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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.

Dorsal view of a Sweltsa (Chloroperlidae) (Sallfly) Stonefly Nymph from the Yakima River in Washington
This species was fairly abundant in a February sample of the upper Yakima.
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 Baetisca lacustris (Armored Mayflies)

Where & when

In 8 records from GBIF, adults of this species have been collected during April (38%), May (13%), February (13%), March (13%), June (13%), and July (13%).

In 1 record from GBIF, this species has been collected at elevation of 618 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

Abdominal tergites of male imago brown, sternites pale yellowish white; venation wholly pale.

Head and thorax brown; thoracic sternum pale brown. Legs pale yellowish white. Wings hyaline, venation wholly pale. Abdominal tergites brown; sternites pale yellowish white, contrasting strongly with the dark upper surface. Forceps yellowish white. Tails whitish, not darker at joinings.


The nymph has lateral but no dorsal spines on the mesonotal shield; the genae are not produced into spines; the frontal projections are small and rounded. The lateral mesonotal spines are very long and sharp. This species is distinguished from others in the genus by the wholly pale venation and the striking contrast in color between the dorsal and ventral surfaces of the abdomen.

Start a Discussion of Baetisca lacustris


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

Mayfly Species Baetisca lacustris (Armored Mayflies)

Species Range
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