May I add suggestions that anytime you want to know what some critter is you give geographical info on where you saw it? Also average temp, season and size, mm & inches if possible? Does it hang near/at surface of water, stay on bottom, try to burrow in detritus? Is it segmented? A 10mm black squiggly thing that is thriving in 45F water in Nova Scotia in April probably is not the same as the 2.5 cm irridescent blue-black thing which is living in 95F water in August in Laredo. One way to find out what larva is - get large jar with water & cover with old nylon stocking then see what it hatches into. There's an entomologist gentleman they call Baldo the bug man on my rose newsgroup, he's got a web page, if anyone is curious I'll look up his address & we can see if he'd be interested in trying to answer our questions on alien aquatic life forms.
I'm not trying to be a pain, just need as much accurate info as possible to make an educated guess.
Kat khandzik at starnetinc_com
BTW, Thanks for doing this live food digest!