Dr. Chapman also writes me: “Arctians resemble bears (Arctos), polar and others, in having long hairs to protect them during winter, and are, in fact, typically hibernators. Many of them have to half-hibernate, having warmth enough to keep them awake, but not enough food for growth, but their tissues, at least the chitinous ones of the cutis, and also probably, and perhaps especially, of the alimentary canal, become old and effete, and require the rejuvenescence acquired by a moult. Other smooth-skinned hibernators have similar capabilities.”
Chapman has shown in his paper on Acronycta that these caterpillars of this genus illustrate how larvæ may lose a moult, and they do so to acquire a sudden change of plumage.
The number of moults in insects of different orders.—It will be seen from the data here presented that the number of moults is as a rule greatest in holometabolic insects with the longest lives, and that an excessive number of ecdyses may at times be due to some physical cause, such as lack of food combined with low temperature.
In Campodea there is a single fragmentary moult (Grassi), while the Collembola (Macrotoma plumbea) shed their skin throughout life. (Sommer.)
In the winged insects, especially Lepidoptera, the number of moults is dependent on climate. Insects of wide distribution growing faster in warmer climates consequently shed their skins oftener; for example, the same species may moult once oftener in the southern than in the northern States, as in the case of Callosamia promethea, which in West Virginia is double-brooded. Hibernating larvæ moult once oftener than those of the summer brood. (W. H. Edwards.) Weniger by rearing the larvæ of Antheræa mylitta and Eacles imperialis, and which, when reared under normal conditions, actually have six stages, found that when reared in a warm moist atmosphere of about 25° C. they have but five stages, i.e. moult but four times. In the hot and moist climate of Ceylon, A. mylitta has but five stages. (Psyche, v, p. 28.)
Among Orthoptera Acrydians moult five times; Diapheromera femorata but twice (Riley); a katydid (Microcentrum retinervis) moults four times (Comstock). Mantis religiosa, according to Pagenstecher, moults seven times, having eight stages, including that before the amnion is cast, but the first “moult” being an exuviation of the amnion, the number of stages is seven. Cockroaches (Periplaneta americana) are said by Marlatt to “pass through a variable number of moults, there being sometimes as many as seven.”
In the Homoptera there are, in general, from two to four moults; thus in Typhlocyba there are five stages, and in Aphis at least three, and in Psylla four during the nymphal state. Psocus has four. Riley states that the nymph of the female coccid, Icerya purchasi, sheds its skin three times, and that of the male twice. Notwithstanding its slow growth, Riley says, the 17–year Cicada moults oftener than once a year, and the number of larval stages probably amounts to 25 or 30 in all. The bed-bug sheds its skin five times; and with the last moult appear the minute wing-pads characteristic of the adult. In Conorhinus sanguisuga there are “at least two larval stages and pupal stages.” (Marlatt.)
In the dragon-flies moulting occurs, Calvert thinks, many times, since the rudiments of wings are said by Poletaiew to only appear in odonate nymphs after the third or fourth moult.
In the May-fly, Chloëon, the number of ecdyses is 20. The neuropterous Ascalaphus (Helecomitus) insimulans of Ceylon moults three times before pupating. Among the Mecoptera Felt has shown that Panorpa rufescens moults seven times.
In Coleoptera the normal or usual number is not definitely known; Meloë moults five times, but this is a hypermetamorphic insect; Tribolium confusum has been carried by Mr. Chittenden through seven moults. Phytonomus punctatus, the clover-leaf weevil, moults three times, according to Riley, who has observed that Dermestes vulpinus passes through seven larval stages.