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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 Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
This is an interesting one. Following the keys in Merritt R.W., Cummins, K.W., and Berg, M.B. (2019) and Jacobus et al. (2014), it keys clearly to Ephemerella. Jacobus et al provide a key to species, but some of the characteristics are tricky to interpret without illustrations. If I didn't make any mistakes, this one keys to Ephemerella mucronata, which has not previously been reported any closer to here than Montana and Alberta. The main character seems to fit well: "Abdominal terga with prominent, paired, subparallel, spiculate ridges." Several illustrations or descriptions of this holarctic species from the US and Europe seem to match, including the body length, tarsal claws and denticles, labial palp, and gill shapes. These sources include including Richard Allen's original description of this species in North America under the now-defunct name E. moffatae in Allen RK (1977) and the figures in this description of the species in Italy.
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 Ameletus vernalis (Brown Duns)

Where & when

In 4 records from GBIF, adults of this species have been collected during May (50%), June (25%), and March (25%).

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: 12 mm
Wing length: 12 mm

A large clear-winged species with faint mid-ventral ganglionic marks on the abdomen.

Head and thorax deep blackish brown, the sutures yellowish. Fore femur deep blackish brown, a faint yellowish tinge at the base and a yellow line along the upper edge; tibia and tarsus black. Femora of middle and hind legs marked like fore femur, but slightly paler; tibiae and tarsi light brown. Wings hyaline, the fore wings shaded with brown on the apex and with a slight brown tinge at the base of R3, where there is a group of 6 cross veins. Veins and cross veins brown except 3 or 4 costal cross veins beyond the bulla, which are pale and indistinct. Stigmatic cross veins anastomosed to form a series of smaller costal cells with larger ones below.

Abdominal segments 2-6 translucent, dull greyish white; segments 7-10 opaque yellowish. A dorsal series of deep purple-brown subtriangular patches is present on the tergites, the bases on the posterior margins, and with apices almost attaining the anterior margin. A similar series of lateral patches are brown in color, thus leaving pale anterior triangles between the darker ones. Tergite 9 is almost entirely brown. Sternites 2-6 show faint traces of mid-ventral brown spots connected by a fine line. Sternites 7 and 8 have brown lateral patches based on the posterior margins. Sternite 9 largely brown, yellowish on the posterior margin. Forceps deep smoky brown. A long spine below the penes on the inner side (see fig. 115).

Female Spinner

Venter of female largely tinged with purplish brown.

Specimens of the Mayfly Species Ameletus vernalis

3 Male Duns
2 Male Spinners
2 Nymphs

Start a Discussion of Ameletus vernalis


Mayfly Species Ameletus vernalis (Brown Duns)

Species Range
Common Name
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