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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 Skwala (Perlodidae) (Large Springfly) Stonefly Nymph from the Yakima River in Washington
This Skwala nymph still has a couple months left to go before hatching, but it's still a good representative of its species, which was extremely abundant in my sample for a stonefly of this size. It's obvious why the Yakima is known for its Skwala hatch.
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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