296 



NORTH AMERICAN DIPTKRA. 



16. Front projecting forward anteriorly in a high angular eminence 



(15) Traginops. 



Front not so projecting. ....... 17 



17. Vibrissa not well differentiated from the adjacent hairs; rather 



thickly haired species; eyes densely pubescent. Arctobiella. 

 Vibrissas distinctly differentiated. ...... 18 



18. Mesonotum with bristles on the sides only. . . Cacoxenus. 

 Mesonotum with bristles in the middle also. . . 19 



19. Only two pairs of fronto-orbital bristles. . . Parodinia. 

 Three or four pairs of fronto-orbitals 20 



20. In addition to the usual fronto-orbital bristles there is an inner 



row of frontal bristles or bristly hairs (13). . Rhicnoessa. 

 No additional row of bristles Odinia. 



21. Head triangular in outline, the front strongly projecting, the face 



much retreating, almost horizontal. . . Acrometopia. 

 Head not of such form 22 



22. No orbital or ocellar bristles. .... Leucopis 

 Orbital bristles present 23 



23. Front with a transverse groove near the middle. Pseudodinia. 

 Front not with transverse groove (17). . Ochthiphila. 



