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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 Hexagenia atrocaudata (Late Hexes)

This species is slightly smaller than Hexagenia limbata and it occurs later in the year. It is only mentioned in passing by a few authors, but it can be locally important.

Where & when

Time of year : July and August

I have heard rumors of fishable hatches of this species in both Wisconsin and the Catskills. The duns and spinners have been a common sight for me on several Wisconsin rivers during August, but they have been too sparse to cause good rises.

I have had some luck with an imitation. Last year a very nice smallmouth bass rose to my atrocaudata spinner pattern as subtly as any trout.

In 8 records from GBIF, adults of this species have been collected during August (75%) and September (25%).

In 3 records from GBIF, this species has been collected at elevations of 2526, 2690, and 3051 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: 22 mm
Wing length: 20 mm

A large dark species; abdomen blackish brown with olivaceous areas; outer margin of hind wing widely purplish brown; penes short, broad, bluntly pointed.

Head light red-brown; carina and frontal margin pale, vertex yellow. Eyes deep orange; very large, almost contiguous above. Pronotum blackish, median and lateral areas paler. Mesonota and metanota and pleura dark reddish to olive brown; a paler reddish median streak on mesonotum; yellowish markings on pleura below wing roots, above leg bases. Sternum deep reddish to purplish brown. Fore legs deep reddish black, middle tarsal joints a pale yellowish basally. Middle and hind legs greenish yellow; distal tarsal joint, tarsal joinings and claws purplish black. Wings hyaline; all veins blackish. Costal margin of fore wing “heavily tinged with brown in the apical half and between veins 2 and 3 in the basal half” (McDunnough). Cross veins in costo-apical space tend to anastomose; only & few cross veins in either wing are narrowly dark-margined, no conspicuous dark spots. Outer margin of hind wing with a very wide purplish black border.

Abdominal tergites dark blackish brown; 1 and 2 largely dark, only a narrow paler median line. Median yellowish olive triangular areas, widened posteriorly, on tergites 3-6; on 7 and 8, a pale median streak at anterior margin only. Pale antero-lateral triangles on tergites 3-10, separated from pale postero-lateral triangles by the blackish-brown lateral stripes, on the middle tergites. “The triangular dorsal portion of segment 10 is dull brown, edged with black and with geminate black medio-dorsal streaks” (McD.). Median triangles on sternites dark brown, the base of each extending over most of the posterior margin of the sternite; the apices attain the anterior margins. A small dark spot in the antero-lateral angle of each sternite, so that only Iateral triangles of the pale olivaceous yellow background remain. Near center of each sternite are two small pale dots, one on each side. Genitalia largely dark red-brown, with a few yellow markings; penes distinctive, being broad, short and bluntly pointed (see fig. 84). Tail joints blackish brown, each with a narrow yellow basal ring. See frontispiece. The characteristic genitalia separate this species from all others in the genus.

Specimens of the Mayfly Species Hexagenia atrocaudata

1 Female Dun
2 Male Spinners

Start a Discussion of Hexagenia atrocaudata


Mayfly Species Hexagenia atrocaudata (Late Hexes)

Species Range
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