FROM EGG TO INSECT SNODGRASS 



403 



other organs, and these nerves, carrying stimuli outward, are col- 

 lectively the motor nerves. To receive information from the ex- 

 terior, it is necessary that there should be another set of sensory 

 nerves. These nerves ordinarily ha\'e their origin in cells of the 

 ectoderm and grow inward to the central ganglia, but the develop- 

 ment of the sensory nerves in insects is not definitely known. 



THE BODY WALL 



The body wall of an insect deserves especial consideration for 

 several reasons : First, it serves as a protection against the outer 

 world, and its modifications of form distinguish the great host of 

 insect species from one another; second, it serves as a skeleton for 

 the attachment of the body muscles; and third, it is the seat of a 

 great variety of sense organs. 



The ectoderm layer of the embryo event uallj^ covers the entire 

 surface of the body and forms also various ingrowths that become 



Fig. 19 — Structure of the body wall of an insect 

 A, piece of body wall, showing outer chitinous cuticula {Ct), cellular hypo- 

 dermis (Hy), and inner basement membrane {DM). B, movable "joints" of 

 the body wall: a, fold, as between segments, where cuticula is soft and flexible; 

 bj a membranous suture. C, a ball-and-socket articulation between bard parts 

 otherwise united by membrane. 



organs or parts of organs of the interior. The nervous system, as we 

 haA^e just seen, is one of these external derivatives of the ectoderm; 

 others, we shall see presently, are the anterior and posterior parts 

 of the alimentary canal, the tracheal s^^stem, various glands, and 

 the outlet ducts of the reproductive organs. The part of the ecto- 

 derm that remains at the surface to form the body wall is commonly 

 know^n in insect anatomy as the hypodermis (fig. 19 A, Hy). The 

 cellular hypodermis is lined by a thin basement membrane (BM), 

 and it forms over its outer surface a chitinous covering, the cuticula 

 (Ct), the three layers constituting the body wall. 



The character of the body wall of an insect is due to the texture 

 of the cuticula. In caterpillars, maggots, and other soft-bodied in- 

 sects, the cuticula is soft and flexible ; in most adult insects it is hard 

 and hornlike. The body of the insect, however, is not incased in a 

 continuous shell, the areas of hard cuticula being interrupted by 

 lines of soft cuticula. At some places the dividing lines are but 



