272 Annals Entomological Society of America [Vol. VI, 



18. Mesonotum opaque gray; center of disk between the rows of dorso-central 



bristles with a yellowish-brown, longitudinal vitta, which extends on to 

 disk of scutellum; three pairs of orbital, and fours pair of dorso-central 



bristles present 15 immaculata Coquillett. 



Mesonotum shining, or, if opaque grayish there is no indication of a central 

 brown vitta 19 



19. Antennae entirely yellow, or third joint only darkened at insertion of arista.* 



20 

 Antennae with at least the third joint black, or dark brown, never yellow.. 23 



20. Head, including antenna, clear lemon yellow, only ocellar region, orbits 



posteriorly, and back of head black, or brown; pleurae and legs lemon 

 yellow with black or brown marks; lateral margins of mesonotum brown. 



16 citreifrons, new species. 

 Frons and face mostly, or entirely, reddish yellow; lateral margins of meso- 

 notum pale yellow, or black and concolorous with disk of mesonotum... 21 



21. Five equally strong orbital bristles present; frons one-half as broad as head; 



orbits not differentiated from center stripe; mesonotum with four pairs 



of dorso-centrals 17 pruinosa Coquillett. 



Four orbital bristles present; orbits differentiated from center stripe 22 



22. Lateral margins of mesonotum pale yellow; wings narrow; outer cross vein 



before wing middle; last section of fourth vein three times as long as the 



two preceding sections together 18 indecisa, new species. 



Lateral margins of mesonotum not pale 3'ellow; wings broad; outer cross vein 

 at near wing middle; last section of fourth vein twice as long as two 

 preceding sections together 19 varifrons Coquillett. 



23. Lateral margins of mesonotum broadly pale yellow 24 



Lateral margins of mesonotum narrowly, or not at all, yellow, the pale color 



conlined almost entirely to the suture, or to the extreme upper margin of 

 the pleurae r 25 



24. Frons with the center stripe clear yellow; orbits posteriorly, sometimes, 



blackened; legs black, or brown, the knees never distinctly yellow. . . . 



20 platyptera Thomson. 



Frons with the center stripe more or less blackened; legs with the knees 



distinctly pale yellow 21 coquilletti, new species 



25. Palpi yellow 22 longipennis Loew. 



Palpi black 26 



26. Larger species — 3-33^ mm. — dull gray-black in color; apices of femora and 



iDases of tibiae narrowly yellow 23 coloradensis, new species. 



Smaller species — 1^-2 mm. — shining black in color; apical half of each femur 

 yellow, tibiae brownish yellow 24 marginata Loew. 



27. Mesonotum with four, or more, pairs of dorso-central bristles 28 



Mesonotum with two or three pairs of dorso-central bristles 35 



28. The pair of bristles between the posterior pair of dorso-central almost of 



equal strength with them; basal two joints of antennae, legs mostly, 

 pleurae, humeri, and abdomen reddish yellow; outer cross vein beyond 



wing middle 25 canadensis, new species. 



The pair of bristles mentioned above much weaker than posterior dorso- 

 centrals, or absent; much darker species; only sometimes a narrow side 

 line on pleurae, knee joints more or less broadly, and posterior margins 

 of abdominal segments narrowly yellow; or entire thorax, abdomen and 

 legs black, cross vein generally at or before wing middle or very slightly 

 beyond it .29 



29. Third antennal joint in male enlarged, subquadrate, thickly covered with 



short, silky pilosity; in female the third joint is smaller and not so notice- 

 ably pilose; frontal lunule distinct, whitish pollinose; center stripe of 



frons brownish 26 laterella Zetterstedt. . . 



Third antennal joint normal in size in both sexes, and not noticeably pilose.. 30 



^'"\ ^Sometimes longipennis has the antennae yellowish, in which case the spec- 

 imens will run down to indecisa when a comparison of the descriptions will be 

 necessary. 



