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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 Neoleptophlebia (Leptophlebiidae) Mayfly Nymph from the Yakima River in Washington
Some characteristics from the microscope images for the tentative species id: The postero-lateral projections are found only on segment 9, not segment 8. Based on the key in Jacobus et al. (2014), it appears to key to Neoleptophlebia adoptiva or Neoleptophlebia heteronea, same as this specimen with pretty different abdominal markings. However, distinguishing between those calls for comparing the lengths of the second and third segment of the labial palp, and this one (like the other one) only seems to have two segments. So I'm stuck on them both. It's likely that the fact that they're immature nymphs stymies identification in some important way.
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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Great Red Quills

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

Mayfly Species Timpanoga hecuba

These are often called Great Red Quills.
Timpanoga hecuba is not abundant enough, and its emergence not concentrated enough to provide great hatches, but where it is locally abundant it creates fishable action because of its large size. This species seems subject to substantial fluctuations in population densities, possibly in relation to the amount of silted habitat they prefer. When silt builds up in drought years, their numbers appear to increase. It is the largest species in the Ephemerellidae family, often rivaling Drunella grandis (Western Green Drake) in length but appearing even stouter due to its dramatic lateral abdominal spines. It contains two subspecies. See the Timpanoga genus hatch page for details.

Anglers call them by a confusing array of names, although many fly shops have fortunately started to clear things up by rightly calling them Hecubas. Great Red Quill and Western Red Drake seem fairly descriptive, and some refer to them as Giant Dark Hendricksons. A fly shop serving Yellowstone out of Gardiner, Montana calls them Drake Mackerels in their hatch chart.
Female Timpanoga hecuba (Ephemerellidae) (Great Red Quill) Mayfly Dun from Mystery Creek #178 in Idaho
This specimen is 14 mm. Technically this is the subspecies T. h. hecuba. The Cascades, Sierras and further West is where the other subspecies, T. h. pacifica is found. The Great Basin seems to have formed a barrier preventing any overlap in their distribution.
Timpanoga hecuba (Ephemerellidae) (Great Red Quill) Mayfly Nymph from the St. Regis River in Montana

References

Great Red Quills

Scientific Name
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