234 NORTH AMERICAN DIPTERA. 



Arista dorsal, face wide, palpi large. ..... 24 



Arista nearly or quite at the tip of the large, pointed third joint. 25 



24. Arista plumose Phylarchus. 



Arista pubescent (d) Diostracus. 



25. Posterior cross- vein distant about its own length from the end of 



the fifth vein (4, i). . . . . Hypocharassus. 



Posterior cross-vein much more than its own length from the end 



of the fifth vein (16, a, b, d) . .... Argyra. 



26. A concave or distinctly flattened bare space before the scutellum. 



27 

 Thoracic dorsum convex as usual. . . . . . 31 



27. Bristles of thorax pale yellow (a) . . . Chrysotimus. 

 Bristles black, rarely brownish. ...... 28 



28. Fourth vein parallel with the third beyond the cross-vein, or near- 



ly so ... 29 



Fourth vein converging towards the third (21, g). Neurigona. 



29. Acrostichal bristles present only at the anterior edge of the thorax, 



very mimite (7, a) Xanthochlorus. 



Acrostichal bristles well developed, two-rowed. ... 30 



30. Very minute, blackish, opaque species (8). . Achalcus. 

 Small, yellow species Xanthina. 



31. Wings elongate, the posterior cross-vein beyond the middle, less 



than its length from tip of fifth vein (Hydrophorince}. 32 



Wings not elongate, posterior cross- vein scarcely beyond the mid- 

 dle, more than its length from end of fifth vein; if less, the 

 hind metatarsus shorter than the following joint. . . 34 



32. Fore femora slender (2, h) Liancalus. 



Fore femora thickened, with spines below (37, 38). . . 33 



33. Spines of fore femora very short, thoracic dorsum without well- 



marked pollinose lines (12, 38, d, e). . Hydrophorus. 



Spines of fore femora long, dorsum with pollinose lines (11, 37, 



c, g) Scellus. 



34. Chiter appendages of the hypopygium long and filiform, arista 



dorsal. Nematoproctus. 



Not with both characters 35 



35. Second joint of antenna with a thumblike projection along the in- 



ner side of the third (39) 3 6 



Second joint not with such projection. 37 



