NORTH AMERICAN DIPTKRA. 



16. Middle tibiae with several long bristles on the outer side (36). 



Paralimna. 

 Middle tibiae not with long bristles exteriorly. . . .17 



17. Face with a large protuberant gibbosity, below which the profile is 



nearly vertical; abdomen broad; legs hairy (23). Gastrops. 

 Face at most only evenly convex; legs not noticeably hairy. 18 



18. Oral opening small. ........ 19 



Oral opening large. ........ 27 



19. Front femora thickened. ....... 20 



Front femora not unusually thickened. . . . . 21 



20. Front femora much thickened, their tibiae with a terminal spine; 



third and fourth veins convergent distally (16, 17). Ochthera- 



Front femora moderately thickened, with spines on the under side 



distally, in the male at least; first posterior cell not narrowed; 



smaller species (18) . . , . . . Ochtheroidea. 



21. Second segment of the abdomen as long as the three following 



together; eyes densely pubescent. . . . Nostima. 

 Second segment of abdomen shorter than the two following to- 

 gether. ........... 22 



22. Fourth abdominal segment unusually long and broad; arista bare; 



sixth vein unusually convex before the cross-vein; third anten- 

 nal segment obtusely angulated above; eyes bare (13, 14, 15) . 



Lytogaster Becker.* 

 Fourth abdominal segment not unusually long and broad. 23 



23. Kyes thickly but short pubescent (9). . . . Hydrellia. 

 Eyes sparsely pubescent or bare. ...... 24 



24. Mesonotutn with four strongly developed dorsocentral bristles on 



each side; arista bare; oral opening small; face without strong 

 bristles (25,35). . .... Pelomyia. 



Mesonotum with three or four dorsocentral bristles. . . 25 



25. Sides of the face wholly without bristles. . . Hyadina. 

 Sides of the face with bristles. ...... 26 



26. Eyes wholly bare; clypeus projecting. .... Pelina. 

 Eyes sparsely pubescent; clypeus hidden (Philygra] (7, 8). 



Hydrina. 



* I refer to this genus two undetermined species, one from Illinois, 

 the other from Brazil. The face, however, is not or but feebly carin- 

 ated above; the fifth segment is also large, the abdomen very convex 

 distally. 



