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.
This one was collected close to a male which, based on size and general structure, was likely the same species. Females are supposed to be lighter-colored and with less distinct markings than the males, although the difference in this pair is particularly stark.
This mayfly was collected from the Bitterroot River in Montana on September 6th, 2021 and added to Troutnut.com by Troutnut on September 8th, 2021.