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.
Some characteristics from the microscope images for the tentative species id: The postero-lateral projections are found only on segment 9, not segment 8. Based on the key in Jacobus et al. (2014), it appears to key to Neoleptophlebia adoptiva or Neoleptophlebia heteronea, same as this specimen with pretty different abdominal markings. However, distinguishing between those calls for comparing the lengths of the second and third segment of the labial palp, and this one (like the other one) only seems to have two segments. So I'm stuck on them both. It's likely that the fact that they're immature nymphs stymies identification in some important way.
I found numerous mature nymphs and one flying adult of this species at the end of July in a small stream at 5,200 feet elevation in the Wenatchee Mountains of Washington.
Although Kogotus and Rickeri nymphs may be difficult or impossible to tell apart, I captured an adult Kogotus nonus specimen in the air on the same day as several of these nymphs, and they are likely the same species.
Described by Needham, J.G. & Claassen, P.W. (1925) A Monograph of the Plecoptera or Stoneflies of America North of Mexico. The Thomas Say Foundation II, 1–397.