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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.
Troutnut is a project started in 2003 by salmonid ecologist Jason "Troutnut" Neuswanger to help anglers and fly tyers unabashedly embrace the entomological side of the sport. Learn more about Troutnut or support the project for an enhanced experience here.

Mayfly Species Ephemerella excrucians (Pale Morning Duns)

For trout (if not anglers), this single species is arguably the most important mayfly in North America. In terms of sheer numbers, breadth of distribution and hatch duration, it has a good argument.

Ephemerella excrucians or Pale Morning Dun usually follows its larger sibling Ephemerella dorothea infrequens with which it shares the same common name. What it often lacks in size by comparison is made up for with it's duration, often lasting for months with intermittent peaks. This close relationship with infrequens has led many anglers to confuse Pale Morning Dun biology with that of the multivoltine Baetidae species, having disparate broods that decrease in size as the season advances. Sharing the same common name has not helped to alleviate this misconception.

Until recently, Ephemerella excrucians was considered primarily an upper MidWestern species of some regional importance commonly called Little Red Quill among other names. Recent work by entomologists determined that it is actually the same species as the important Western Pale Morning Dun (prev.Ephemerella inermis), and the lake dwelling Sulphur Dun of the Yellowstone area, (prev.Ephemerella lacustris). Since all three are considered variations of the same species, they have been combined into excrucians, being the original name for the type species reported as far back as the Civil War. Angler speculation had simmered for some time that the stillwater loving Ephemerella lacustris was much more widespread, inhabiting more water types then previously thought and could account for many large sulfurish ephemerellids found in still to very slow water locations throughout the West. With the revisions, this discussion is now moot.

Ephemerella excrucians variability in appearance, habitat preferences, and wide geographical distribution are cause for angler confusion with the changes in classification. They can be pale yellow 18's on a large Oregon river, creamy orange 14's on western lakes and feeder streams, large olive green on CA spring creeks as well as tiny sulfur ones in many Western watersheds. Then there's the little Red Quill on small streams in Wisconsin. Yet, all are the same species.

Where & when

Time of year : April through October with peaks of a month or more within this period depending on location

Preferred waters: All water types except warm river systems and infertile high country lakes

Altitude: variable

These ubiquitous mayflies are extremely abundant throughout the West and have a wide range of dates for their emergence. There is considerable evidence that on temperature stable spring creeks they can have asynchronous emergences in the spring and fall, not to be confused with many baetids multivoltine life history. In contrast, Eastern emergences are shorter, smaller and far less significant. Anglers would be wise to consult hatch charts and obtain current local information on specific rivers to time this species. Keep in mind that these charts usually combine them with the often larger and earlier hatching Ephemerella dorothea infrequens as they are very difficult to tell apart.

In 25 records from GBIF, adults of this species have mostly been collected during June (44%), April (24%), May (16%), and July (12%).

In 29 records from GBIF, this species has been collected at elevations ranging from 381 to 11453 ft, with an average (median) of 5351 ft.

Species Range

Hatching behavior

Time of day : Late morning and early evening in the West; late afternoon to evening in the East

Habitat: Highly variable, though the greatest concentrations occur in weedy riffles and runs

Water temperature: Varies with location

As noted with Ephemerella dorothea infrequens, excrucians is a classic surface emerger and often engage in "practice runs" exposing the nymphs to trout during extended pre-hatch periods. The main differences are that their smaller size means they struggle a bit more with the water's surface tension and the warmer weather they usually hatch in also means they spend less time on the water preparing their wings for flight. As a result, the observant angler should look for this and be even more ready to use emerger and cripple patterns if high floating dun imitations prove unproductive. Because of this hatches duration and sheer numbers, the fish become increasingly wary as the season progresses. Towards the end of their cycle on heavily fished water, even the most expert angler will be challenged to the limit. Fishing the 'right" fly is usually eclipsed by the need to avoid micro drag and pickup/deliveries that alert the quarry to the attempted fraud.

Spinner behavior

Time of day: Morning and again at dusk in the West; only dusk in the East, where they're not important

Habitat: See notes

The spinner falls are insignificant in some locations while achieving legendary status in others. They can be so dense at times that they are virtually unfishable. They prefer riffles if they exist. Otherwise they seem to pick specific areas for a variety reasons, most often riparian shelter, depth, and substrate related. See comments on Ephemerella dorothea infrequens hatch page for additional information.

Nymph biology

Diet: Detritus and algae

Current speed: Slow to fast in the West; medium to fast in the East

Substrate: All types, but prefer gravel and cobble with weed growth or the edges of weed beds in spring creeks

The nymphs can be important to imitate before and during the emergence. They come in shades of olive or very dark brown, but they also run the gamut of cinnamons like the larger infrequens. Unlike their big sisters though, excrucians nymphs are often patterned with longitudinal stripes and/or a veriagated pattern on the abdominal dorsum. Trout can be selective to these schemes.

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

Body length: 5.5-7.5 mm
Wing length: 7-8.5 mm

A member of the Ephemerella invaria group; second joint of forceps not swollen apically. Eyes (in living male) egg-yellow. Vertex of head, and antennae, reddish brown. Dorsum of thorax reddish brown. Legs pale yellow; first tarsal joint of fore leg, all tarsal joinings of fore leg, and tips of all the tarsi, cloudy. Wings hyaline; venation wholly hyaline; a slight yellowish tinge on the costa of the fore wing. Dorsum of abdomen reddish brown, sometimes almost reddish black. Tails whitish, the joinings brown except near the tips. Second joint of forceps little if any swollen apically; no spines at the apex of the penes.

Described as E. semiflava

Body length 7 mm, wing length 8 mm

A species of the Ephemerella invaria group; second joint of forceps not swollen apically; nymph unknown.

Eyes of living male mustard yellow. Head yellowish, tinged with reddish brown near the ocelli. Thorax olive brown shaded with yellow; light yellowish brown near the scutellum; reddish brown shading anterior to the wing roots. Blackish transverse streak near the posterior margin of the prothorax; pleura and sternum light yellowish brown; an oblique stripe of pure yellow on the pleura from the base of the fore wing to the base of the fore leg. Legs light yellow, becoming whitish on the fore tarsi. Fore tibia about one and one-half times as long as the femur; tarsus about twice the length of the tibia. Wings hyaline; venation hyaline.

Abdomen yellowish olive brown; lateral rows of indistinct oblique brown stripes, more or less fused on tergites 8-10. Tergites 2 and 3 faintly blackish on the posterior margins. Tails white, with faint brownish joinings at the base. Genitalia very similar to Ephemerella excrucians. It is quite possible that this species may be synonymous with excrucians.


Described in Needham et al (1935) as Ephemerella inermis

The nymph is rather dark reddish brown in color; the head and thorax are irregularly mottled with light and dark areas, which are sometimes rather extensive on the pronotum. The lateral margins of the pronotum are pale. Legs blackish brown, the femora with three large pale areas; tibiae pale at apex and near the base; tarsi pale distally. Spines on the joints are very similar to E. mollitia (now a synonym of Ephemerella dorothea infrequens). Lateral extensions of the abdominal segments very well developed. Dorsal spines absent. A rather large pale spot on each side of the median line, on the posterior margin of each tergite. On the middle tergites the median line is often pale, sometimes wholly whitish. Rather extensive pale areas are often present just above the gills on these tergites, most pronounced on tergites 5 and 6. These markings are variable in different specimens. Tails yellowish brown, with several median and apical dark bands. Ventrally yellowish brown, the apical sternites and the lateral margins of the middle ones blackish brown. Traces of dark submedian oblique dashes on the anterior margins, and of dark dots nearer the center of the sternites, may be present.

Specimens of the Mayfly Species Ephemerella excrucians

2 Male Duns
5 Female Duns
3 Male Spinners
4 Female Spinners
10 Nymphs

Discussions of Ephemerella excrucians

PMD Spinner - Egg sack color?
20 replies
Posted by Wbranch on Jan 26, 2010
Last reply on Aug 18, 2020 by Troutnut
Do any of you entomologist types know the true color of the PMD spinner? Dorothea or excrucians. Where I fish in MT there are huge spinner falls, many spents are on the water in the morning and others fall again at various periods during the day. I'd like to tie some with egg sacks as I saw many in July but forgot what color they were. Thanks.

Start a Discussion of Ephemerella excrucians


Mayfly Species Ephemerella excrucians (Pale Morning Duns)

Species Range
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