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.

The trees were leafing out and the trout were rising on this productive Wisconsin stream

From the Namekagon River in Wisconsin

Stream photos

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Onocosmoecus (Great Late-Summer Sedge) Caddisfly Larva Onocosmoecus (Great Late-Summer Sedge) Caddisfly Larva

Lateral view of a Onocosmoecus (Limnephilidae) (Great Late-Summer Sedge) Caddisfly Larva from the Yakima River in Washington

This specimen keys pretty easily to Onocosmoecus, and it closely resembles a specimen from Alaska which caddis expert Dave Ruiter recognized as this genus. As with that specimen, the only species in the genus documented in this area is Onocosmoecus unicolor, but Dave suggested for that specimen that there might be multiple not-yet-distinguished species under the unicolor umbrella and it would be best to stick with the genus-level ID. I'm doing the same for this one.

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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 Ameletus similior (Brown Duns)

Where & when

In 3 records from GBIF, adults of this species have been collected during August (67%) and July (33%).

In 7 records from GBIF, this species has been collected at elevations ranging from 770 to 4856 ft, with an average (median) of 2493 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)

Body length: 9 mm
Wing length: 9 mm

A medium-sized species, distinguished by the dark heart-shaped ganglionic marks on venter of abdomen. Wings with faint clouds.

Head and thorax shiny blackish brown, in the male. Fore legs blackish brown; middle and hind legs lighter brown. Wings hyaline, veins dark, palest along the outer margin; some are faintly clouded with light umber brown, in the male.

Abdomen deep brown with lighter brown shading, the posterior margins definitely brown-ringed. Dark heart-shaped marks on ganglionic areas of sternites. Tubercles on the apical margin of the forceps base, between the forceps, rather poorly developed. Penes rather long and slender, only slightly incurved near the tip: each bears one or more spines on the inner margin (see fig. 117).

Female Spinner

Described in Needham et al (1935)

Body length: 9 mm
Wing length: 9 mm

In the female, the thorax is rather bright reddish brown. Subanal plate of female rather small, covering less than half the tenth segment.

Start a Discussion of Ameletus similior

References

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