86 NORTH AMERICAN DIPTERA. 



25. Front tibiae distally and tarsi of the $ dilated, those of the female 



slightly widened. . . PLATYCHIRUS St. Fargeau and Serv. 

 Front tibiae and tarsi slender in both sexes. MELANOSTOMA Schiner. 



26. Abdomen narrowed toward the base, in outline spatulate or club- 



shaped 27 



Abdomen oval or slender, not club-shaped or spatulate. . . 31 



27. Third longitudinal vein deeply curved into the first posterior cell (Cen- 



tral and South America). . . . SALPINGOGASTKR Schiner. 

 Third longitudinal vein straight or gently curved. ... 28 



28. Hind femora slender; front of female long, narrowed above. . 29 

 Hind femora thickened; abdomen less elongate; front not long and 



narrow above. .......... 30 



29. Alulae rudimentary or wanting. . . BACCHINA Williston, nov. 

 Alulae well developed BACOHA Fabricius. 



30. Epistoma produced anteriorly, the face in profile deeply concave from 



antennae to tip ; third joint of the antennae rounded. 



SPHEGINA Meigen. 



Epistoma produced more downward, in profile gently concave; third 



joint of the antennae not rounded. . . NKOASCIA Williston. 



81. Front long, much narrowed above in the female; cheeks very narrow, 



the eyes approaching each other at the lower part of the head; wings 



usually with a dark picture; abdomen more or less elongate. 



OCYPTAMUS Macquart. 

 Species without the above characters. . . . . . . 32 



32. Dorsum of the thorax with yellow lateral margins. ... 39 

 Dorsum of thorax not with yellow lateral margins. . . . 33 



33. Abdomen with definite yellow cross-bands 34 



Abdomen not with definite yellow cross-bands 42 



34. Hind femora extraordinarily thickened. SYRITTA St. Farg. and Serv. 

 Hind femora slender 35 



35. Sixth abdominal segment in the $ as long as the two preceding to- 



gether, cylindrical ; fifth segment in the 9 one-half as long as the 

 preceding. ...... EUPEODES Osten Sacken. 



Sixth segment not peculiar, the fifth segment of the 9 one-third or one- 

 fourth as long as the preceding. ...... 36 



36. Front very convex ; eyes of $ with an area of enlarged facets above. 



CATABOMBA Osten Sacken. 

 Front not remarkably convex 37 



37. Third longitudinal vein with a distinct curvature into the first posterior 



cell; third joint of the antennae elongate oval. DIUEA Macquart. 

 Third longitudinal vein straight or gently curved ; epistoma not pro- 

 duced (if produced snout-like, Rhingia). .... 38 



