THE EXTERNAL ANATOMY OF INSECTS 35 



tergum; two lateral divisions, one on each side of the body, the pleura; 

 and a ventral division, the sternum. 



Each of these divisions may include several definite areas of' 

 chitinization. In this case the sclerites of the tergum are referred to 

 collectively as the tergites, those of each pleurum, as the pleurites, and 

 those constituting the sternum, as the sternites. 



The division of a segment into a tergum, two pleura, and a sternum 

 are most easily seen in the wing-bearing segments, but it can be 

 recognized also in the prothorax of certain generalized insects. This 

 is especially the case in many Orthoptera, as cockroaches and walking- 

 sticks, where the pleura of the prothorax are distinct from the tergum 

 and the sternum. In the abdomen it is evident that correlated with 

 the loss of the abdominal appendages a reduction of the pleura has 

 taken place. 



The lateral conjunctivas. On each side of each abdominal segment 

 of adults the tergum and the sternum are united by a strip of non- 

 chitinized cuticula; these are the lateral conjunctivas. Like the 

 transverse conjunctivas, the lateral ones are more or less infolded. 



The sclerites. Each definite area of chitinization of the cuticula 

 is termed a sclerite. 



The sutures. The lines of separation between the sclerites are 

 termed sutures. Sutures vary greatly in form ; they may be infolded 

 conjunctivas ; or they may be mere lines indicating the place of union 

 between two sclerites. Frequently adjacent sclerites grow together 

 so completely that there is no indication of the suture; in such cases 

 the suture is said to be obsolete. 



The median sutures. On the middle line of the tergites and also of 

 the sternites there frequently exist longitudinal sutures. These are 

 termed "the median sutures. They represent the lines of the closure 

 of the embryo, and are not taken into account in determining the 

 number of the sclerites. 



The dorsal median suture has been well-preserved in the head and 

 thorax, as it is the chief line of rupture of the cuticula at the time of 

 molting. 



The pilif erous tubercles of larvae. The setas of larvae are usually 

 borne on slightly elevated annular sclerites; these are termed pittf- 

 erous tubercles. 



The homologizing of the sclerites. While it is probable that the 

 more important sclerites of the body in winged insects have been 

 derived from a common winged ancestor and, therefore, can be 

 homologized, many secondary sclerites occur which can not be thus 

 homologized. 



