DEVELOPMENT 165 
the stomodzeum, proctodzeum, tracheze, integumentary glands, 
etc. The process of moulting, ot 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 exruvie. 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. Acridiidz 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 
I. 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 larvee. 
Adaptations of Larve.—Larve exhibit innumerable con- 
formities of structure to environment. The greatest variety 
of adaptive structures occurs in the most active larvee, such as 
predaceous forms, terrestrial or aquatic. These have well- 
