I don't know for sure that this is
Drunella tuberculata, but that's my best guess for now.
It certainly has a different look and much more robust body shape from
Drunella lata duns I photographed a couple weeks earlier, so I doubt it's that species. Using distribution records to eliminate other choices narrows this down to
Drunella tuberculata or
Drunella walkeri.
Markings described for the
abdominal sternites of the male spinner of
Drunella tuberculata are suspiciously similar to those on this female dun. Also, this dun is 9.5mm long (my ruler pic isn't very good, but I'm basing this on measuring the real thing). The size range given in the old Allen & Edmunds keys for
walkeri females is 7-8mm, while
tuberculata is 9-11mm. For these reasons I'm sticking it in
tuberculata for now.
This is the only
Drunella mayfly I saw all day. I scooped it off the water as it emerged at around 7pm from a big Catskill tailwater.