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Lateral view of a Female Hexagenia limbata (Ephemeridae) (Hex) Mayfly Dun from the Namekagon River in Wisconsin
Hex Mayflies
Hexagenia limbata

The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.

Dorsal view of a Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
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: "Abdominal terga 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.
27" brown trout, my largest ever. It was the sub-dominant fish in its pool. After this, I hooked the bigger one, but I couldn't land it.
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Cherylkorca has attached these 5 pictures to aid in identification. The message is below.
Cherylkorca
Cherylkorca's profile picture
Posts: 4
Cherylkorca on Feb 18, 2020February 18th, 2020, 4:17 pm EST
Hello, I did recently sampled our home water the Bitterroot river in SW Montana and I collected 2 stoneflys specimens and I know for fact one of them is Skwala nymph but the smaller one wanted to verify if is Nemoura nymph , the problem I am having is that this stone fly nymph appear to be much larger in length that my good reference book Aquatic insects by Rick Hafele shows ?
The do overlap each other i emergance time , Capnia first ( smallest of 3 ) than Nemoura and than biggest Skwala

Thank you for anyone who could help out with this nymph ID , I did include collage with adult Nemoura and hope to be a nymph in same order , but not so sure about the nymph ??
Millcreek
Healdsburg, CA

Posts: 344
Millcreek on Feb 19, 2020February 19th, 2020, 2:05 am EST
My guess is that it's not a Nemouridae, but a Perlodidae. The easiest way to tell the difference is to look at the mouth. If you have access to a microscope or a magnifying glass take a look at the glossae and paraglossae, they should be more or less the same size in Nemouridae. In Perlodidae the paraglossae should be much larger than the glossae.
"If we knew what it was we were doing, it would not be called research, would it?"
-Albert Einstein
Leskorcala
Posts: 16
Leskorcala on Feb 24, 2020February 24th, 2020, 4:39 pm EST
Hello , Thank you for a great tip to distinguish two however i have bad eyes ( blind on one ) and use magnifying glass 8X but i can not see it.I did take several closeup images with my camera , perhaps I can show it to you.
I need to figure out first how to attach some images.

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