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. 
