Bats use the Doppler Principle to catch insects. They emit a screech, and wait to see if the echo of the screech comes back quickly , or takes a while. This is called The Doppler Principal. Imagine standing in a train station, perfectly still, listening to an approaching train's continuous whistle. As the train nears you, the sound becomes louder and louder, because the sound waves compress as the train approaches you. Now the train passes, and you notice the train's whistle, although still continuous, becomes lower in pitch. This is because the sound waves have moved farther apart. Same thing with the insect:The density of the body of a close insect will return a high pitched screech heard from the sound the bat emits. If the insect is far away, the sound becomes lower in pitch to the bat, because the sound waves are now farther apart. So then the bat goes to another restaurant.