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.
I think this one might be Leptophlebia intermedia.
This mayfly was collected from the Bois Brule River in Wisconsin on February 5th, 2004 and added to Troutnut.com by Troutnut on January 25th, 2006.