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Artistic view of a Male Pteronarcys californica (Pteronarcyidae) (Giant Salmonfly) Stonefly Adult from the Gallatin River in Montana
Pteronarcys californica

The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.

Dorsal view of a Pycnopsyche guttifera (Limnephilidae) (Great Autumn Brown Sedge) Caddisfly Larva from the Yakima River in Washington
This specimen appears to be of the same species as this one collected in the same spot two months earlier. The identification of both is tentative. This one suffered some physical damage before being photographed, too, so the colors aren't totally natural. I was mostly photographing it to test out some new camera setting idea, which worked really well for a couple of closeups.
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 dorothea infrequens (Pale Morning Duns)

Ephemerella dorothea infrequens (formerly Ephemerella infrequens), together with its often smaller and later hatching sibling Ephemerella excrucians, make up the most important Western hatches. They go by several common names but are best known as Pale Morning Duns (PMD's). They are rivaled only by the many baetid species that typically bookend them. In terms of availability, consistency and abundance (not to mention their convenient timing and preference for beautiful weather), they have no rival. They can run in size from a large 14 to a small 16 and various shades of illusive yellowish creams, sulfurs, and even yellowish greens, depending on the system they inhabit.

This taxon used to be considered the separate species Ephemerella infrequens, but entomologists now regard it as a subspecies together with the small eastern Pale Evening Dun hatch Ephemerella dorothea dorothea. There is another related listing of significance common in California and the Southwest that has undergone revision. The large (often exceeding 10 mm) Ephemerella mollitia is now considered synonymous with d. infrequens.

While it is not the normal policy of TroutNut to list subspecies as separate taxa, d. infrequens and d. dorothea are so important and distinct from each other in terms of geography, appearence, and angling tradition that they warrant an exception.

Where & when

Time of year : March-August, best in May-June in the coastal states;July-August in the Rockies

Preferred waters: All types, but especially abundant in alkaline spring creeks.

Altitude: From near sea level on some West Coast tailwaters to as high as 8,000 ft.

This Western subspecies hatching cycle is more difficult to predict because of the high local variation in climate and the varieties of habitat including spring creeks, tailwaters and snow melt fed freestones. Hatches generally occur in the Rocky Mountain states from late Spring through the Summer months depending largely on elevation, latitude, and water type. They have been reported as early as March on some West Coast watersheds with mid-May to mid-June being the prime time at higher elevations. Despite the wide range of emergence dates, this hatch is usually prevalent on a given stretch of water for less than a few weeks or so, often mixing with the other Pale Morning Dun species Ephemerella excrucians as it begins to taper off. This often gives the impression of a much longer lived hatch. Because of this mix, some spring creeks and tailwaters in the West can have at least sparse activity of "PMD's" virtually the entire season. Caucci and Nastasi note in Hatches II that they can extend well into October on stable spring creeks. Fred Arbona writes in Mayflies, the Angler, and the Trout that they may appear during the last few weeks of June. Knopp and Cormier report their hatches during May in California.

In 13 records from GBIF, adults of this species have been collected during June (46%), July (23%), August (15%), and May (15%).

In 15 records from GBIF, this species has been collected at elevations ranging from 5066 to 9193 ft, with an average (median) of 6670 ft.

Species Range

Hatching behavior

Time of day : Usually midday often moving to evenings during warm weather

Habitat: Highly adaptable from freestone riffles to spring creek weedbeds

Water temperature: variable depending on location

Classic surface emergers with long sedate floats in optimal weather. Have been reported to emerge subsurface in faster flows. They usually engage in "practice runs" exposing the nymphs to trout during extended pre-hatch periods.

Spinner behavior

Time of day: Usually morning and often again near dusk

Habitat: Riffles are preferred in freestone environments. Spring creek dwellers prefer specific locations determined by a variety of factors.

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

Females often, 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 bright yellow dots near the tail and upright wings as well. The gravid females are often active and far from "spent". Spent spinners usually cause the most feeding activity. Both male and female are usually similar enough that the same pattern can work for both, though the angler should take note of the differences in size between the sexes as the fish can become selective to one or the other. thogh often seen in the evening, mid- morning spinner falls have achieved legendary status at many locations.

Nymph biology

Diet: Detritus, algae

Current speed: Can be found in all current speeds, but especially near vegetation.

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. They can be found in the benthic drift at almost any time of the day but concentrated behavioral drift can occur in the evening. Color is not a good way to distinguish them from other Ephemerellids they often share habitat with. In addition to their larger size, another way to often tell them from their close relative excrucians is their lack of veriagation or striping of the abdominal dorsum (top). With a few less common exceptions, their lack of paired abdominal tubercles (little parallel spikey projections along the top of the abdomen) is an easy way (for young eyes) to tell them apart from other common Ephemerellids. They run in colors from a dark chocolate brown through various shades of rust, cinnamon or tan. They can also be olive hued. The three tails and legs are usually speckled to some degree and similarly shaded as their bodies.

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 infrequens
Body length: 8 mm
Wing length: 9 mm

A species of the Ephemerella invaria group; second joint of forceps swollen at apex; nymph without dorsal spines, and with no pale spots on posterior margins of tergites.

Eyes of living male light orange. Head light brown, clypeus and bases of antennae pale yellow. Pronotum and the lateral edges of the mesonotum anterior to the wing base purplish brown. Mesonotum deep olive brown, becoming reddish brown posteriorly; two slightly raised blackish lines anterior to the scutellum. Bases of wings and the pleural sutures tinged with yellowish brown. Prosternum yellowish; remainder of sternum brown. Legs light yellow, the fore tibiae and tarsi rather duller than the femora; tarsal joints slightly marked with brown. Wings hyaline; venation pale; cross veins indistinct, especially in the basal portion of the costal space.

Abdomen light reddish brown dorsally; posterior margins smoky. A narrow yellowish line along the pleural fold. Dull purplish brown ventrally, shading into yellowish brown on the apical sternites. The middle sternites show traces of pale semi-translucent areas along the anterior margins. Forceps yellowish, penes similar, with a darker transverse band near the base. Tails yellowish at the base, whitish beyond; joinings narrowly deep brown. Usually four apical spines on the penes, and four or five others lower down, in two irregular groups (see fig. 152).

Described as E. mollitia

Body length 10 mm, wing length 10 mm

A species of the Ephemerella invaria group; spines on the penes more numerous than in any other species of this group; venation pale.

Eyes salmon pink in life. Antennae purplish black at base, second joint pale; filament dark brown (in alcoholic specimens, rather pale.) Thorax deep yellow, shading into deep orange on the median line, the lateral margins of the mesonotum above the wing roots, and the lateral margins of the scutellum. Scutellum margined narrowIy with brown. Anterior and posterior margins of pronotum, the median line, and markings on each side of the median line, purplish black. Deep pinkish to orange at the base of each leg. Legs pale (in life the fore leg may have had a ruddy tinge); tarsal joinings of the fore leg narrowly brownish. Wings hyaline; venation pale yellowish to hyaline.

Basal and middle abdominal tergites deep reddish; posterior margins rather widely purplish black, the anterior margins paler (probably the “white or translucent joinings” of the original description). Apical tergites yellowish, tinged with reddish laterally. Pleural fold pale on the extreme margin; an interrupted dark line parallels it closely on each tergite. Ventrally paler, the basal and middle sternites tinged with reddish. Forceps smoky brown, penes pale. Tails white, the joinings black. 24 to 28 spines on the penes; an apical cluster consisting of 4 to 6 spines on each side, and others irregularly arranged further back toward the base (see fig. 152).


Described in Needham et al (1935) as Ephemerella infrequens

The nymph is dark reddish to blackish brown in color. Antennae dark brown at base, remainder yellowish. Vertex of head, each side of the pronotum, and the mesonotum anterior to and between the wing roots, indistinctly mottled with darker brown; these mottlings are much less apparent than in the very similar nymph of E. inermis (now a synonym of Ephemerella excrucians). Legs reddish brown; femora blotched irregularly with yellowish; tibiae and tarsi each pale-banded near the base and again at the apex. Femora with the usual posterior spines; fore femur with spines as usual on the upper surface near the apex. 5 to 6 denticles on each claw. No dorsal abdominal spines. Lateral extensions present, but slightly less developed than in E. inermis, and lacking the dark mark across the middle, which is present in the latter species. No pale spots at the posterior margins of the tergites, near the median line, such as are present in E. inermis and E. mollitia (now a synonym of Ephemerella dorothea infrequens). A pale mid-dorsal line is usually present. Pale lateral areas, if present, are inconspicuous and hidden beneath the gills; often these are entirely wanting. Venter without conspicuous markings. Tails reddish brown, crossed by several yellowish bands.

Described as E. mollitia

Nymph reddish brown; very similar to E. inermis (now a synonym of Ephemerella excrucians), but with fewer bands on the tails. Vertex and occiput of head, the pronotum on each side, and the mesonotum anterior to and between the wing roots, mottled with irregular dark reddish brown spots, which are surrounded by a yellowish ground color. Lateral margins of the pronotum pale yellowish. Genae show a slight tendency toward lateral development. Legs pale reddish to greyish brown; femora indistinctly mottled. A brownish band at the base of tibia and tarsus; another dark band beyond the middle of the tibia. Short spines on the posterior margins of the femora, and the anterior margins of tibiae and tarsi; shorter spines on the upper surface of the fore femur near the apex. 8-10 denticles on each claw. Lateral extensions of the abdominal segments well developed. Dorsal spines absent. Two pale spots on the posterior margin of each tergite, one on each side of the median line. Faint indications of a pale median line and of pale submedian streaks, on the middle tergites. Larger pale areas are usually present laterally, just above the gills, on tergites 3-7; most extensive on tergite 5. Tails yellowish. In mature nymphs, each joining in the basal half appears black-ringed, due to the subimaginal tail within. In younger nymphs, tails pale except for one or two dark basal rings.

Specimens of the Mayfly Species Ephemerella dorothea infrequens

1 Male Dun
3 Male Spinners
1 Female Spinner
1 Nymph

Discussions of Ephemerella dorothea infrequens

So is Ep Infrequens now known as Ep Dorothea?
20 replies
Posted by Wbranch on Feb 17, 2008
Last reply on Jul 1, 2014 by Crepuscular
These mayflies look more like the Sulfurs I see on the Delaware system than the PMD's I see in Montana. The Montana mayfly has a distinct yellow leading edge to an overall light dun gray wing and the abdomen and thorax have a more light greenish/yellow cloration so how is it that Infrequens is now known as Ep Dorothea Dorothea?

Start a Discussion of Ephemerella dorothea infrequens


Mayfly Species Ephemerella dorothea infrequens (Pale Morning Duns)

Species Range
Common Name
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