The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
Although the identification is not certain because the nymph is not yet mature, the configuration of tubercles on this one--particularly the forefemora--seems to best match Drunella flavilinea.
This mayfly was collected from the Gallatin River in Montana on July 2nd, 2019 and added to Troutnut.com by Troutnut on July 18th, 2019.