It can tell the difference between a bat that is closing in for the kill and one that is just cruising by. And it can pick out bat sounds from the cacophony of crickets and katydids on a warm summer night. Remarkably, it can do all this with no ability to distinguish between frequencies, or pitches, of sound. When a tympanate moth hears bat ultrasound and takes evasive action, it can react in one of two ways. These behaviors can be explained by looking at the relative speed and sound detection abilities of moths and bats.
Moths are able to detect bats at a greater distance than bats can detect moths.
This gives them the opportunity to rapidly fly away if they hear a distant bat. If, however, the bat has also detected the moth, the bat has the advantage because it flies faster. Therefore, when a moth detects a close bat, flying away will do it little good, because the bat has probably detected the moth and is closing in for the kill. In this case the moth chooses to drop rapidly, often erratically, to the ground, hopefully eluding the bat with its unexpected movements. The arms race between bats and moths becomes apparent when you look at bat and moth relationships worldwide.