FLIES OF GENUS RIVELLIA — NAMBA 39 



with yellowish brown lateral strip on each side which terminates just 

 before oral margin; parafacials and part of orbits ventral to each eye 

 brownish yellow. Antenna with first and second segments brownish 

 yellow; third segment with tip, outside, and anterior side of inside 

 blackened, remainder yellow, sometimes almost wholly blackened, the 

 tip pointed anteriorly. Anteclypeus concolorous with medio ventral 

 portion of face. Palpi brown. Postcranium black, pruinose. 



Thorax: Mesonotum black or dark brown; strong humeral and 

 posterior dorsocentral setae present. Scutellum and postscutellum 

 concolorous with mesonotum. Pleura dark brown or black, mostly 

 shining. Mesopleuion with scattered pale setae on posterior half; 

 pruinose area at dorsal posterior corner in form of a right triangle, 

 one side reaching two-thhds distance to ventral posterior corner and 

 the other reaching two-thirds distance to anterior spu-acle. Stemo- 

 pleuron with narrow pruinose strip on ventral margin and a narrower 

 strip on posterior margin; disc with scattered pale setae. Disc of 

 pteropleuron with scattered pale setae, black setae present or absent 

 ventral to subalar. Hypopleuron with disc shining, borders narrowly 

 prumose. Laterotergite completely and densely pruinose. Proster- 

 num black. 



Thoracic appendages: Front legs: Coxae, femora, and tibiae 

 yellow; tarsi with fifth segment slightly blackened, remaining seg- 

 ments yellow. Middle legs: Wholly yellow. Hind legs: Coxae 

 yellow; femora wholly yellow or brown at tip on inside, sometimes on 

 outside also; tibiae yellow at tip, remainder brownish yellow or brown; 

 tarsi wholly yellow or fifth segment slightly blackened. 



Wing (fig. 4,e) length 2.1-3.0 mm. Bands dark brown or black, nar- 

 row. Fhst and second bands widely separated at costal margin ; third 

 and fom-th bands definitely separated at costal margin. Costal cell and 

 the part of first basal cell contiguous to second basal cell completely 

 hyaline or at most very faintly banded basally. Section of fourth 

 vein proximad of anterior crossvein with slight bend. Halteres with 

 pedicel brownish yellow, capitulum dark brown. 



Abdomen: Tergites shining, smooth, black or dark brown. Disc of 

 first tergite with pale setae. Genitalia (fig. 4:,g,h) with apical part of 

 aedeagus somewhat conical in shape; the two filamentous terminal 

 tubes little longer than base of apical part. The apical part appressed 

 at base to basal part of aedeagus. 



Specimens examined: Illinois (Icf): Savanna; June 12. Mary- 

 land (299, 2d'cf): Cabin John, Forest Glen.; May 16-July 29. 

 Michigan (19): Detroit; June 24. Minnesota (27999, 174 cfd'): 

 St. Paul Water Works (Anoka County), Hennepin County, Missis- 

 sippi Bluff one-half mile north of state line (Houston County), Lake 



