1924] North American Species of Opacifrons and Pteremis 125 
interfrontal bristles strong, in four pairs, the anterior pair 
reduced, the other three increasing in size anteriorly, with the 
third pair from above, cruciate; antennae prominent, second joint 
with strong bristles, third joint oval, larger than second; arista 
two and one-half times antennal length, its pubescence short 
and dense; face concave in profile, slightly gray dusted; carina 
developed between the antennae only; clypeus broadly visible 
from in front; cheeks gray dusted, one-fourth eye height; 
buccae with a strong anterior upcurved bristle and three setae; 
oral setulae hair-like, Mesonotum with three distinct pairs of 
dorsocentral bristles, increasing in size posteriorly; acrostichal 
setulae arranged in six rows between the anterior pair of dorso- 
central bristles, the middle rows longer than the others; post- 
humerals absent; scutellum large, triangular, marginal bristles 
four, the posterior pair very long; upper sternopleurae with 
two anterior setae and a long posterior bristle. Legs with knees 
browned; front femora with five hair-like extensor and four 
preapical flexor bristles, middle femora with six preapical an- 
terior bristles; middle tibiae with five extensor, two before 
middle and two on apical third, and two flexor bristles, one at 
middle and one at near apex (preapical bristle absent in male); 
middle metatarsi with a flexor bristle at basal third and a number 
of apical bristles; hind tibiae with a row of hair-like bristles on 
outer side; first and second joints of hind tarsi with a brush of 
stiff hairs beneath; hind metatarsi one-half as long as next joint. 
Abdomen broad, shorter than thorax; second segment one and 
one-half times as long as third; hypopygium moderately large 
and hairy. Wings slightly browned; second section of costa 
distinctly longer than third; first section weakly bristly and 
equal to third; penultimate section of third and fourth veins 
subequal and two times outer cross vein; last section of third 
vein nearly straight; fourth vein traceable to wing-margin; 
fifth vein produced but little beyond outer crossvein. Halteres 
white. Length: 2.5-3mm. 
Type. — Female; Friday Harbor. June 25, 1909. (Melander) 
Paratypes. — Fifty-one specimens of this species, distributed 
as follows : 
