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Pteronarcys californica

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

Mayfly Species Epeorus longimanus (Slate Brown Dun)

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.

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.

Specimens of the Mayfly Species Epeorus longimanus

1 Male Dun
Male Epeorus longimanus (Slate Brown Dun) Mayfly Dun
Identification of this one was as follows. Body 9 mm, wing 11 mm.
Both Epeorus albertae and Epeorus dulciana should have a conspicuously darkened humeral crossvein in the forewing. This one doesn't.
The foretarsal claws are dissimilar (one sharp, one blunt), which also rules out the Epeorus albertae group.
The dark macula on the forefemora rules out Epeorus deceptivus, which is also supposed to be a little bit smaller.
Both Epeorus grandis and Epeorus permagnus should be much, much larger.
Of the species known to be present in Washington, this leaves only Epeorus longimanus, which is exactly the right size. The key to male spinners in Traver (1935) describes distinctive markings that are visible (although more faintly) in this dun: “Black posterior margins of tergites do not extend laterally to pleural fold, but an oblique black line form this margin cuts across poster-lateral triangle to pleural fold.”
3 Nymphs
1 Female Spinner

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