DEVELOPMENT . 165 



the stomodseum, proctodaeum, tracheae, integumentary glands, 

 etc. The process of moulting, or ecdysis, in caterpillars is 

 briefly as follows. The old skin becomes detached from the 

 body by an intervening fluid of hypodermal origin; the skin 

 dries, shrinks, is pushed backward by the contractions of the 

 larva, and at length splits near the head, frequently under the 

 neck ; through this split appear the new head and thorax, and 

 the old skin is worked back toward the tail until the larva is 

 freed of its cxuvia. The details of the process, however, are 

 by no means simple. Ecdysis is probably something besides 

 a provision for growth, for Collembola continue to moult long 

 after growth has ceased, and the winged May fly sheds its 

 skin once after emergence. The meaning of this is not 

 known, though perhaps ecdysis has an excretory importance 

 in the case of Collembola, which are exceptional among in- 

 sects in having no Malpighian tubes. 



Number of Moults. The frequency of moulting differs 

 greatly in different orders of insects. Acridiidae have five 

 moults; Lepidoptera usually four or five, but often more, as 

 in Isia (Pyrrharctia) Isabella, which moults as many as ten 

 times (Dyar) ; Musca domestica has three (Packard) ; the 

 honey bee probably six (Cheshire) ; and the seventeen-year 

 locust about twenty-five or thirty (Riley). Packard suggests 

 that cold and lack of food during hibernation in arctians (as 

 /. Isabella) and partial starvation in the case of some beetles, 

 cause a great number of moults by preventing growth, the 

 hypodermis cells meanwhile retaining their activity. 



The appearance of the insect often changes greatly with 

 each moult, particularly in caterpillars, in which the changes 

 of coloration and armature may have some phylogenetic sig- 

 nificance, as Weismann has attempted to show in the case of 

 sphingid larvae. 



Adaptations of Larvae. Larvae exhibit innumerable con- 

 formities of structure to environment. The greatest variety 

 of adaptive structures occurs in the most active larvae, such as 

 predaceous forms, terrestrial or aquatic. These have well- 



