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Lateral view of a Male Baetis (Baetidae) (Blue-Winged Olive) Mayfly Dun from Mystery Creek #43 in New York
Blue-winged Olives

Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.

Case view of a Pycnopsyche guttifera (Limnephilidae) (Great Autumn Brown Sedge) Caddisfly Larva from the Yakima River in Washington
It's only barely visible in one of my pictures, but I confirmed under the microscope that this one has a prosternal horn and the antennae are mid-way between the eyes and front of the head capsule.

I'm calling this one Pycnopsyche, but it's a bit perplexing. It seems to key definitively to at least Couplet 8 of the Key to Genera of Limnephilidae Larvae. That narrows it down to three genera, and the case seems wrong for the other two. The case looks right for Pycnopsyche, and it fits one of the key characteristics: "Abdominal sternum II without chloride epithelium and abdominal segment IX with only single seta on each side of dorsal sclerite." However, the characteristic "metanotal sa1 sclerites not fused, although often contiguous" does not seem to fit well. Those sclerites sure look fused to me, although I can make out a thin groove in the touching halves in the anterior half under the microscope. Perhaps this is a regional variation.

The only species of Pycnopsyche documented in Washington state is Pycnopsyche guttifera, and the colors and markings around the head of this specimen seem to match very well a specimen of that species from Massachusetts on Bugguide. So I am placing it in that species for now.

Whatever species this is, I photographed another specimen of seemingly the same species from the same spot a couple months later.
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 Eurylophella bicolor (Chocolate Duns)

According to the Leonards in Mayflies: An Angler's Study of Trout Water Ephemeroptera , this species and Eurylophella bicolor are the two most likely hatches for fly fishermen to encounter in Michigan. Based on my sampling, it seems other species may be more prevalent elsewhere.

Where & when

Time of year : May and June

In 15 records from GBIF, adults of this species have been collected during June (47%), July (33%), May (13%), and August (7%).

In 5 records from GBIF, this species has been collected at elevations ranging from 26 to 1270 ft, with an average (median) of 699 ft.

Species Range

Hatching behavior

Time of day : Early afternoon

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 bicolor
Body length: 5-7 mm
Wing length: 6-7 mm

A member of the bicolor (now a synonym of Eurylophella bicolor) group; imago paler than other related species.

Head of male light brown. Thorax dark reddish brown. Legs pale yellow; coxae tinged with brown, and with black spots. Tinges of red may be present near the apex of each femur. Wings hyaline; venation hyaline. Stigmatic cross veins anastomosed. Abdominal tergites pale reddish brown with a distinct tinge of olive-brown. Blackish streaks near the pleural fold, and in some specimens a paler mid-dorsal line margined by interrupted blackish submedian streaks. Ventrally pale, the basal sternites slightly tinged with brown. Tracheae appear as fine dark lines. Tails pale yellowish white, the joinings purplish red.

Female Spinner

Described in Needham et al (1935) as Ephemerella bicolor

Head of female pale yellow.


Described in Needham et al (1935) as Ephemerella bicolor

Nymph variable in both size and color. The occipital tubercles are wanting in the male nymph, and much reduced in the female. The rows of dorsal spines are distinctly divergent toward the rear, with a rather abrupt increase in the space between them on tergite 5. The spines on tergites 1-3 are short and chunky, and barely tapering. Postero-lateral spines on segments 2 and 3 are very poorly developed; these spines are well developed on the posterior segments, tipped with black and gracefully curved. The margins are set with short spinules, between which are long hairs. Color very variable, as is also the size. Pale specimens are strikingly marked with dark brown on the thorax and the posterior abdominal tergites. Legs distinctly banded. An oval patch is often present near the middle of the operculum. The usual four dots in a curved row, and the dark lateral dashes, are present in most specimens. Tails brownish, with indications of pale cross bands. Fore leg as in fig. 62e.

Start a Discussion of Eurylophella bicolor


Mayfly Species Eurylophella bicolor (Chocolate Duns)

Species Range
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