Development and Metamorphosis 



43 



development the young have to develop wings and make what other change 



is necessary to reach the adult type, but the life is continually free and active 



and the change is only a simple gradual transformation of the various parts 



in which differences exist. A common locust is an excellent example of 



an insect with such incomplete metamorphosis. Fig. 72 shows the develop- 



ing locust at different successive ages, or stages, as these periods are called 



because of their separation from each other by the phenomenon, common 



to all insects, of moulting. As the insect grows it finds its increase of girth 



and length restrained by the firm 



inelastic external chitinized cuticle, 



or exoskeleton. So at fixed periods 



(varying with the various species 



both in number and duration) this 



cuticle is cast or moulted. From 



a median longitudinal rent along 



the dorsum of the thorax and head, 



the insect, soft and dangerously 



helpless, struggles out of the old 



skin, enclosed in a new cuticle 



which, however, requires some little 



time to harden and assume its 



proper colors (often protective). F IG. 74. Metamorphosis, incomplete, of an 



After each moulting the young 

 locust appears markedly larger and 



with its wing-pads better developed 



r 



(Fig. 73). But not until the final 

 moulting in the case of the locust 

 this is the fifth are the wings usable as organs of flight. So that there 

 is after all likely to be a rather marked difference between the habits of 

 the young and those of the adult of an insect with incomplete metamor- 

 phosis, that difference being primarily due to structural differences. The 

 young are confined to the ground, and their locomotion is limited to walking 

 or hopping. The adults can live, if they like, a life in the air, and they 

 have a means of locomotion of greatly extended capability. 



The insects with complete metamorphosis are the beetles, the two- 

 winged flies, the butterflies and moths, the ichneumons, gall-flies, ants, 

 bees, and wasps, the fleas, the ant-lions, and several other small groups 

 of insects with less familiar names. In the case of all the thousands of 

 species in these groups, the young when hatched from the egg differ very 

 much in structure and appearance, and also in habits and general economy, 

 from the parents. Familiar examples of such young are the caterpillars 

 and "worms" of the moths and butterflies, the grubs of beetles, the mag- 



eggs; 5, young after first moulting, showing 

 ^ginning wing-pads; c, older stage with 

 complex wing-pads; D, adult with fully 

 developed wings. (One-half larger than 

 natural size.) 



