The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
It does look a little chunky I originally though maybe a Ephemerella because of that, but I don't think it is. Do any Ephemerella hatch that late in the session ???
Eric what was the length? It appears to be on a finger or ridge of the hand, and very small. Perhaps it's an undercover Trico trying to get some information about the Leptophlebidae drug cartel.
Do we think that this could be a female?
The separation of the genera Leptophlebia and Paraleptophlebia is simple with one exception... Leptophlebia johnsoni is the smallest North American Leptophlebia (7mm) and its wings do not appear to differ in any character from that of Paraleptophlebia. Within its geographic range, most Paraleptophlebia can be smaller and their caudal filaments are usually uniformly pale rather than tan with reddish brown rings at the joinings as found in L. johnsoni.
Above the photo I have the length listed. Are you not seeing that? it was 7-8 mm