ii] GROWTH AND CHANGE 9 



those simpler cases in which the differences between 

 the young and adult insect are comparatively slight. 

 We shall then be in a position to understand better 

 the meaning of the more puzzling and complex 

 cases in which the differences between the stages are 

 profound, 



In the first place it is necessary to realise that 

 the changes which any insect passes through during 

 its life-story are essentially accompaniments of its 

 growth. The limits of this little book allow only 

 slight reference to features of internal structure; 

 we must be content, in the main, to deal with the 

 outward form. But there is an important relation 

 between this outward form and the underlying living 

 tissues which must be clearly understood. Through- 

 out the great race of animals the Arthropoda of 

 which insects form a class, the body is covered out- 

 wardly by a cuticle or secretion of the underlying 

 layer of living cells which form the outer skin or 

 epidermis^ (see fig. 10 ep, cu, p. 39). This cuticle 

 has regions which are hard and firm, forming an 

 exoskeleton, and, between these, areas which are 

 relatively soft and flexible. The firm regions are 

 commonly segmental in their arrangement, and the 

 intervening flexible connections render possible ac- 

 curate motions of the exoskeletal parts in relation 



1 The term ' hypodermis ' frequently applied to this layer is mis- 

 leading. The layer is the true outer skin ectoderm or epidermis. 



