On the question of what a hellgrammite is,
assuming I'm attaching the URL correctly...

If they were not dragonfly nymphs, I wonder what they were? The giant lake caddis is a possibility, but I don't think they necessarily live under rocks.

[URL=http://search.atomz.com/search/?sp-q=hellgrammite&submit.x=67&submit.y=8&sp-a=sp1000104e][/URL]