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Artistic view of a Male Pteronarcys californica (Pteronarcyidae) (Giant Salmonfly) Stonefly Adult from the Gallatin River in Montana
Pteronarcys californica

The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.

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.
Troutnut is a project started in 2003 by salmonid ecologist Jason "Troutnut" Neuswanger to help anglers and fly tyers unabashedly embrace the entomological side of the sport. Learn more about Troutnut or support the project for an enhanced experience here.

Mayfly Species Siphlonurus alternatus (Gray Drakes)

This species occasionally produces important spinner falls. Its spinners may join the swarms of Siphlonurus quebecensis or Siphlonurus rapidus, amplifying the importance of all three species.

Where & when

Time of year : Early May through early September; peaking from late June through July

In 61 records from GBIF, adults of this species have been collected during July (44%), June (26%), August (23%), and May (7%).

In 285 records from GBIF, this species has been collected at elevations ranging from -30 to 2933 ft, with an average (median) of 689 ft.

Species Range

Spinner behavior

Time of day: Evening

Habitat: Riffles

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 rather large, somewhat pale species, with conspicuous ventral markings (see fig. 119).

Head whitish. Thorax yellowish brown. Wide reddish brown median stripe on metanotum and lateral margins of mesoscutellum. Scutella dark brown. Pleural sutures brown. Purplish red marks anterior to wing roots. Prosternum white, marked with two dark brown triangles. White band across anterior portion of mesosternum, enclosing a dark median spot.

Fore leg of male longer than body; yellowish. Apical dark band on femur; knee brown; apex of tibia, and tarsal joinings, brown. Femora of middle and hind legs whitish, with apical brown band. Tibiae and tarsi yellowish, joinings brown. Longitudinal veins of fore wing, and costal margin of hind wing, purplish black. Humeral cross vein purplish black. Costal cross veins before bulla pale, almost invisible; in stigmatic area pale brown, numerous, somewhat irregular, a few anastomosed. All other veins brown, the longitudinal veins all pale at the wing root.

Abdomen pinkish white, marked with chestnut brown. Posterior margins of tergites widely banded with brown; wide median brown streak; usual brown triangles and oval spots, the latter very prominent. Pale areas on anterior margin extensive. Tergites 8-10 with large powdery white areas. Ventrally, no lateral patch. Oblique lines extend from near the postero-lateral angle almost to the anterior margin near the median line, ending in a black dot. A pair of smaller black dots near center of each sternite, on each side of median line. A dark brown triangle on median line at the anterior margin. Sternites 7-9 of the female, and 9 of the male, largely yellow with reddish markings. Dark lateral mark on sternite 9 of the male.

Forceps yellowish. Penes yellow tipped with black. Tails reddish brown at the extreme base, becoming yellow and finally white in the distal portion. Joinings rather widely dark purplish brown. Penes as in fig. 123.


Nymph has double gills on all abdominal segments, one member of the 7th pair being very small (see 120). Femur with an apical brown band; tibia banded near base; tarsus twice banded. Usual black band across tail, beyond the middle. Ventral markings comparable to those of the imago. This species, originally described from the middle west, is common throughout eastern North America.

Specimens of the Mayfly Species Siphlonurus alternatus

1 Female Spinner
1 Nymph

Start a Discussion of Siphlonurus alternatus


Mayfly Species Siphlonurus alternatus (Gray Drakes)

Species Range
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