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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 Setvena wahkeena (Perlodidae) (Wahkeena Springfly) Stonefly Nymph from Mystery Creek #199 in Washington
As far as I can tell, this species has only previously been reported from one site in Oregon along the Columbia gorge. However, the key characteristics are fairly unmistakable in all except for one minor detail:
— 4 small yellow spots on frons visible in photos
— Narrow occipital spinule row curves forward (but doesn’t quite meet on stem of ecdysial suture, as it's supposed to in this species)
— Short spinules on anterior margin of front legs
— Short rposterior row of blunt spinules on abdominal tergae, rather than elongated spinules dorsally
I caught several of these mature nymphs in the fishless, tiny headwaters of a creek high in the Wenatchee Mountains.
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 Ephemerella tibialis (Little Western Dark Hendrickson) Mayfly Dun Pictures

I'm fairly confident calling this dun Ephemerella tibialis because it emerged from one of several similar nymphs in my bucket, which I was able to ID with quite a bit of certainty. However, it looks very different from the other duns I currently have listed, which makes me rethink their IDs.

Ruler view of a Female Ephemerella tibialis (Ephemerellidae) (Little Western Dark Hendrickson) Mayfly Dun from the East Fork Big Lost River in Idaho The smallest ruler marks are 1 mm.
Lateral view of a Female Ephemerella tibialis (Ephemerellidae) (Little Western Dark Hendrickson) Mayfly Dun from the East Fork Big Lost River in Idaho
Female Ephemerella tibialis (Ephemerellidae) (Little Western Dark Hendrickson) Mayfly Dun from the East Fork Big Lost River in Idaho
Female Ephemerella tibialis (Ephemerellidae) (Little Western Dark Hendrickson) Mayfly Dun from the East Fork Big Lost River in Idaho
Dorsal view of a Female Ephemerella tibialis (Ephemerellidae) (Little Western Dark Hendrickson) Mayfly Dun from the East Fork Big Lost River in Idaho
Ventral view of a Female Ephemerella tibialis (Ephemerellidae) (Little Western Dark Hendrickson) Mayfly Dun from the East Fork Big Lost River in Idaho

This mayfly was collected from the East Fork Big Lost River in Idaho on August 3rd, 2020 and added to Troutnut.com by Troutnut on August 18th, 2020.


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Female Ephemerella tibialis (Little Western Dark Hendrickson) Mayfly Dun Pictures

Collection details
Location: East Fork Big Lost River, Idaho
Date: August 3rd, 2020
Added to site: August 18th, 2020
Author: Troutnut
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