The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
Notes from the microscope on the ID: Maxillary palp is present, distinctly 2-segmented, but very small. Gills on segment 4-7. This specimen has some unfortunate damage to the abdomen, but it's the only one I found in my sample.
This mayfly was collected from the East Fork Big Lost River in Idaho on August 3rd, 2020 and added to Troutnut.com by Troutnut on August 19th, 2020.