286 Mr. C. H. T. Townsend on Diptera 
species. They differ from my 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 antenne. Mesonotum of male in front 
of suture deeply golden-yellow pollinose, with two median 
vitte and a more or less elongate spot near suture velvet- 
black, the vittee more or less united, except at posterior ends. 
In one of the sinaller males the markings of mesonotum 
appear silvery with a golden shade. The elongate lateral 
spots are about half as long as the vitte, or little more, and 
represent another pair of vitte 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 this species. The golden of hind 
border of mesoscutum is more or less extensive, and may be 
rather silvery. In the female the vitte 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 vittee 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. Inthe 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 large 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, 
for 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 antenne vary a little in length, from slightly more 
to considerably more than one-half 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 
