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 either Neoleptophlebia or Paraleptophlebia; I know of no way to be sure from a female dun alone. The genus is only a guess based on the fact that I found more Neoleptophlebia nymphs in the general area.
This mayfly was collected from the Neversink River in New York on May 6th, 2007 and added to Troutnut.com by Troutnut on May 10th, 2007.