SYRPHID^. 253 



Front and face wholly black in ground color (Chrysogaster, Rhy- 

 sops). . .......... 19 



14. Face black in ground color. . . . . . . .15 



Face more or less yellow or yellowish in ground-color. . 26 



15. Abdomen with only four visible segments, very convex, the venter 



excavated; first two joints of the antennae very short, the third 

 large, subquadrate, with a short subterminal arista. 



Nausigaster. 

 Abdomen with more than four visible segments. . . .16 



16. Hind femora distinctly thickened. ...... 17 



Hind femora but little or not at all thickened. ... 18 



17. Scutellum unusually large, nearly square; males dichoptic. 



Chalcomyia. 



Scutellum not unusually large, considerably broader at its base; 



males holoptic and with a facial tubercle (27). Myiolepta. 



18. Face rounded, not tuberculate, the oral margin not projecting(30) 



Pipiza. 

 Face tuberculate or the oral margin projecting. ... 19 



19. Epistoma projecting; small, black species. .... 20 

 Face tuberculate, the oral margin not projecting. ... 21 



20. Front in the female and usually the face in both sexes with trans- 



verse wrinkles; spurious vein obsolete; antennae short or long 



(2,3,4) Chrysogaster. 



Front and face not wrinkled; face pilose. . . . Psilota. 



21. Metallic green, metallic green and black, or black species; facial 



orbits separated by a slender parallel groove (8). Chilosia. 

 Black with more or less metallic green or blue, with yellow, yel- 

 lowish or metallic cross-bands on the abdomen ; face not with 

 orbital grooves. ......... 22 



22. Face with transverse grooves in the middle; (antennae elongate). 



Rhysops.* 

 Face not with transverse grooves or wrinkles. ... 23 



23. Wings not longer than the abdomen; ocellar tubercle large; abdo- 



men depressed, long elliptical, somewhat narrowed toward its 

 base, the markings ferruginous or orange yellow. Pyrophaena. 



* filelanostoma scitulum, rugonasus. melanocerum. 



