272 SYSTEMATIC ZOOLOGY. 



17. The thorax consists of three somites and bears three 

 pairs of legs and usually two pairs of wings. 



18. The abdomen lacks distinct appendages. 



19. The head usually has a pair of compound eyes and 

 three ocelli. The mouth-parts are either mandibulate or 

 haustellate. 



20. Wingless insects usually have a direct development 

 (Ametabola); winged insects may have an incomplete 

 metamorphosis (Hemimetabola), or a complete meta- 

 morphosis (Holometabola) . 



21. Classification of the Hexapoda is based on the char- 

 acter of the metamorphosis, and the structure of the 

 mouth-parts and the wings. 



22. The Hexapoda are divided into Thysanura, Orthop- 

 tera, Pseudoneuroptera, Neuroptera, Coleoptera, Hymen- 

 optera, Hemiptera, Lepidoptera, and Diptera. 



