A. First of all, I'd be very hesitant (foolish?) trying to identify an organism without actually seeing it, but in your case, I'm pretty sure what you found. It sounds like a classic description of a rock covered with black fly larvae (Family Simuliidae). The larvae can literally cover rocks having a thin film of water going over them from which the larvae filter out fine organic particles with basket-like fans on their mouthparts. They are about a quarter of an inch long, attached by anal hooks to a silken pad they attach to the rock or vegetation surface, and can let themselves into the water column and then "reel" themselves back by a silk strand. They appear as tiny "bowling pins," with swollen abdomens; older ecologists, like me, who remember Lil Abner, liken them to schmoos. Trout do feed on them.