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.

Firehole River Sunset

From the Firehole River in Wyoming

Stream photos

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Female Sweltsa borealis (Boreal Sallfly) Stonefly Adult Female Sweltsa borealis (Boreal Sallfly) Stonefly Adult

Lateral view of a Female Sweltsa borealis (Chloroperlidae) (Boreal Sallfly) Stonefly Adult from Harris Creek in Washington

I was not fishing, but happened to be at an unrelated social event on a hill above this tiny creek (which I never even saw) when this stonefly flew by me. I assume it came from there. Some key characteristics are tricky to follow, but process of elimination ultimately led me to Sweltsa borealis. It is reassuringly similar to this specimen posted by Bob Newell years ago. It is also so strikingly similar to this nymph from the same river system that I'm comfortable identifying that nymph from this adult. I was especially pleased with the closeup photo of four mites parasitizing this one.

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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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Ephemerella nymph

Ephemerella aurivillii Mayfly Nymph

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Dorsal view of a Ephemerella aurivillii (Ephemerellidae) Mayfly Nymph from the Foss River in Washington

This is a puzzling one to identify and I'm not sure about the species. The maxillary palp is present and segmented, and the maxillary canines are not strongly serrate laterally. I think it's Ephemerella, not Serratella. The ventral lamellae of the gills on abdominal segment 6 have a clear median notch with a depth at least half the length of the lamellae, which points toward a couple of uncommon species (most likely Ephemerella alleni), but the abdominal tubercles and coloration don't fit that species. To add to the confusion, none of the above species are expected to emerge in the fall, as far as I know. I'm going to call this one Ephemerella aurivillii for now, but that's highly uncertain.

Collected September 9, 2020 from the Foss River in Washington

Added by Troutnut on September 18, 2020

Collected September 9, 2020 from the Foss River in Washington and added by Troutnut on September 18, 2020

Millcreek
Healdsburg, CA

Posts: 344

Millcreek on Sep 24, 2020September 24th, 2020, 5:28 pm EDT

Jason,
I think this nymph may be Ephemerella tibialis. I am basing this on the following papers by Allen and Edmunds.

http://www.ephemeroptera-galactica.com/pubs/pub_a/puballenr1963p583.pdf

http://www.ephemeroptera-galactica.com/pubs/pub_a/puballenr1965p244.pdf

Ephemerella tibialis is in the paper on Serratella.

"If we knew what it was we were doing, it would not be called research, would it?"
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Troutnut

Administrator
Bellevue, WA

Posts: 2758

Troutnut on Sep 26, 2020September 26th, 2020, 6:53 am EDT

I was using this more recent key from Jacobus et al (not sure where to find a PDF online at the moment):

Jacobus, L. M., N. A. Wiersema and J. M. Webb. 2014. Identification of Far Northern and Western North American mayfly larvae (Insecta: Ephemeroptera), north of Mexico. Joint Aquatic Science meeting, Portland, OR. 176 pp. + suppl. Unpublished workshop manual.

Here's the relevant key couplet:

81e) Maxillary canines strongly serrate laterally; thoracic nota with small, brown excrescences (sometimes difficult to detect), especially between forewingpads = Ephemerella (Vittapallia) tibialis (very similar to E. nuda)

81e’) Maxillary canines not obviously serrate laterally; thoracic nota without small, brown excrescences = 81f

I think I did find a number of E. tibialis nymphs earlier this year in Idaho. Here's one of them. I remember being able to make out the highly serrate canines pretty clearly and possibly the excresences, and I couldn't find either one on this specimen. However, I don't have the greatest microscope and am hoping to get a better one soon, so I might take a closer look.

Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist