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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 Kogotus (Perlodidae) Stonefly Nymph from Mystery Creek #199 in Washington
This one pretty clearly keys to Kogotus, but it also looks fairly different from specimens I caught in the same creek about a month later in the year. With only one species of the genus known in Washington, I'm not sure about the answer to this ID.
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 Iswaeon rubrolaterale (Little Olives)

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

Described in Needham et al (1935) as Pseudocloeon rubrolaterale
Body length: 4 mm
Wing length: 4 mm

Abdominal tergites 2-6 of male imago pale yellowish white; large lateral maroon patches on each.

Turbinate eyes of moderate size, almost circular; reddish brown in dried specimen. Head and thorax deep blackish; pleural membranes paler brown. Legs pale yellowish white; fore femur with smoky tinges. Wings hyaline, venation pale. Abdominal tergites 2-6 pale yellowish white; very distinct large lateral maroon or wine-colored patches on each; sternites pale, each with a small brown dot near the center, on the mid-ventral line. Tergites 7-10 sepia brown; lateral patches visible in certain lights; sternites paler brown, with mid-ventral dark dots. Tails and forceps pale.

The prominent wine-colored lateral patches on the tergites distinguish this species from all others thus far described.

Start a Discussion of Iswaeon rubrolaterale


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

Mayfly Species Iswaeon rubrolaterale (Little Olives)

Species Range
Common Name
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