1909.] 



NATURAL SCIENCES OF PHILADELPHIA. 



a 



39 



Fig. 21. 



n 



x 



5 3 



be prothoracic in some insects and mesothoracic in others, but this 

 view does not seem to have a very wide acceptance. 



The embryological proof seems to be in favor of Heymons view, 

 and it is certainly the case that the musculature of such insects as the 

 Blattidse indicates that the spiracles belong to the segment behind 

 them rather than to the preceding one. It is possible, however, to 

 designate the spiracles as the first thoracic spiracle, second thoracic 

 spiracle, etc., without specifying to which segment they belong, and 

 this usage has been here adopted. 



In the thorax of Japyx there occurs an extra spiracle, which has 

 given rise to much discussion, but all speculation as to its homology 

 can be of no value until the musculature and embryology of Japyx 

 have been carefully studied with a view to determining this point. 



In insects other than Japyx, most investigators now agree in desig- 

 nating the third spiracle as the first abdominal one. It would appear 

 that Latreille's, '20-'22, designating the first abdominal segment the 

 "segment mediaire" is responsible for much of the dispute which 

 later arose concerning this segment, especially in the Diptera and 

 arculeate Hymenoptera ; and it is hard to understand why certain 

 modern systematists — Schmiedeknecht, '07, for example — persist in 

 using Latreille's confusing terminology. The first abdominal segment 

 is the first abdominal segment, no matter where it is located, and if 

 it be designated by its proper name, there can be no dispute as to its 

 homology or that of its spiracle. 



The Cervictjm. — Between the head and the prothorax is a narrower 

 neck region whose softer walls give a greater freedom of motion for 

 the head. Embedded in the membranous integument of this region 



