For many stream groups, the fall sampling season is over. But if you're still out with your tables and nets, or if you plan to look at some streams in the next 2-3 months, there's a good chance that you'll find Leuctrids (rolled-winged stoneflies) and Capniids (small winter stoneflies) at the same time in the same place. These nymphs can be confused, but you should be able to tell them apart. Best advice -- look at the wing pads.
1. Rolled-winged stoneflies: genus Leuctra
The wing pads on Leuctrids on long and slender -- finger-like -- with those in the rear being shorter than those in the front. All four wing pads are uniform in width from beginning to end: the sides are essentially parallel. However, the front wing pads are widely separated, those in the rear are not, they're close together. The front wing pads are so wide apart that the inner edges of those pads overlap the outer edges of the rear wing pads.
I've found two genera of Capniidae in our streams, so we must look at them one at a time.
2. Small winter stoneflies: genus Allocapnia
Allocapnia small winter wing pads differ from those of Leuctra in two significant ways. Note that those in the rear are not the same shape as those in the front: they are wider than those in the front, and the bottoms are flat/truncated. In addition, while those in the front are further apart than those in the back, the inner edges of the front wing pads do not normally overlap the outer edges of those in the rear.
3. Small winter stoneflies: genus Paracapnia
Paracapnia wing pads are actually pretty close in appearance to those of Leuctra. The front and the rear are much the same shape. However, they tend to look more like "tear drops" than "fingers": they are wider at the top than they are at the bottom.
There is one other thing you can look at when you're trying to distinguish Leuctrids and Capniids. Note the length and shape of the abdomens. The abdomen of the Leuctrid is long and thin, and like the wing pads, it's the same width from beginning to end -- parallel-sided, pencil-like. The abdomen of the Capniid (both genera) is somewhat shorter, and it tends to be slightly wider in the middle than it is at either end -- i.e. it's tapered, cigar-like.
Now, the wing pads and abdomens tell the tale if you've got a nymph that's mature or fairly mature. What if you find one that's tiny, one where the wing pads are not very well formed? Answer: you can't ID them in the field. Preserve them and take them back to the lab. And if you're the one who's stuck with the ID work back at the lab, what then do you look for?
Well, you can hope that the shape of the wing pads and/or the shape of the abdomen will be more easily seen with a microscope. But certain ID depends on seeing "pleural" or "ventrolateral" folds on the abdominal segments. It can be tough. But here's what you're trying to see. (Put the nymph on its back or its side.)
1. Capniids -- all Capniids -- have ventrolateral folds on all nine abdominal segments (See Beaty, "The Plecoptera of North Carolina," p. 1.) They look like this.
2. Leuctrids have ventrolateral folds on the first four abdominal segments only. (Beaty, p. 2)
Best I can do. Good luck!