PROC. ENT. SOC. WASH., VOL. 23, NO. 8, NOVEMBER, 1921 177 



Superficial examination of any sawfly larva immediately 

 suggests that area A is at the cephalic end of any abdominal 

 segment but more substantial indications that the anterior 

 margin of this area marks the cephalic limit of each tergum are 

 obtained from certain features of the musculature and from the 

 construction of the prothorax and the tenth abdominal segment. 



An examination of the musculature of the larva of Pteronidea 

 ribesii (Plate XIV, fig. 3), as well as that of such other sawfly 

 larvae as have been studied shows that there is a greater number 

 of muscles extending -posteriorly from the anterior margin of area 

 A, and linking it with the segment caudad than there are muscles 

 extending anteriorly from the posterior margin of the same area 

 A, and serving to unite it with the cephalad segment. Area A 

 is also closely linked to the three posterior areas by the small, 

 short, intrasegmental muscles which extending caudad always 

 start from margin D A, and which extending cephalad start at 

 the same margin. The significance of these two points of 

 musculature can be more readily understood by an examination 

 of accompanying illustration (Plate XIV, fig. 3), and they seem 

 to the author to indicate quite clearly that the infolding D A is 

 the line of intersegmental division and that area A belongs to 

 the segment caudad of the line. 



The anal segment (Plate XIV, fig. 6) is not of the same con- 

 struction as the other abdominal segments, and seems to be 

 dorsally composed of a single, fused, segmental plate named the 

 epiproct. This plate extends from the posterior margin of the 

 unarmed area D of the ninth abdominal segment to the anus at 

 the apex of the abdomen. In those larvae in which the anal 

 segment retains traces of the ornamentation of the preceding 

 segments, this ornamentation is aborted and the portion of the 

 segment on which it occurs, although occupying the same rela- 

 tive position as the ornamented parts in preceding segments, is 

 not separated from the remainder of the epiproct by any such 

 constriction or infolding of the skin as marks the posterior 

 margins of similar areas in the preceding segments. The areas 

 of the anal segment are never distinct but are occasionally 

 indicated by the vestiges of ornamentation mentioned above, 

 and when thus indicated they occupy the same relative position 

 as in any preceding abdominal segment. The unarmed, narrow 

 area (D) immediately preceding the unified anal segment can not 

 be considered as part of the last segment, but must be looked 

 upon as the terminal fold of the ninth abdominal segment. The 

 sharp, well defined infolding or constriction separating this 

 area (D) from the anterior of the anal segment adds proof to the 

 muscular indications that area D is the posterior subdivision of 

 the segment. 



Viewing the larva from the exterior there seems to be a reduc- 

 tion in the number of areas in the prothorax (Plate XV, fig. 9). 



