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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.

Lateral view of a Female Sweltsa borealis (Chloroperlidae) (Boreal Sallfly) Stonefly Adult from Harris Creek in Washington
I was not fishing, but happened to be at an unrelated social event on a hill above this tiny creek (which I never even saw) when this stonefly flew by me. I assume it came from there. Some key characteristics are tricky to follow, but process of elimination ultimately led me to Sweltsa borealis. It is reassuringly similar to this specimen posted by Bob Newell years ago. It is also so strikingly similar to this nymph from the same river system that I'm comfortable identifying that nymph from this adult. I was especially pleased with the closeup photo of four mites parasitizing this one.
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 Heterocloeon amplum

Where & when

In 3 records from GBIF, adults of this species have been collected during June (33%), August (33%), and July (33%).

In 3 records from GBIF, this species has been collected at elevations of 440, 480, and 981 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

Described in Needham et al (1935) as Baetis amplus
Body length: 7 mm
Wing length: 7 mm

Abdominal tergites 2-6 of male imago yellowish brown, the posterior margins hyaline whitish; hind wing two-veined, costal projection wanting.

Turbinate eyes large, almost circular; orange in color. A black mark across the face. Antennae light brown. Thorax reddish brown. Longitudinal dark brown streaks on the mesonotum, median and submedian in position. Metanotum distinctly reddish. Sternites light reddish brown. A red spot on the coxa of the fore leg; fore femur distinctly tinged with red, a band of this color near its apex. Fore tibia yellowish, tarsus yellowish white. Middle and hind legs yellowish, the femora light yellowish brown ; tarsal joinings may be very narrowly reddish. Wings hyaline, venation yellowish brown. Cross veins of the stigmatic area of the fore wing 7-8 in number; only a slight tendency toward anastomosis; almost no granulations between them. Intercalaries absent from the first interspace, well developed in other spaces. Hind wing very small (1 mm. in length), long and narrow; two veins only, the third entirely absent. Costal projection entirely wanting.

Abdominal segments 2-6 yellowish brown, tinged with grey. The posterior margins rather widely opaque, giving this part of the abdomen an annulate appearance. Segments 7-10 light reddish brown, rather darker dorsally. Basal sternites slightly paler than their tergites. Genitalia of the modified moffati type (now a synonym of Baetis tricaudatus), the second joint of the forceps being cylindrical. The distal forceps joint is relatively very long, fully three times as long as wide; it is almost one-half the length of the preceding joint.

This species is very close to B. frivolus (now a synonym ofHeterocloeon frivolum), but may be distinguished from it by the larger size, the color of the eyes and thorax, the paler abdomen and the distinctly brownish venation.


Start a Discussion of Heterocloeon amplum

References

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

Mayfly Species Heterocloeon amplum

Species Range
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