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Lateral view of a Male Baetis (Baetidae) (Blue-Winged Olive) Mayfly Dun from Mystery Creek #43 in New York
Blue-winged Olives
Baetis

Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.

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.
Troutnut is a project started in 2003 by salmonid ecologist Jason "Troutnut" Neuswanger to help anglers and fly tyers unabashedly embrace the entomological side of the sport. Learn more about Troutnut or support the project for an enhanced experience here.

Whirling Duns

Like most common names,"Whirling Dun" can refer to more than one taxon. They're previewed below, along with 9 specimens. For more detail click through to the scientific names.

Mayfly Genus Leptophlebia

These are very rarely called Whirling Duns.
Leptophlebia mayflies do not generate superhatches, but their medium-large size and other properties make them a relevant part of the early season.

The information below was mostly discovered in Leptophlebia cupida, the most important species, but it is not known to differ in the others.
Lateral view of a Male Leptophlebia cupida (Leptophlebiidae) (Black Quill) Mayfly Dun from the Teal River in Wisconsin
This Leptophlebia cupida dun was extremely cooperative, and it molted into a spinner for me in front of the camera. Here I have a few dun pictures and one spinner picture, and I've put the entire molting sequence in an article.
Lateral view of a Female Leptophlebia cupida (Leptophlebiidae) (Black Quill) Mayfly Spinner from the Namekagon River in Wisconsin
Leptophlebia cupida (Leptophlebiidae) (Black Quill) Mayfly Nymph from the Namekagon River in Wisconsin

Mayfly Species Ephemerella invaria

These are very rarely called Whirling Duns.
This species, the primary "Sulphur" hatch, stirs many feelings in the angler. There is nostalgia for days when everything clicked and large, selective trout were brought to hand. There is the bewildering memory of towering clouds of spinners which promise great fishing and then vanish back into the aspens as night falls. There is frustration from the maddening selectivity with which trout approach the emerging duns--a vexing challenge that, for some of us, is the source of our excitement when Sulphur time rolls around.

Ephemerella invaria is one of the two species frequently known as Sulphurs (the other is Ephemerella dorothea). There used to be a third, Ephemerella rotunda, but entomologists recently discovered that invaria and rotunda are a single species with an incredible range of individual variation. This variation and the similarity to the also variable dorothea make telling them apart exceptionally tricky.

As the combination of two already prolific species, this has become the most abundant of all mayfly species in Eastern and Midwestern trout streams.
Lateral view of a Male Ephemerella invaria (Ephemerellidae) (Sulphur) Mayfly Dun from Penn's Creek in Pennsylvania
Lateral view of a Male Ephemerella invaria (Ephemerellidae) (Sulphur) Mayfly Spinner from the Teal River in Wisconsin
Dorsal view of a Ephemerella invaria (Ephemerellidae) (Sulphur) Mayfly Nymph from the Beaverkill River in New York
This small Ephemerella invaria nymph was at least a month away from emergence.

Mayfly Species Ephemerella subvaria

These are very rarely called Whirling Duns.
The Hendrickson hatch is almost synonymous with fly fishing in America. It has been romanticized by our finest writers, enshrined on an untouchable pedestal next to Theodore Gordon, bamboo, and the Beaverkill.

The fame is well-deserved. Ephemerella subvaria is a prolific species which drives trout to gorge themselves. Its subtleties demand the best of us as anglers, and meeting the challenge pays off handsomely in bent graphite and screaming reels.
Lateral view of a Female Ephemerella subvaria (Ephemerellidae) (Hendrickson) Mayfly Dun from the Beaverkill River in New York
I collected this female Hendrickson dun and a male in the pool on the Beaverkill where the popular Hendrickson pattern was first created. She is descended from mayfly royalty.
Ruler view of a Female Ephemerella subvaria (Ephemerellidae) (Hendrickson) Mayfly Spinner from the Bois Brule River in Wisconsin The smallest ruler marks are 1/16".
This one's a bit bedraggled because it was rainy and I had a hard time keeping anything dry, including the container I was putting mayflies in. I was practically juggling up there balanced on a rock trying to catch mayflies and trout at the same time.
Ephemerella subvaria (Ephemerellidae) (Hendrickson) Mayfly Nymph from unknown in Wisconsin
This is another unusual brown Ephemerella nymph. The "fan-tail" which defines the Ephemerella genus is particularly evident on this specimen.

References

Whirling Duns

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