PROC. ENT. SOC. WASH., VOL. 23, NO. 8, NOVEMBER, 1921 179 



Bl, B2, C, D: Abdomen (urites 1-9) A, Bl, B2, Cl, C2, C3, 



D -(urite 10) epiproct. 



Thus, in all sawfly larvae the segments are divided into four 

 areas, three of which are usually armed, and always distinct and 

 prominent, for which reasons and others which will follow I 

 have called them segmental areas and a fourth, unarmed area 

 which is occasionally infolded or not found unless sought for. 

 The first area is recognized by bearing on its anterior and pos- 

 terior margins the greatest number of muscles. The second is 

 above the spiracle in the abdomen and the third is above the 

 alar area. All three of these areas are always present and 

 usually ornamented more or less conspicuously but the fourth 

 area is never ornamented, not infrequently reduced in size or 

 appearing wanting and occurs between the third area of the 

 segment of which it is a part and the first area of the following 

 segment. Because the fourth area is unlike the segmental areas 

 in never being ornamented and in not always possessing a similar 

 prominent, easily seen position, and because it is apparently 

 used chiefly to connect the segment of which it is a part with the 

 segment following, I have called it a connective area. 



Composition of Adult Thorax. 



Having thus briefly characterized the composition and defined 

 the limitation of the larval segment of Pteronidea ribesii the 

 salient parts of the segments in the adult may be called to mind 

 before entering into a discussion of the possible homologies 

 existing between larval and adult areas. The three thoracic 

 segments of adult sawflies are not equal in size or composition 

 (Plate XV, fig. 20). The mesothorax is much larger than 

 either of the other thoracic segments and is the part usually 

 considered as the typical thoracic segment. It is divided into 

 four parts: the prescutum, scutum, scutellum (including 

 posttergite) and postscutellum. The three anterior parts are 

 fused together and constitute the most conspicuous part of 

 the segment. The posterior plate is rather distinct, being 

 separated from the anterior part by a membrane, although 

 laterally it usually fuses with the pleurum. The prothorax 

 differs remarkably from the mesothorax and is much smaller. 

 The metathorax while much smaller than the mesothorax 

 differs from it chiefly in the lack of a prescutum. 



Comparing the construction of the thorax of the adult with 

 that of the larva two striking differences are evident: 1 The 

 mesothorax of the adult is composed of four, dorsal, segmental 

 areas and a connective membrane, while in the larva there are 

 only three dorsal segmental areas ami a connecting area. 2 

 The thoracic segments of the adult arc of unequal construction, 

 while in the larva each thoracic segment has an equal number 



