Was There Something New at the Rapidan Yesterday? Baetis tricaudatus?

It seems like I always find something new when I go to the Rapidan River.  So I was a bit disappointed with the report that I had to post yesterday.  But I had preserved a few insects for closer inspection, including the tiny small minnow mayfly in the picture above, one that I found towards the end of the day.
I knew that the genus was Baetis: I could see that it had three tails.  But it took the microscope to show me that this was a species I had not seen before: Baetis tricaudatus.

Baetis tricaudatus nymphs can be confused with Baetis pluto nymphs, but the "caudal filaments," or "cerci," or "tails" help us to distinguish the two from one another.  Here's a Baetis pluto that I found last summer, and focus on the tails.

Let me read you what Steven Beaty says about the tails (caudal filaments) of Baetis pluto.  "[the] middle caudal filament [is] 3/4 to subequal to [the] lateral filaments, usually with [a] distinct dark band on [the] caudal filaments medially." (The Ephemeroptera of North Carolina," p.6).    A perfect description of the tails of the nymph in our photo.

Now, here is a microscope view of tails of the tiny nymph I found yesterday, the one in the picture at the top of the page.

There is no dark medial band, and that middle caudal filament is certainly not 3/4 the length of the lateral filaments -- i.e. the tails on the side.  Back to Beaty:

B. tricaudatus -- nymphs 5-8mm; distinct palpal "thumb"; antennal scape and pedicel with robust setae; gill margins without large robust setae and serrate; caudal filament shading gradual, less dark than in B. intercalaris; middle caudal filament less than half as long as lateral filaments.  Primarily a Mountain species.  Typically collected spring through fall.  Relatively common and intolerant.  ("The Ephemeroptera of North Carolina," p.6).    (North Carolina assigns a TV of 1.5 to Baetis tricaudatus.)

The nymph in our photo was not quite 5mm -- but it's a young one.  On the "distinct palpal 'thumb," have a look.

Not my best photo, but the "palpal" thumbs are quite well defined: the palpal lobes look like "mittens"!
I could see the setae on the pedicels of the antennae -- but I could not get a good microscope photo.  The same is true for the edges of the gills.  But, the shading of the caudal filaments is indeed gradual, turning darker as we move toward the tips, and, the middle filament/tail is, indeed, "less than half as long as [the] lateral filaments."  I have no trouble calling this Baetis tricaudatus.  Always fun to find something new!

Another look.

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Postscript:  Yikes!  I just had another look at the nymph I found at South River on 1/12.  That, too, could well be B. tricaudatus.  Very short middle tail and no medial banding.  I'll have to check these three-tailers with greater care.