The growth is most rapid after the last moult. “Thus a larva of Sphinx ligustri, which at its last change weighed only about 19 to 20 grains, at the expiration of eight days, when it was fully grown, weighed nearly 120 grains.” (Newport.)
d. The process of moulting (ecdysis)
Insects periodically shed the exoskeleton, together with the chitinous lining of their internal organs of ectodermal origin, which thus sloughed off are called the exuvia. The process in the locust has been described by Riley.[[95]] It occupies from half to three-quarters of an hour (Fig. 565). This process has naturally, from the ease with which it can be observed, been most frequently examined in the Lepidoptera, though careful and detailed observations of the inner and outer changes are still greatly needed, especially in other orders. In the caterpillar of most moths, especially one of the more generalized bombycine moths, on slipping out of its egg-shell the head is of enormous size as compared with the body, but the latter soon fills out after the creature has eaten a few hours; the head, of course, does not during this time increase in size, and the larvæ of different instars may be exactly distinguished, as Dyar has shown, by the measurements of the head.
Fig. 565.—Process of moulting from nymph to imago in the locust (M. spretus): a, nymph with skin just split on the back; b, the imago drawing itself out, at c, nearly free; d, the imago, with wings expanded; e, the same with all parts perfect.—After Riley.
Before the caterpillar moults, it stops feeding, and the head is now small compared with the body; the head of the second instar is now large, situated partly under the much-swollen prothoracic segment, and pushes the head of the first instar forward.
Newport has well described the mode of shedding the skin in Sphinx ligustri, and his detailed description will apply to most lepidopterous larvæ.
The whole body is wrinkled and contracted in length, and there are occasionally powerful contractions and twitchings of its entire body; the skin becomes dry and shrivelled, and is gradually separated from the new and very delicate one of the next instar beneath. After several powerful efforts of the larva the old skin cracks along the middle of the dorsal surface of the mesothoracic segment, and by repeated efforts the fissure is extended into the 1st and 3d segment, while the covering of the head divides along the vertex and on each side of the clypeus. “The larva then gradually presses itself through the opening, withdrawing first its head and thoracic legs, and subsequently the remainder of its body, slipping off the skin from behind like the finger of a glove. This process, after the skin has once been ruptured, seldom lasts more than a few minutes. When first changed the larva is exceedingly delicate, and its head, which does not increase in size until it again changes its skin, is very large in proportion to the rest of the body.” (Art. Insecta, etc.)
Trouvelot’s account is more detailed and an advance on that of Newport’s view. He explicitly states, and we know that he was a very close observer, that the old skin is detached by “a fluid which circulates between it and the worm.” His account is as follows: The polyphemus worm, like all other silkworms, changes its skin five times during its larval life. The moulting takes place at regular periods, which comes around about every 10 days for the first four moultings, while about 20 days elapse between the fourth and fifth moulting. The worm ceases to eat for a day before moulting, and spins some silk on the vein of the under surface of a leaf; it then secures the hooks of its hind legs in the texture it has thus spun, and there remains motionless; soon after, through the transparency of the skin of the neck, can be seen a second head larger than the first, belonging to the larva within. The moulting generally takes place after four o’clock in the afternoon; a little before this time the worm holds its body erect, grasping the leaf with the two pairs of hind legs only; the skin is wrinkled and detached from the body by a fluid which circulates between it and the worm; two longitudinal bands are seen on each side, produced by a portion of the lining of the spiracles, which at this moment have been partly detached; meanwhile the contractions of the worm are very energetic, and by them the skin is pulled off and pushed towards the posterior part; the skin thus becomes so extended that it soon tears just under the neck, and then from the head. When this is accomplished the most difficult operation is over, and now the process of moulting goes on very rapidly. By repeated contractions the skin is folded towards the tail, like a glove when taken off, and the lining of the spiracles comes out in long white filaments. When about one-half of the body appears, the shell still remains like a cap, enclosing the jaws; then the worm, as if reminded of this loose skull-cap, removes it by rubbing it on a leaf; this done, the worm finally crawls out of its skin, which is attached to the fastening made for the purpose. Once out of its old skin, the worm makes a careful review of the operation, with its head feeling the aperture of every spiracle, as well as the tail, probably for the purpose of removing any broken fragment of skin which might have remained in these delicate organs. Not only is the outer skin cast off, but also the lining of the air-tubes and intestines, together with all the chewing organs and other appendages of the head. After the moulting, the size of the larva is considerably increased, the head is large compared with the body, but 8 or 10 days later it will look small, as the body will have increased very much in size. This is a certain indication that the worm is about to moult. Every 10 days the same operation is repeated. From the fourth moulting to the time of beginning the cocoon the period is about 16 days. (Amer. Naturalist, i, pp. 37, 38.)
Little has been recorded as to the exact mode of casting the larval skin in Coleoptera. Slingerland states that Euphoria inda when pupating sheds the larval skin off the anal end in the same way as in caterpillars, while in Pelidnota punctata the larval skin splits down the whole length of the back, retains the larval shape, and forms a covering for the pupa which remains inside. (Can. Entomologist, xxix, p. 52.) The old larval skin in the Coccinellidæ and certain Chrysomelidæ is retained crumpled up at the end of the body, while in Dermestes, Anthrenus, etc., it cloaks the pupa.