Fig. 588.—Mandibles (md) of Micropteryx purpuriella, enlarged.—Author del. A, pupal head of a hydropsychid caddis-fly, showing the large mandibles.—After Reaumur, from Miall.
In Myrmeleon the pupa pushes its way half out of the cocoon, and then remains, while the imago ruptures the skin and escapes (Fig. 589, a).
Thus in the Neuroptera and Trichoptera we have already established the more fundamental methods of escape from the cocoon, which we see carried out in various ways in the more generalized or primitive Lepidoptera.
The most primitive method in the Lepidoptera of escaping from the cocoon seems to be that of Micropteryx.
Fig. 589.—Larva of Myrmeleon with (a) its cocoon and cast pupa-skin.
“In this genus,” says Chapman, “though it is nominally the pupa that escapes from the cocoon, it is in reality still the imago, the imago clothed in the effete pupal skin. To rupture the cocoon it uses not its own jaws, but those of the pupal skin, energizing them, however, in some totally different way from ordinary direct muscular action, their movements being the result of the vermicular movements of the pupa, acting probably by fluid pressure on the articular structure of the jaws, by some arrangement not altogether different perhaps from the frontal sac of the higher Diptera. In the Micropteryges the jaws of the pupa not only rupture the cocoon, but appear to be the most active agents in dragging the pupa through the opening in the cocoon and through any superincumbent earth, being merely assisted by the vermicular action of the abdominal segments, and we find in accordance with this circumstance that the pupal envelope is still very thin and delicate, and has little or no hardening or roughness by which to obtain a leverage against the walls of the channel of escape.” (Trans. Ent. Soc. London, 1896, pp. 570, 571.)
Fig. 590.—Pupa of Talæporia: a, cocoon-cutter; with vestiges of four pairs of abdominal legs, and the cremaster.
Some sort of a beak or hard process, more or less developed, according to Chapman, adapted for breaking open the cocoon exists in nearly all the Lepidoptera with incomplete pupæ (pupæ incompletæ), except the limacodid and nepticulid section. “In all these instances the pupa emerges from the cocoon precisely as in the Micropteryges, that is, the moth it really is that emerges, but does so encased in the pupal skin. To achieve this object, it seems to have been found most efficient to have three, four, or five abdominal segments capable of movement, but to have the terminal sections (segments) soldered together.”