MOLTING, the change of skin, or of such cutaneous or partially cutaneous appendages as hair or feathers, which occurs annually or periodically in animals of many sorts, such as arthropods, birds, reptiles, amphibians, etc. It is a process often perilous to the animal, being not infrequently attended by loss of life. In mammals it may be gradual, as when the pelage or hair is changed. Examples of analogous processes are the annual shedding of the antlers of deer in spring, or the autumnal dropping of the horns of the pronghorn antelope.
Molting in Amphibia and Reptiles.— Here the entire skin is cast. The newt in early spring sheds its skin by detaching it from around the jaws, then pulling it back over the head and the limbs. The toad in molting causes the old skin to split along the back, it then pulls it off as one would a coat by working the muscles of the back; it is detached by movements of the head from around the lips, face, eyes and mouth; the skin is more readily pulled off from the legs to the tips of the toes. The skin, as in salamanders, is a thin film, and it comes off in four pieces. Immediately after molting the creature is shy and active. Sharp describes the molting of the common striped snake (Euternia sirtalis). One came out of the Water in a vivarium, gliding on to the grassy sod; it then shrugged itself for a moment, when the skin parted at the jaws. The skin on the head, how ever, remained fixed, so that when the creature crawled out the old skin was inside out. The operation took less than one minute. The rat tle of the rattlesnake (q.v.) consists of those parts of the successive molts which are re tained by a long knob made by the coalescence of the last few caudal certebrw..
Molting in Birds.— The feathers may all be cast or only a part of them. Young birds molt several times before adult age. The passerine birds undergo a complete ecdysis after the breeding season is over (post-nuptial), when the worn-out plumage is restored; and they may also before the breeding time pass through an incomplete molt, when their wedding dress is put on. The process is so gradual as a rule that few birds, except the ducks, etc., are un able to fly, or go unprotected. The height of the molting season is in August, though the feathers drop out in nearly every month of the year. The complete molt is undergone in from a month to six weeks' time. Besides their
feathers the pelican sheds a horny projection on the ridge of the bill, and the puffin and certain allied species shed the horny sheath of the bill, etc. For further information see PLUMAGE.
Molting in Arthropods.— In most arthro pods the various developmental stages become indicated in the external appearance of the animal by means of the successive molts, for no modification of form is possible without the removal of the rigid exoskeleton. The Arach nid Limulus, the common horsefoot crab (q.v.), frecfuently sheds, its skin opening around the edge of the head; this is also the case with the fresh-water crustacean Apus. In the crayfish and lobster the skin splits open be tween the thorax and abdomen, and the animal draws itself out of the transverse rent thus formed. The skin is cast entire, while the chitinous lining of the mouth, throat, fore stomach and of the rectum is also shed. The process of exuviation in the crayfish has long been known to be aided by the outgrowth of little delicate papilla: called casting-hairs; these serve to loosen the old integument; recently Packard has discovered similar papillae on the new or under-skin (hypodermis) of the lobster, and similar undergrowths aid the serpent in shedding its scaly epidermis.
In insects molting is frequent, especially in the larva stage, most caterpillars molting four or five times. The body moves convulsively and splits along the back, at the same time, casting hairs being usually absent, a molting fluid is poured out which serves to detach the old skin. The head is molted separately, the shell falling off by itself, then the body-skin is shuffled off, being pulled back toward the tail, and with the outer integuments all the lining of the digestive canal is shed (except that of the stomach and beginning of the intestine), and the lining of the spiracles, as well as the cuticle of all the hairs, and the spines.
Consult Dwight, The Sequence of Plumage and Molts of the Passerine Birds' (Annals New York Academy of Sciences, Vol. XIII. 1900) ; Mivart, St. J., The Elements of Ornithology> (London 1892) ; Newton, 'Dictionary of Birds> (1893-96) ; Packard, of Entomology' (1898). Further details will be found in the articles in this encyclopedia on specific groups of animals and the authorities mentioned therein.