Our common pronggilled mayfly appears to be Paraleptophlebia assimilis

One of the things that our new key has helped me ID is the pronggilled mayfly in the photo above.  This can now be ID'd with some confidence as Paraleptophlebia assimilis.  It's the most common pronggilled nymph that I see.  This one was found in South River (Greene County), but the Rapidan River is filled with these nymphs in the winter and spring.  And it seems to be a winter/spring species: I have photos of this one from December (young) to April (fully mature).

We can confirm this ID with our new key, and find the nymph fully described in the article by Randolph and McCafferty -- "First Larval Descriptions of Two Species of Paraleptophlebia," Entomological News 107 (1996), pp. 225-229.  We can also confirm it with a quick and easy deduction.   There are only four species of pronggilled mayfles with "branched" gills (very clear in our photos): swannanoa, assimilis, adoptiva and mollis.   Swannanoa is out -- it hasn't been found in Virginia.  As for adoptiva and mollis, both have posterolateral projections on segment 9, but not on 8: assimilis, on the other hand, has projections on both 8 and 9.  That's what we've got.

Now lets look at our key:  Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species, p. 140.

263 Gills 2-7 forked at one-fourth or more length from base; gill trachea with distinctly pigmented branches at least in unforked region..........264
263' Gills 2-7 forked near base, usually not more than one-sixth length from base; gill trachea without distinctly pigmented branches, but often with very short and faint lateral branches.........267

Gills on our nymph fork about 1/3 the way down from the base, and, as we've seen, they're clearly branched above the fork.

On to couplet 264.

264  Mandibles relatively elongate, with about half length of angulate (left) mandible beyond angulate shelf.........Paraleptophlebia swannanoa
264' Mandibles not elongated as above, with less (usually much less) than half length of angulate mandible beyond angulate shelf.....265

At the moment, I don't have a specimen in my reference collection, so I can't examine this feature.  (I'll remedy this in the winter.)  However, as we've already seen, it can't be swannanoa since that species doesn't occur in our state.  Swannanoa has only been found in NC, SC, and GA.  On to 265.

265  Posterolateral projections present on abdominal segments 8 and 9.....Paraleptophlebia assimilis
265' Posterolateral projections present on abdominal segment 9 only ...... 266

Bingo.  P. assimilis.

Now let's look at the description in Randolph and McCafferty (pp. 225-227).  Unfortunately, I can't go through their description in a lot of detail: again, I don't have a specimen that I can examine.  But, there are features that help to confirm our ID.

1. "Head capsule brown with small, oval, pale medial spot bewtween antennal bases."  "Thorax brown with pair of pale, circular spots medially on mesonotum anterior to forewingpad bases."  Yes.

2. "Abdominal terga brown with pale, oval area sublaterally on terga 2-8, and with white, oval spot medially within posterior third to one-half and sometimes extending anteriorly on terga 2-8 or 2-9."  Here are the sublateral ovals.

But I think the medial ovals show up better on this mature specimen that I found in 2015.

I can't say that I'm crazy about calling those ovals, or ignoring the pale crescent shaped marks on some of the terga, but since everything else is a fit, I'm still content to accept our ID.  Paraleptophlebia assimilis.  However, I'll come back to this ID in the winter when I can examine the morphological features that Randolph and McCafferty describe.
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What about this other pronggilled nymph that I found in May, 2012, also up at South River?  (Must be a spring/summer species, much like guttata and strigula.)  Any help from our new key?

Not really.  At the first couplet (see above, 263- 263'), we'd clearly move to 267 since these gills are forked near the base and lacking in pigmented branching.  But that leads us into a measurement of the maxillary palps, something I can't do without finding another nymph next spring.   That being said, I'm leaning towards an ID of Paraleptophlebia jeanae, based on the description in Randolph and McCafferty.

"Abdominal coloration variable: abdominal tergum 1 brown; terga 2-9 often with paired crescent-shaped pale markings submedially on each tergum; submedial tergal markings less often coalescing, forming larger pale area in posterior area of terga .... tergum 10 brown, pale medially." (p. 227)  Pretty good match.  (crescent-shaped marks on 4-6; coalesced on 7 and 8)

But two of the nymphs my friend found in a small stream in Sugar Hollow fill the bill even better.

Time will tell if this diagnosis is right.  Just have to find some more of these nymphs next spring!