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.
Probably Callibaetis ferrugineus.
This mayfly was collected from Flathead Lake in Montana on May 25th, 2005 and added to Troutnut.com by Bnewell on June 27th, 2011.