Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
This nymph keys pretty well to Neoleptophlebia. Under the microscope, the fork on the first gill is more symmetrical than it looks in the picture here. Following the species key in Jacobus et al. (2014), the lack of tusks on the head or postero-lateral spines on segment 8 leads to adoptiva or heteronea, but both refer to a 3-segmented labial palp as a key characteristic, and this one only seems to have two segments, just like another, different-looking specimen from the same sample. So I'm stuck.
I thought the end of the abdomen of this one was damaged during capture and transport, but I photographed it anyway because it looked different from the others. However, it became clear under the microscope that it was actually just mid-way through molting and the abdomen was intact underneath. I got a couple nice microscope photos showing that. The pattern of markings on the abdomen is still distinctive compared to the other specimen, which was more representative of a common nymph in my sample. This one was unique.
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.