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- 



FIG. 2. Longitudinal section of anterior half of an insect, Menopon titan, to show chitin- 

 ized exoskeleton, with muscles attached to the inner surface. (Much enlarged.) 



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 



p IG ^ git 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 which 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. 



