The amount of fat is sometimes very great. Newport removed from the larva of Cossus ligniperda 42 grains of fat, being more than one-fourth of the whole weight of the insect, he adds that the supply is soon nearly exhausted during the rapid development of the reproductive organs, “since, when these have become perfected, the quantity that remains is very inconsiderable.”
Although the larval skin of a lepidopterous insect is suddenly cast off, the pupa quickly emerging front it, yet there are several intermediate stages, all graduating into each other. If a caterpillar of a Clisiocampa, which, as we have observed, is much shortened and thickened a day or two before changing to a pupa, is hardened in alcohol and the larval skin is stripped off, the semipupa (pro-nymph, pro-pupa of different authors) is found to be in different stages of development, and the changes of the mouth-parts are interesting, though not yet sufficiently studied.
Newport attributes the great enlargement and changes in the shape of the thoracic segments of the larva of Vanessa urticæ at this time, to the contraction or shortening of the muscles of the interior of those segments, “which are repeatedly slowly extended and shortened, as if the insect were in the act of laborious respiration.” This, he adds, generally takes place at short intervals during the two hours immediately preceding the change to the pupa, and increases in frequency as that period approaches. He thus describes the mode of moulting the larval skin: “When the period has arrived, the skin bursts along the dorsal part of the 3d segment, or mesothorax, and is extended along the 2d and 4th, while the coverings of the head separate into three pieces. The insect then exerts itself to the utmost to extend the fissure along the segment of the abdomen, and, in the meantime, pressing its body through the opening, gradually withdraws its antennæ and legs, while the skin, by successive contortions of the abdomen, is slipped backwards, and forced towards the extremity of the body, just as a person would slip off his glove or his stocking. The efforts of the insect to get entirely rid of it are then very great; it twirls itself in every direction in order to burst the skin, and, when it has exerted itself in this manner for some time, twirls itself swiftly, first in one direction, then in the opposite, until at last the skin is broken through and falls to the ground, or is forced to some distance from it. The new pupa then hangs for a few seconds at rest, but its change is not yet complete. The legs and antennæ, which when withdrawn from the old skin were disposed along the under surface of the body, are yet separate, and do not adhere together as they do a short time afterwards. The wings are also separate and very small. In a few seconds the pupa makes several slow, but powerful, respiratory efforts; during which the abdominal segments become more contracted along their under surface, and the wings are much enlarged and extended along the lateral inferior surface of the body, while a very transparent fluid, which facilitated the slipping off of the skin, is now diffused among the limbs, and when the pupa becomes quiet dries, and unites the whole into one compact covering.”
The changes in the head and mouth-parts.—The changes of form from the active mandibulate caterpillar to the quiescent pupa, and then to the adult butterfly, are, as we have seen, in direct adaptation to their changed habits and surroundings, and they differ greatly in details in insects of different orders. In many Lepidoptera and certain Diptera the pupa and imago are without the mandibles of the larva, and, instead, the 1st maxillæ in the former order, and the 2d maxillæ in the latter, are highly developed and specialized. The changes in the shape of the head, with the antennæ, the latter rudimentary in the larvæ of the two orders named, are noteworthy, and will be referred to under those orders. The same may be said of the thorax with the legs and wings, and the abdomen with the ovipositor. Every part of the body undergoes a profound change, though in the Coleoptera, Trichoptera, and the more generalized and primitive Diptera, each segment and appendage of the larva are directly transformed into the corresponding parts of the pupa, and subsequently of the imago. We shall see, however, beyond, that this general statement does not apply to the Hymenoptera, in which there is a process of cephalization or transfer of parts headward, peculiar to that order.
Fig. 598.—Internal organs of Sphinx ligustri: 1, head; 2–4, thoracic, 5–13, abdominal segments; V, fore-, M, mid-, E, hind-intestine; gs, brain; gi, infraœsophageal ganglion; n, ventral ganglion; vm, urinary tubes; c, heart; G, testis; o, œsophagus; a, anus; m, alary muscles of the heart.
Fig. 599.—Pupa of the same.
Fig. 600.—Imago of the same.—This and Figs. 598 and 599 after Newport, from Gegenbaur.