The 2003 catalog by Evenhuis & Greathead(1) lists only three species for Colorado: nudus, aridus, and snowi.

From the key in Painter (1932):

   nudus has no pile (fine hairs) on the face (area between antennae and mouthparts);
   aridus has all tibiae yellowish; and
   snowi has all legs the same dark-brown to black color.

From your last (flash) photo, I think I see hairs on the face (in silhouette), which would seem to eliminate nudus.

Assessing leg color is trickier, as the apparent color is often dependent (and seems to change) with lighting. For instance, in your 1st image (with little direct light) the legs look mostly dark, with a grayish cast on front and back femora. In your 3rd (flash) image, the middle tibia looks dark-reddish-brown, the other tibiae blackish, and the front and back femora again look "dusted gray".

From the gray on the tibiae, I'd say the legs aren't all the same color, which would eliminate snowi. While the tibiae don't appear yellowish, that might be accounted for by lighting, variation, and/or ambiguity in the author's perception or intended meaning for "yellowish". The description of G. aridus in Painter is an OK match, but not strikingly good.

All the above isn't saying G. aridus is necessarily the ID here...it's just an attempt to narrow to the best candidate, given the limitations of the images and the details available in Painter's key and descriptions. It may be that this genus is under-collected and that there are more than three species in CO, or that Painter's descriptions (and the "typological" approach inherent therein) don't adequately address variation within the species. In other words, as is so often the case, it may be the subgenus needs more work and revision.

At any rate, whatever the species, I think Geron are neat little bee flies :-)

Moved from Geron.

Geron.

Keep after them! We're getting quite a collection here.