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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.

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 cooki (Brown Duns)

Where & when

In 8 records from GBIF, adults of this species have been collected during June (38%), July (38%), and August (25%).

In 4 records from GBIF, this species has been collected at elevations of 3500, 4564, 4682, and 4856 ft.

Species Range

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: 7 mm
Wing length: 8 mm

A small clear-winged species. Head blackish. Prothorax and postero-lateral portions of the mesothorax and metathorax also blackish. Mesothorax brown dorsally, with orange shading before the wing root and on and before the scutellum. Pleura and sternum largely blackish, shaded with orange at the bases of the legs, most prominent around the middle legs.

Fore legs blackish brown; middle and hind legs lighter brown, the basal joints tinged with orange-brown. Wings hyaline; longitudinal veins light amber, the cross veins pale and indistinct, except in the stigmatic area, where they are smoky.

Dorsum of abdomen orange-brown. Suffused brown shading along the lateral margin, tending to form obscure semi-triangular patches in the postero-lateral corners; sometimes extending narrowly along the posterior margin. Trachea of the anterior segments forming a black network. Sternites 2-6 dull hyaline whitish, with traces of orange on the ganglionic areas. Sternites 7 and 8 opaque, bright yellowish; sternite 9 largely blackish brown shaded with orange. Forceps base yellowish. Penes long and rather slender, each bearing a spine on the inner margin near the middle; penes curved only at tip (see fig. 116). Tails dirty whitish, the joinings dark.

Specimens of the Mayfly Species Ameletus cooki

1 Nymph

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Mayfly Species Ameletus cooki (Brown Duns)

Species Range
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