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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 Glossosoma (Glossosomatidae) (Little Brown Short-horned Sedge) Caddisfly Larva from the Yakima River in Washington
I caught this tiny larva without a case, but it seems to key pretty clearly to to Glossosomatidae. From there, the lack of sclerites on the mesonotum points to either Glossosoma or Anagapetus. Although it's difficult to see in a 2D image from the microscope, it's pretty clear in the live 3D view that the pronotum is only excised about 1/3 of its length to accommodate the forecoxa, not 2/3, which points to Glossosoma at Couplet 5 of the Key to Genera of Glossosomatidae Larvae.
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 Ephemerella aurivillii Mayfly Dun Pictures

This dun was collected in proximity to this spinner, and to this female that looks like the same species.

Among western species, it most closely resembles Ephemerella aurivillii, of which I reared a specimen from a positively-identified nymph and photographed the male dun previously in Alaska. In the photos, this dun does not seem to have the same evidence of the nymph's dorsal stripe, but that's due to the lighting. A faint stripe is apparent under the microscope, as it is in the female linked above.

They do appear to be a bit short for aurivillii, with the male at 8 mm and the female at 9 mm, but the spinners could be a bit longer.

Among the other Ephemerella mayflies documented in Montana, Ephemerella alleni and Ephemerella mucronata are the only two others that seem to not be ruled out.

Lateral view of a Male Ephemerella aurivillii (Ephemerellidae) Mayfly Dun from the Madison River in Montana
Male Ephemerella aurivillii (Ephemerellidae) Mayfly Dun from the Madison River in Montana
Male Ephemerella aurivillii (Ephemerellidae) Mayfly Dun from the Madison River in Montana
Dorsal view of a Male Ephemerella aurivillii (Ephemerellidae) Mayfly Dun from the Madison River in Montana
Ruler view of a Male Ephemerella aurivillii (Ephemerellidae) Mayfly Dun from the Madison River in Montana The smallest ruler marks are 1 mm.
Artistic view of a Male Ephemerella aurivillii (Ephemerellidae) Mayfly Dun from the Madison River in Montana
Ventral view of a Male Ephemerella aurivillii (Ephemerellidae) Mayfly Dun from the Madison River in Montana

This mayfly was collected from the Madison River in Montana on July 1st, 2019 and added to Troutnut.com by Troutnut on July 18th, 2019.


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Male Ephemerella aurivillii Mayfly Dun Pictures

Collection details
Location: Madison River, Montana
Date: July 1st, 2019
Added to site: July 18th, 2019
Author: Troutnut
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