6 NORTH AMERICAN DIPTERA. 



41. Between the third and fourth longitudinal veins and sub-parallel with 



them, crossing the anterior cross-vein, a spurious longitudinal vein, 

 or, when rarely not present, the first posterior cell closed a consid- 

 erable distance from the border SYRPHID.*:. 



Spurious vein never present; first posterior cell not closed before the 

 margin; eyes very large ; small species. . . PIPUNCULID^E. 



42. Proboscis horny and elongate, often folding back near the middle; ab- 



domen never with bristles; tibiae without terminal spurs or if with 

 spurs, the face with a median ridge, and the proboscis folding; first 



posterior cell closed or narrowed CONOPID^E. 



Proboscis fleshy and not elongate ; if elongate and slender, with or with- 

 out a median hinge, the body with distinct bristles, or the bristle 

 of the antennas plumose. .... MUSCID^E sens. lat. 



43. Wingless flies, parasitic upon bats; head not flattened, folding back 



upon the dorsum of the thorax; first joint of tarsi not abbreviated. 



NYCTERIBIDjK. 



Winged or wingless flies, parasitic upon birds or mammals ; head flat- 

 tened, sunk into a shallow emargination of the thorax ; first joint of 

 the four anterior or of all the tarsi abbreviated. . HIPPOBOSCID^;. 



