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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.

Lateral view of a Female Sweltsa borealis (Chloroperlidae) (Boreal Sallfly) Stonefly Adult from Harris Creek in Washington
I was not fishing, but happened to be at an unrelated social event on a hill above this tiny creek (which I never even saw) when this stonefly flew by me. I assume it came from there. Some key characteristics are tricky to follow, but process of elimination ultimately led me to Sweltsa borealis. It is reassuringly similar to this specimen posted by Bob Newell years ago. It is also so strikingly similar to this nymph from the same river system that I'm comfortable identifying that nymph from this adult. I was especially pleased with the closeup photo of four mites parasitizing this one.
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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Slate-Winged Mahogany Duns

This common name refers to only one genus. Click its scientific name to learn more.

Mayfly Genus Paraleptophlebia

These are sometimes called Slate-Winged Mahogany Duns.
There are many species in this genus of mayflies, and some of them produce excellent hatches. Commonly known as Blue Quills or Mahogany Duns, they include some of the first mayflies to hatch in the Spring and some of the last to finish in the Fall.

In the East and Midwest, their small size (16 to 20, but mostly 18's) makes them difficult to match with old techniques. In the 1950s Ernest Schwiebert wrote in Matching the Hatch:

"The Paraleptophlebia hatches are the seasonal Waterloo of most anglers, for without fine tippets and tiny flies an empty basket is assured."

Fortunately, modern anglers with experience fishing hatches of tiny Baetis and Tricorythodes mayflies (and access to space-age tippet materials) are better prepared for eastern Paraleptophlebia. It's hard to make sense of so many species, but only one is very important and others can be considered in groups because they often hatch together:
In the West, it is a different story. For starters the species run much larger and can be imitated with flies as large as size 12, often size 14, and rarely smaller than 16. Another difference is the West has species with tusks! Many anglers upon first seeing them think they are immature burrowing nymphs of the species Ephemera simulans aka Brown Drake. With their large tusks, feathery gills, and slender uniform build, it's an easy mistake to make. Using groups again:
Female Paraleptophlebia debilis (Leptophlebiidae) (Mahogany Dun) Mayfly Dun from the Fall River in California
Size: 9mm. These photos really highlight the brown pigmentation of the wing venation, but in the hand the wings look to be a uniform smokey gray. - Entoman
Lateral view of a Male Paraleptophlebia sculleni (Leptophlebiidae) Mayfly Spinner from Mystery Creek #249 in Washington
This specimen (and a few others I collected but didn't photograph) appear to represent the first finding of Paraleptophlebia sculleni outside the Oregon Cascades, although it is not a monumental leap from there to the Washington Cascades. The key characteristics are fairly clear.
Paraleptophlebia (Leptophlebiidae) (Blue Quill) Mayfly Nymph from Big Brook in Wisconsin

References

Slate-Winged Mahogany Duns

Scientific Name
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