Thorax. — The thorax is the second or intermediate region 

 of the body. It is readily distinguished by its appendages ; 

 which are three pairs of legs and two pairs of wings. It 

 consists of three segments ; but as each segment is com- 

 posed of several distinct pieces, it requires considerable 

 study to trace the outlines of each segment. We will return 

 to this subject later. 



Abdomen. — The abdomen is the third or caudal region 

 of the body. The segments of which it is composed are 

 more simple, distinct, and ring-like than those of the other 

 regions. 



STRUCTURE OF THE BODY-WALL. 



Chitin. — In studying the anatomy of insects it is found 

 that in the adult stage the greater portion of the body-wall, 

 that part of the insect which corresponds in position to the 

 skin of higher animals, is hard. 



This hardness is due to the deposition of a horny sub- 

 stance, called chitin, in the membrane which constitutes the 

 body-wall. 



Sclerites. — The chitin is not evenly distributed through- 

 out this membrane. Pull the head of a locust so as to sepa- 

 rate it from the thorax as far as possible without breaking 

 the skin. Note that the head is joined to the thorax by a 

 soft flexible membrane, in which but little, if any, chitin 

 has been deposited. 



Examine the sides of the thorax with a lens and observe 

 that the body-wall appears to be made up of many distinct 

 pieces. The integument is, however, really continuous ; and 

 in each case what appears as a distinct piece is simply a por- 

 tion of the body-wall in which considerable chitin has been de- 

 posited. Such a portion of the body-wall is called a sclerite* 



' The sclerites are analogous to the centers of ossification in the bones of the 

 higher animals. 



