NORTH AMERICAN DIGGER WASPS— EERNALD. 301 



developed, these last being- sometimes very sharp and giving to the 

 groove the appearance of a gutter. (Jecasionalh^ a trace of a short 

 lateral groove parallel with the central one may be seen lying a short 

 distance from the base of the wing, and representing the parapsidal 

 groove. Directly behind the mesonotum lies the scutellum, at the 

 sides of which the hind wings are attached. This plate is much broader 

 than long-, convex, and with a more or less developed central elevation 

 which is often partially divided into right and left halves by a weak 

 central groove. As a general thing the central elevation is higher than 

 the highest part of the mesonotum. 



The mesothoracic pleuron is large and lies below the wing, its pos- 

 terior edge being approximately indicated by an oblic(ue groove extend- 

 ing downward and backward from beneath the wing nearly to the 

 anterior side of the mesocoxa, where it ends at a fswelling which 

 apparently serves to prevent too great a dorsal flexure of this segment 

 of the leg. Near the base of the fore coxa a groove is also present which 

 passes from beneath upward through the plearon somewhat behind the 

 prothoracic lobe. This is known as the episternal groove, and it varies 

 in amount of development in difl'erent species. Immediately around and 

 behind the prothoracic lobe the mesopleuron is noticeably hollowed out 

 as though for the accommodation of this lobe. There is no suture or 

 other mark of separation between the pleuron and sternum, and no 

 characters have been observed on the latter which are useful for the 

 distinction of species except a short longitudinal incision about half- 

 way from the median sternal suture to the angle where the surface 

 curves upward to foi'm the side of the body. 



Metathorax.- — The postscutelhun, which lies inunediatel}^ behind the 

 scutellum, is a somewhat similar but narrower plate. Its antei'ior 

 margin is nearly straight, but its posterior margin curves backward 

 slightly, the plate having its greatest antero-posterior length in the 

 middle. The central portion is the highest, though not as high as the 

 scutellum, and like the latter it may have a slight median groove. The 

 metapleuron has a small, rounded, swollen area or metapleural lobe 

 near its middle dorsally, a little below the place of origin of the hind 

 wing. This area is often pubescent when the remainder of the plate 

 is not so, in which case it is very noticeable. The meta})leuron nar- 

 rows ventially, its narrowest point being a little below the middle. 

 Here it appears to turn and extend horizontally V)ack to the base of 

 the petiole, the sternum of the median segment not being visible. 

 The lines or sutures separating it from the mesopleuron in front and 

 the median segment behind disappear near the base of the mesocoxa, 

 and the dorsal line separating its lower part from the pleuron of the 

 median segment above is very faint or may even be absent. The real 

 Imiits of the pleura of the meso- and metathorax and of the median 

 segment can, indeed, be hardly regarded as having been tinally settled, 

 Proc. N. M. vol. xxxi— 06 20 



