The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
This spinner was collected in proximity to this dun and this other dun, so one of them is likely the same species. Note for ID purposes that this one is missing one of its cerci.
This mayfly was collected from the Madison River in Montana on July 1st, 2019 and added to Troutnut.com by Troutnut on July 18th, 2019.