It keys pretty clearly to Rhithrogena undulata using the key in Traver 1935, although the size is larger than expected for that species in that source.
W. C. Day, Aquatic Insects of California differentiates morrisoni nymphs by submedian pale dots on tergites (
One tergite of this Isonychia bicolor mayfly spinner is highlighted in red.
Tergite: The top (dorsal) part of a single segment on an insect's abdomen when it consists of a single chitinous plate (sclerite), or an individual sclerite if the segment has more than one.) 8 & 9, which look to be in evidence. This is one of the "super-hatches" on this river that can be prolific March through April.
One tergite of this Isonychia bicolor mayfly spinner is highlighted in red.
Tergite: The top (dorsal) part of a single segment on an insect's abdomen when it consists of a single chitinous plate (sclerite), or an individual sclerite if the segment has more than one.) this may be Rhithrogena virilis.
Rhithrogena virilis Mayfly SpinnerView 2 PicturesBefore I reared this specimen the nymphs were unknown. See Western North American Naturalist for the paper on the description of this species' nymph.
The relative angle of the penes (Penes: The paired genital structures of most male insects, which vary widely in form and are one of the main characteristics used for species identification.) is a bit shallower in this specimen, but I photographed another specimen from the same collecting trip (and I think even the same swarm, although I don't recall for sure) as the other one, and it had the shallower angle seen on this specimen. I'm guessing it's just variation within the species.