NoKTii Amekican Dh'tkra. 49 



11. Face black in ground color. ....... 12 



Face more or less j'ellow in ground color. ... 20 



12. Abdomen with only four visible segments ; first two joints of 



the antenna; very short, third large, subquadrate, with asiiort, 



subterminal arista Nadskjastkr 



Abdomen with more than four visible segments. . . 18 



13. Hind femora distinctly thickened 14 



Hind femora but little or not at all thickened. . . .15 



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



Chalco.aiyia 



Scutellum not unusually large, considerably broader at the base; 



males holoptic and with a facial tubercle. . Myiolepta 



15. Epistoma projecting ; small black species. ... 16 

 Face tuberculate, the oral margin not projecting. . . 17 

 Face rounded, pilose, not tuberculate, the oral margin not pro- 

 jecting PiPlZA 



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

 verse wrinkles Chrysogaster 



Face and front not wrinkled ; face pilose, without tubercle. 



Psilota 



17. Metallic green, metallic green and black, or black species; ab- 



domen oval, never with entire shining cross-bands. Ciiilosia 



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



yellowish abdominal markings, or if not, at least with entire 



, shining cross-bands ; abdomen usually elongate. . . 18 



18. Wings not longer than the abdomen ; ocellar tubercle large ; 



abdomen depressed, long elliptical, somewhat narrowed to- 

 ward the base, the lighter markings ferruginous or orange- 

 yellow Pyhoph.ena 



Wings longer thaji the abdomen, usually more slender species, 

 abdominal cross-bands yellow or greenish yellow, or some- 

 times shining metallic 19 



19. Front tibiae, distally, and tarsi of the male dilated, those of the 



female a little widened Platychirds 



Front tibisB and tarsi slender in both sexes. Melanostoma 



20. Dorsum of thorax with yellow lateral stripes. ... 29 

 Dorsum of thorax not with yellow lateral stripes. . . 21 



21. Abdomen narrowed toward the base, in outline club-shaped or 



spatulate. 22 



Abdomen oval or slender, not spatulate or club-shaped. 24 



