24 NORTH AMERICAN DIPTERA 



45. Second antennal segment with a longitudinal seam along the upper 



outer edge extending almost the whole length; posterior calli defi- 

 nitely formed by a depression extending from behind the base of the 

 wings to above the base of the scutellum (Calypteratse; Muscoidea) . .85 

 Second antennal segment rarely with a well developed dorsal seam, the 

 posterior calli not differentiated (except in Gasterophilus) ; squamae 

 small (Acalypteratse) 46 



46. Mouth parts vestigial, sunken in a very small oral pit. 



(p. 400) GASTEROPHILID^ 

 Mouth parts well developed, the oral opening large 47 



47. Posterior spiracle with several hairs on the border in addition to the 



pubescence, (visible only with high magnification) 50 



Posterior spiracle with pubescence only 48 



48. Subcosta complete, free from the first vein and ending in the costa 



( cf . Trypaneidae ) 64 



Subcosta incomplete, or not ending in the costa or the apex curved 

 forward beyond the bend 49 



49. Subcosta curved forward at nearly a right angle and weakened beyond 



the bend, the costa fractured at the apex of the subcosta; wings 



almost always pictured (p. 285) TRUPANEID^ 



Subcosta not angularly curved and weakened 51 



50. Head broad, flattened above; scutellum and thorax flattened, the former 



elongate; large flies (p. 299) ROPALOMERIDJE 



Head rather spherical; scutellum usually convex, not elongate; small, 

 cylindrical flies (p. 313) SEPSID^ 



51. First segment of the posterior tarsi shortened and incrassate. 



(p. 360) BORBORID^ 

 First segment of the posterior tarsi normal, usually longer than the 

 second 52 



52. Anal cell absent 53 



Anal cell present 55 



53. Postocellar bristles divergent (345) EPHYDRID^ 



Postocellars convergent or absent 54 



54. Ocellar triangle large; fifth vein with a distinct curvature near the 



middle of the discal cell (p. 339) CHLOROPID^ 



Ocellar triangle small; fifth vein straight or without a sharp curvature. 



(p. 325) DROSOPHILID^ 



55. Costa broken at the humeral crossvein 56 



Costa not broken at the humeral crossvein 58 



56. Postocellars divergent, if absent the arista absent. 



(p. 332) AGROMYZID^ 

 Postocellars convergent, parallel or absent, arista present 57 



57. A pair of convergent frontal bristles anteriorlv; interfrontals often 



present (p. 334) PHYLLOMYZIDJE 



Anterior frontals not convergent (p. 325) DROSOPHILID^ 



58. Postocellar bristles convergent (p. 329) OPOMYZID^ 



Postocellar bristles divergent or absent 59 



