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 is a subspecies M. terminatum terminatum, formerly called Stenonema. Found mostly in larger rivers in western Montana. This is an immature nymph.
This mayfly was collected from the Flathead River-lower in Montana on November 23rd, 2005 and added to Troutnut.com by Bnewell on June 26th, 2011.