8 



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 sderite* 



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

 higher animals. 



