THE ARTHROPODS 



387 



in some low crustaceans. In some species of gall-insects and 

 scale-bugs males have not been found ; and permanent par- 

 thenogenesis is supposed to be the rule. This is not known to 

 occur in any other group of animals ; but there are well-known 

 cases of flowers whose egg-cells always develop without 

 fertilization. Parthenogenesis of both plants and animals is 

 interesting because sexual reproduction is usually provided 

 for in all types of organisms. 



326. Classification of Insects. The group Insecta is a 

 class, and it is divided into about twenty orders. Most of 

 these, however, are not represented by many common insects, 

 and so will be omitted from the following account. 



In classifying insects, as all other animals, it is necessary 

 to consider all points of external and in- 

 ternal stucture in order to determine which 

 species are most alike ; but it is a fortunate 

 fact that it is often possible to identify by 

 means of some one structure. In the case 

 of many insects, the wings happen to be the 

 convenient parts for general classification ; 

 but similar wings should not be taken as 

 indicating close relationship if other organs 

 are not homologous (i.e., of corresponding 

 structure). 



The most common insects in United 

 States are conveniently grouped in the 

 following orders:- 



Aptera. Name means without wings. 

 Example: the " silver-moth " or "silver- 



r 



fish " (Fig. 136) and the " spring-tails." 

 These are the oldest insects now living. 



m, . .^. , . , ,1 L 



They are primitively wingless; that is, 

 there is no evidence that their ancestors had wings. Fleas 

 are secondarily wingless, because of degeneracy caused by 

 parasitism. Entomologists conclude that the ancestors of 



"* 



pisma. A modern 

 Representative of 



the ancient pnmi- 



tive insects which 

 had ****** P ai . rs of 



Ie 8 s but no wings. 



