Fig. 1, Larva of the Cockchafer (Melolontha). (Westwood, Int. to the Modern Classification of Insects, vol. i. p. 194.). 2, Larva of Cetonia. 3, Larva of Trox. 4, Larva of Oryctes. 5, Larva of Aphodius (Chapuis and Candèze, Mém. Soc. Roy. Liège, 1853). 6, Larva of Lucanus. (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 animals or 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 of Sirex (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.
Fig. 7, Larva of Brachytarsus (Ratzeburg, Forst. Insecten). 8, Larva of Crioceris (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 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 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 of Brachytarsus (Fig. [7]). On the other hand, the larvæ 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 larvæ 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]
Fig. 9, Larva of Sitaris humeralis (Fabre, Ann. des Sci. Nat., sér. 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.