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 
and mouth-parts, and other parts. The small 
portions of chitinized cuticle thus isolated or 
made separate by the thin interspaces or sutures 
v.n.'c. 
Fig. 4. Fig. 5. 
Fig. 4.—Diagram of cross-section through the thorax of an insect to show leg and wing 
muscles and their attachment to body-wall, h., heart; al.c., alimentary canal; v.n.c. 
ventral nerve-cord; w., wing; leg; m., muscles. (Much enlarged; after Graber.) 
Fig. 5.—Left middle leg of cockroach with exoskeleton 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. 1 and 7), which may be pretty distinct and similar, as in a caterpillar 
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 
