NORTH AMERICAN DIPTERA. 



wing; palpi small or wanting. Large to very large, bristleless 

 flies Mydaidae, 190 



Proboscis without fleshy labella at tip, horny and rigid; five pos- 

 terior cells almost always present; palpi usually prominent. 

 Mostly large, bristly, predaceous flies. . AsilidaB, 192 



26. Five posterior cells in the wing; basal cells large. . . 27 

 Not more than four posterior cells.* ..... 29 



27. Venation intricate; empodia and pulvilli membranous, biit fre- 



quently minute (see 21).. 

 Venation not intricate, the anterior cross-vein always apparent. 28 



28. The fourth vein terminates before tip of wing; male sexual organs 



prominent; males dichoptic. . . . Apioceridae , 188 

 The fourth vein terminates beyond the tip of the wing; male sex- 

 ual organs small; males often holoptic. . There vidae, 205 



29. Third antennal joint without bristle or style; three posterior cells, 



the first narrowed or closed; the fourth vein terminating at or 



before the tip of the wing. . . . Scenopinidae, 208 



Third antennal joint usually with a terminal style; four or three 



posterior cells; the fourth vein terminates beyond tip of wing. 30 



30. Anal cell narrowly open or closed near border of wing (if discal 



cell wanting, see Hilarimorpha p. 157, footnote) . 



Bombyliidae, 210 



Anal cell closed remote from border, sometimes absent; discal cell 

 not rarely absent. ..... Empididae, 218 



31. Wings pointed; no cross-veins, save at base; second basal cell 



short; arista terminal; small flies. . Lonchopteridae, 240 

 Wings not lanceolate. ........ 32 



32. Anal cell elongate, acute, closed toward or near the border of the 



wing; second basal cell usually long. .... 36 

 Anal cell, if present, short, closed remote from the border of the 

 wing, not acutely produced (rarely produced in a narrow, lobe- 

 like prolongation.) ........ 33 



33. Second basal cell confluent with discal cell, or the discal cell ab- 



sent; auxiliary vein usually vestigial or indistinct; anal cell 

 often absent; usually small flies. ..... 34 



Second basal cell separated by a cross-vein from a complete discal 

 cell; auxiliary vein and anal cell usually complete. . 40 



* Five in a very few species of Rombyliidse. 



