98 GENERAL ZOOLOGY 



nearly equal in size. The species probably developed from 

 the egg without metamorphosis. The lack of special adapta- 

 tions or modifications of the various organs marks the ances- 

 tral insects as generalized forms, as distinguished from their 

 more or less specialized descendants of to-day, in which the 

 organs have become modified to perform different functions. 

 Thus the greatly developed hind legs of the locusts are a 

 specialization in structure, fitting the insect to progress by 

 leaps as well as by walking. 



Very different degrees of specialization often exist in the 

 organs of the same species ; thus the digestive system of 

 the locust is quite complex, while the separate prothorax is 

 a generalized character, which shows the locust to be allied 

 in this respect more closely to the primitive type than are 

 insects like the wasps, for example, where the three divisions 

 of the thorax are grouped in one mass. Within the limits of 

 every order there are different degrees of specialization, and 

 there are cases in every order where loss or decline of parts 

 (called degeneration) has been brought about through various 

 causes. Among the May-flies the mouth-parts of the imago 

 have become degenerate in connection with the short adult 

 life, which lasts only long enough for mating and the laying 

 of eggs for a new generation. In some scale-insects the female 

 becomes degenerate in connection with the quiescent life 

 beneath a protecting scale. She loses eyes, antennae, and legs, 

 becoming very little more than a bag capable of feeding and 

 reproducing. Parasitism also brings about degeneration. 



From the study of the forms in which different organs 

 appear in the orders of insects (that is, from the study of 

 morphology, the science of form) the naturalist is able to say 

 which kinds of insects have, on the whole, become most spe- 

 cialized in structure, and those which have, on the whole, 

 varied least from the primitive generalized type. The evi- 

 dence from this source points to the fact that the more 



