Mayfly Family Ameletidae (Brown Duns)

This specimen is close to Ameletus vernalis but the wing coloration is missing.

Keying this one out using Zloty & Pritchard 1997:
-No ganglionic markings on sternites 2–8
-Posterior margins of sternits 6–8 without numerous spines
-Mesal gill extension well developed (I really don't like the "well developed" language when it's subjective, but in this case the other option doesn't lead anywhere productive)
-Tails pale in basal 1/3
-Larger species (13–16 mm) — this one is just shy of 12 mm, but closer to 13 than to <10
-Spring emergence
This all points to Ameletus vernalis.

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.

I spent ages trying to identify this one but ultimately couldn't narrow it down to species. I'm guessing it's either a species that has not yet been reported from Idaho or a species with some variation in characteristics not accounted for in the current key (Zloty 1997), which is only for Alberta but happens to contain all the species documented in Idaho except for one (which is rare and only in a different part of the state from this one).

Here are my raw notes from the microscope session:

8. Ameletus nymph (genus 100 % based on mouth parts under microscope)
1. This is probably a species with the nymph either not described yet or not reported in Idaho (or Alberta).
2. There is a key to the species of nymphs in Alberta (Zloty 1997) which includes all but one (A. tolae) of the species listed in Idaho by IDFG (https://idfg.idaho.gov/species/taxa/8607), and A. tolae is only listed from one drainage in north-central Idaho. So my specimen should be in that key. However, it doesn’t fit any of them.
1. The antennae are pale with brown at the apex. This doesn’t fit any of the species they described.
2. The labrum is almost completely dark brown, maybe a bit paler toward the apex.
3. Following the key in Zloty 1997 basically rules out every species reported in Idaho except for tolae, which would be outside its range:
1. Couplet 1 : There definitely aren’t strong ganglionic markings on sternites 2-8 (100 % rules out similior and celer) —> 3
2. Couplet 3 : Posterior margins of sternites 6-8 lack large spines (80 % sure) but other characteristics rule out the species if there were spines (validus, oregonensis, subnotatus) —> 6
3. Couplet 6 : Mesal gill extension present, but pretty slim… similar to Fig. 23B or 23G —> 7
4. Couplet 7 : Obviouly gos to 8
5. Couplet 8 : Small size and time of year rules out velox, tergite patterna and gill shape rules out pritchardi (which is not reported in Idaho anyway). Additional features (antennae, labrum color) rule out a small velox.
6. Backtrack to call the mesal gill extension “well developed” —> 9
7. Couplet 9 : Tail coloration obviously —> 10
8. Couplet 10 : Supposing it’s a small specimen of a “larger species” leads to 11, in which femora coloration and timing rule out vernalis, and color pattern rules out bellulus. Mesal extension on gills from species description very conclusively rules out bellulus. Therefore, calling it a “smaller species” is the correct path —> 12
9. Couplet 12 : Sternites without well-defined longitudinal stripe —> cooki. However, tergite color patterns don’t even come close to matching ANY of the 3 species from this point on (cooki, sparsatus, suffusus). From the species descriptions:
1. cooki: Antenna and labrum colors don’t fit. Mesal extension should be larger and tracheation lighter, to be this species.
2. sparsatus: Antenna and labrum colors don’t fit. Mesal extension should be larger. Postero-lateral spines should be very prominent, not barely noticeable.
3. suffusus: Also bad fit to antenna and labrum colors, mesal extension, and especially gill tracheation.

I keyed this one out using the Alberta species key from Zloty & Pritchard 1997.

Notes from the ID include:

1. Posterior margin of sternites 6-8 without spines
2. Mesal gill extension "well developed"
3. Basil third of caudal filaments pale
4. Anterior surface of front femora mostly pale
5. Dark band on caudal filaments begins around segment 20
6. Final instars early season

This one keys out pretty cleanly to Ameletus vernalis except the color pattern on the tergites doesn't match. However, two species known in Washington aren't included in the key. Of those two, Ameletus vancouverensis would be too small (adult body length 9 mm), but Ameletus andersoni (currently documented only from a spring in Cowlitz County) has a wide range of sizes and emergence times that could be compatible with this one. So I can't rule that species out, but Ameletus vernalis seems the most likely.

I think this is a pretty early instar Ameletus nymph. It's certainly a striking one.

These two exuviae are my sad little consolation prize after collecting a couple of really cool, large, mature mayfly nymphs I didn't recognize from a tiny, probably fishless, spring-fed tributary of a slightly less tiny trout stream. I saw the nymphs in the kicknet samples and carefully transferred each one to my holding cooler before going to collect more samples. When I got home, I carefully went through the whole sample and couldn't find either one of them. Getting worried, I did it again and even more carefully. This time, I found both exuviae. By process of elimination, pretty much the only possibility is that both of them hatched out of my cooler and flew away during the 10 minutes or so that I had the lid off to collect more samples.

I was really excited about them for the whole three-hour drive home and seriously bummed that they disappeared. In the field I thought they might be one of the families of swimmer nymphs I don't have yet, like Ametropodidae, but keying the exuviae takes me to Ameletus, so at least I didn't miss some great rarity. Still, they were much larger and darker than any other Ameletus I've collected and probably a species I don't have yet.

Edit: I returned four years later and caught a specimen of Ameletus velox in this spot (actually several, just photographed one), which I think solves the mystery of this one's ID.

This Ameletus has puzzled me since I found their exuviae four years ago. Using the key in Larvae and adults of Ameletus mayflies (Ephemeroptera: Ameletidae) from Alberta this appears to key to Ameletus velox. It also sort of matches the color pattern on abdominal segments 6-7 from their figure 20C, which their text mentions as another identifying characteristic. However, one characteristic ("incisor area of left mandible with second denticle much smaller than first") doesn't seem to match. Also, velox is reportedly among the largest Ameletus species, but not quite as large as this nymph. My best guess is still that it represents a bit of undocumented variation on velox.