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Lateral view of a Female Hexagenia limbata (Ephemeridae) (Hex) Mayfly Dun from the Namekagon River in Wisconsin
Hex Mayflies
Hexagenia limbata

The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.

Dorsal view of a Limnephilidae (Giant Sedges) Caddisfly Larva from the Yakima River in Washington
This specimen resembled several others of around the same size and perhaps the same species, which were pretty common in my February sample from the upper Yakima. Unfortunately, I misplaced the specimen before I could get it under a microscope for a definitive 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 Rhithrogena flavianula

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: 14 mm
Wing length: 15 mm

A large species of the brunnea group, having basal lateral spines on penes.

Clypeus dull grey, “crossed by a black band at its base” (McD.). Head ochreous at bases of antennae and around ocelli. Thorax deep brown. Anterior to the wing roots a pale ochreous patch, within which are two blackish streaks. Pleural sutures and bases of wings with ochreous markings. Fore leg blackish. Middle and hind legs golden brown; a black streak on basal half of each femur; tarsi blackish. Wings hyaline; stigmatic area tinged with pale amber. Venation blackish-brown. Abdominal segments deep reddish brown; lateral and posterior margins of tergites narrowly light yellow, so that abdomen appears annulate. Posterior sternites darker in color, the posterior margins yellow-banded as on the dorsum. Forceps deep brown. Tails blackish. Penes distinctly outcurved at tip, each bearing a series of very small spines near apex and on lateral margin. A large spine laterally at base on outer margin (see fig. 100).

The annulate abdomen, large size and genitalic structures should serve to distinguish this from other allied species.

Start a Discussion of Rhithrogena flavianula


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

Mayfly Species Rhithrogena flavianula

Species Range
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