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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.

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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Lateral view of a Female Epeorus frisoni (Heptageniidae) Mayfly Dun from Mystery Creek #23 in New York
I collected this female dun together with a female spinner, a male dun, and a larger, damaged male dun.
Site Editor
"Bear Swamp," PA

Posts: 1681
GONZO on Apr 8, 2009April 8th, 2009, 11:07 am EDT
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.

Troutnut's profile picture
Bellevue, WA

Posts: 2758
Troutnut on Apr 8, 2009April 8th, 2009, 4:53 pm EDT
Sounds like a good fit. The size difference leaves a pretty big question mark over this ID, but for a relatively obscure species it's quite possible the full size range isn't fully documented. In such an isolated system high in the Catskills.

I can't tell the general audience where exactly these flies are from, but it's a small stream high in the Catskills that matches the description Gonzo found pretty well.
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist

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