FORM, GROWTH, AND CHANGE 7 



(or epidermis l ) it cannot grow, and there are limits beyond which 

 it cannot be stretched. From this it follows that an insect 

 or other arthropod must needs shed its cuticle periodically 

 during its growth, a new cuticle, at first thin and delicate, being 

 formed beneath the old one, to be revealed after the " moult ' 

 (or ecdysis) is over. Where, as is often the case, successive 

 layers of the cuticle can be distinguished, the older (primary) 

 layer (Fig.2 ciij} is at the surface, the newer layers (cu 2 ) deeper 

 next the skin. The cuticular nature of the insect's outer 

 envelope (or exoskeleton) and the consequent necessity for its 

 periodic renewal and casting, are most important factors in 

 determining the course of insect transformations. 



FIG. 2. SECTION THROUGH THE SKIN (ep) AND CUTICLE (Cu) OF 

 AN INSECT. 



cuj, primary, and cu s , secondary cuticle ; sp, a rigid spine : 



s, a flexible " bristle " or " hair " ; somewhat diagrammatic 



and highly magnified. From Comstock, " Introduction to 

 Entomology ". 



Closer study of the cuticle of our grasshopper shows that it 

 is not uniformly thick and firm ; were it so the whole exoskeleton 

 would be rigid, whereas there are many parts that can be 

 readily moved in relation to each other. If we examine the 

 hind-body (or abdomen) 01 the insect, we see that the cuticle 

 may be roughly described as consisting of a series of firm skeletal 

 pieces (or sclerites) connected by short regions of thin and 

 flexible membrane. These sclerites of the exoskeleton indicate 

 the segments of which the abdomen is built up, and in arthro- 

 pods generally the body is segmented, the segmentation being 



1 Most writers on the structure of insects use the term hypodermis for 

 this cell-layer. 



