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.
This mayfly was collected from the Yakima River in Washington on February 18th, 2023 and added to Troutnut.com by Troutnut on February 21st, 2023.