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.

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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Male Eurylophella temporalis (Chocolate Dun) Mayfly Spinner Pictures

This male spinner molted from a dun I photographed a few days earlier, as did a female of the same species that I captured with it.
It keys confidently to Eurylophella temporalis using Allen & Edmunds (1963), primarily based on the shape of the penes, which is more distinctive for this species than for most others in the genus. However, the remainder of the description of the male spinner in that paper fits this specimen perfectly as well.

Lateral view of a Male Eurylophella temporalis (Ephemerellidae) (Chocolate Dun) Mayfly Spinner from the West Fork of the Chippewa River in Wisconsin

Dorsal view of a Male Eurylophella temporalis (Ephemerellidae) (Chocolate Dun) Mayfly Spinner from the West Fork of the Chippewa River in Wisconsin

Ruler view of a Male Eurylophella temporalis (Ephemerellidae) (Chocolate Dun) Mayfly Spinner from the West Fork of the Chippewa River in Wisconsin The smallest ruler marks are 1 mm.

Ventral view of a Male Eurylophella temporalis (Ephemerellidae) (Chocolate Dun) Mayfly Spinner from the West Fork of the Chippewa River in Wisconsin

The weird fibers obscuring the view aren't a part of the insect here, obviously.

Male Eurylophella temporalis (Ephemerellidae) (Chocolate Dun) Mayfly Spinner from the West Fork of the Chippewa River in Wisconsin

Full terminalia of the male spinner

Male Eurylophella temporalis (Ephemerellidae) (Chocolate Dun) Mayfly Spinner from the West Fork of the Chippewa River in Wisconsin

Lateral view of penes, ventral side down

Male Eurylophella temporalis (Ephemerellidae) (Chocolate Dun) Mayfly Spinner from the West Fork of the Chippewa River in Wisconsin

Dorsal view of the penes. The apparent projections on the side are just unrelated debris.

Male Eurylophella temporalis (Ephemerellidae) (Chocolate Dun) Mayfly Spinner from the West Fork of the Chippewa River in Wisconsin

Ventral view of the penes detached from the claspers.

Male Eurylophella temporalis (Ephemerellidae) (Chocolate Dun) Mayfly Spinner from the West Fork of the Chippewa River in Wisconsin

This mayfly was collected from the West Fork of the Chippewa River in Wisconsin on June 21st, 2021 and added to Troutnut.com by Troutnut on June 24th, 2021.

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References

Allen, R.K., and Edmunds, George F. Jr. 1963. A Revision of the Genus Ephemerella (Ephemeroptera: Ephemerellidae) VII. The Subgenus Eurylophella. Canadian Entomologist 95: 597-623.