Identification: Easy Angler's Key to Common Mayfly Nymphs, Couplet 5

This specimen keys to the Neoleptophlebia memorialis group, which means it could be Neoleptophlebia memorialis or Neoleptophlebia temporalis. In the absence of a clear species key, I'm just guessing memorialis for now.

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.

I caught this Ameletus nymph with several others of the same kind. This was the most vivid example, but they all had quite a bit of striking and unusual red shading, especially on the last few abdominal segments.

I keyed it out under the microscope using Larvae and adults of Ameletus mayflies (Ephemeroptera: Ameletidae) from Alberta with slightly larger (10 mm), mature specimen with darkened wingpads. Microscope pictures are from that specimen. The characteristics in the key and most of the verification table point pretty clearly to Ameletus cooki, except that the coloration of the antennae more closely resembles Ameletus sparsatus. However, on other characteristics in which these species differ (spines on the dorsal surface of the front femora, which seem very short in this specimen; length of posterolateral spines on segments 8–9; length of spines on posterior edge of tergites 6–9), this is a better match for cooki, and that's probably the correct ID.

A few of these nymphs turned up in my kick net sample, with this being the only one fully mature and ready to emerge. It keys pretty straightforwardly to Ameletus suffusus, particularly because of the triangular dark patch on the labrum. The abdominal markings are close but don't quite match other drawings, likely just individual variation.

This specimen was collected along with a female which was quite a bit larger, different in color, but otherwise seemed the same morphologically.

These nymphs were highly abundant in my early April kick net samples from the Yakima Canyon, and one of them emerged into a dun, which I photographed immediately. Similar-looking nymphs but with distinctly brighter color patterns were also abundant. I just photographed one. After extensive views under the microscope, it's clear the bright ones are males and the dull ones are females of the same species.

The most likely guess at the species is Baetis tricaudatus, which may be a complex of related species that haven't all been sorted out yet. It isn't a perfect fit to every key characteristic (and I never seem to find a Baetis that matches the expected pronotum color patterns, but that seems to be the closest.

The microscope images here were taken with different specimens from the main photos (so I could dissect them while preserving that one intact), but clearly the same species.