22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



on which the fibers of the longitudinal muscles are attached, become 

 the antecostae of the definitive terga and sterna (fig. 3). In the 

 thorax, on the other hand, the ventral intersegmental sclerotizations 

 either remain as small, free sclerites (fig. i, 1st), or they unite with 

 the posterior parts of the segmental plates preceding. The interseg- 

 mental stemites, or intersternites, of the thoracic region are the 

 spinasternites (fig. 18 D, Ss), so-called because each usually bears a 

 small median apodemal process, the spina (fig. 4). A spinaster- 

 nite occurs typically between the prothorax and the mesothorax, and 

 between the mesothorax and the metathorax ; there is never a free 

 spinasternite following the metasternum because the corresponding 

 intersegmental element goes with the first abdominal sternum to form 

 the antecosta of the latter, except where it is lost as a result of the 

 degeneration of the first abdominal sternum. The first spinasternite 

 is more commonly persistent than the second which is usually fused 

 into the posterior part of the mesosternum, where it may become 

 entirely obliterated. 



The second structural difference between the thoracic and abdomi- 

 nal sterna accompanies the difference in the relations of the interseg- 

 mental sclerites to the segmental plates, but is not necessarily corre- 

 lated with it. It consists of a reversal in the overlapping of the sterna. 

 The successive abdominal sterna overlap regularly in a posterior direc- 

 tion, as do the terga of both the abdomen and the thorax (fig. 3). The 

 sterna of the thorax, on the other hand, overlap anteriorly. The meta- 

 thoracic sternum, therefore, stands as a dividing plate between the 

 anteriorly overlapping sterna of the thorax and the posteriorly over- 

 lapping sterna of the abdomen (fig. 3, S3). 



This reversal in the overlapping of the sternal plates as between the 

 thorax and the abdomen is probably the oldest structural differentia- 

 tion between the two regions of the body, for it is well shown in some 

 of the Apterygota, particularly in Japyx, and is exhibited by all 

 pterygote insects in which the thoracic sterna remain free from each 

 other. It was probably, therefore, established when the thorax was 

 first set apart as the locomotor center of the body, and has nothing 

 to do with the development of the wings. Just what advantage accrues 

 to the thoracic mechanism from the reversed relations of its sternal 

 plates is not clear, but presumably it gives a better device for the 

 movement of the legs or for the movement of the successive seg- 

 mental plates on each other. 



The third distinction between the thorax and the abdomen occurs 

 in adult pterygote insects, and pertains to the attachments of the ven- 

 tral muscles. We have assumed that the primitive attachments of the 



