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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.

Lateral view of a Clostoeca disjuncta (Limnephilidae) (Northern Caddisfly) Caddisfly Larva from the Yakima River in Washington
This one was surprisingly straightforward to identify. The lack of a sclerite at the base of the lateral hump narrows the field quite a bit, and the other options followed fairly obvious characteristics to Clostoeca, which only has one species, Clostoeca disjuncta.
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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Female Baetis tricaudatus (Blue-Winged Olive) Mayfly Nymph Pictures

These nymphs were highly abundant in my early April kick net samples from the Yakima Canyon, and one of them emerged into a dun, which I photographed immediately. Similar-looking nymphs but with distinctly brighter color patterns were also abundant. I just photographed one. After extensive views under the microscope, it's clear the bright ones are males and the dull ones are females of the same species.

The most likely guess at the species is Baetis tricaudatus, which may be a complex of related species that haven't all been sorted out yet. It isn't a perfect fit to every key characteristic (and I never seem to find a Baetis that matches the expected pronotum color patterns, but that seems to be the closest.

The microscope images here were taken with different specimens from the main photos (so I could dissect them while preserving that one intact), but clearly the same species.

Dorsal view of a Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Nymph from the Yakima River in Washington
Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Nymph from the Yakima River in Washington
Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Nymph from the Yakima River in Washington
Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Nymph from the Yakima River in Washington
Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Nymph from the Yakima River in Washington
Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Nymph from the Yakima River in Washington
Ventral view of a Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Nymph from the Yakima River in Washington
Ruler view of a Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Nymph from the Yakima River in Washington The smallest ruler marks are 1 mm.
Foreleg closeup

Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Nymph from the Yakima River in Washington
One of the mandibles. The outer incisors aren't fused, indicating this isn't in the Baetis piscatoris complex.

Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Nymph from the Yakima River in Washington
Labium and labial palps removed from the mouth. I think all that's left of the palps is the clear exuvium from a nymph that was perhaps on its way to emerging any minute, but the shape is still pretty evident.

Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Nymph from the Yakima River in Washington
The dark spots along the anterior edge of tergum 10 here seem to be the "stout setae" referenced in the species key.

Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Nymph from the Yakima River in Washington
Closeup of the other mandible.

Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Nymph from the Yakima River in Washington

This mayfly was collected from the Yakima River in Washington on April 9th, 2021 and added to Troutnut.com by Troutnut on April 12th, 2021.


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Female Baetis tricaudatus (Blue-Winged Olive) Mayfly Nymph Pictures

Collection details
Location: Yakima River, Washington
Date: April 9th, 2021
Added to site: April 12th, 2021
Author: Troutnut
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