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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 Pycnopsyche guttifera (Limnephilidae) (Great Autumn Brown Sedge) Caddisfly Larva from the Yakima River in Washington
This specimen appears to be of the same species as this one collected in the same spot two months earlier. The identification of both is tentative. This one suffered some physical damage before being photographed, too, so the colors aren't totally natural. I was mostly photographing it to test out some new camera setting idea, which worked really well for a couple of closeups.
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.
Troutnut is a project started in 2003 by salmonid ecologist Jason "Troutnut" Neuswanger to help anglers and fly tyers unabashedly embrace the entomological side of the sport. Learn more about Troutnut or support the project for an enhanced experience here.

Female Epeorus frisoni Mayfly Dun Pictures

I collected this female dun together with a female spinner, a male dun, and a larger, damaged male dun.

Female Epeorus frisoni (Heptageniidae) Mayfly Dun from Mystery Creek #23 in New York
Ruler view of a Female Epeorus frisoni (Heptageniidae) Mayfly Dun from Mystery Creek #23 in New York The smallest ruler marks are 1 mm.
Lateral view of a Female Epeorus frisoni (Heptageniidae) Mayfly Dun from Mystery Creek #23 in New York
Female Epeorus frisoni (Heptageniidae) Mayfly Dun from Mystery Creek #23 in New York
Female Epeorus frisoni (Heptageniidae) Mayfly Dun from Mystery Creek #23 in New York
Dorsal view of a Female Epeorus frisoni (Heptageniidae) Mayfly Dun from Mystery Creek #23 in New York
Ventral view of a Female Epeorus frisoni (Heptageniidae) Mayfly Dun from Mystery Creek #23 in New York

This mayfly was collected from Mystery Creek #23 in New York on September 6th, 2006 and added to Troutnut.com by Troutnut on October 3rd, 2006.

Discussions of this Dun

Probably E. fragilis, but...
1 replies
Posted by GONZO on Apr 8, 2009
Last reply on Apr 8, 2009 by Troutnut
Both the process of elimination and reading published descriptions of Eastern Epeorus (pleuralis group) seem to leave fragilis as the only likely possibility for this specimen (as well as the related ones--#630, #631, and #633). The collection date is near the emergence range given for fragilis, and they match the description of this species in nearly every way except one: size.

Burian, et al. (2008) provide a key for Northeastern Epeorus adults that uses size as a significant means of distinguishing between fragilis and frisoni. They describe fragilis as less than 6.80mm. All of these specimens are larger, ranging from about 7mm-8.5mm. This puts them squarely within the range given for (the very similar) frisoni--6.83mm-9.42mm.

Although frisoni is described as being lighter in color than these specimens, two comments from the Burian publication are particularly interesting in this regard:

At least one of us believes that a thorough search of suitable, higher elevation, cold streams in the White Mountains of New Hampshire, the Adirondacks of New York, and the Gaspe area of Quebec should result in the discovery of several new records [of frisoni].


The habitat association of E. frisoni with cold (even in summer), high gradient, minimally disturbed streams at or above 1200' (~365m) could be used to identify some of the best high elevation lotic habitats for conservation purposes. Variation among the New England subset of the eastern Nearctic genus Epeorus strongly points to the need for a full revision of this genus.


It would probably be reasonable to place these specimens as (size-anomalous) fragilis under the current taxonomic scheme. However, the stream in which they were captured seems to fit the above criteria pretty well. It would be very interesting to see someone like Konchu investigate this population further.


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Female Epeorus frisoni Mayfly Dun Pictures

Collection details
Location: Mystery Creek #23, New York
Date: September 6th, 2006
Added to site: October 3rd, 2006
Author: Troutnut
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