HEXAPODA. 77 



In arranging the orders in a linear series, as must be done in a 

 book, it is impossible to indicate in a satisfactory way either the rela- 

 tion of the orders to each other or the relative rank of the orders. 

 An effort is made to place near together closely allied orders, and to 

 treat first those that are more simple or primitive or generalized in 

 structure, and last those that are more specialized. But this plan 

 could be fully carried out only by having several parallel columns on 

 the pages of the book, each representing a distinct line of descent, 

 an arrangement which, to say the least, is impracticable. 



What has been done in this work is to place first the Thysanura, 

 which is doubtless the most primitive order. Then follow first the 

 orders that undergo an incomplete metamorphosis, and last, those 

 that undergo a complete metamorphosis. Within these two groups of 

 orders those with biting mouth-parts are placed first, and these are 

 followed by those with sucking mouth-parts, except that in the 

 second group the Coleoptera and Hymenoptera are placed last for 

 want of a better position. We do not intend to indicate by this that 

 these two orders are closely related, or that they are more specialized 

 than the Diptera. 



In fact, with regard to at least five of the ordersof insects (Hemip- 

 tera, Lepidoptera, Diptera, Coleoptera, and Hymenoptera), it seems 

 idle to us to discuss which is the more highly specialized. Each has 

 been specialized in a direction peculiar to itself ; and to attempt to 

 describe which is the " highest " seems as futile as the discussion by 

 children of the question : " Which is better, sugar or salt? " 



We give below a table for use in classifying specimens. This table 

 is merely intended to aid the student in determining to which of the 

 orders a specimen that he is examining belongs. No effort has been 

 made to indicate in the table the relation of the orders to each other. 



TABLE FOR DETERMINING THE ORDERS OF HEXAPODA.* 

 (This table includes only adult insects.') 



A. Wingless or with rudimentary wings. 



B. Mandibles and maxillae retracted within the cavity of the head 



so that only their apices are visible, p. 82 THYSANURA. 



BB. Mandibles and maxillae more or less prominent and fitted for 

 biting. (See BBB also.) 

 C. Head with long, trunk-like beak. (Boreus.} p. 184. 



MECOPTERA. 



* See note at bottom of p. 10. 



