178 [August, 



33 (2). Halterex i/ellotv. 



Tnvol Ufa, already included in the group with black halteres, some- 

 times has these organs yellow, but its thickened fore-tarsi should 

 prevent its being mistaken for any of the species under this lieading. 



34 (39). Costa conspicuously short, except in female albipennis, where it nenrttf 



reaches the'wing middle. 



35 (36). One of the pleural bristles very large and conspicuous. Tibial cilia 



fairly strong and sparse li-lf ""i^- nlbipennis. 



36 (35). Bristles equal or nearly so. Tibial cilia wealc and numerous, or quite 



absent. 



37 (38). Wings nearly clear ; fringe very, hit not extremely, long ; first costal divi- 



sion double the other ftoo. Tibial cilia fine and numerous. Moderately 

 large species l^-lf mm. simplex. 



38 (37). Wings yellowish hrotvn ; fringe extremely long; first costal division 



barely half as long again as the other two. A very small species. 



...f mm. superciliata. 



39 (34). Costa long, sometimes just short of the tving^middle {altifrons and sub- 



pleuralis), hut usually beyond it. 



40 (41). First thin vein well curved at its origin, especially in the female. Hind 



femora of male densely fringed on the basal half with long hairs. 

 Hypopygium curiously clothed behind with long and overhanging 

 hairs 2| mm. sordida, Ztt. 



41 (40). The vein only gently curved. Hind femora and hypopygium without 



such hairs. 



42 (43). Frons high, scarcely broader than long I5 mm. altifrons. 



43 (42). Frons low, about \ broader than long. 



Four closely allied species come under this heading. Common 

 to all of them is a moderately broad frons with full-sized antennae, 

 closely approximated svipra-antennal bristles, large and neai-ly equal, 

 and large and prominent yellow palpi ; the costal fringe is very long, 

 the ciirve at the origin of the first thin vein gentle in the male, biit 

 more pronounced, though still moderate, in the female ; and the tibial 

 cilia always distinct and generally large, fore-tarsi somewhat thick in 

 both sexes of variabilis and in the female of pleuralis. 



44 (49), First costal division not longer or barely longer than the .lecond, and 



distinctly .shorter than the .second and third together. 



45 (46). 7' Male hypopygium armed at the corners and on the sides with large and 



conspicuous bristles, almost identical in number and distribution with 

 those in humilis 1^-2 mm. pleuralis, 



46 (45). Hypopygium without bristles. 



47 (48). Wings clear ; angle at fork rather variable but never acute. 



...1^-2 mm. diversa. 



48 (47). Wings deeply tinged with yellowish brown; angle at fork acute but 



rather variable 2-2-i mm. variabilis. 



49 (44\ First costal division about twice as long as the 2nd, and distinctly longer 



than the 2nd and 3rrf together. Hypopygium armed with some small 

 and inconspicuous bristles, somewhat similar to those of pleuralis. 



...lA mm, subpleuralis. 



