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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 Ephemerella dorothea dorothea (Pale Evening Duns)

Ephemerella dorothea consists of two subspecies, which both produce excellent action. Ephemerella dorothea dorothea is a small species of Sulphur in the East, and Ephemerella dorothea infrequens (formerly Ephemerella infrequens) is one of the two main Pale Morning Dun hatches of the West. The remainder of this page focuses on the dorothea dorothea subspecies, and Ephemerella dorothea infrequens is discussed separately on its own page.

This is one of the most challenging mayfly hatches on Eastern waters. On many streams, it follows or overlaps hatches of the larger, lingering Ephemerella invaria.

Where & when

Time of year : May-July, often best in June.

Preferred waters: Perhaps most common in mountain streams, but especially good in some alkaline spring creeks.

This Eastern subspecies begins to emerge in late May in Pennsylvania. It progresses through the Catskills in early June and peaks in mid-June farther north, lasting as late as early July in some places.

In 16 records from GBIF, adults of this species have been collected during June (31%), July (25%), May (13%), October (13%), April (6%), September (6%), and February (6%).

In 2 records from GBIF, this species has been collected at elevations of 200 and 341 ft.

Species Range

Hatching behavior

Time of day : Flexible, but typically mid-late evening

Habitat: Slow water

Water temperature: 60-65°F

These insects are actually too perfect for dry fly fishing, which makes matching their hatches difficult.

The nymphs may drift for a while just below the surface before trying to break through. When they do, it takes them a long time to crawl out of their shucks. After that, they ride the water for an exceptionally long time to dry their wings, and low-floating patterns like the Comparaduns are preferred. And as if that weren't enough, they are also one of the most cripple-prone of all mayfly species, and trout may feed selectively on their cripples and stillborns. This may all take place at the same time as the spinner fall, especially in the East.

These Sulphurs emerge from smooth, slow water, which allows the trout maximum time to inspect their prey. Because a rising trout may be selective to either floating nymphs, emergers, duns, cripples, or spinners, this is one of the most puzzling hatches in all fly fishing. It is also difficult because the flies are small, hook size 16 to 18, and such small imitations are prone to microdrag. There is no better time to hone your powers of observation and presentation.

Spinner behavior

Time of day: Near dusk

Habitat: Riffles are preferred, but on spring creeks without riffles any broken water will do.

Hatched duns typically return to the stream within two days as spinners. After mating, both genders fall spent on the water.

Females usually, but not always, drop their eggs from the air above the stream. When they do end up on the water with egg sacs still attached, trout may become selective to spinners with little yellow dots near the tail. Patterns are sold to imitate this.

Nymph biology

Current speed: Some in riffles and runs, but best in slower stream sections and pools.

Substrate: Gravel, sand, vegetation

The nymphs display the usual Ephemerellidae habits of high activity in the hours and days before they hatch, and trout claim many of them before they're anywhere near the surface.

Ephemerella dorothea dorothea Fly Fishing Tips

See the section on Hatching Behavior above regarding the many challenges this hatch poses. The best approach is extremely keen observation, and the standby is rapid-fire trial and error. Do not stick with one thing for very long if it's not working.

Caucci and Nastasi note in Hatches II that some light-bodied species of Epeorus, such as Epeorus vitreus, may emerge at the same time as dorothea in the East. It's just one more entry in the long list of complications of the dorothea Sulphurs.

Physical description

Most physical descriptions on Troutnut are direct or slightly edited quotes from the original scientific sources describing or updating the species, although there may be errors in copying them to this website. Such descriptions aren't always definitive, because species often turn out to be more variable than the original describers observed. In some cases, only a single specimen was described! However, they are useful starting points.

Male Spinner

Described in Needham et al (1935) as Ephemerella dorothea
Body length: 5-6 mm
Wing length: 7-8.5 mm

A small very pale species, of the Ephemerella invaria group; the second joint of the forceps enlarged distally; 12-14 spines on the penes.

Eyes of male bright red. Thorax and legs pale yellowish. Wings hyaline, iridescent, the venation wholly pale. Abdomen pale yellowish. Tails white, without dark markings. Genitalia yellowish, the second joint of the forceps enlarged distally. Spines on the penes number from 12 to 14, and are usually ventral and lateral in position (see fig. 152).

Female Spinner

Described in Needham et al (1935) as Ephemerella dorothea

The head of the female is often reddish on the vertex. Abdomen pale yellowish; in the female, often suffused with ruddy on the dorsum.


Described in Needham et al (1935) as Ephemerella dorothea

The small yellowish brown nymph is speckled with pale dots. The lateral extensions of the abdomen are poorly developed, the postero-lateral spines short except on segments 8 and 9. There are no dorsal spines on the abdomen. Claws are strongly curved; each bears 8-9 denticles. Tails pale, rather darker at the tips; may be faintly banded.

Specimens of the Mayfly Species Ephemerella dorothea dorothea

1 Male Dun
1 Nymph

Discussions of Ephemerella dorothea dorothea

Wow, they really can take forever to get off the water
36 replies
Posted by Troutnut on Jun 13, 2006
Last reply on Jun 15, 2008 by Falsifly
I watched quite a few of these guys emerge tonight. (I think -- they seemed too small and light to be Ephemerella invaria, though I didn't bring one home to check under the microscope.) It was a cool evening but not cold, and they were emerging on the slow flats of a large midwestern spring creek. I watched several of them drift 50+ feet on the very slow-moving water, slowly rising up out of the surface film. Their emergence was sporadic and lucky for them the trout were also sporadic. Many were eaten but others went ignored for their entire lengthy drifts.

Later in the evening I was bested by a half-dozen rising trout. The sulphurs were still emerging, and a mix of spinners was starting to appear on the water, but I didn't get so much as a splashy refusal from several rising fish, even in the low light of dusk. My best guess is that they were picky feeders keying on a stage of Ephemerella dorothea mayflies.

Start a Discussion of Ephemerella dorothea dorothea


Mayfly Species Ephemerella dorothea dorothea (Pale Evening Duns)

Species Range
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