The Structure and Special Physiology of Insects 5 



of the body bearing the wings and legs. Where the cuticle is not strongly 



chitinized it is flexible (Fig. 6), thus permitting 



the necessary movement or play of the rings 



of the body, the segments of the legs, antennae W\ , // 



and mouth-parts, and other parts. The small ^"^ <-' 



portions of chitinized cuticle thus isolated or 



made separate by the thin interspaces or sutures 



Fig. 5. 

 Fig. 4. — Diagram of cross-section through the thora.i; of an insect to show leg and wing 



muscles and their attachment to body-wall, /i., heart; a/.c, alimentary canal; v.ii.c. 



ventral nerve-cord; ;c'., wing; /., leg; m., muscles. (Much enlarged; after Graber.) 

 Fig. 5. — Left middle leg of cockroach with cxoskeleton partly removed, showing muscles. 



(Much enlarged; after Miall and Denny.) 



are called sclerites, and many of them have received specific names, while 

 their varying shape and character are made use of in distinguishing and 

 classifying insects. 



Fig. 6. — Chitinized cuticle from dorsal wall of two body segments of an insect, showing 

 sutures (the bent places) between segmental sclerites. Note that the cuticle is not 

 less thick in the sutures than in the sclerites, but is less strongly chitinized (indi- 

 cated by its paler color). 



The whole body is composed fundamentally of successive segments 

 (Figs. I and 7), which may be pretty distinct and similar, as in a caterpilla:- 

 or termite or locust, or fused together, and strongly modified, and hence 

 dissimilar, as in a house-fly or honey-bee. The segments, originally five 

 or six, composing the head, are in all insects wholly fused to form a single 

 box-like cranium, while the three segments which compose the thorax are 

 in most forms so fused and modified as to be only with difficulty distinguished 

 as originally independent body-rings. On the other hand, in most insects 



