286 Mr. C. 11. T. Tovvnsend on Diptera 



species. They differ from ray description of atripennis as 

 follows : — 



Whole face and sides of front of female silvery-white 

 pollinose. The frontal stripe is sometimes but little widened 

 anteriorly at base of anteniise. Mesonotum of male in front 

 of suture deeply golden-yellow pollinose, with two median 

 vittai and a more or less elongate spot near suture velvet- 

 black, the vittffi more or less united, except at posterior ends. 

 In one of the smaller males the markings of mesonotum 

 appear silvery with a golden shade. The elongate lateral 

 spots are about half as long as the vittse, or little more, and 

 represent another pair of vittas anteriorly obsolete. These 

 black markings may be so approximated as to give the ap- 

 pearance of a golden fascia along the suture, with its anterior 

 border irregular or jagged, as given in my description of 

 atripennis ; though this is not the case in the present speci- 

 mens, and probably not in tliis species. The golden of hind 

 border of mesoscutum is more or less extensive, and may be 

 rather silvery. In the female the vittge and golden pollen of 

 mesonotum are very faint, except that the humeri are dis- 

 tinctly and rather broadly golden, following inward along 

 front border of suture a short distance. The rest of thoracic 

 dorsum appears more or less shining black, thinly shaded 

 with silvery, the vittaj sometimes being faintly outlined. 

 Scutellum is more or less faintly silvery. The males agree 

 fairly well in colour of abdomen with description of atri- 

 pennis. In the female the black of abdomen is much more 

 extensive, leaving only a large more or less irregularly de- 

 fined oblong lateral yellow spot on second and third segments. 

 The black covers third segment in a lai'ge triangle, and on 

 second segment appears as a narrow but sometimes much 

 widened median vitta. In the female only the basal half or 

 third, or even less, of hind femora is yellow; in male some- 

 times only basal half; other femora often only narrowly 

 yellow on bases. (See my paper, above referred to, p. 143, 

 tor the colorational and abdominal differences in the sexes of 

 arcuata, drawn from Illinois specimens.) The variation in 

 colour of bases of femora just mentioned cannot be depended 

 on as a specific character. Claws and pulvilli of female 

 short. The antennaj vary a little in length, from slightly more 

 to considerably more than one-lialf length of face. The sides 

 of abdominal segments in female are conspicuously silvery 

 pollinose in oblique lights. The frontal bristles are moderately 

 strong, stronger than in Giglio-Tos's figure of X. articulata. 

 Wings quite abruptly hyaline on somewhat more than inner 

 half. Abdomen of male yellow, with basal segment more or 



