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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 Neoleptophlebia (Leptophlebiidae) Mayfly Nymph from the Yakima River in Washington
Some characteristics from the microscope images for the tentative species id: The postero-lateral projections are found only on segment 9, not segment 8. Based on the key in Jacobus et al. (2014), it appears to key to Neoleptophlebia adoptiva or Neoleptophlebia heteronea, same as this specimen with pretty different abdominal markings. However, distinguishing between those calls for comparing the lengths of the second and third segment of the labial palp, and this one (like the other one) only seems to have two segments. So I'm stuck on them both. It's likely that the fact that they're immature nymphs stymies identification in some important way.
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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Chocolate Duns

Like most common names,"Chocolate Dun" can refer to more than one taxon. They're previewed below, along with 8 specimens. For more detail click through to the scientific names.

Mayfly Genus Eurylophella

These are often called Chocolate Duns.
None of the species in this genus are recognized as significant hatches. Nevertheless, I have found their nymphs in my samples in many streams in both the East and Midwest.
Lateral view of a Male Eurylophella (Ephemerellidae) (Chocolate Dun) Mayfly Dun from the Teal River in Wisconsin
Lateral view of a Male Eurylophella minimella (Ephemerellidae) (Chocolate Dun) Mayfly Spinner from the Namekagon River in Wisconsin
The genus ID on this specimen is confident but species is very tentative, based on the tentative ID of a seemingly-identical specimen from a nearby river a few days apart.
Dorsal view of a Eurylophella temporalis (Ephemerellidae) (Chocolate Dun) Mayfly Nymph from the Marengo River in Wisconsin

Mayfly Species Eurylophella bicolor

These are often called Chocolate Duns.
According to the Leonards in Mayflies: An Angler's Study of Trout Water Ephemeroptera , this species and Eurylophella bicolor are the two most likely hatches for fly fishermen to encounter in Michigan. Based on my sampling, it seems other species may be more prevalent elsewhere.

Mayfly Species Ephemerella needhami

These are very rarely called Chocolate Duns.
This small and slightly noteworthy mayfly appears during the finest hours of the year. Ernest Schwiebert describes an Ephemerella needhami day in Matching the Hatch:

"It was a wonderul morning, with a sky of indescribable blue and big, clean-looking cumulus clouds, and the water was sparkling and alive. You have seen the water with that lively look; you have also seen it dead and uninviting in a way that dampens the enthusiasm the moment you wade out into the current."


I have not fished a needhami emergence, but the exquisite nymphs show up often (though never abundantly) in my samples.
Lateral view of a Male Ephemerella needhami (Ephemerellidae) (Little Dark Hendrickson) Mayfly Dun from the West Branch of the Delaware River in New York
See the comments for an interesting discussion of the identification of this dun.
Dorsal view of a Ephemerella needhami (Ephemerellidae) (Little Dark Hendrickson) Mayfly Nymph from the Bois Brule River in Wisconsin
I photographed three strange striped Ephemerella nymphs from the same trip on the same river: this one, a brown one, and a very very striped one. I have tentatively put them all in Ephemerella needhami for now.

Mayfly Species Ephemera simulans

These are very rarely called Chocolate Duns.
The Brown Drakes are a favorite hatch of many in the Midwest, and they make a good showing on localized waters across the country. They are usually the first in a series of big drakes which bring large trout to the surface at twilight and into the early hours of the night. The experience can be much like fishing the Hexagenia limbata hatch, except that the nymphs seem to emerge from slightly more wadeable, sandy bottoms instead of the boot-sucking mud underlying the best Hex water. It will draw big trout out from the depths of a big pool to feed in the shallow tailout after dark.
Male Ephemera simulans (Ephemeridae) (Brown Drake) Mayfly Dun from Flathead Lake in Montana
Lateral view of a Male Ephemera simulans (Ephemeridae) (Brown Drake) Mayfly Spinner from the Namekagon River in Wisconsin
Ephemera simulans (Ephemeridae) (Brown Drake) Mayfly Nymph from the Namekagon River in Wisconsin

References

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