286 DIPTERA OF NORTH AMERICA. [PART IV. 



straight, only the anterior branch of the second longitudinal vein 

 is somewhat arcuated (especially in B. tenuipes, where the 

 posterior branch is also, but very slightly, arcuated); the mar- 

 ginal cross-vein is very near the tip of the first longitudinal vein 

 (at this very tip in R. tenuipes) ; the tip of the first longitudinal 

 vein is opposite the tip of the third branch of the fourth longi- 

 tudinal vein. Both branches of the fourth longitudinal vein are 

 forked ; the anterior fork is very short, the second posterior cell, 

 which it incloses being about one-third the length of the first 

 posterior cell ; the basis of the third posterior cell in R. tenuipes 

 (which has no rascal cell), is in one line with the small cross-vein, 

 and rather pointed ; in R. Jlaveola the third posterior cell is 

 divided in two by the cross-vein, which forms the subtriangular 

 discal cell ; the fourth posterior cell is about half the length of 

 the first ; the fifth is somewhat longer than the fourth ; the great 

 cross-vein is a little beyond the first forking of the fourth longi- 

 tudinal vein ; the fifth, sixth, and seventh longitudinal veins are 

 nearly straight, somewhat, but not conspicuously, arcuated. 



The abdomen is elongated and slender ; the male genitals 

 rather club-shaped, consisting of the usual basal pieces, with 

 horny appendages ; one of the latter, in R. tenuipes, is elongated, 

 needle-shaped, and conspicuous in living specimens, although not 

 visible in dry ones ;^ the ovipositor (R. Jlaveola) has rather long, 

 broad, arcuated upper valves, and blunt, without being actually 

 rounded at the tip ; the lower valves are shorter, but also rather 

 broad ; the ovipositor of R. tenuipes is likewise comparatively 

 long, and arcuated, but narrower and more pointed. 



This genus, described here for the first time, although it was 

 mentioned by name in the Proc. Entom. Soc. Philad. 1865, p. 

 225, is closely allied to Plectromyia, but easily distinguished 

 from it by the greater slenderness of the body and especially of 

 the abdomen, which has the male genitals distinctly club-shaped ; 

 by the structure of the male genitals, and by the venation of the 

 wings, which have five, instead of four posterior cells. 



Although R. Jlaveola differs from R. tenuipes by the presence 



' I add this detail from memory, as the description of the forceps, taken 

 down from a living specimen, has been lost with my original manuscript. 

 Although I have caught a specimen since, I have omitted to describe its 

 forceps. 



