XXIY BULLETIN 31, L^^ITED STATES XATIOXAL MUSEUM. 



This group is a natural oue, but it is not distinctly separated from the 

 followiug. I^ristaUs shows a considerable diversity in coloration, but 

 less in structure. In many of the species the hind femora are slender, 

 iu others sometimes much thickened {Simoides Lw.) ; this character will 

 not, however, serve for generic separation. Xor can the absence of the 

 moderate plumosity of the arista {ErisUdomyia Rond.) serve to divide 

 the genus. It throws together unrelated species. The pilosity of the 

 eyes seems to be equally unsatislactory. The relationship oi' Plagiocera, 

 Senaspis, Priomerus, BolioiiyrpJiifs, M€gaspis,ete.,ciiuuoteLl\\dysheiiir€iip\y 

 defined. Two of these ai)pear in our fauna, and will readily be distin- 

 guished by the general habitus, structure of the head, etc. In Xorth 

 America, ^m^aZis is sharply distinguished from Eelophilus by the closure 

 of the marginal cell. In Euroi)e and Africa, however, the two genera 

 coalesce, some species of EristaUs having dichoptic males {Eristaliuus 

 Rond.) 5 other species of Helophilus holoptic males and without any of 

 the characteristic markings, and with the marginal cell only narrowly 

 open. So, too, Eelophilus is ultimately distinguished from Mallofa only 

 by a very slight diflereuce in the third joint of the antennae.* The group 

 of genera immediately surrounding Mallota is an unsatisfactory one, and 

 very difficult to define. Pferallastes will be easily distinguished by its 

 general habitus, and the presence of dense pollen on the upper surface 

 of the thorax and scutellum, but Ttiiclioenemis and Triodonta are not so 

 easily disposed of; both of them difier from Mallota in being thinly 

 pilose, but there is no other tangible character. Teuchocnemis was based 

 upon the presence of an internal hind tibial spur in the male, but the 

 same feature occurs in the true Mallotcc. The two species placed in this 

 genus are not homogeneous. Triodonta cannot be i)laced under J/erof7on, 

 on account of the anteunjB, termination of the fourth vein, etc.; its di- 

 choptic male is common to European species oi Mallota. It is very singu- 

 lar that the genus Merodon, so richly represented in Eurojie, does not 

 occur iu our fauna. The relationship of Mallota is evident with Xylota. 

 (See Schiner, Nov. Exped., 300.) 



XYLOTINI. 



Third longitudinal vein only gently curved, marginal cell open; anterior 

 cross-vein near or beyond the middle of discal cell, usually oblique ; antennce 

 short; thorax without yelloto marlcings. 



Xylotini. — Abdomen more or less elongnte ; hind femora more or less thickened, with 



projections or spines below ; face without tubercle; nearly bare species; eyes large. 



a. Face strongly curiuate; hind femora thickened, with an angular i)rotuberance 



below near (he end Tropidia Meig. 



aa. Face not carinate. 

 h. Third segment of the abdomen iu the male very much contracted, cylindrical, 

 the hind femora much thickened, with a bifid spine below at the tip. 

 Eyes very large, face small Senogaster Macq. 



* Even this difference is not accepted by Schiner in Helophilus antipodus Sch.— No- 

 vara Exped., 359.— Auckland. 



