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Baetis

Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.

Mayfly Species Epeorus deceptivus

This western species distribution somewhat overlaps both Epeorus longimanus and Epeorus albertae and they often occur together. This species is not as common or important.

Where & when

Time of year : July through early October

Preferred waters: Fast, cold, mid-size rivers

Altitude: Above 5,000 feet

Hatching behavior

Specimens of the Mayfly Species Epeorus deceptivus

1 Male Dun
Male Epeorus deceptivus  Mayfly Dun
The lack of a darkened humeral crossvein rules out Epeorus albertae and Epeorus dulciana. The lack of a dark macula on the forefemora rules out Epeorus longimanus. The small size rules out Epeorus grandis and Epeorus permagnus. That leaves as the only possibility known in Washington state Epeorus deceptivus. It is a small species, although not reportedly quite as small as this specimen. I couldn't find anything in the species description in Traver (1935) to definitively confirm or rule out the species ID, given that I don't have the preserved specimen to check under a microscope, but it has to be either deceptivus or something not yet reported in Washington.

It was collected at the same time as a similar-sized female dun.
1 Female Dun
Female Epeorus deceptivus  Mayfly Dun
I'm guessing this female is of the same species as this male dun, because they came from the same pool at the same time and the size matches, although the males and females would look very different in this case.
2 Nymphs

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References

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