48 Annals Entomological Society of America [Vol. IX, 



behind the other. The dorsally situated sympleura are termed, 

 aero-, pro-, meso-, and meta-sympleurites, while the sclerites 

 composing the more ventrically situated row are termed the 

 aero-, pro-, meso-, and meta-pleurites. The pleura and sym- 

 pleura, he thinks, are detached portions of the region, which he 

 terms the zygoterga. The zygoterga is the region comparable 

 to the dorsal or tergal half of the abdominal segment. Thus he 

 considers that each thoracic segment is composed of four sub- 

 segments designated by the terms aero-, pro-, meso-, and meta- 

 subsegments, that the dorsum is composed of four tergites and 

 the sternum of four sternites, designated by the above mentioned 

 prefixes. He regards the lateral region as consisting of two 

 longitudinal rows of sclerites (i. e., the upper row or sympleura 

 and the lower row or pleura) and considers that they are 

 detached portions of the zygoterga. 



DISCUSSION OF THE THORACIC SCLERITES. 



Tergum. The term dorsum has been used to designate 

 various parts of the dorsal surface of the thorax in different 

 insects, but when correctly applied it refers to the entire dorsal 

 surface and this usage will be followed here. The terms tergum 

 and notum will be used interchangeably to designate the entire 

 dorsal surface of any one thoracic segment. These terms are 

 generally used in this sense and if given any other meaning, 

 would lead to confusion. 



In the nymphs of the lower insects as well as in many larvae 

 and pupae, the tergum is a single undivided plate, the scuto- 

 scutellum (Crampton, '09). This is probably the primitive 

 condition of the tergum, which has possibly persisted in a more 

 or less modified condition in the pronotum of adult insects. The 

 fact that no postscutellum has ever been described as occurring 

 in the pronotum of insects, supports this view. The post- 

 scutellum in adult insects, develops in the intersegmental mem- 

 brane of the mesonotum and metanotum, its development being 

 apparently parallel with the development of the wings, which is 

 possibly due to anatomical causes such as the stimulus of 

 muscle stress, etc. Snodgrass, '10, calls the postscutellum the 

 "postnotum" or "pseudonotum." There is no reason for 

 rejecting Audouin's term postscutellum, which is suitable, 

 widely used and generally accepted. Hence it will be retained 

 in this paper. 



