ANATOMY OF INSECTS EXTERNAL 



21 



the mesothorax and 

 metathorax of most 

 adult insects bear a 

 pair of wings. The 

 prothorax is usually 

 smaller than the 

 two posterior seg- 

 ments, the relative 

 size of which de- 

 pends upon which 

 pair of wings is the 

 better developed. 

 The dorsal surface, 

 or back, of a tho- 

 racic segment is 

 called the tergum, 

 or nottim, the ven- 

 tral or under sur- 

 face is the sternum, 

 and each side is 

 a pletirum. These 

 parts are further 

 divided by sutures 

 into distinct plates, 

 or sclerites, to which 

 the appendages are 

 articulated. The de- 

 velopment, shape, 

 size, and position of 

 these sclerites are 

 characters of such 

 uniformity tjiat the 

 sclerites are used 

 in classifying in- 

 sects, in much the 

 same way as are 

 the bones of the 

 vertebrate animals. 



