The original post is a stonefly nymph. The damselfly nymph has 3 FEATHER shaped gills on the end of it's abdomen. The stonefly has 2 antennae.
When it swims, does it wiggle back and forth and slowly sink? Damselfly nymphs don't use their legs to swim. All of them in the great white north don't, anyways...
There are some other deadly aquatic larva besides dragonfly nymphs... Water beetle larva, backswimmers and giant water bugs all catch and eat small fish. They would be interesting subjects for a small aquarium, I would think. I kept a beetle larva for a few months, but it died because it couldn't pupate.
My tank sucks now
Fraternity of Dirt # 22