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 is an interesting one. Following the keys in Merritt R.W., Cummins, K.W., and Berg, M.B. (2019) and Jacobus et al. (2014), it keys clearly to Ephemerella. Jacobus et al provide a key to species, but some of the characteristics are tricky to interpret without illustrations. If I didn't make any mistakes, this one keys to Ephemerella mucronata, which has not previously been reported any closer to here than Montana and Alberta. The main character seems to fit well: "Abdominalterga with prominent, paired, subparallel, spiculate ridges." Several illustrations or descriptions of this holarctic species from the US and Europe seem to match, including the body length, tarsal claws and denticles, labial palp, and gill shapes. These sources include including Richard Allen's original description of this species in North America under the now-defunct name E. moffatae in Allen RK (1977) and the figures in this description of the species in Italy.
I fished from 7:30 am to 2:00 pm in the area I had hoped to fish the night before, if I hadn't see too many vehicles. I was the first one there, but a couple other guys arrived soon. I walked pretty far from the parking area, and I started fishing. upstream. I worked upstream with dries and nymphs, seeing no early action, but spooking a few good fish. Finally I saw one before it saw me, and I successfully targeted it with a perdigon-like nymph ("Spanish Bullet") under an indicator. That was the beginning of a great morning chasing big cutthroat.
My uncanny success with a nymph made for tight-line or Euro nymphing, despite using it on my usual 5-weight with a typical indicator, motivated me to finally give my new Euro nymphing rod and reel a try at my evening stop, the Big Lost River near Mackay, Idaho. I left my normal rod in the car and walked a good distance down to the river, committing to flailing around with the new technique and seeing what happens. Some good fish were rising, and nothing was touching my nymphs. I tied on a dry, flailed it around on the end of a very long leader of lightweight monofilament, and somehow managed to fool a 15" rainbow. Interesting way to inaugurate the new rod!
This "specimen" is actually two different duns, one missing the middle front and back left legs, the other missing the terminal filament i.e. middle tail. I added them together by accident, but since they're the same species, stage, and gender, I might as well leave them together.