CHAPTER V 



THE SUBCLASSES AND THE ORDERS OF THE 

 CLASS HEXAPODA 



Insects constitute one of the classes of the Arthropoda, that 

 division of the animal kingdom in which the body is composed of a 

 series of more or less similar segments and in which some of these 

 segments bear jointed legs. This class is known as the Hexapoda. 



The distinctive characteristics of the Class Hexapoda and its 

 relation to the other classes of the Arthropoda are discussed in the 

 first chapter of this work; we have now to consider the division of 

 this class into subclasses and orders. 



The orders that constitute the Hexapoda represent two well-marked 

 groups; this class is divided, therefore, into two subclasses. This 

 division was first proposed by Brauer ('85), who recognized the fact 

 that while the wingless condition of certain insects, the fleas, lice, 

 bird-lice, and the wingless members of orders in which the wings are 

 usually present, is an acquired one, the wingless condition of the 

 Thysanura and Collembola is a primitive one. In other words, from 

 the primitive insects, which were wingless, there were evolved on the 

 one hand the orders Thysanura and Collembola, which remained 

 wingless, and on the other hand, a winged form from which have 

 descended all other orders of insects. 



An extended study of the wings of insects has shown that the 

 wings of all of the orders of winged insects are modifications of a 

 single type ; it is believed, therefore, that all of the orders of winged 

 insects have descended from a common winged ancestor. As to the 

 lice, bird-lice, and fleas, the relation of each of these groups to certain 

 winged insects, as shown by their structure, has led to the belief that 

 their wingless condition is an acquired one, being the result of parasitic 

 habits. The lice or Anoplura are commonly regarded as closely 

 allied to the Homoptera and Heteroptera ; the bird-lice or Mallophaga 

 to the Corrodentia; and the fleas or Siphonaptera to the Diptera. 

 Hence these wingless insects are placed with the winged insects in a 

 single subclass. 



The two subclasses thus recognized were named by Brauer the 

 Apter^^gogenea and the Pterygogenea respectively. The cumber- 

 someness of these names led to the substitution for them of the 

 shorter names Apterygota and Pter3'gota. The Apterygota includes 

 the orders Thysanura and Collembola; and the Pterygota, all other 

 orders of insects. Some writers regard the Th\'sanura and Collem- 

 bola as suborders of a single order, which they term the Aptera. 



The distribution of insects into orders is based on the classification 

 of Linnaeus, as set forth in his "Systema Naturse" (i 735-1 768). 

 Linnaeus, who has been called the Adam of zoological science, divided 



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