33Q 



NORTH AMERICAN' DIPTERA. 



7. Head noticeably longer than high; face much retreating (2). 



Acicephala. 

 Head scarcely longer than high; face gently retreating in profile. 



Cordylura. 



8. One sternopleural bristle; eyes but little higher than broad; anten- 



nae as long as the face; oral margin with but one, anteriorly di- 

 rected principal bristle on each side; arista distinctly haired; 

 five dorsocentrals; moderate sized, shining yellow flits. 



Megophthalma. 

 Two sternopleural bristles: antennae as long as the face, third joint 

 rounded, strongly pubescent; hind tibia with two pairs of bris- 

 tles on outer side; shining black, small species. 



Hexamitocera. 

 Three sternopleural bristles; five dorsocentrals ; arista distinctly 

 hairy; four scutellar bristles; hind tibiae with three pairs of 

 bristles; moderate sized, thickly hairy, gray flies. 



Orthochaeta. 



9. Eyes and head round; five dorsocentral bristles. ... 10 

 Eyes and head oval, higher than long; a single dorsocentral bris- 

 tle; cheeks broad; proboscis thick and swollen; oral border with 

 one short bristle on each side; antennae short distallv rounded; 

 abdomen flat; hypopygium only a little prominent; rather lar^e; 

 bare, ashy gray "flies (7). . . . • Hydromyza. 



10. Front tibiae on the inner side with a short, rectangular spine in 



addition to the terminal bristles. . . Acanthocnema. 

 Front tibiae without such bristle; male hypopygium with long 

 tufts of hairs; third antennal joint somewhat elongated; with a 

 sharp, upper distal angle; oral border with many thickly stand- 

 ing bristles. Pogonota. 



Front tibiae without -uch bristles; male hypopygium without long 

 tufts " . . . 11 



11. Third antennal joint rounded anteriorly; eyes somewhat obliquely 



oval; cheeks small; face very short; oral border with a single 



bristle on each side; proboscis thickened; legs but little bristly; 



femora somewhat thickened. . . . Microprosopa. 



Third antennal joint with a sharp anterior angle. . . 12 



12. Acrostical bristles in more than two rows: thorax and legs thickly 



haired; bristles of oral border numerous; femora strong, with 

 fine hair and weak bristles; cheeks broad (5). Spathiophora. 

 Acrostical bristles in two rows; niesonotum and legs sparsely and 

 not long haired; antennae small, with sharp tip; cheeks not 

 broad; oral border with bristles on each side; small, short, 

 gray-dusted flies {Cheztosa. Opsiomyia). . Tricopalpus. 



13. Under side of head with about ten bristles. . Plethochseta. 

 Under side of head with about four bristles. . PycnoglOSSa. 



