CLASSIFICATION OF INSECTS 303 



KEY TO THE ORDERS OF INSECTS 



The principles underlying the classification of insects have already 

 been discussed in the text. In arranging this key an attempt has 

 been made to eliminate all useless characters and to include only 

 those families commonly encountered. Possibly this elimination has 

 been carried too far for some of the extreme forms of the different 

 orders ; however, in an elementary textbook it is not deemed prac- 

 tical to include material that would be of use only to the specialist. 



Several families are included in the key which are not mentioned 

 in the text. This becomes necessary for the complete arrangement 

 of the key, and may be of use in aiding students to determine the 

 more uncommon families which they may collect. In giving out 

 specimens for determination the teacher should, if possible, use 

 only those families described in the text. 



The following key is intended only for the identification of typical 

 adult forms. An attempt has been made to produce a usable key in 

 preference to a strictly natural one. An ideal key should, of course, 

 combine these two characteristics, but it has been found necessary 

 many times to sacrifice the natural arrangement for clearness. 



In the production of these keys the authors are indebted to all 

 previous workers in entomology. Due credit is given in every case 

 where a key has been adapted from another author's work. 



KEY TO THE ORDERS 



A. Mouth-parts adapted for biting. 



B. Without wings, or rudiments of wings. 



C. Mouth-parts retracted within the head. (Page 73) . . APTERA 

 CC. Mouth-parts not retracted within the head. 



D. Abdomen joined to thorax by slender petiole. Ants. (Page 



243) HYMENOPTERA 



DD. Abdomen broadly joined to thorax. 



E. Insects small, body antlike or louselike in form. Bird-lice ; 



book-lice; white ants. (Page 103) . . . PLATYPTERA 



EE. Insects of medium or large size. Body not antlike or 



louselike in form. 



F. Head prolonged into beak, at the end of which are 

 the biting mouth-parts. Scorpion-flies. (Page 93) 



MECOPTERA 

 FF. Head not prolonged into beak. 



