The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
The positive species ID on this dun comes from both the spinner that it (or possibly one other dun just like it) molted into and the overwhelming abundance of nymphs of this species in my kicknet samples from the same site.
This mayfly was collected from Mystery Creek #199 in Washington on July 28th, 2019 and added to Troutnut.com by Troutnut on July 30th, 2019.