66 NORTH AMERICAN DIPTERA. 



The contact of the discal cell with the third posterior at least twice as 

 long as its contact with the fourth posterior, the latter contact often 

 merely punctiform. ANTHRAX Scopoli. 



13. Wings with three posterior cells. 39 



Wings with four posterior cells. . . . . . . . 14 



14. First posterior cell closed 15 



First posterior cell open. : . . . - . . . . 22 



15. Three submarginal cells present. 16 



Two submarginal cells. 17 



16. Head broader than the thorax; posterior orbits not excised. 



PANTARBES Osten Sacken. 

 Head narrower than the thorax ; posterior orbits excised. 



TRIPLASIUS Loew. 



17. First basal cell longer than the second. . . . . . 18 



First basal cell not longer than the second. . . . . 21 



18. Proboscis very short, not protruding beyond the oral margin ; anal cell 



usually closed; large species (Cen. America). ANISOTAMIA Mat-quart. 

 Proboscis more or less elongate ; anal cell open. ... 19 



19. First posterior cell closed at some distance from the border of the 



wing. 20 



First posterior cell narrowed, or closed in or near the margin of the 

 the wing; small species (5-6 mm.) . THLIPSOGASTER Rondani. 



20. Head comparatively small ; the emargination of the occipal orbits al- 



most imperceptible BOMBYLIUS Linne. 



Head broad ; emargination of the occipital orbits distinct. 



HETEROSTYLUM Macquart. 



21. Face thickly clothed with pile. . ANASTOSCHUS Osten Sacken. 

 Face sparsely clothed with pile. . . . SYSTXECHUS Loew. 



22. Two submarginal cells present 27 



Three submarginal cells present. 23 



23. Bare species; tibiae without bristles. . AMPHICOSMUS Coquillett. 

 More or less pilose species ; tibiae with bristles 24 



24. Antennas as long as the head, the third joint not longer than the first 



two together 25 



Antennae shorter than the head, the third joint twice as long as the first 

 two together EXEPACMOS Coquillett. 



25. First antennal joint not thickened 26 



First antennal joint extraordinarily thickened. . PLOAS Latreille. 



26. "Scutellum deeply sulcate longitudinally". GEMINARIA Coquillett. 

 Scutellum convex, not sulcate LORDOTUS Loew. 



