THE X UMBER OF MOULTS 615 



changes of skin, it seems that growth may go on and still be accompanied by 

 considerable changes in shape of the body without change of skin. Frequent 

 ecdyses appear, then, to be the result of the great and sudden changes of the 

 body, necessitated by the adaptation of the animal to new or unusual conditions 

 of life. In young Daphnia. a cladocerous crustacean, as many as eight moults 

 were observed in a period of 17 days, and spiders frequently moult even after 

 reaching t'.ieir full size. The swollen bodies of the gravid female of Gastrophysa, 

 Meloe, or of Termites, and of the honey ant show that the skin can stretch to 

 a great extent, but in the metamorphoses of Crustacea and of insects, whose 

 young are more or less worm-like or generalized in form, with fewer segments 

 and appendages, or with appendages adapted for quite different uses from 

 those of* mature life, the necessity for a change of skin is seen to be necessary 

 for mechanical reasons. Hence Crustacea and insects moult most frequently 

 early in life, when the changes of form are most thoroughgoing and radical, while 

 simple growth and increase in size are most rapid at the end of larval life, as seen 

 both in shrimps and crabs, and in insects. 



The hibernating caterpillars of certain butterflies are known to moult once 

 oftener than those of the summer brood. Mr. W. H. Edwards has discussed the 

 subject with much detail. "There seems," he says, "to be a necessity with 

 the hibernators of getting rid of the rigid skin in which the larva has passed the 

 winter ; that is, if the hibernation has taken place during the middle stages, 

 as it does in Apatura and Limenitis. In these cases very little food is taken 

 between the moult which precedes hibernation and the one which follows it, 

 and the larva while in lethargy is actually smaller than before the next previous 

 moult. The skin shrinks, and has to be cast off before the awakened larva can 

 grow. Those species (observed) whose larva moults five times in the winter 

 brood require but four moults during the summer." He adds that while the 

 larva is in lethargy, it is actually smaller than before the next previous moult. 

 Dr. Dyar writes: "I think there is no doubt about the number of stages of 

 arctian larvre. They seem to have a great capacity of spinning out their life- 

 history by interpolated stages (as regards width of head). I think it is because 

 so many of them hibernate, and only a single brood extends throughout the 

 season." (Psyche iii, p. 161.) 



On the other hand, it is difficult to understand why the caterpillars of arctians 

 moult so frequently, nearly twice as often as in most other caterpillars, though 

 the changes of form and armature are so slight. 



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, typi- 

 cally 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 larvae 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. 



