Have you ever seen a butterfly abruptly start fluttering acrobatically around? Well, it may be because they're hearing high frequency pulses, much like the sounds a bat makes in using echolocation. They start to fly all crazy, performing loops and dives and spins in the night air.
Butterflies actually have ultrasonic ears on their wings like moths, and also like moths, they're night flying. A lot of moths have ultrasonic ears because it enables them to detect incoming bats and get out of the way fast, preventing them from becoming the bat's next meal.
Scientists aren't exactly sure how butterflies evolved this way, but evidence suggest that these ears might be evidence that bats created butterflies by driving moths into the daylight. The idea is that with the evolution of bat echolocation, moths had to find some way of avoiding the predator's jaws. One way was to evolve ultrasonic ears, but another possibility might have been to switch to the daylight lifestyle of the butterfly.