Identification of this one was as follows. Body 9 mm, wing 11 mm.
Both Epeorus albertae
and Epeorus dulciana
should have a conspicuously darkened humeral crossvein
in the forewing. This one doesn't.
The foretarsal claws are dissimilar (one sharp, one blunt), which also rules out the Epeorus albertae
The dark macula on the forefemora rules out Epeorus deceptivus
, which is also supposed to be a little bit smaller.
Both Epeorus grandis
permagnus should be much, much larger.
Of the species known to be present in Washington, this leaves only Epeorus longimanus
, which is exactly the right size. The key to male spinners in Traver (1935) describes distinctive markings that are visible (although more faintly) in this dun: “Black posterior
margins of tergites
do not extend laterally to pleural fold, but an oblique black line form this margin cuts across poster-lateral
triangle to pleural fold.”