112 GENERAL ZOOLOGY 



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

 limits of every order there are different degrees of specializa- 

 tion, 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 ap- 

 pear in the orders of insects (that is, from the study of mor- 

 phology, 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 which have, on the whole, varied 

 least from the primitive generalized type. While all zoolo- 

 gists agree that the Thysanura and Collembola are the low- 

 est orders of insects, there are many differences of opinion 

 on the arrangement of the other orders. In an introduction 

 to zoology it is not necessary to consider some of the orders 

 which contain only very unusual or uncommon insects. 



Generalized Insects. The generalized, or lower, orders 

 which have been taken up in this book are the Thysanura, 

 Collembola, Ephemerida, Odonata, Orthoptera, Isoptera, 

 Homoptera, and Hemiptera. The insects of all these orders 

 agree in having incomplete metamorphosis or no metamor- 

 phosis at all. 



Specialized Insects. The insects which have complete met- 

 amorphosis are generally recognized as the more highly 

 developed. The Coleoptera, Lepidoptera, Diptera, and Hy- 

 menoptera are specialized, or higher, orders. 



