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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 Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
This is an interesting one. Following the keys in Merritt R.W., Cummins, K.W., and Berg, M.B. (2019) and Jacobus et al. (2014), it keys clearly to Ephemerella. Jacobus et al provide a key to species, but some of the characteristics are tricky to interpret without illustrations. If I didn't make any mistakes, this one keys to Ephemerella mucronata, which has not previously been reported any closer to here than Montana and Alberta. The main character seems to fit well: "Abdominal terga with prominent, paired, subparallel, spiculate ridges." Several illustrations or descriptions of this holarctic species from the US and Europe seem to match, including the body length, tarsal claws and denticles, labial palp, and gill shapes. These sources include including Richard Allen's original description of this species in North America under the now-defunct name E. moffatae in Allen RK (1977) and the figures in this description of the species in Italy.
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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Mayfly Species Epeorus grandis

Where & when

Time of year : June through early August

Preferred waters: Small streams

Altitude: Above 5,000 feet

In 9 records from GBIF, adults of this species have been collected during July (33%), May (22%), June (22%), August (11%), and April (11%).

In 7 records from GBIF, this species has been collected at elevations ranging from 131 to 8500 ft, with an average (median) of 3363 ft.

Species Range

Nymph biology

Current speed: Fast

Environmental tolerance: Requires very cold streams

Specimens of the Mayfly Species Epeorus grandis

3 Nymphs

Start a Discussion of Epeorus grandis

References

Mayfly Species Epeorus grandis

Species Range
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