bats have bad eyesight... they will send out a screech and listen for the sound to bounce off an object. they can tell how long it takes for the sound to hit, to tell them how far an object is away from them... most bats are highly adept at doing this, so they can use sound to tell the shape location and everything else from this method. they can continually send out sound waves, one after another so make somewhat of a movie inside their head... they can tell when something is moving and the size of that something... hence they can tell what is a bug,where it is, where its going, and they use this information to their advantage, by flying down and eating.