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 nymph keys out to Osobenus yakimae, and a comparison of the markings with a specimen on flyfishingentomology.com provides a good verification. The size is about right, as that species has been described as 12 mm. The markings generally fit the original description of the nymph of that species (Jewett 1955) perfectly.