NORTH AMERICAN DIPTERA. 



Wings longer than the abdomen, abdomen with yellow or green- 

 ish-yellow, or shining metallic cross-bands ; usually elongate 

 species. .......... 24 



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



female slightly widened (31). . . . Platycheirus. 

 Front tibiae and tarsi slender in both sexes. .... 25 



25. Rather large, blackish species, with a large flat, elliptieal ab io- 



men. ...... Xanthandrus Verrall."* 



More elongate and slender species; abdomen not elliptical in out- 

 line. . Melanostoma. 



26. Abdomen narrowed toward the base, distinctly club-shaped or 



spatulate in outline. ........ 27 



Abdomen oval or slender, not spatulate or club-shaped in out- 

 line. ........... 30 



27. Third longitudinal vein bent deeply into the first posterior cell. (11) 



Salpingogaster, 

 Third longitudinal vein straight or gently curved. . . 28 



28. Hind femora slender; front of female long, narrowed above; the 



cheeks very narrow below the eyes; abdomen often very slender. 



(37,39) ... Baccha. 



Hind femora thickened; front not unusually long in female. 29 



29. Epistoma produced anteriorly, the face in profile deeply concave 



from antennae to tip; third joint of antennae rounded. 



Sphegina. 



Epistoma produced more downward, in profile gently concave; 

 third joint of antennae not rounded. . . . Neoascia. 



30. Front long, much narrowed above in the female; cheeks very nar- 



row, the eyes approaching each other at the lower third of the 

 head; wings usually with dark picture; abdomen more or less 

 elongate (compare Baccha when in doubt as to shape of abdo- 

 men). ........ Ocyptamus. 



Flies not having the above assemblage of characters. . . 31 



31. Mesonotum with distinct yellow lateral margins. ... 38 

 Mesonotum not with yellow lateral margins. ... 32 



32. Abdomen with definite yellow cross-bands. 33 

 Abdomen not with definite yellow cross-bands. ... 42 



* Melanostouia bncephalus Wied. 



