This specimen represents a common find in a late-April sample from the far upper Yakima River. It seems to be the same species as another one I collected previously. Of the species keyed in Szczytko & Stewart 1979, it probably matches Isoperla fusca closest, but there's a good chance it's a species that wasn't in the key. The leg segments have a fringe of fine hairs which is supposed to be absent in Isoperla fusca, and the four dark stripes of the mesonotum and metanotum don't continue as 4 separate stripes on the pronotum as they should in the description of fusca. It's possible fusca is more variable than previously described, or this is a different species not included in that key. It's also worth noting there's definitely no fringe of fine setae on any part of the cerci, just the whorls of little stout ones around segment bases.

This is a difficult one. It keys convincingly to either Calineuria or Doroneuria. There is no mesal longitudinal row of silky hairs on thorax and abdominal dorsum, in either my pictures or anything I can see up close under the microscope. Lacking those hairs should indicate Calineuria. But abdominal sternum 7 has an incomplete posterior setal fringe, which would indicate Doroneuria, although the key says "usually" on that characteristic. For now I'm going with Calineuria, but it's far from certain. Maybe it's a younger Doroneuria and they don't grow that row of hairs until they're older. The markings on the head, especially the shape of the bright spot around the posterior ocelli, match online images of Calineuria californica better than Doroneuria.

This one took an embarrassingly long time to identify, because the key I was following sent me on a wild goose chase when I interpreted the longitudinal dorsal stripe on the abdomen of this specimen as a "longitudinal pigment band." I've updated the key text on this site so that mistake isn't so easy for others to make.

I'm tentatively calling this specimen Rhithrogena virilis, based on the incomplete key in The Biology of Mayflies which describes the correct body length (12 mm) and a purplish tinge on the dorsal half of the platelike gills. An early specimen posted by Bob Newell describes markings on the last couple abdominal tergites which also match this nymph.

This one was very puzzling to identify. It's clearly Heptageniidae. It keys pretty easily to couplet 58 in Merritt, Cummins, & Berg 5th Edition, but that's where things get tricky. 58: Male penes distinctly L-shaped; Segment 1 of foretarsi usually 1/3 to 2/3 the length of segment 2 58': Males penes not distinctly L-shaped; Segment 1 of foretarsi usually 1/5 to 1/2 the length of segment 2 The characteristics conflict here, as I'd say the penes ARE L-shaped, but the fore tarsal ratio is less than 1/3. Maybe the weasel word "usually" allows for that, and it's some kind of Stenonema, for which I'm not aware of any records in Montana, let alone western Montana. Going by the tarsal ratio and assuming it's not Stenonema, everything in couplet 60 points to Afghanurus: contiguous compound eyes, genitalia similar to fig 13.222 in MC&B, and weakly developed basal costal crossveins in the forewings. But I looked up the original descriptions of the three Afghanurus species, and the body sizes (flowersi 5-7 mm, inconspicua 4 mm, joernensis 6-7 mm) are way below this one's 10 mm. This led me to notice that the closely related Ecdyonurus is not included in the key at all, so I checked the descriptions of those species and found that Ecdyonurus criddlei fits very well: the color of the legs, the markings on the tergites and sternites, the short fore tarsus and ratios of the segments, etc. It was originally described as 7 mm, smaller than the 10 mm body of this specimen, but synonyms have been described with larger sizes including wing lengths up to 11 mm for females for Heptagenia salvini. This specimen was found with a female nearby, which I think is probably the same species, although it is difficult to be certain.

This specimen was collected along with a female. The forewing venation and styliger plate of the male give a confident identification as Tricorythodes explicatus according to Baumgardner 2009.

This one was collected close to a male which, based on size and general structure, was likely the same species. Females are supposed to be lighter-colored and with less distinct markings than the males, although the difference in this pair is particularly stark.

This female Trico was collected with an associated male and many others.