I took my time keying it out when I returned home, but in the end, I was forced to conclude that it was Cinygmula subaequalis just like the nymph in Sugar Hollow on Sunday. Digging around, I found the following comment in Unzicker and Carlson: "maxillary palp usually partially visible at sides of head in dorsal view." (Aquatic Insects and Oligochaetes of North and South Carolina, 3.70) So "usual" is the operative word.
In the end, it was the dorsal abdominal pattern that led me to my conclusion. Though no key comments on this, there does seem to be a common pattern on these nymphs. 1) There are paired submedian dots on each of the segments, and 2) there are 3 light dots on tergite 10.
If we look back at the previous nymphs I have found, we can see the same thing. Here's the nymph that I found on Sunday,
and it's also clear on this nymph from 2014.
You'll also note that on occasion, there are pale marks on segment 9. I suspect keys omit these details because the maxillary palpi are sufficient to establish ID. As you can see, the nymph that I found on Monday is fully mature. It probably would have hatched out that night. Perhaps this is why the maxillary palpi are no longer easy to see.
No shortage of insects in the Rapidan at this time of year. I was not surprised to see an Isoperla orata, one on which the wingpads are starting to blacken,
and as expected, I saw numerous specimens of the nymph we're calling -- for the moment at least -- a "variant form" of Isoperla orata.
I also picked up some small nymphs which I couldn't ID on the spot. But they turned out to be young Remenus bilobatus. See more of those later on in the spring.