This is the first of it's family I've seen, collected from a tiny, fishless stream in the Cascades. The three species of this genus all live in the Northwest and are predators that primarily eat stonefly nymphs Merritt R.W., Cummins, K.W., and Berg, M.B. (2019).
Several of these pictures show larvae of the midge Epoicocladius (Diptera: Churonomidae: Orthocladiinae) attached to the gills of the nymph. This species is an obligate commensal that feeds on organic material that accumulates on hair tufts at the base of the gills. They are almost always present where burrowing mayflies are present in large numbers. Different species of Epoicocladius are found on Ephemera spp., Hexagenia spp., and Litobrancha sp.