372 



NORTH AMERICAN DIPTERA 



KEY TO GENERA 



1. All the tibiae with preapical bristles 2 



Tibiae without preapical bristles 4 



2. Cheeks half as wide as the third antennal segment 3 



Cheeks not over one-third as wide as the third antennal segment. 



Spilochroa Williston 



3. Cheeks not half as wide as the eye-height; bristles black 5 



Cheeks almost as wide as the eye-height; bristles wholly yellow (3, 4). 



Zagonia Coquillett 



4. Anterior pair of orbitals convergent (5, 6) Chyromya Desvoidy 



All the orbitals reclinate Aphaniosoma Becker 



5. Mesopleura with hairs and a strong bristle (1, 2). . . .Trixoscelis Rondani 

 Mesopleura bare Neossos Malloch 



5 ■'•-- ..:: ■ 6 



Chyiomyidae. — 1, 2, Trixoscelis; 3, 4, Zagonia flava; 5, 6, Chiromya flava. 



