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.
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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.
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Landscape & scenery photos from the Icicle River
Closeup insects by Troutnut from the Icicle River in Washington
Keying this one out using Zloty & Pritchard 1997:
-No ganglionic markings on sternites 2–8
-Posterior margins of sternits 6–8 without numerous spines
-Mesal gill extension well developed (I really don't like the "well developed" language when it's subjective, but in this case the other option doesn't lead anywhere productive)
-Tails pale in basal 1/3
-Larger species (13–16 mm) — this one is just shy of 12 mm, but closer to 13 than to <10
-Spring emergence
This all points to Ameletus vernalis.
-No ganglionic markings on sternites 2–8
-Posterior margins of sternits 6–8 without numerous spines
-Mesal gill extension well developed (I really don't like the "well developed" language when it's subjective, but in this case the other option doesn't lead anywhere productive)
-Tails pale in basal 1/3
-Larger species (13–16 mm) — this one is just shy of 12 mm, but closer to 13 than to <10
-Spring emergence
This all points to Ameletus vernalis.