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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 Setvena wahkeena (Perlodidae) (Wahkeena Springfly) Stonefly Nymph from Mystery Creek #199 in Washington
As far as I can tell, this species has only previously been reported from one site in Oregon along the Columbia gorge. However, the key characteristics are fairly unmistakable in all except for one minor detail:
— 4 small yellow spots on frons visible in photos
— Narrow occipital spinule row curves forward (but doesn’t quite meet on stem of ecdysial suture, as it's supposed to in this species)
— Short spinules on anterior margin of front legs
— Short rposterior row of blunt spinules on abdominal tergae, rather than elongated spinules dorsally
I caught several of these mature nymphs in the fishless, tiny headwaters of a creek high in the Wenatchee Mountains.
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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Western Slate Olive Duns

Like most common names,"Western Slate Olive Dun" can refer to more than one taxon. They're previewed below, along with 6 specimens. For more detail click through to the scientific names.

Mayfly Species Drunella spinifera

These are pretty much always called Western Slate Olive Duns.
This taxon prefers cold water and does not flourish where water temperatures exceed 60 degrees. As with the slightly larger Drunella coloradensis, this species prefers cold tailwaters, high elevation headwaters and spring creeks.
Female Drunella spinifera (Ephemerellidae) (Western Slate Olive Dun) Mayfly Dun from Fern Creek in Montana
Male Drunella spinifera (Ephemerellidae) (Western Slate Olive Dun) Mayfly Spinner from Fern Creek in Montana
Lateral view of a Male Drunella spinifera (Ephemerellidae) (Western Slate Olive Dun) Mayfly Nymph from Mystery Creek #199 in Washington
In a bucket full of Drunella coloradensis nymphs, this was the only specimen of Drunella spinifera (and the first one I've found anywhere).

Mayfly Species Drunella flavilinea

These are very rarely called Western Slate Olive Duns.
The Flavs pick up about a week after the closely related but larger Western Green Drakes (Drunella grandis and Drunella doddsii) finish hatching on most Western waters.

Their hatches may be complemented by simultaneous hatches of two less prolific species, Drunella coloradensis and Drunella spinifera.
Artistic view of a Male Drunella flavilinea (Ephemerellidae) (Flav) Mayfly Dun from the Cedar River in Washington
This dun is smaller than would normally be expected for Drunella flavilinea, but it seems to fit the physical description of that species fairly well. I wasn't sure from the dun pictures alone, so I was hoping it would molt into a spinner. It never made it out of the dun's shuck, but it did begin the process, which allowed me with great care to tease out the last couple abdominal segments of the spinner from the still tightly attached dun shuck, enabling a more confident ID based on the shape of the spinner's penes and sub-genital plate. I also collected a nymph of flavilinea in the same river on the same evening, as well as a shed exuvium that looked large enough to belong to an emerging dun.

The other possibility I considered for a while based on the dun was Drunella pelosa, which would be the right size, but the other characteristics don't fit.
Male Drunella flavilinea (Ephemerellidae) (Flav) Mayfly Spinner from the Flathead River-lower in Montana
Dorsal view of a Drunella flavilinea (Ephemerellidae) (Flav) Mayfly Nymph from the Cedar River in Washington

Western Slate Olive Duns

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