14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



insects as a free sclerite, being usually fused into the lower part of 

 the episternum, or limited to the region between the episternum, 

 precoxal bridge, and the coxa. It is clear that the trochantin is a 

 sclerite of the pleuron that has played a more important role in 

 primitive insects, and one now in a state of becoming obliterated in 

 the higher insects. The evidence bearing on its past history will be 

 discussed in the next section of this paper. 



Fig. 5. — Inner surfaces of typical dorsal or ventral segmental plates of insect 

 with secondary segmentation. 



Ac, antecosta, or anterior submarginal ridge of segmental plate ; LMcl, longi- 

 tudinal muscles, attached to antecostse ; Mb, secondary " intersegmental " mem- 

 brane ; Pc, precosta, narrow anterior lip of plate, before the antecosta. 



EVOLUTION OF THE THORACIC SCLERITES 



It is one thing to formulate in a general wa}' a working plan for 

 the study of the thoracic sclerites as they occur in modern adult 

 insects ; it is quite another to understand how the structure repre- 

 sented by this plan has been evolved from a more primitive one, and 

 to determine what the primitive structure itself may have been. In 

 pursuing this line of investigation a study of the thorax of the Aptery- 

 gota should be of assistance, though, as we shall find, the apterygote 

 thorax gives little evidence of having evolved into the pterygote 

 thorax. Yet, the thorax of apterygote insects has preserved certain 

 characters, which, though degenerate in some respects, afford better 

 evidence of the structure of the primitive insect thorax, and therefore 



