I didn't have the best riffle to look in -- but I had no trouble locating nymphs -- lots and lots of stoneflies. In the short while I lifted up rocks and sorted through leaf packs, I found about a half dozen large Perlodid stoneflies, genus Clioperla -- as in the photo above. Note that the wingpads are almost fully developed: I'd say the one in the photo above was about 3/4" long. I also found lots of large winter stoneflies, both genera -- Taeniopteryx and Strophoteryx. All of the Taeniopteryx nymphs were big, I'd call them fully mature; the Strophopteryx nymphs were all different sizes.
I took some photos of the Taeniopteryx nymphs, and one cooperated nicely by rolling over on its back and lying still. The photos below are of the dorsal and ventral views of that nymph, and note how clearly the "coxal gills" show up in the ventral view. They were quite visible to the naked eye. There is no need to guess about the family or genus ID of this insect!
I also found a Sowbug, and there were still a few small winter stoneflies around: I don't expect to see them much longer.
Finally, a photo to gross us all out: a large colony of black flies. A lot of the rocks that I lifted were covered with black fly larvae. In fact, there was no need to pick the rocks up. You could see them on top of the rocks, bending with the current. These were all genus Prosimulium. I reported on another site in the Mechunk on December 15th of last year where large colonies were already in place. This is one of the streams where they're found in large numbers just about every year.
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