56 THE STUDY OF INSECTS. 



grow no larger, except that in case of females the body 

 may be distended by the growth of eggs within it. While 

 many adults eat more or less, it is only to sustain life, and 

 not for growth. Indeed, many adult insects take very little 

 food, and some have lost their mouth-parts entirely, through 

 disuse. The adult stage usually lasts for a considerably 

 shorter time than the larval or nymph stages. In fact, it 

 seems planned in the economy of nature that the grown-up 

 insects should live only long enough to lay eggs, and thus 

 secure the perpetuation of the species. 



THE EXTERNAL ANATOMY OF INSECTS. 



The subject of insect anatomy is separated into two divi- 

 sions : one, treating of the structure of the body-wall or 

 skeleton ; the other, of the internal organs. The former is 

 termed external anatomy ; the latter, internal anatomy. 



In our own bodies we find a central framework or skele- 

 ton, about which are arranged the muscles, blood-vessels, 

 nerves, and other organs. But insects are constructed on 

 an entirely different plan : with them the supporting skele- 

 ton is outside, and the muscles, nerves, and other organs 

 are within this skeleton. The difference can be well seen 

 if the figure showing the internal structure of the leg of a 

 May-beetle (Fig. 67) be compared with one of our own 

 limbs, either arm or leg. 



FIG. 67. Leg of May-beetle. (After Straus-Durckheim.) 



The body of an insect is built on the same plan as are 

 its legs. The outside of the body is more or less firm, being 

 hardened by chitine ; and this firm outer wall supports the 

 muscles and other organs, thus serving as a skeleton. The 

 skeleton is therefore, in general outline, a hollow cylinder. 



