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.
It has the segment 4 gill and abdominal segment 9 is longer than segment 8. I couldn't see abdominal tubercles, but they may not yet be developed in such an early instar.
This mayfly was collected from unknown in Wisconsin on March 10th, 2004 and added to Troutnut.com by Troutnut on January 19th, 2006.