2 5 y NORTH AMERICAN DIPTERA. 



66. Third joint of antennae broad; niesonotum not vittate. . 68 

 Third joint of antennae oval. 



Males holoptic; femora with protuberance below. Merodon. 

 Males dichoptic; femora not with protuberance. . . 67 



67. Ocelli remote from each other; niesonotum not vittate. 



Asemosyrphus . 

 Ocelli not remote from each other; niesonotum vittate (18l. 



Helophilus. 



68. Thickly pilose species. Mallota. 



Thinly pilose. ......... 69 



69. Hind tibiae of male with an internal spur (compare Mallota, sp.) 



Teuchocnemis. * 



Hind tibiae of male not with such spur (Triodonla, Polydonta) 



(34) Pterallastes. 



70. Thorax with distinct yellow markings other than on the humeri; 



wasp-like flies. ......... 81 



Thorax not with distinct yellow markings of the ground-color 

 other than rarely on the humeri. . . . . . 71 



71. Hind femora swollen and with a protuberance or spur below dis- 



t ally; face carinate or subcarinate. ..... 72 



Hind femora not with such protuberance or spur. . . 73 



72. Face concave in profile; femoral spur bifid. Senogaster.t 

 Face protuberant in profile; femoral protuberance not spur-like 



(19, 36 1 Tropidia. 



73. Abdomen narrowed basally; slender species (see 27). 

 Abdomen in no wise club-shaped. ..... 74 



74. Face transversely arched, not produced, not tuberculate; abdo- 



men more or less elongate and nearly bare. ... 75 

 More or less thickly pilose species; often large. ... 76 



75. Hind femora extraordinarily thickened; anterior cross-vein rec- 



tangular, and before the middle of discal cell (Syritta). 

 Hind femora distinctly thickened, but the cross-vein distinctly 

 oblique, and near or beyond middle of discal cell. XylotaJ 



* Of doubtful validity; I think that both this and Polydonta should 

 be united with Pterallastes. 



t Of doubtful occurrence in the United States. 



Xylota may be confounded with BrachypalpUS. The nearly bare 

 abdomen of the one. rather thickly pilose of the other, will distinguish 



them. 



