y 2 NORTH AMERICAN DIPTKRA. 



40. Head small; proboscis more or less elongated; alula of wings usu- 



ally vestigial (jEmpididte, pt. 218). 



Head not unusually small; proboscis rarely elongated; arista al- 

 most invariably dorsal. . . . . . . . 41 



A. MYODARIA. 



Never more than three posterior cells present, the first of which only may be 

 closed or narrowed in the margin ; none of the longitudinal reins furcate ; 

 marginal and submarginal cells never closed; anal cell very rarely produced 

 toward the margin of the icing (Micropezidce pt. Ortalididie, pt. etc.) Antennae 

 three-jointed, simple, with a bare, pubescent, pectinate or plumose arista, which 

 is almost always dorsal in position, never thickened into a terminal style. More 

 or less bristly Jlies. 



41. Squamae large; front of male narrowed or eyes contiguous. 59 

 Squamae small; eyes of male not more approximated than those of 



female, or if so the narrowing is due to the diminished width of 

 the median stripe, the borders remaining the same, the males 

 never holoptic; posterior callus of thorax almost always not dis- 

 tinct. .42 



B. ACALYPTERJZ. 



Squamae always small or vestigial. Auxiliary vein often indistinct or vestigial, 

 or closely approximated or fused with the first vein. First longitudinal vein 

 shortened, often very short. Basal cells small, the posterior ones often indistinct 

 or wanting. Males never holoptic, the front in this sex never markedly narrowed. 

 Thorax without complete transverse suture; posterior callosity usually absent. 

 Never large flies, usually small or very small. 



42. Auxiliary vein present, separated from first longitudinal vein and 



terminating distinctly in the costa; the first vein usually ends 

 near or beyond the middle of the wing; posterior basal cells 

 present. .......... 43 



Auxiliary vein absent, vestigial or incomplete; the first vein usu- 

 ally ends in the costa before the middle of the wing. . 49 



43. A distinct bristle on each side of the face near the oral margin, i. 



e. oral vibrissae present. ....... 44 



No oral vibrissae. ......... 45 



44. Mesonotum and scutellum flat; front bristly; cheeks and face bris- 



tly; all the tibiae spurred and with preapical bristle; seashore 



flies Phycodromidse, 317 



Mesonotum not flattened, convex; no costal spine; more than 

 four abdominal segments visible. . Cordyluridae, 327 



