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.
This specimen keys to the Epeorus albertae group of species. Of the five species in that group, the two known in Washington state are Epeorus albertae and Epeorus dulciana. Of the two, albertae has been collected in vastly more locations in Washington than dulciana, suggesting it is far more common. On that basis alone I'm tentatively putting this nymph in albertae, with the large caveat that there's no real information to rule out dulciana.
Anal cell: The anal cell (denoted ac) is a cell formed by crossveins closing the space between anal veins, including sometimes at their bases, creating of a loop from which the veins appear to arise. Some sources may refer to the two cells basal of the one labeled in the forewing illustration as the first and second anal cells.
These other words reference the same concept: anal cells, first anal cell, first anal cell, second anal cell, second anal cell.
