On the Origin and Metamorphoses of Insects Part 3

CHAPTER II.

_THE INFLUENCE OF EXTERNAL CONDITIONS ON THE FORM AND STRUCTURE OF LARVae._

The facts recapitulated briefly in the preceding chapter show, that the forms of insect larvae depend greatly on the group to which they belong.

Thus the same tree may harbour larvae 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 larvae extremely similar in form, yet very different in habits. Those, for instance, of the common c.o.c.kchafer (Fig.

1) feed on the roots of gra.s.s; those of _Cetonia aurata_ (Fig. 2) inhabit ants' nests; the larvae of the genus _Trox_ (Fig. 3) are found on dry animal substances; of _Oryctes_ (Fig. 4) in tan-pits; of _Aphodius_ (Fig. 5) in dung; of _Luca.n.u.s_ (the stag-beetle, Fig. 6) in wood.

[Ill.u.s.tration: FIG. 1, Larva of the c.o.c.kchafer (_Melolontha_).

(Westwood, Int. to the Modern Cla.s.sification of Insects, vol. i. p.

194.). 2, Larva of _Cetonia_. 3, Larva of _Trox_. 4, Larva of _Oryctes_.

5, Larva of _Aphodius_ (Chapuis and Candeze, Mem. Soc. Roy. Liege, 1853). 6, Larva of _Luca.n.u.s_. (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 larvae which are internal parasites, whether in animals or plants, are vermiform, as are those which live in cells, and depend on their parents for food. On the other hand, larvae 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 larvae of the Ichneumons, &c., which live in animals,-those of the Cynipidae, 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 larvae of bees were ever hexapod, but that bees are descended from ancestors which had hexapod larvae, and that the present apod condition of these larvae is not original, but results from their mode of life.

On the other hand, the larvae of _Sirex_ (Fig. 14) being wood-burrowers, possess well-developed thoracic legs. Again, the larvae of the Tenthredinidae, which feed upon leaves, closely resemble the caterpillars of Lepidoptera, even to the presence of abdominal pro-legs.

[Ill.u.s.tration: FIG. 7, Larva of _Brachytarsus_ (Ratzeburg, Forst.

Insecten). 8, Larva of _Crioceris_ (Westwood, loc. cit.).]

The larvae 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 a _Cynips_ or gall-fly (Pl. I., Fig. 7), an ant (Pl. I., Fig. 8), and wasp (Pl. I., Fig. 9).

Nor is _Balaninus_ the only genus of Coleoptera which affords us examples of this fact. Thus in the genus _Scolytus_ (Pl. I., Fig. 4), the larvae (Pl. II., Fig. 4), which, as already mentioned, feed on the bark of the elm, closely resemble those just described, as also do those of _Brachytarsus_ (Fig. 7). On the other hand, the larvae of certain beetles feed on leaves, like the caterpillars of Lepidoptera; thus that of _Crioceris Asparagi_ (Fig. 8)-which, as its name denotes, feeds on the asparagus-closely resembles the larvae of certain Lepidoptera, as for instance of _Thecla spini_. From this point of view the transformations of the genus _Sitaris_ (Pl. III., Fig. 4), which have been very carefully investigated by M. Fabre, are peculiarly interesting.[12]

[Ill.u.s.tration: FIG. 9, Larva of _Sitaris numeralis_ (Fabre, Ann. des Sci.

Nat., ser. 4, tome vii.). 10, Larva of _Sitaris humeralis_, in the second stage. 11, Larva of _Sitaris humeralis_, in the third stage. 12, Larva of _Sitaris humeralis_, in the fourth stage. 13, Pupa of _Sitaris_.]

The genus _Sitaris_ (a small beetle allied to Cantharis, the blister-fly, and to _Meloe_, the oil-beetle) is parasitic on a kind of Bee (Anthophora), which excavates subterranean galleries, each leading to a cell. The eggs of the _Sitaris_, which are deposited at the entrance of these galleries, are hatched at the end of September or beginning of October; and M. Fabre not unnaturally expected that the young larvae, 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 the _Anthophoras_, but also by direct observation of some young larvae 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 larvae or pupae 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 larvae, 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 experience," he says, "n'a eprouve pareille deconfiture. 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 Leon Dufour on one of the wild bees, and named by him Triungulinus, was, in fact, the larva of _Meloe_. The larvae of _Sitaris_ much resembled Dufour's Triungulinus; and acting on this hint, M. Fabre examined many specimens of Anthophora, and found on them at last the larvae of his _Sitaris_. The males of Anthophora emerge from the pupae sooner than the females, and M. Fabre ascertained that, as they come out of their galleries, the little _Sitaris_ larvae 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 pa.s.s 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 the _Sitaris_. The mystery was solved. At the moment when the egg is laid the _Sitaris_ larva 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 the _Sitaris_, therefore, at its first meal, relieves itself from its only rival. After eight days the egg is consumed, and on the empty sh.e.l.l the _Sitaris_ undergoes 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: "grace a l'embonpoint du ventre," says M. Fabre, "la larve est a l'abri de l'asphyxie." In this state it remains until the honey is consumed; then the animal 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 pa.s.sed in this condition varies much.

When it has elapsed, the animal moults again, again changes its form, and a.s.sumes 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 perfect _Sitaris_ (Pl.

III., Fig. 4) makes its appearance.

On the other hand, there are cases in which larvae 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, Tenthredinidae and Siricidae (Fig. 14), are caterpillars, more or less closely resembling those of Lepidoptera. There is, however, a group of minute Hymenoptera, the larvae of which reside within the eggs or larvae of other insects. It is difficult to understand why these larvae 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 larvae was observed by De Filippi,[13] who, 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 Pteromalidae. Fig. 15 shows the egg of the Beetle, with the parasitic larva, which is represented on a larger scale in Fig. 16.

[Ill.u.s.tration: FIG. 14, Larva of _Sirex_ (Westwood, loc. cit.). 15, Egg of _Rhynchites_, showing the parasitic Larva in the interior. 16, the parasitic Larva more magnified.]

More recently this group has been studied by M. Ganin,[14] who thus describes the development of _Platygaster_. The egg, as in allied Hymenopterous families, for instance in _Cynips_, 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 ma.s.s, thus giving rise to the embryo (Fig. 22).

[Ill.u.s.tration: FIG. 17, Egg of _Platygaster_ (after Ganin). 18, Egg of _Platygaster_ showing the central cell. 19, Egg of _Platygaster_ after the division of the central wall. 20, Egg of Platygaster more advanced. 21, Egg of _Platygaster_ more advanced. 22, Egg of Platygaster showing the rudiment of the embryo.]

Ganin met with the larvae of _Platygaster_ in 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 of _Platygaster _differ considerably in form, as shown in Figs. 23-25. They creep about within the larva of _Cecidomyia_ by 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, a.s.suming 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 also the 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 being indicated only by the arrangement of the muscles. _slkf_ is the sophagus leading into a wide stomach which occupies nearly the whole body, _gsae_ is the rudiment of the supra-sophageal ganglia, _bsm_ the ventral nervous cords. The ventral nervous ma.s.s 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.

[Ill.u.s.tration: FIG. 23, Larva of _Platygaster_ (after Ganin)-_mo_, mouth; _a_, antenna; _kf_, hooked feet; _z_, toothed process; _lfg_, lateral process; _f_, branches of the tail. 24, Larva of another species of _Platygaster_. The letters indicate the same parts as in the preceding figure. 25, Larva of a third species of _Platygaster_. The letters indicate the same parts as in the preceding figures. 26, Larva of _Platygaster_ in the second stage-_mo_, mouth; _slkf_, sophagus; _gsae_, supra-sophageal ganglion; _lm_, muscles; _bsm_, nervous system; _ga_, _gh_, rudiments of the reproductive glands. 27, Larva of _Platygaster_ in the third stage-_mo_, mouth; _md_, mandibles; _gsae_, supra-sophageal ganglion; _slk_, sophagus; _ag_, ducts of the salivary glands; _bnm_, ventral nervous system; _sp_, salivary glands; _msl_, stomach; _im_, imaginal discs; _tr_, tracheae; _fk_, fatty tissue; _ed_, intestine; _ga_, rudiments of reproductive organs; _ew_, wider portion of intestine; _ao_, posterior opening.]

At the next moult the larva enters its third state, which, as far as the external form (Fig. 27) is concerned, differs from the second only in being somewhat more elongated. The internal organs, however, are much more complex and complete. The tracheae have made their appearance, and the mouth is provided with a pair of mandibles. From this point the metamorphoses of _Platygaster_ do not appear to differ materially from those of other parasitic Hymenoptera.

An allied genus, _Polynema_, has also very curious larvae. The perfect insect is aquatic in its habits, swimming by means of its wings; flying, if we may say so, under water.[15] It lays its eggs inside those of Dragon-flies; and the embryo, as shown in Fig. 28, has the form of a bottle-shaped ma.s.s of undifferentiated embryonal cells, covered by a thin cuticle, but without any trace of further organization. Protected by the egg-sh.e.l.l of the Dragon-fly, and bathed in the nouris.h.i.+ng fluid of the Dragon-fly's egg, the young _Polynema_ imbibes nourishment through its whole surface, and increases rapidly in size. The digestive ca.n.a.l gradually makes its appearance; the cellular ma.s.s forms a new skin beneath the original cuticle, distinctly divided into segments, and provided with certain appendages. After a while the old cuticle is thrown off, and the larva gradually a.s.sumes the form shown in Fig. 29.

The subsequent metamorphoses of _Polynema_ offer no special peculiarities.

[Ill.u.s.tration: FIG. 28, Embryo of _Polynema_ (after Ganin). 29, Larva of _Polynema_-_asch_, rudiments of the antenna; _flsch_, rudiments of the wings; _bsch_, rudiments of the legs; _vfg_. lateral projections; _gsch_, rudiments of the ovipositor; _fk_, fatty tissue.]

From these facts-and, if necessary, many more of the same nature might have been brought forward-it seems to me evident that while the form of any given larva depends to a certain extent on the group of insects to which it belongs, it is also greatly influenced by the external conditions to which it is subjected; that it is a function of the life which the larva leads and of the group to which it belongs.

The larvae of insects are generally regarded as being nothing more than immature states-as stages in the development of the egg into the imago; and this might more especially appear to be the case with those insects in which the larvae offer a general resemblance in form and structure (excepting of course so far as relates to the wings) to the perfect insect. Nevertheless we see that this would be a very incomplete view of the case. The larva and pupa undergo changes which have no relation to the form which the insect will ultimately a.s.sume. With a general tendency to this goal, as regards size and the development of the wings, there are coincident other changes having reference only to existing wants and condition. Nor is there in this, I think, anything which need surprise us. External circ.u.mstances act on the insect in its preparatory states, as well as in its perfect condition. Those who believe that animals are susceptible of great, though gradual, change through the influence of external conditions, whether acting, as Mr.

Darwin has suggested, through natural selection, or in any other manner, will see no reason why these changes should be confined to the mature animal. And it is evident that creatures which, like the majority of insects, live during the successive periods of their existence in very different circ.u.mstances, may undergo considerable changes in their larval organization, in consequence of forces acting on them while in that condition; not, indeed, without affecting, but certainly without affecting to any corresponding extent, their ultimate form.

I conclude, therefore, that the form of the larva in insects, whenever it departs from the hexapod _Campodea_ type, has been modified by the conditions under which it lives. The external forces acting upon it are different from those which affect the mature form; and thus changes are produced in the young which have reference to its immediate wants, rather than to its final form.

And, lastly, as a consequence, that metamorphoses may be divided into two kinds, developmental and adaptional or adaptive.

CHAPTER III.

_ON THE NATURE OF METAMORPHOSES._

In the preceding chapters we have considered the life history of insects after they have quitted the egg; but it is obvious that to treat the subject in a satisfactory manner we must take the development as a whole, from the commencement of the changes in the egg, up to the maturity of the animal, and not suffer ourselves to be confused by the fact that insects leave the egg in very different stages of embryonal development. For though all young insects when they quit the egg are termed "larvae," whatever their form may be (the case of the so-called Pupipara not const.i.tuting a true exception), still it must be remembered that some of these larvae are much more advanced than others. It is evident that the larva of a fly, as regards its stage of development, corresponds in reality neither with that of a moth nor with that of a gra.s.shopper. The maggots of flies, in which the appendages of the head are rudimentary, belong to a lower grade than the grubs of bees, &c., which have antennae, mandibles, maxillae, labrum, labium, and, in fact, all the mouth parts of a perfect insect.

The caterpillars of Lepidoptera are generally cla.s.sed with the vermiform larva of Diptera and Hymenoptera, and contrasted with those of Orthoptera, Hemiptera, &c.; but, in truth, the possession of thoracic legs places them, together with the similar larvae of the Tenthredinidae, on a decidedly higher level. Thus, then, the period of growth (that in which the animal eats and increases in size) occupies sometimes one stage in the development of an insect, sometimes another; sometimes, as for instance in the case of _Chloeon_, it continues through more than one; or, in other words, growth is accompanied by development. But, in fact, the question is even more complicated than this. It is not only that the larvae of insects at their birth offer the most various grades of development, from the grub of a fly to the young of a gra.s.shopper or a cricket; but that, if we were to cla.s.sify larvae according to their development, we should have to deal, not with a simple case of gradations only, but with a series of gradations, which would be different according to the organ which we took as our test.

Apart, however, from the adaptive changes to which special reference was made in the previous chapter, the differences which larvae present are those of gradation, not of direction. The development of a gra.s.shopper does not pursue a different course from that of a b.u.t.terfly, but the embryo attains a higher state before quitting the egg in the former than in the latter: while in most Hymenoptera, as for instance in Bees, Wasps, Ants, &c., the young are hatched without thoracic appendages; in the Orthoptera, on the contrary, the legs are fully developed before the young animal quits the egg.

Prof. Owen,[16] indeed, goes so far as to say that the Orthoptera and other h.o.m.omorphous insects are, "at one stage of their development, apodal and acephalous larvae, like the maggot of the fly; but instead of quitting the egg in this stage, they are quickly transformed into another, in which the head and rudimental thoracic feet are developed to the degree which characterizes the hexapod larvae of the _Carabi_ and _Petalocera_."

[Ill.u.s.tration: FIG. 30, Egg of _Phryganea_ (Mystacides)-_A_, mandibular segment; _C_ to _C_5, maxillary, l.a.b.i.al, and three thoracic segments; _D_, abdomen (after Zaddach). 31, Egg of _Phryganea_ somewhat more advanced-_b_, mandibles; _c_, maxillae; _cfs_, rudiments of the three pairs of legs.]

I quite believe that this may have been true of such larvae at an early geological period, but the fact now appears to be, so far at least as can be judged from the observations yet recorded, that the legs of those larvae which leave the egg with these appendages generally make their appearance before the body-walls have closed, or the internal organs have approached to completion. Indeed, when the legs first appear, they are merely short projections, which it is not always easy to distinguish from the segments themselves. It must, however, be admitted, that the observations are neither so numerous, nor in most cases so full, as could be wished.

Fig. 30 represents an egg of a May-fly (_Phryganea_), as represented by Zaddach in his excellent memoir,[17] just before the appearance of the appendages. It will be seen that a great part of the yolk is still undifferentiated, that the side walls are incomplete, the back quite open, and the segments merely indicated by undulations. This stage is rapidly pa.s.sed through, and Zaddach only once met with an egg in this condition; in every other specimen which had indications of segments, the rudiments of the legs had also made their appearance, as in Fig. 31, which, however, as will be seen, does not in other respects show much advance on Fig. 30.

Again in _Aphis_, the embryology of which has been so well worked out by Huxley,[18] the case is very similar, although the legs are somewhat later in making their appearance. When the young was 1/140th of an inch in length, he found the cephalic portion of the embryo beginning, he says, "to extend upwards again over the anterior face of the germ, so as to const.i.tute its anterior and a small part of its superior wall. This portion is divided by a median fissure into two lobes, which play an important part in the development of the head, and will be termed the 'procephalic lobes.' I have already made use of this term for the corresponding parts in the embryos of Crustacea. The rudimentary thorax presents traces of a division into three segments; and the dorso-lateral margins of the cephalic blastoderm, behind the procephalic lobes, have a sinuous margin. It is in embryos between this and 1/100th of an inch in length, that the rudiments of the appendages make their appearance; and by the growth of the cephalic, thoracic, and abdominal blastoderm, curious changes are effected in the relative position of those regions."

In _Chrysopa oculata_, one of the Hemerobiidae, Packard has described[19]

and figured a stage in which the body segments have made their appearance, but in which he says "there are no indications of limbs. The primitive band is fully formed, the protozorites being distinctly marked, the transverse impressed lines indicating the primitive segments being distinct, and the median furrow easily discerned." Here also, again, the dorsal walls are incomplete, and the internal organs as yet unformed.