I was fascinated by this tiny fly when I first spotted it on a willow leaf (likely Salix exigua
from gestalt, reinforced by this nearby record
...but perhaps not especially relevant for a predatory fly, although small gall insects often abundant on willows could provide a plausible food resource).
Unfortunately it flew after the first (poor quality) photo I got...and despite further searching in the field, we were unable to find another. (I was wondering whether the myriad damselflies in the area might have found them before we did!)
In the field, it was too small for me to get an idea of what it might be...but upon seeing the photo on a computer screen, it appeared to be a tiny robber fly. And after scanning BugGuide and other web resources, and in particular having the good fortune of finding the thorough and excellent 1975 revision of Dioctria
and related genera by Adisoemarto & Wood...I'm confident this is Dioctria
, and specifically D. pusio
In going through the keys in Adisoemarto & Wood, a close second candidate was D. vera
. Both pusio
have records closer to the location here than any other Dioctria
species in the revision...but comparing Osten-Sacken's original 1877 description of D. pusio
and Back's original 1909 description of D. vera
with the photo here further supports D. pusio
. In particular, D. vera
is described as having "a proportionately broad band (of light golden pubescence) extending over the anterior portion of the meta- and sternapleura and reaching to the front coxae" which, even with the poor resolution of the photo here, seems evidently absent.
While Adisoemarto & Wood give the size for D. pusio
as 6-7mm for males and 7-8mm for females, Osten-Saken's value of 4.2mm seems more in line with my impression from the field. (Though field estimates of size are often inaccurate, the impression is reassuring :-).
If I had to guess, I'd say this is a female (from the bulging abdominal venter)...but I really don't know.