140 Annals Entomological Society of America [Vol. XII, 



12. Hypopygium from dorsal view conspicuously widened at apex inflatus Hine 

 Hypopygium not widened eximius Bellardi 



13. Bristles and hair of body and legs largely pale. Hypopygium from dorsal 



view distinctly widened at apex varipes Williston 



Bristles of the legs largely black. Hypopygium not widened apically. ... 14 



14. Hypopygium with a rather conspicuous tuft of hair below at apex, furcation 



of the third vein very near the middle of the distance between base of 



second posterior cell and small cross vein canus Hine 



Hypopygium without conspicuous tuft of hair apically, furcation of the third 

 vein plainly before the middle of the distance between the base of the 

 second posterior cell and the small cross- vein 15 



15. Hypopygium plainly notched on apical border 16 



Hypopygium not notched on apical border 17 



16. Thorax yellowish pollinose, first two segments of male abdomen colored 



like the thorax argentifrons Hine 



Thorax gray pollinose, first two male abdominal segments colored like the 

 following ones argyrosoma Hine 



17. Hypopygium truncate at apex truncatus Hine 



Hypopygium rounded at apex pallidulus Hine 



18. Femora largely red, especially posteriorly 19 



Femora entirely black 20 



19. Male abdominal segments two, three and four with black markings. 



argyrogaster Macquart 

 Male abdominal segments two, three, four, five and six with black markings, 



triton Osten Sacken 



20. Mystax white, scutellum with long white hair and bristles 21 



Mystax yellow, scutellum with yellow or black hair and bristles 22 



21. Very small species, 12 millimeters, mesothorax posteriorly white haired, 



pilosus n. sp. 



Larger species, not imder 15 millimeters, mesonotum posteriorly black 



haired. coquillettii n. sp. 



22. Mystax bright yellow, scutellum with long yellow hair and bristles, 



auripilus Hine 

 Mystax pale yellow, scutellum with black hair and bristles 23 



23. Male, first three abdominal segments without silvery markings, female 



uniformly yellow pollinose plenus Hine 



Male, only first two abdominal segments without silvery markings; female, 

 abdomen with a series of large black markings dorsally, one to each seg- 

 ment nemoralis Hine 



Erax stramineus Williston. 



Total length 15 to 18 millimeters. Mystax, beard and hairs of the 

 palpi pale but distinctly yellow, bristles of the front and occipito- 

 orbital bristles sometimes black, but they may be yellow and in most 

 specimens examined at least part of them are pale yellow. Dorsum 

 of the mesothorax with black hairs and bristles in many specimens, 

 but in some the bristles posteriorly, at least are pale; scutellum with a 

 row of many long black or pale bristles on the margin. Wings hyaline, 

 narrow, furcation of the third vein distinctly before the middle of the 

 distance between the anterior cross-vein and the base of the second 

 posterior cell, stump usually near twice as long as the basal section. 

 Legs nearly entirely black in some specimens, tibias reddish in others. 

 In many species of the genus the tibias normally are widely red basally 

 and narrowly black apically. Here it may be said that the normal 

 color of the legs is uniformly black and the variation toward red appears 

 to be due to the lack of varying amounts of the dark pigment, thus 



