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

Blue-winged Olives
Baetis

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.

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Psychodidae True Fly Larva Psychodidae True Fly Larva

Lateral view of a Psychodidae True Fly Larva from Mystery Creek #308 in Washington

This wild-looking little thing completely puzzled me. At first I was thinking beetle or month larva, until I got a look at the pictures on the computer screen. I made a couple of incorrect guesses before entomologist Greg Courtney pointed me in the right direction with Psychodidae. He suggested a possible genus of Thornburghiella, but could not rule out some other members of the tribe Pericomini.

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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 verisimilis (Chocolate Duns)

Where & when

In 22 records from GBIF, adults of this species have mostly been collected during June (45%), May (32%), and July (18%).

In 19 records from GBIF, this species has been collected at elevations ranging from 20 to 2723 ft, with an average (median) of 620 ft.

Species Range

source

Map of specific collection sites

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 Emphemerella verisimilis

Body length: 7 mm
Wing length: 8 mm

A species of the bicolor group (now a synonym of Eurylophella bicolor), intermediate between E. bicolor (now a synonym of Eurylophella bicolor) and E. temporalis (now a synonym of Eurylophella temporalis); abdominal sternites darker and legs deeper yellow than in these species. Occipital tubercles of nymph well developed; rows of dorsal spines evenly divergent to rearward.

Eyes orange. Head reddish brown with purplish brown shading around the ocelli. Thorax deep brown. Legs rather bright yellow, with traces of orange patches at the base, the middle and the apex of each femur. Wings hyaline, venation hyaline. Abdominal tergites deep brown, with no indication of yellow or orange shading. Abdominal sternites darker than in bicolor or temporalis. Faint indications of the usual dark dots and oblique streaks. Tails smoky, the joinings narrowly darker.

Nymph

Described in Needham et al (1935) as Ephemerella verisimilis

The occipital tubercles of the nymph are quite well developed in both sexes. The rows of dorsal spines are gradually and evenly divergent to rearward; those of tergites 1-3 are slightly longer and more erect than in E. bicolor, the tips more pointed. The postero-lateral spines on segments 2 and 3 are considerably better developed than in bicolor (now a synonym of Eurylophella bicolor). Tails brown with no pale bands.

Start a Discussion of Eurylophella verisimilis

References

Needham, James G., Jay R. Traver, and Yin-Chi Hsu. 1935. The Biology of Mayflies. Comstock Publishing Company, Inc.