The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
I have been told this is the only western aquatic moth, Petrophila confusalis, see here on milkweed blossoms.
This insect was collected from the Flathead River-lower in Montana on July 21st, 2005 and added to Troutnut.com by Bnewell on June 28th, 2011.