I found this little nymph at Buck Mt. Creek last Thursday (4/11/19), and since we have a new key to use for species ID -- Morse, McCafferty, Stark, and Jacobus, Editors, Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species, 2017 -- I thought I might see if I could work out the species ID. My conclusions should be seen as tentative since I know from Beaty that work still needs to be done on this genus, and the descriptions in our new book are indeed rather tentative. Nonetheless, using this key, I'd conclude that, to date, I've found three different species.
1. Rhithrogena uhari (the nymph above)
The description of R. uhari reads as follows: "Abdominal terga light yellow brown to light chestnut brown to cinnamon brown; conspicuous dorsal abdominal marking not present in darker forms but some diffuse pairs of markings sometimes evident in lighter-colored forms, including pairs of cloudy patches laterally." (p. 122)
The color sure seems right, and I do see pale paired dots on the terga as well as interesting lateral markings on all of the terga. Can't say if they're "cloudy," since I'm not sure what that the editors have in mind.
So, possibly R. uhari.
2. Rhithrogena exilis
Beaty told me sometime ago that this one would key out to exilis, even though he cautioned to take that ID with a grain of salt. Here's the description of exilis in our new key. "Abdominal terga 1 and 2 pale yellow; terga 3-6 dark reddish brown and unmarked except for yellow or yellow-hyaline lateral margins; terga 7-9 yellowish, unmarked." (p. 122) This one works out pretty well: terga 1 and 2 are indeed pale, as are 8 and 9 -- can't say that's true for tergite 7. But, I do see pale markings on the terga.
3. Rhithrogena fasciata
This nymph is clearly darker than the two we've looked at above. Here's the description of R. fasciata. "Abdominal terga medium to dark brown or orange brown; terga 1 and 2 sometimes pale; terga 4-8 sometimes darker anteriorly and paler posteriorly; terga 7-9 often shade lighter than preceding terga." (p. 122)
Certainly true that terga 1 and 2 are pale, and terga 8 and 9 are clearly a "shade lighter than the preceding terga" -- not sure I could say that of segment 7. It is not the case that terga 4-8 are "darker anteriorly and paler posteriorly," rather, it seems that the posterior edge is darker. That character doesn't apply to any of the species that our key describes, so maybe it's a variant with this species. But that exception should be noted.
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Results and conclusions are tentative for sure. Still, I'd argue that we've clearly found three different species, and they key out pretty closely to uhari, exilis, and fasciata. Be in touch if anything new shows up on this one.