4 The Structure and Special Physiology of Insects 
mostly on resemblances and differences in corresponding parts of the body, 
apparent in the various insect kinds. What these parts are, with their names 
and general characters, and what their particular use and significance are, 
may be got partly from the following brief general account, and partly from 
the special accounts given in connection with special groups of insects else¬ 
where in this book. A little patience and concentration of attention in 
the reading of the next few pages will make the reader’s attention to the 
rest of the book much simpler, and his understanding of it much more 
effective. 
The outer layer of the skin or body-wall of an insect is called the cuticle, 
and in most insects the cuticle of most of the body is firm and horny in char¬ 
acter, due to the deposition in it, by the cells of the skin, of a substance called 
chitin. This firm external chitinized * cuticle (Fig. 2) forms an enclosing 
exoskeleton which serves at once to protect the inner soft parts from injury 
F IG . 3.—Bit of body-wall, greatly magnified, of larva of blow-fly, Calliphora erythrocephala, 
' to show attachment of muscles to inner surface. 
and to afford rigid points of attachment (Figs. 2, 3 and 4) for the many small 
but strong muscles wdiich compose the insect’s complex muscular system. 
Insects have no internal skeleton, although in many cases small processes 
project internally from the exoskeleton, particularly in the thorax or part 
* It is not certainly known whether the cuticle is wholly secreted by the skin cells, or 
is in part composed of the modified external ends of the cells themselves. 
