N OLTING, OR ECDYSIS IN THE LOWER CLASSES. In animals like Crustacea, Myriapoda, insect•A, and spiders, with a more o• less solid exoskele ton, too dense to permit of gradual growth o• in crease in volume, there must be a periodical east ing of the skin, or ecdysis. The same occurs in salamanders. and especially in the scaly reptiles, ns snakes and lizards.
In the Crustaeea, insects, and similar animals the old skin dies and the new int egmnent is formed by a secretion thrown out by the inner layer of epithelial cells called the hypoderm's. On removing the shell of the lobster the hypo dermis is seen to be a richly colored reddish soft layer. The formation of the new layer of chitin arrests the supply of nourishment to the old skin. so that it dries and hardens. and is filially slinfiled off. In the crayfish and presumably in the lobster (q.v.). as well as in lizards and snakes. the casting of the old skin is materially induced and aided by the growth on the surface of the hypodermis of a layer of very fine and equally distributed hairs, called 'casting hairs,' which, by their rigidity and position, mechanic ally loosen and raise the bid skin. These re main as vestiges, forming spikes, ridges, or warts, serving to ornament the crust of crabs, or the outer edges of the skin-scales of snakes and lizards.
In insects, where casting hairs are exceptional, the loosening and casting of the skin is brought about by the secretion of a fluid by the cells of the hypodermis. This fluid spreads over the whole surface when the cells are still soft and not hardened by exposure to the air. This same fluid, when the pupa extricates itself from the larval skin. extends anuind the whole pupa and serves to protect it and to glue together the wings, legs, etc., in their new position.
The process of molting as seen in caterpillars (e.g. Teleo Polyphemus) is thus. It changes its skin five times before changing to a chrysalis; and the ecdysis occurs at regular periods, i.e. about every ten days fur the first four moltings, while about twenty days elapse between the fourth and fifth casting. The operation usually takes place after 4 P.M. A little before this time it holds its body erect, grasping a leaf with the two pairs of hind legs only; the skin is wrinkled and detached from the body by the fluid prevMusly mentioned; the body now energetically contracts, so that the skin is pulled off and pushed toward the end of the body; the skin thus becomes so stretched that it soon tears just under the neck, and then separates from the head. When this is accom
plished the most difficult operation is over, and now the process of casting goes on very rapidly. By repeated contractions the skin is folded to ward the tail. like a glove when taken off, and the lining of the spiracles. month, fore stomach, and rectum comes out in long filaments. When about one-half of the body appears, the shell still remains like a cap, inclosing the then the worm, says Trouvelot. as if reminded of this louse 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. Directly after the ecdysis the larva increases in size, the head hieing distinctly larger as well as the body, which soon fills out.
The lobster frequently molts after maturity. But in insects which undergo a complete meta morphosis and have the longest lives the number of molts is greatest. When the numher is excessive this seems to he due to some physical cause, such as lack of food combined with low temperature. In caterpillars the number of eedyses appears to lie dependent on climate.
BIBLIOGRAPHY. For birds, eonsult Dwight, Bibliography. For birds, eonsult Dwight, Jr., "The Sequence of Plumage and Molts of the Passerine Birds," in Annals of the York .rodeo, of Sciences, vol. xiii.. part T. (New York, 1900), with a full bibliography as regards passerine birds; Newton (with (.adow), Diction ury of Birds (London. 1A13-961: also the au thorities mentioned muter C's CE- BIRDS.
For mammals: Bedard. 1/ammaih, (London, 1902) : C'aton. Antelope and Deer of America (New York, 1877).
For reptiles and Amphibia, authorities men tioned under _1:V1'111111A ; SN A NE.
Fo• insects, etc.: Simper. .inimul Life us Af fected by the Naturol Conditio»s of Existence (New York, 1titi1) ; Packard, Text-book of Ento mology (New York, 1898) ; Max Braun, Ueber die Ionyitnye Lei der lliintang con Astacus Flurottilis (Semper's Arbeiten acs thin zoologiselo a institut iu War:burg. vol. ii., 1875), with the writing' of Trmtvel°t, Riley, Newport, Bugnion. Guilin. and Tower.