Gnats are somewhat disorganized compared to a school of fish in which all fish turn at exactly the same split second. If something gets in the way, the school splits, moves around the obstruction and then merges back into the same formation as before. Unless their reaction time is zero, there is no way they could be using visual clues. It's like they all had little radios in their head and they are listening to the head fish saying "to the right flank, MARCH!" But even a platoon of human soldiers can't match the precision of a bunch of dumb fish.

Then there are fireflies. Most of them flash whenever the mood strikes them. But there are some species of firefly that all flash at the same instant. They live in deep forests where it gets really dark. You can't see your hand in front of you face, and then suddenly 5000 tiny flash bulbs go off at the same time. Strange indeed.
Howard