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.

The North Fork Couer d'Alene River in Idaho

Stream photos

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Limnephilidae (Giant Sedges) Caddisfly Larva Limnephilidae (Giant Sedges) Caddisfly Larva

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.

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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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Ameletus vernalis (Brown Dun) Mayfly Nymph Pictures

I keyed this one out using the Alberta species key from Zloty & Pritchard 1997.

Notes from the ID include:

1. Posterior margin of sternites 6-8 without spines
2. Mesal gill extension "well developed"
3. Basil third of caudal filaments pale
4. Anterior surface of front femora mostly pale
5. Dark band on caudal filaments begins around segment 20
6. Final instars early season

This one keys out pretty cleanly to Ameletus vernalis except the color pattern on the tergites doesn't match. However, two species known in Washington aren't included in the key. Of those two, Ameletus vancouverensis would be too small (adult body length 9 mm), but Ameletus andersoni (currently documented only from a spring in Cowlitz County) has a wide range of sizes and emergence times that could be compatible with this one. So I can't rule that species out, but Ameletus vernalis seems the most likely.