276 BULLETIN' ol, UNITED STATES NATIONAL MUSEUM. 



more slender, iiiul with gTeater convexity above. In all those genera 

 ((7e/m, Baccita, i^iphcgina^ eic.) where the abdomen is clubbed this char- 

 acte. is more })ronoanced in the male. In the genera allied to Mallota 

 the male abdomen is found more cylindrical toward the end. 



The most important sexual characters are in the male legs, and 

 jirobably subscn-ve some purpose in the sexual relations, although such 

 must often be difficult; to uuderstaud, where otherwise closely allied 

 species differ markedly in their i)resence or absence. In the front pair, 

 PlaUjchirus and Pi/rophwaa ocl/nil are the only ones known to me where 

 structural differences are seen, the tibia3 and tarsi being more or less 

 dilated in the male, and the same sex sometimes i)rov]ded with femoral 

 bristles. Differences in the middle pair are confiued to Pyrophcena ocymi 

 and some species of riatt/chirus, where likewise may occur a dilatation 

 of the tarsi and structural peciiliarities of thetibiai. When we come 

 to the hind pair the most numerous and strik ing plastic differences pre- 

 sent themselves. In the cox* there is sometimes a more or less prom- 

 inent spur or process on the inner hind side in the male. Jn Fijjba 

 {Cnemodon) this may be long and slender, but usually it is conical, as 

 will be observed in Triodonta, XyJota sp., etc. In the femora, wherever 

 there is thickening it is usually more pronounced in the male, as in 

 Mesoyrapta sp., and genera of the Eristalini and Xylotini. In some of 

 these, as for instance Mesograpta gemmaia and Mallota^ the male femora 

 are, in addition, more arcuate, sometimes very much so. In not a few 

 genera the male is provided with one or more projections on the under 

 side of the thickened femora. (In Milesia and Spilomyia (9,5) there is 

 a conical projection near the outer end. In Senogaster it is flattened 

 and bilid; in others it takes the shape of an angular ])rojection, most 

 prominent in Tropidia. In some genera (9,5) (as Xylota, Brachypal- 

 pus, Brachyopa, and especially Syritta) there is a row of spines or short 

 bristles on the under side; in Lepromyia and species of Myiolepta this 

 character is found below all the femora, which are also all thickened). 

 In only one case {Tropidia mamillata) do I know of a long and cylindri- 

 cal process near the basal end of the femur. In the hind tibire there 

 are also differences. The most common are greater curvature and com- 

 pression, especially in those cases where the femora are thickened. An 

 excellent example will be found in si)ecies of Mallota. In some, as in 

 Spatigaster, etc., the bending may be sharp, almost angular; in others 

 the tibioe are provided with more bushy and long pile in the male. In 

 Helophilus spp., Eristalis spp., Xt/lota spp., Triodonta, and other genera, 

 the tibijB ( S ) are provided with a more pronounced spur at the tip. 

 In Mallota sp. and Teuchocnemia there is, in addition to the flattened 

 tibiae, a strong projecting spur in the middle in front, w hich lies on the 

 inner side of the femora when flexed. In Brachypalpus frontosus, and 

 in certain exotic genera, there is a smaller projection near the base of 

 the tibia). In the hind tarsi the only sexual character with which 1 am 

 acquainted consists in a thickening and elongation of the hind meta- 

 tarsi. This peculiarity finds its greatest and remarkable development 



