turn out to be halli . Granted, the squamae are not margined in black, but marginatus is also described having black or brown legs, except for the knees.
And there's the considerably longer proboscis.
Meanwhile, I'll take a look at the link you just provided under the first photo.

The drawing of the head of E. halli in "Manual of Nearctic Diptera, Vol. 1" conforms nicely to the specimen pictured laying on its back. Proboscis length is notoriusly variable within the species of Eulonchus, so I wouldn't put too much faith in that as a defining character. The dark legs with paler knees seem to be a common thread in marginatus, halli, tristis, and the as yet undescribed species from the southern Sierra Nevadas. The length of the proboscis on the specimen pictured above looks perfect to me for halli. The prominent ocellar tubercle would also match and separate your specimen from E. "sierraensis" but not E. marginatus. Have fun trying to untangle this one!