, thus assuming the appearance of an oval brownish shell or case, within which the insect changes into a chrysalis. The pupæ of the gnats, on the contrary, have the limbs distinct and enclosed in sheaths. They are generally inactive, but some of the aquatic species continue to swim about.
One group of Flies, which is parasitic on horses, sheep, bats, and other animals, has been called the Pupipara, because it was supposed that they were not born until they had arrived at the condition of pupæ. They come into the world in the form of smooth, ovate bodies, much resembling ordinary dipterous pupæ, but as Leuckart has shown,11they are true, though abnormal, larvæ.
The next order, that of the Aphaniptera, is very small in number, containing only the different species of Flea. The larva is long, cylindrical, and legless; the chrysalis is motionless, and the perfect insect is too well known, at least, as regards its habits, to need any description.
The Heteroptera, unlike the preceding orders of insects, quit the egg in a form differing from that ofthe perfect insect principally in the absence of wings, which are gradually acquired. In their metamorphoses they resemble the Orthoptera, and are active through life. The majority are dull in colour, though some few are very beautiful. The species constituting this group, though very numerous, are generally small, and not so familiarly known to us as those of the other large orders, with indeed one exception, the well-known Bug. This is not, apparently, an indigenous insect, but seems to have been introduced. The word is indeed used by old writers, but either as meaning a bugbear, or in a general sense, and not with reference to this particular insect. In this country it never acquires wings, but is stated to do so sometimes in warmer climates. The Heteroptera cannot exactly be said either to sting or bite. The jaws, of which, as usual among insects, there are two pairs, are like needles, which are driven into the flesh, and the blood is then sucked up the lower lip, which has the form of a tube. This peculiar structure of the mouth prevails throughout the whole order; consequently their nutriment consists almost entirely of the juices of animals or plants. The Homoptera agree with the Heteroptera in the structure of the mouth, and in the metamorphoses. They differ principally in the front wings, which in Homoptera are membranous throughout, while in the Heteroptera, the front part is thickened and leathery. As in the Heteroptera, however, so also in the Homoptera, some species do not acquire wings. The Cicada, celebrated for its chirp, and the lanthorn fly, belong to this group. So also does the so-called Cuckoo-spit, so common in our gardens, which has the curiousfaculty of secreting round itself a quantity of frothy fluid which serves to protect it from its enemies. But the best known insects of this group are the Aphides or Plant-lice; while the most useful belong to the Coccidæ, or scale insects, from one species of which we obtain the substance called lac, so extensively used in the manufacture of sealing-wax and varnish. Several species also have been used in dyeing, especially the Cochineal insect of Mexico, a species which lives on the cactus. The maleCoccusis a minute, active insect, with four large wings; while the female, on the contrary, never acquires wings, but is very sluggish, broad, more or less flattened, and in fact, when full grown, looks like a small brown, red, or white scale.
The larva of the order Lepidoptera are familiar to us all, under the name of caterpillars. The insects of this order in their larval condition are almost all phytophagous, and are very uniform both in structure and in habits. The body is long and cylindrical, consisting of thirteen segments; the head is armed with powerful jaws; the three following segments, the future prothorax, mesothorax, and metathorax, each bears a pair of simple articulated legs. Of the posterior segments, five also bear false or pro-legs, which are short, unjointed, and provided with a number of hooklets. A caterpillar leads a dull and uneventful life; it eats ravenously, and grows rapidly, casting its skin several times during the process, which generally lasts only a few weeks; though in some cases, as for instance that of the goat-moth, it extends over a period of two or three years, after which the larva changes into a quiescent pupa or chrysalis.
The facts recapitulated briefly in the preceding chapter show, that the forms of insect larvæ depend greatly on the group to which they belong. Thus the same tree may harbour larvæ of Diptera, Hymenoptera, Coleoptera, and Lepidoptera; each presenting the form typical of the family to which it belongs.
If, again, we take a group, such, for instance, as the Lamellicorn beetles, we shall find larvæ extremely similar in form, yet very different in habits. Those, for instance, of the common cockchafer (Fig.1) feed on the roots of grass; those ofCetonia aurata(Fig.2) inhabit ants' nests; the larvæ of the genusTrox(Fig.3) are found on dry animal substances; ofOryctes(Fig.4) in tan-pits; ofAphodius(Fig.5) in dung; ofLucanus(the stag-beetle, Fig.6) in wood.
Figs. 1-6Fig. 1, Larva of the Cockchafer (Melolontha). (Westwood, Int. to the Modern Classification of Insects, vol. i. p. 194.). 2, Larva ofCetonia. 3, Larva ofTrox. 4, Larva ofOryctes. 5, Larva ofAphodius(Chapuis and Candèze, Mém. Soc. Roy. Liège, 1853). 6, Larva ofLucanus. (Packard, Guide to the Study of Insects, Fig. 403).
Fig. 1, Larva of the Cockchafer (Melolontha). (Westwood, Int. to the Modern Classification of Insects, vol. i. p. 194.). 2, Larva ofCetonia. 3, Larva ofTrox. 4, Larva ofOryctes. 5, Larva ofAphodius(Chapuis and Candèze, Mém. Soc. Roy. Liège, 1853). 6, Larva ofLucanus. (Packard, Guide to the Study of Insects, Fig. 403).
On the other hand, in the present chapter it will be my object to show that the form of the larva depends very much on the conditions of its life. Thus, those larvæ which are internal parasites, whether in animalsor plants, are vermiform, as are those which live in cells, and depend on their parents for food. On the other hand, larvæ which burrow in wood have strong jaws and generally somewhat weak thoracic legs; whilst those which feed on leaves have the thoracic legs more developed, but less so than the carnivorous species. Now, the Hymenoptera, as a general rule, belong to the first category: the larvæ of the Ichneumons, &c., which live in animals,—those of the Cynipidæ, inhabiting galls,—and those of ants, bees, wasps, &c., which are fed by their parents, are fleshy, apodal grubs; though the remarkable fact that the embryos of bees in one stage of their development possess rudiments of thoracic legs which subsequently disappear, seems to show, not indeed that the larvæ of bees were ever hexapod, but that bees are descended from ancestors which had hexapod larvæ, and that the present apod condition of these larvæ is not original, but results from their mode of life.
On the other hand, the larvæ ofSirex(Fig.14) being wood-burrowers, possess well-developed thoracic legs. Again, the larvæ of the Tenthredinidæ, which feed upon leaves, closely resemble the caterpillars of Lepidoptera, even to the presence of abdominal pro-legs.
figs. 7-8Fig. 7, Larva ofBrachytarsus(Ratzeburg, Forst. Insecten). 8, Larva ofCrioceris(Westwood, loc. cit.).
Fig. 7, Larva ofBrachytarsus(Ratzeburg, Forst. Insecten). 8, Larva ofCrioceris(Westwood, loc. cit.).
The larvæ of most Coleoptera (Beetles) are active, hexapod, and more or less flattened: but those which live inside vegetable tissues, such as the weevils, are apod fleshy grubs, like those of Hymenoptera. Pl.II., Fig. 6, represents the larva of the nut-weevil,Balaninus(Pl.I., Fig. 6), and it will be seen that it closely resembles Pl.II., Fig. 5, which represents that of a fly (Anthrax), Pl.I., Fig. 5, and Pl.II., Figs. 7, 8, and 9, which represent respectively those of aCynipsor gall-fly (Pl.I., Fig. 7), an ant (Pl.I., Fig. 8), and wasp (Pl.I., Fig. 9). Nor isBalaninusthe only genus of Coleoptera which affords us examples of this fact. Thus in the genusScolytus(Pl.I., Fig. 4), the larvæ (Pl.II., Fig. 4), which, as already mentioned, feed on the bark of the elm, closely resemble those just described, as also do those ofBrachytarsus(Fig.7). On the other hand, the larvæ of certain beetles feed onleaves, like the caterpillars of Lepidoptera; thus that ofCrioceris Asparagi(Fig.8)—which, as its name denotes, feeds on the asparagus—closely resembles the larvæ of certain Lepidoptera, as for instance ofThecla spini. From this point of view the transformations of the genusSitaris(Pl.III., Fig. 4), which have been very carefully investigated by M. Fabre, are peculiarly interesting.12
Figs. 9-13Fig. 9, Larva ofSitaris humeralis(Fabre, Ann. des Sci. Nat., sér. 4, tome vii.). 10, Larva ofSitaris humeralis, in the second stage. 11, Larva ofSitaris humeralis, in the third stage. 12, Larva ofSitaris humeralis, in the fourth stage. 13, Pupa ofSitaris.
Fig. 9, Larva ofSitaris humeralis(Fabre, Ann. des Sci. Nat., sér. 4, tome vii.). 10, Larva ofSitaris humeralis, in the second stage. 11, Larva ofSitaris humeralis, in the third stage. 12, Larva ofSitaris humeralis, in the fourth stage. 13, Pupa ofSitaris.
The genusSitaris(a small beetle allied to Cantharis, the blister-fly, and toMeloë, the oil-beetle) is parasitic on a kind of Bee (Anthophora), which excavates subterranean galleries, each leading to a cell. The eggs of theSitaris, which are deposited at the entrance of these galleries, are hatched at the end of September or beginning of October; and M. Fabre notunnaturally expected that the young larvæ, which are active little creatures with six serviceable legs (Fig.9), would at once eat their way into the cells of the Anthophora. No such thing: till the month of April following they remain without leaving their birthplace, and consequently without food; nor do they in this long time change either in form or size. M. Fabre ascertained this, not only by examining the burrows of theAnthophoras, but also by direct observation of some young larvæ kept in captivity. In April, however, his captives at last awoke from their long lethargy, and hurried anxiously about their prisons. Naturally inferring that they were in search of food, M. Fabre supposed that this would consist either of the larvæ or pupæ of the Anthophora, or of the honey with which it stores its cell. All three were tried without success. The first two were neglected, and the larvæ, when placed on the latter, either hurried away, or perished in the attempt, being evidently unable to deal with the sticky substance. M. Fabre was in despair: “Jamais expérience,” he says, “n’a éprouvé pareille déconfiture. Larves, nymphes, cellules, miel, je vous ai tous offert; que voulez-vous donc, bestioles maudites?” The first ray of light came to him from our countryman, Newport, who ascertained that a small parasite found by Léon Dufour on one of the wild bees, and named by him Triungulinus, was, in fact, the larva ofMeloë;. The larvæ ofSitarismuch resembled Dufour’s Triungulinus; and acting on this hint, M. Fabre examined many specimens of Anthophora, and found on them at last the larvæ of hisSitaris. The males of Anthophora emerge from thepupæ sooner than the females, and M. Fabre ascertained that, as they come out of their galleries, the littleSitarislarvæ fasten upon them. Not, however, for long: instinct teaches them that they are not yet in the straight path of development; and, watching their opportunity, they pass from the male to the female bee. Guided by these indications, M. Fabre examined several cells of the Anthophora: in some, the egg of the Anthophora floated by itself on the surface of the honey; in others, on the egg, as on a raft, sat the still more minute larva of theSitaris. The mystery was solved. At the moment when the egg is laid theSitarislarva springs upon it. Even while the poor mother is carefully fastening up her cell, her mortal enemy is beginning to devour her offspring: for the egg of the Anthophora serves not only as a raft, but as a repast. The honey which is enough for either, would be too little for both; and theSitaris, therefore, at its first meal, relieves itself from its only rival. After eight days the egg is consumed, and on the empty shell theSitarisundergoes its first transformation, and makes its appearance in a very different form, as shown in Fig.10.
The honey which was fatal before is now necessary; the activity which before was necessary is now useless; consequently, with the change of skin, the active, slim larva changes into a white, fleshy grub, so organized as to float on the surface of the honey, with the mouth beneath, and the spiracles above the surface: “grâce à l’embonpoint du ventre,” says M. Fabre, “la larve est à l’abri de l’asphyxie.” In this state it remains until the honey is consumed; then theanimal contracts, and detaches itself from its skin, within which the further transformations take place. In the next stage, which M. Fabre calls the pseudo-chrysalis (Fig.11), the larva has a solid corneous envelope and an oval shape; and in its colour, consistency, and immobility reminds one of a Dipterous pupa. The time passed in this condition varies much. When it has elapsed, the animal moults again, again changes its form, and assumes that shown in Fig.12; after this it becomes a pupa (Fig.13) without any remarkable peculiarities. Finally, after these wonderful changes and adventures, in the month of August the perfectSitaris(Pl.III., Fig. 4) makes its appearance.
On the other hand, there are cases in which larvæ diverge remarkably from the ordinary type of the group to which they belong, without, as it seems in our present imperfect state of information, any sufficient reason.
Thus the ordinary type of Hymenopterous larva, as we have already seen, is a fleshy apod grub; although those of the leaf-eating and wood-boring groups, Tenthredinidæ and Siricidæ (Fig.14), are caterpillars, more or less closely resembling those of Lepidoptera. There is, however, a group of minute Hymenoptera, the larvæ of which reside within the eggs or larvæ of other insects. It is difficult to understand why these larvæ should differ from those of Ichneumons, which are also parasitic Hymenoptera, and should be, as will be seen by the accompanying figures, of such remarkable and grotesque forms. The first known of these curious larvæ was observed by De Filippi,13who,having collected some of the transparent eggs of a small Beetle (Rhynchites betuleti), to his great surprise found more than half of them attacked by a parasite, which proved to be the larva of a minute Hymenopterous insect belonging to the Pteromalidæ. Fig.15shows the egg of the Beetle, with the parasitic larva, which is represented on a larger scale in Fig.16.
Figs. 14-16Fig. 14, Larva ofSirex(Westwood, loc. cit.). 15, Egg ofRhynchites, showing the parasitic Larva in the interior. 16, the parasitic Larva more magnified.
Fig. 14, Larva ofSirex(Westwood, loc. cit.). 15, Egg ofRhynchites, showing the parasitic Larva in the interior. 16, the parasitic Larva more magnified.
More recently this group has been studied by M. Ganin,14who thus describes the development ofPlatygaster. The egg, as in allied Hymenopterous families, for instance inCynips, is elongated and club-shaped (Fig.17). After a while a large nucleated cell appears in the centre (Fig.18). This nucleated cell divides (Fig.19) and subdivides. The outermost cells continue the same process, thus forming an outer investing layer. The central, on the contrary, enlarges considerably, and develops within itself a number of daughter cells (Figs.20 and 21), which gradually form a mulberry-like mass, thus giving rise to the embryo (Fig.22).
Figs. 17-22Fig. 17, Egg ofPlatygaster(after Ganin). 18, Egg ofPlatygastershowing the central cell. 19, Egg ofPlatygasterafter the division of the central wall. 20, Egg ofPlatygastermore advanced. 21, Egg ofPlatygastermore advanced. 22, Egg ofPlatygastershowing the rudiment of the embryo.
Fig. 17, Egg ofPlatygaster(after Ganin). 18, Egg ofPlatygastershowing the central cell. 19, Egg ofPlatygasterafter the division of the central wall. 20, Egg ofPlatygastermore advanced. 21, Egg ofPlatygastermore advanced. 22, Egg ofPlatygastershowing the rudiment of the embryo.
Ganin met with the larvæ ofPlatygasterin those of a small gnat,Cecidomyia. Sometimes as many as fifteen parasites occurred in one gnat, but as a rule only one of these attained maturity. The three species ofPlatygasterdiffer considerably in form, as shown in Figs.23-25. They creep about within the larva ofCecidomyiaby means of the strong hooked feet,kf, somewhat aided by movements of the tail. They possess a mouth, stomach, and muscles, but the nervous, vascular, and respiratory systems do not make their appearance until later. After some time the larva (Fig.23) changes its skin, assuming the form represented in Fig.26. In this moult the last abdominal segment of the first larva is entirely thrown off: not merely the outer skin, as in the case of the other segments, but alsothe hypodermis and the muscles. This larva, as will be seen by the figure, resembles a barrel or egg in form, and is .870 mm. in length, the external appendages having disappeared, and the segments beingindicated only by the arrangement of the muscles.slkfis the œsophagus leading into a wide stomach which occupies nearly the whole body,gsaeis the rudiment of the supra-œsophageal ganglia,bsmthe ventral nervous cords. The ventral nervous mass has the form of a broad band, with straight sides; it consists of embryonal cells, and remains in this undeveloped condition during the whole larval state.