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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 Limnephilidae (Giant Sedges) Caddisfly Larva from the Yakima River in Washington
This specimen resembled several others of around the same size and perhaps the same species, which were pretty common in my February sample from the upper Yakima. Unfortunately, I misplaced the specimen before I could get it under a microscope for a definitive ID.
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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Male Epeorus longimanus (Slate Brown Dun) Mayfly Dun Pictures

Identification of this one was as follows. Body 9 mm, wing 11 mm.
Both Epeorus albertae and Epeorus dulciana should have a conspicuously darkened humeral crossvein in the forewing. This one doesn't.
The foretarsal claws are dissimilar (one sharp, one blunt), which also rules out the Epeorus albertae group.
The dark macula on the forefemora rules out Epeorus deceptivus, which is also supposed to be a little bit smaller.
Both Epeorus grandis and Epeorus permagnus should be much, much larger.
Of the species known to be present in Washington, this leaves only Epeorus longimanus, which is exactly the right size. The key to male spinners in Traver (1935) describes distinctive markings that are visible (although more faintly) in this dun: “Black posterior margins of tergites do not extend laterally to pleural fold, but an oblique black line form this margin cuts across poster-lateral triangle to pleural fold.”

Male Epeorus longimanus (Heptageniidae) (Slate Brown Dun) Mayfly Dun from Mystery Creek #295 in Washington
Male Epeorus longimanus (Heptageniidae) (Slate Brown Dun) Mayfly Dun from Mystery Creek #295 in Washington
Lateral view of a Male Epeorus longimanus (Heptageniidae) (Slate Brown Dun) Mayfly Dun from Mystery Creek #295 in Washington
Ruler view of a Male Epeorus longimanus (Heptageniidae) (Slate Brown Dun) Mayfly Dun from Mystery Creek #295 in Washington The smallest ruler marks are 1 mm.
Ventral view of a Male Epeorus longimanus (Heptageniidae) (Slate Brown Dun) Mayfly Dun from Mystery Creek #295 in Washington
Male Epeorus longimanus (Heptageniidae) (Slate Brown Dun) Mayfly Dun from Mystery Creek #295 in Washington
Male Epeorus longimanus (Heptageniidae) (Slate Brown Dun) Mayfly Dun from Mystery Creek #295 in Washington
Male Epeorus longimanus (Heptageniidae) (Slate Brown Dun) Mayfly Dun from Mystery Creek #295 in Washington
Dorsal view of a Male Epeorus longimanus (Heptageniidae) (Slate Brown Dun) Mayfly Dun from Mystery Creek #295 in Washington
Male Epeorus longimanus (Heptageniidae) (Slate Brown Dun) Mayfly Dun from Mystery Creek #295 in Washington
Male Epeorus longimanus (Heptageniidae) (Slate Brown Dun) Mayfly Dun from Mystery Creek #295 in Washington
Male Epeorus longimanus (Heptageniidae) (Slate Brown Dun) Mayfly Dun from Mystery Creek #295 in Washington

This mayfly was collected from Mystery Creek #295 in Washington on May 31st, 2022 and added to Troutnut.com by Troutnut on June 1st, 2022.


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Male Epeorus longimanus (Slate Brown Dun) Mayfly Dun Pictures

Collection details
Location: Mystery Creek #295, Washington
Date: May 31st, 2022
Added to site: June 1st, 2022
Author: Troutnut
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