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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.

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 Procloeon venosum (Tiny Sulphur Duns)

This taxon can be found in large western rivers.

Where & when

In 1 records from GBIF, adults of this species have been collected during August (100%).

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 Centroptilum venosum
Body length: 5.5-6 mm
Wing length: 5.5-6 mm

Abdominal tergites 2-6 of male imago yellowish white, the posterior margins reddish brown; a distinct rounded bulge on inner margin of second forceps joint.

Turbinate eyes large; quite bright orange, usually paler along margin. Face light brown; paler areas around bases of antennae and on clypeal margin near the eye. Antennae pale brown, the second joint and tip of filament whitish. Pronotum reddish brown, its margins darker. Mesonotum reddish brown (yellowish, in pale specimens); sutures darker; usually a pale median area near the anterior margin; antero-lateral and postero-lateral margins creamy. Scutellum creamy white, more or less shaded with pinkish. Lateral areas below scutellum dark reddish brown. Faint greyish markings before the wing. Metanotum red-brown, the median projection and lateral and posterior margins darker, and with distinct red shading. Pleura and sternum yellowish to olive brown, sutures darker; yellowish areas below wing and around leg bases.

Legs yellowish white, fore leg slightly amber-tinged. Joinings sometimes narrowly darker; claws orange-brown. Wings hyaline; a slight cloud in stigmatic area. Longitudinal veins of fore wing yellowish brown; 6-10 cross veins in stigmatic space; marginal intercalary usually absent from first two interspaces. Hind wing wide for its length; two longitudinal veins and a distinct intercalary below the second vein, extending basad for half or more than half the wing length. Distinct cross veins are present; 3 to 5 in the costal space; 1 or 2 near the center of the space between veins 1 and 2; and 3 to 4 in the last space (see fig. 165). In the latter area, the cross veins may all extend from vein 2 to the intercalary, or the basal ones may reach the hind margin. Costal projection strongly curved.

Abdominal tergites 2-6 yellowish white, semi-translucent; posterior margins reddish or orange-brown. A wedge-shaped mark of the same color extends forward halfway to the anterior margin, above the spiracular area, at the end of each dark band, on each side. Indications of pale brown oblique submedian streaks on the anterior margin. An interrupted black line, almost straight, marks the spiracular area; near the posterior margin of each tergite, a short oblique branch extends downward, with a short red-brown dash at its extremity. Tergites 7-10 opaque, light red-brown, posterior margins darker; median area often marked with whitish or pinkish opaque streaks. Sternites yellowish white; 2-6 semi-translucent, the posterior margins sometimes faintly darker; 7-9 opaque, tinged more or less with pinkish, especially on the posterior margins. Tails whitish or yellowish white. Distal joint of forceps short; third (long) joint not strongly bowed; a distinct rounded bulge on inner margin of second joint. Plate between forceps bases broad, its posterior margin almost straight (see fig. 166).

The broad hind wing with intercalary and cross veins distinguishes this species from C. bellum (now a synonym of Procloeon bellum); the conspicuous abdominal banding separates it from C. bifurcatum (now a synonym of Anafroptilum bifurcatum). The distinctive venation of the hind wing should separate it from other species heretofore described.

Female Spinner

Described in Needham et al (1935) as Centroptilum venosum

Head and thorax of female yellowish brown; longitudinal reddish streaks on vertex; pronotum much marked with reddish brown. Abdominal tergites light reddish, yellower on apical tergites; posterior margins bright reddish or red-brown. Sternites pale reddish yellow. Black markings along the tracheae.

Start a Discussion of Procloeon venosum


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

Mayfly Species Procloeon venosum (Tiny Sulphur Duns)

Species Range
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