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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.

Artistic view of a Perlodidae (Springflies and Yellow Stones) Stonefly Nymph from the Yakima River in Washington
This one seems to lead to Couplet 35 of the Key to Genera of Perlodidae Nymphs and the genus Isoperla, but I'm skeptical that's correct based on the general look. I need to get it under the microscope to review several choices in the key, and it'll probably end up a different Perlodidae.
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 Epeorus longimanus (Slate Brown Duns)

Epeorus longimanus and Epeorus albertae are the two most common Epeorus species in the West.

Where & when

Time of year : June through mid-August, peaking in mid-June to mid-July, for about one week in any given location

Altitude: Above 5,000 feet

According to Ernest Schweibert in Matching the Hatch, the trout "respond very well" to this hatch. Fred Arbona in Mayflies, the Angler, and the Trout called the hatches disappointing, noting that they may be overshadowed by simultanenous hatches of Rhithrogena hageni.

In 25 records from GBIF, adults of this species have been collected during July (32%), August (32%), June (20%), and September (16%).

In 42 records from GBIF, this species has been collected at elevations ranging from 259 to 10840 ft, with an average (median) of 6713 ft.

Species Range

Hatching behavior

Time of day : Anytime; sometimes concentrated in late morning

The nymphs emerge sporadically on the bottom in the shallows and rise to the surface as duns.

Spinner behavior

Time of day: Midday

Habitat: Fast water

Nymph biology

Current speed: Fast

This species is found more in the cold headwaters than Epeorus albertae and Epeorus grandis.

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 Iron longimanus
Body length: 9-10 mm
Wing length: 10-11 mm

A rather pale brownish species; genitalia of the longimanus type (now a synonym of Epeorus longimanus). (see fig. 105).

Head light reddish brown. Thoracic notum light yellowish to reddish brown ochre, pleura and sternum very similar in color. Fore femur and tibia “translucent raw umber” (Eaton); other femora similar, tibiae yellowish brown. All femora with a round black spot near the middle; fore tibia blackish at apex. Fore tarsus dull yellowish brown, middle and hind tarsi slightly paler in color; tarsal joinings may appear somewhat darker. Wings hyaline; venation light brownish amber, the humeral cross vein, base of subcosta and other longitudinal veins behind it, paler; veins in hind wing rather pale.

Abdominal tergites 2-9 “tinged extensively with light brownish” (Eaton); 9 and 10 often light brown-ochre. Tergites 2-7 semi-hyaline; posterior margins deep purplish brown, this band not extending around the postero-lateral angle, but a short dark oblique streak arising from this dark band runs to the spiracular area. Purplish brown shading along mid-dorsal line; median line narrowly pale, with darker streak on each side. Pale lunate submedian marks on each side lie within the shaded area. Segments 8-10 opaque. Sternites pale yellowish; nerve cord and ganglia whitish, opaque; sometimes traces of pale submedian oblique streaks on basal and middle segments. Tails “light warm sepia brown” (Eaton). Apical margin of forceps base projects backward as a dome-shaped structure, concealing the bases of the penes. Each division of the penes has an erect apical lobe and a larger laterally-directed one, bearing a sharp lateral spine. A median pair of strong spines is present, their tips divergent.

In type of genitalia this western species is allied to the eastern I. fraudator (now a synonym of Epeorus pleuralis), I. fragilis (now a synonym of Epeorus fragilis) and I. tenuis (now a synonym of Epeorus fragilis). It is larger and somewhat darker in color than either of the two latter species. The lobes of the penes are likewise much less deeply cut than in any of the three. The darkest specimens we have seen are slightly paler than typical fraudator, which is likewise a little larger than I. longimanus.

Described as I. proprius

Body length 9-10 mm, wing length 10 mm

Similar in general appearance to I. longimanus (now a synonym of Epeorus longimanus), but with genitalia of the pleuralis (now a synonym of Epeorus pleuralis) type.

The only specimens available are so faded from long immersion in alcohol that it is impossible to be certain of the original color. However, they seem to have been very similar to I. longimanus, from which species they are distinguishable mainly by the very different type of genitalia. A comparison of these with specimens of longimanus which likewise have been in alcohol for several years, shows that the dark spot on each femur is smaller and less distinct than in the latter species. The oblique black streak present on the postero-lateral angles of the tergites in I. longimanus, is apparently lacking in I. proprius (now a synonym of Epeorus longimanus). Genitalia very similar to I. pleuralis, but smaller; chitinized appendage attached to dorsal lobe of penes is narrower and more distinctly sickle-shaped than in that species. (See fig. 106).

Specimens of the Mayfly Species Epeorus longimanus

1 Male Dun
1 Female Spinner
3 Nymphs

Start a Discussion of Epeorus longimanus

References

Mayfly Species Epeorus longimanus (Slate Brown Duns)

Taxonomy
Species Range
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