419 



15. Thorax and abdomen entirely black, the former covered with dense 



prninescence ; basal joint of fore tarsi slightly longer than fore 



tibia3 (50: 45) ; hypopygium as in Figure 16, Plate XXXIII 



23. hasalis. 



— Thorax or abdomen with reddish or yellowish marks; basal joint of 



fore tarsi much longer than fore tibise 16 



16. Thorax with the central vitta and generally the anterior half of the 



lateral pair reddish, contrasting sharply with the velvety black 



ground-color of thorax ; abdomen entirely velvety black 



24. palliatus. 



— Thorax black, vittge concolorous; abdomen with yellow post- 



marginal bands to segments 25. riparius. 



17. Legs except the coxas entirely yellow, without any black markings 



except the usual black apical comb on mid and hind tibige 18 



— Legs not entirely yellow except the coxae 19 



18. Thorax glossy black ; mid and hind coxa blackened ; basal joint of 



fore tarsi one fifth longer than fore tibiae (72: 60) . .13. nigricans. 



— Thorax opaque black, whitish pruinescence between the vittse; mid 



and hind coxte slightly browned ; basal joint of fore tarsi one fifth 

 longer than fore tibiae (75 : 60) 40. dimorplius. 



19. Legs pale yellow, fore femora except the bases and apices black .... 



14. fallax. 



— Legs with more than fore femora black or fuscous 20 



20. Second joint of fore tarsi shorter than third 16. harhipes. 



— Second joint of fore tarsi longer than third 21 



21. Thorax black; abdomen pale yellowish or greenish with the apical 



3 segments black or brown 15. pedellus. 



— Thorax black ; abdomen either entirely black or with pale posterior 



margins to the segments 22 



22. Abdomen without noticeable pale posterior margins to segments. 23 



— Abdomen with conspicuous pale posterior margins to segments. .25 



23. Thorax glossy black ; halteres black ; small species, 2 mm. in length 



21. nigrolialteralis. 



— Thorax opaque black ; halteres yellow ; larger species, at least 3 mm, 



in length 24 



24. Thorax densely covered with gray pruinescence ; legs black ; abdo- 



men subshining, black 23. basalts. 



— Thorax with slight whitish pruinescence; legs mostly pale yellow, 



femora and tibiae usually slightly browned; abdomen opaque 

 black 24. palliatus. 



25. Basal joint of fore tarsi one half longer than fore tibiae 



25. riparius. 



— Basal joint of fore tarsi less than one half longer than fore 



tibia; 26 



26. Large species, 7 mm. in length, or more; basal joint of fore tarsi 



more than a third longer than fore tibiae 19. fasciventris. 



