INSECTS AND THEIR NEAR RELATIVES 23 



and families, although the differences between the members 

 of the several classes are often very striking. 



22. Hexapoda. The class Hexapoda, or the insects, is 

 by far the most important group in the branch, and as it 

 is the one which claims the major part of our atten- 

 tion, a discussion of the characters upon which their 

 arrangement into orders and families is based is here 

 given. 



The grouping of insects into orders is based largely 

 upon variations in three characters namely, those of the 

 wings, of the mouth parts and of the metamorphosis. A 

 combination of the description of the wings, the type of 

 mouth and the nature of the development, whether direct 

 or indirect, will place any insect in its proper order. 

 Tables for the identification of adult insects may employ 

 certain other characters on account of the fact that the 

 nature of the transformations cannot be determined from 

 a specimen in the cabinet. 



Families are identified by means of characters that 

 vary within each order. In some cases the only characters 

 that can be used appear to be those of the wing- venation. 

 Since these present difficulties that are too great for the 

 beginner they will be omitted. In other orders, families 

 are distinguished by characters of the antennae, of the 

 tarsi and of various parts of the head and thorax. Greater 

 refinements of these same characters serve to define the 

 genera and to some extent the various species. Species 

 may be separated in many cases by differences in color 

 and size, these characters being not available for use in 

 the case of the larger groups because the color and size in a 

 genus, for instance, may vary as greatly as it does in the 

 entire family. No attempt is made here to separate the 

 insects into their natural groupings lower than families 



