STRUCTURE AND DEVELOPMENT OF INSECTS 23 



Harvest-mites, or " daddy-long-legs," sow-bugs, thousand- 

 legged worms, and similar vermin are often popularly called 

 insects, but all of them can readily be distinguished from true 

 insects by their possessing more than six legs, the harvest-mites 

 and spiders having eight and the others many more. 



How Insects Grow 



With rare exceptions insects hatch from eggs laid by the 

 adult females. Upon hatching they are but little larger than 

 the eggs, and often bear but little resemblance to their parents. 

 Thus the young caterpillar would never be recognized as the 

 immature stage of the butterfly by one unfamiliar with its transfor- 

 mations. Grasshoppers and some other insects, however, upon 

 hatching from the egg bear a marked resemblance to the adult 

 form, except that they lack wings. 



Complete Metamorphosis. When the caterpillar hatches 

 from the egg it at once commences to feed and grows very 

 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 body, but the outer skin becomes 

 hardened and to it the muscles and ligaments are attached. This 

 hardening 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 develop more 

 fully. Thus the skins of insects are shed several times (see Fig. 15, 

 6), 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 staje, and during which it is called a 

 larva, it is an 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 segment, and probably 



