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.
Here's another nymph with the strange brownish coloring rather than the olive I usually see for subvaria. I suspect it's the same species and it's just wide individual variation.
This mayfly was collected from unknown in Wisconsin on February 7th, 2004 and added to Troutnut.com by Troutnut on January 25th, 2006.