22 INSECT PESTS OF FARM, GARDEN AND ORCHAR 



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 always occurs 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 stage, 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 

 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. 



With the next molt the insect changes in appearance most 

 radically, becoming a pupa, or chrysalis, as this stage is termed 

 for butterflies. During the pupal stage the insect remains 

 dormant either in a small cell slightly under the surface of the 

 earth, or in a silken cocoon spun by the caterpillar, or merely 

 attached to the food-plant by a strand of silk or the cast larval 

 skin. In many of the Diptera, the order including flies, mos- 

 quitoes, gnats, etc., however, the last larval skin is not shed, 

 but hardens and forms a case called a puparium within which 

 the pupal stage is passed. 



The typical pupa (Fig. 15, c, d) of a butterfly or moth re- 

 sembles neither the adult insect nor the larva, is of a more or 

 less oval shape, with the wings and antennae tightly folded at 

 the sides, the legs drawn up snugly together under them, and the 

 head and mouth-parts bent upon the breast, or sternum, though 



