This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
In order fully to understand the circumstances connected with this part of our subject, it is necessary to premise that the outer integument of most larvae is of a dense corneous texture, coriaceous in some parts, but quite hard and horny in others. In the second place, it is but very slightly extensible; and, moreover, as is always the case with epidermic structures, it is not permeated by any vascular apparatus, and consequently is absolutely incapable of growth when once formed. This epidermis encases every portion of the larva: the body, the legs, the antennae, the jaws, and all external organs are closely invested with a cuticular envelope, such as, from its want of extensibility, would form an insuperable obstacle to development, were there not some extraordinary provision made to meet the necessity of the case. The plan adopted is, to cast off at intervals the old cuticle by a process termed moulting - an operation which is repeated several times during the life of the insect in its larva condition, and is accomplished in the following manner: - The caterpillar becomes for a few days sluggish and inactive, leaves off eating, and endeavours to conceal itself from observation.
The skin, or more properly the cuticle, becomes loosened from the subjacent tissues, and soon a rent appears upon the back of the animal, which gradually enlarges in a longitudinal direction, and the imprisoned insect, after a long series of efforts, at length succeeds in extricating itself from its old covering, and appears in a new skin of larger dimensions than the one it replaces, which, however, in all other particulars it closely resembles. With the old epidermis the larva throws off all external appendages to the cuticle: the horny coverings of the jaws, the corneae of the eyes, the cases of the claws, are all removed; and many writers have even found attached to the exuviae an epidermic pellicle that had formed a lining to the rectum, and delicate prolongations of the cuticle derived from the interior of the larger ramifications of the air-tubes. Absurd, indeed, have been the explanations given by various writers of the nature of the process under consideration. Swammerdam and Bonnet, nay, even our own illustrious entomologists Kirby and Spence, believed that even at the birth of the caterpillar all these skins existed ready-formed one beneath the other, and that the most external being removed at intervals displayed in succession the skins placed underneath.
Surely the advocates of this extraordinary theory could scarcely have reflected upon the real object of the moults in question (namely, to provide a succession of larger coverings proportioned to the continually increasing bulk of the larva) when they advocated this strange doctrine, alike at variance with observation and sound physiological principles. The epidermis and all cuticular structures are mere secretions from the subjacent cutis or true skin; and it can be no more necessary to suppose the pre-exist-ence of so many skins in order to explain the moults of a larva, than to imagine that because, when in our own persons the cuticle is removed by the application of a blister, a new layer of epidermis is again and again produced, man should possess as many skins, one beneath the other. Nothing, in fact, can be more simple and free from the miraculous than the whole process: at certain periods, when the old cuticle has grown too small for the rapidly-enlarging dimensions of the insect, it becomes gradually loosened and separated from the vascular and living skin or cutis by which it was originally secreted; and, a new secretion of corneous matter taking place, a fresh and more extensive layer of cuticle is slowly formed, and then the old, dry and dead epidermis, being quite detached, is split by the exertions of the larva, and the newly-secreted layer placed beneath it appears.
When the old skin is at length completely thrown off, the newly-formed one soon hardens by exposure, and the re-clothed caterpillar assumes again its former activity and habits.
Fig. 179. Process whereby a chrysalis becomes suspended by the tail. (See note, p. 354).
Fig. 180. Head of a Caterpillar from beneath: a a, antennae; b b, horny jaws; c, thread of silk emitted from the conical fusulus, which is seen surrounded by four rudimentary palpi pupa now lays hold of the old skin, nipping it between the rings of the abdomen, and hanging in this posture inserts the apex of the tail, which is covered with hooks for the purpose, into the silk previously deposited, and thus remains fixed in safety (d).
(923). Neither is the change from the larva to the pupa or chrysalis less easily explained, although regarded by our forefathers as being so mysterious and astonishing a phenomenon. According to the hypothesis above alluded to, after removing three or four skins in the embryo larva, the anatomist ought to have arrived at the totally different pupa-case ready-formed, and only waiting for the removal of the coats above it to exhibit its characteristic form. Leaving, however, such visionary notions, let us examine the real nature of this portion of the metamorphosis. The reader will bear in mind that, whatever the form of the exterior or epidermic crust, it is merely a dead and extra-vascular secretion, unchangeable when once deposited. But the living skin or cutis, beneath it, is, during the whole process of the metamorphosis, undergoing great and important changes - increasing only in size during the larva condition, but when perfectly organized, developing itself at different points, and expanding into variously-shaped organs which did not previously exist.