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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.

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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Little Slate-Winged Brown Quills

Like most common names,"Little Slate-Winged Brown Quill" can refer to more than one taxon. They're previewed below, along with 2 specimens. For more detail click through to the scientific names.

Mayfly Species Baetis intercalaris

These are sometimes called Little Slate-Winged Brown Quills.

Mayfly Species Baetis brunneicolor

These are very rarely called Little Slate-Winged Brown Quills.
This is the largest common species of Baetis on our trout streams, and it can hatch in incredible numbers, drawing impressive rises of selective trout.

Anglers may have read in books about Baetis hiemalis, which is now a synonym of Baetis brunneicolor. It appears to have been a name for the fall-hatching brood of this species, which was reported to prefer slow water and weedy habitat instead of the gravelly riffles of the early summer brood.

Mayfly Species Plauditus dubius

These are very rarely called Little Slate-Winged Brown Quills.
This species may produce good hatches.

Mayfly Species Baetis flavistriga

These are very rarely called Little Slate-Winged Brown Quills.
This is one of the most widespread and abundant Baetis species, and it may produce fishable hatches under a variety of conditions.

This species is known in angling books by several old synonyms, including Baetis quebecensus, Baetis levitans, Baetis cingulatus, and Baetis phoebus, in addition to the correct name.
Dorsal view of a Male Baetis flavistriga (Baetidae) (BWO) Mayfly Nymph from the Dosewallips River in Washington
This nymph keys to Baetis assuming the villipore is present (hard to see in my photos or scope), and within that genus it tentatively keys to the flavistriga species complex, of which Baetis flavistriga itself is by far the most common in Washington state, so that's the most likely ID.

Mayfly Species Acentrella turbida

These are very rarely called Little Slate-Winged Brown Quills.
Although these mayflies are tiny, in places their numbers compensate for their small size and make for excellent hatches.

Rick Hafele and Dave Hughes in Western Mayfly Hatches rate turbida as one of the three most "key" western species of Baetidae, alongside Baetis tricaudatus and Diphetor hageni. In the West, turbida is more variable in size and appearance than its eastern iteration, in keeping with the large and varied regions it inhabits. It can run as small as 3.5 mm and as large as 5 mm, the larger sizes tending to be more brownish. It is often confused with the smaller broods of Diphetor hageni, but its conical mesonotal projection, lack of hind-wings, exaggerated turbinate eyes (hence its name) and stockier build help to differentiate it.

They are often found on the water with a mix of other Baetidae mayflies, making for very challenging fishing.

References

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