258 



NORTH AMERICAN DIPTERA. 



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

 Third joint of antennae oval. 



Males holopiic; femora with protuberance below. Merodon. 

 Males dichoptic; femora not with protuberance. . . 67 



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



Asemosyrphus . 

 Ocelli not remote from each other; mesonotum vittate (18). 



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 ( Triodonta, Polydonta] 

 (34) Pterallastes. 



70. Thorax with distinct yellow markings other than on the humeril 



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- 



tally; 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) 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 (Syriila}. 

 Hind femora distinctly thickened, but the cross-vein distinctly 

 oblique, and near or beyond middle of discal cell. Xylota.+ 



* 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. 



