Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
Hi Roger,
As I understand it, Baetis bicaudatus has only 2 longitudinal veins in the hind wing. This one has three, with the third being only about half-length and very close to the margin of the wing, but clearly present (especially under my new microscope -- harder to see in the pictures).
This specimen seems to fit very well the description of Baetis alius, but I also don't have information to rule out tricaudatus or flavistriga. In fact I can't find much of a description of the adults of either species, except for the fairly un-detailed original descriptions, which probably aren't terribly relevant now given all the synonyms that have been combined in Baetis since then.
Of the species known to be present in central Idaho, these three seem like the most plausible options. Do you see anything to narrow it down from there?