_ 4 NORTH AMERICAN DIPTERA. 



50. Hind metatarsi incrassate and usually shorter than the second 



joint; oral vibrissa present; second basal cell distinct or not; 

 about excrement or near water. . . Borboridae, 315 



Hind metatarsi not incrassate and always longer than the follow- 

 ing joint. .......... 5 1 



51. Discal and basal cells united, the separating cross-vein vestigial 



or absent 52 



Discal and second basal cells separated. .... 54 



52. Anal cell absent 53 



Anal cell complete, though often small 54 



53. Front bare, or at the most bristly above; small, usually light color- 



ed flies Oscinidse, 310 



Front often bristly; mouth cavity usually large; no oral vibrissa;; 

 rather small to very small black or dark-colored flies about water. 



Ephydridae, 303 



54. Scutellum elongate, triangular; with spines or protuberances on 



its margin; femora thickened. . Rhopalomeridae, 280 

 Flies not having all the al>ove characters 55 



55. Oral vibrissa; present 5° 



No oral vibrissa.- , ... 58 



56. Arista long plumose, or pectinate above. Drosophilidae, 299 

 Arista bare, pubescent or short plumose 57 



57. Front bare or bristly at vertex only; small, black flies, witb nar- 



row, usually contracted abdomen. . . Sepsidae, 269 



Front bristly at least as far as the middle; often light colored, small 



or very small flies. Agromyzidae, 291; Geomyzidae, 297 



58. The auxiliary vein is evanescent at its tip, where it turns sharply 



forward at some distance before the tip of the first vein; wings 

 almost always pictured; anal cell angular, or drawn out into 

 a narrow acute lobe; no preapical tibial bristle. 



Trypetidas, 2S2 



Auxiliary vein more or less fused with the first vein; antennae 

 more or less elongate and decumbent; anal cell not produced; 

 rather small, elongate flies. Psilidae, 2<>7 



Posterior basal and anal cells very small; wings rarely pictured. 

 Small or very small, often silvery gray or whitish gray species. 



Geomyzidae, 297 



