33. Hind femora extraordinarily thickened (28). . . Syritta. 

 Hind femora slender. ........ 34 



34. Sixth abdominal segment in the male as long as the two preceding 



together, cylindrical; fifth segment of the female one-half as 

 long as the preceding. ..... Eupeodes. 



Sixth abdominal segment of the male not peculiar; the fifth seg- 

 ment of the female one-third or one-fourth as long as the pre- 

 ceding. . 35 



35. Front very convex; eyes of male with an area of enlarged facets 



above (Lasiopticns (35). ..... Scaeva."* 



Front not remarkably convex. ...... 36 



36. Third longitudinal vein with distinct curvature into the first pos- 



terior cell; third joint of antennae elongate oval (9, 1O). Didea 



Third longitudinal vein straight or gently curved; epistoma not 



produced (if produced snout-like, Rhingia], ... 37 



37. Males holoptic (32) Syrphus. 



Males broadly dichoptic; arista more or less thickened. 



Chamsesyrphus. 



38. Mesonotum with median cinereous line; ocelli usually remote from 



the vertex. .......... 39 



Mesonotum not with a median cinereous linear stripe. . 40 



39. Hind femora in the male thickened and arcuate, the tibiae dilated 



at the tip Toxomerus. 



Hind femora simple and straight; the tibiae not dilated at tip. 



Mesogramma. 



40. Eyes of male with an area of enlarged facets above; fourth seg- 



ment of abdomen with two median yellow stripes and oblique 



side spots Allograpta.* 



Eyes of male not with an area of enlarged facets above; fourth 

 abdominal segment not so-marked. ..... 41 



41. Face projecting below ; slender species, the hypopygium often 



large Sphserophoria. 



Face receding; abdomen oval. . . . Xanthogramma. 



42. Thickly pilose species; the abdomen black, the basal part light 



yellow. Leucozona. 



Thinly pilose species; abdomen not so marked. ... 43 



43. Hind femora thickened. ... ... 44 



Hind femora slender. ..... . 47 



' Feebly characterized genera. 



