DEVELOPMENT I 33 



growth of the insect, is periodically shed, and along with it go not only 

 such integumentary structures as hairs and scales, but also the chitinous 

 lining, or intima, of the stomoda^um, proctodaeum, tracheae, integumen- 

 tary 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 exuvice. 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 excre- 

 tory importance in the case of Collembola, which are exceptional among 

 insects 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 significance, as Weismann has 

 attempted to show in the case of sphingid larvae. 



Adaptations of Larvae. Larvae exhibit innumerable conformities 

 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-developed sense organs, excellent powers of 

 locomotion, special protective and aggressive devices, etc. In insects 

 as a whole, the environment of the larva or nymph and that of the adult 

 are very different, as in the dragon fly or the butterfly, and the larvae are 

 modified in a thousand ways for their own immediate advantage, with- 

 out any direct reference to the needs of the imago. 



The chief purpose, so to speak, of the larva is to feed and grow, and 



