8 INSECTS INJURIOUS TO STAPLE CROPS. 



Complete Metamorphosis. — But let us return to the cater- 

 pilLir and follow it through its short hut interesting life. 

 Upon hatching from the (igg it at ouce commences to feed 

 and grows Tery rapidly. But before long an obstacle to 

 further growth arises. Unlike higher animals, insects 

 possess no internal skeleton or framework for the organs 

 of the bod}^, but the outer skin becomes hardened and to 

 it the muscles and ligaments are attached. This harden- 

 ing of the skin is best seen in the horny wing-covers of the 

 beetles and is due to the secretion of a hard substance 

 called chitin. This chitin is secreted by all parts of the 

 skin in greater or less degree, and thus forms a sort of 

 shell for the whole body. Though this hardening is not 

 so apparent in larvae as in adult insects, it is always 

 present, and it is for this reason that when the young- 

 caterpillar has made a certain growth it is forced to shed 

 its skin, which refuses to expand further, in order to 

 develoj) more fully. Thus the skins of insects are shed 

 several times (see Fig. 2, h), — usually five or six, but 

 sometimes as many as twenty, — this process being known 

 as ^^ molting." During its life as a caterpillar, which is 

 called the ''larval stage," and during which it is called a 

 ^^arva," it is nn elongate, worm-like creature, with six 

 short, jointed legs on the three thoracic segments, a pair 

 of fleshy false legs or pro-legs on the last abdominal seg- 

 ment, and probably several pairs of pro-legs between these 

 and the true legs. No traces of wings can be seen, but the 

 body is often covered with hairs, spines, or warty tubercles. 



But with the next molt the insect changes in appearance 

 most radically, becoming a joupa, or chrysalis as this stage 

 is termed for butterflies. During the pupal stage the 

 insect remains dormant either in a small cell slightly under 



