It is thought that the hearing organs of moths may have developed to help them detect and evade predatory bats. Scientists have analysed the sounds that moth ears are sensitive to and it seems that moths are able to detect the range of sounds usually produced by hunting bats. Interestingly, in areas where bats are frequent predators of moths, moths often have better hearing than in areas where bats are scarce.

Hearing moths are also thought to be able to judge the direction of a bat’s approach because a moth’s two ‘ears’ receive the sounds of an approaching bat at slightly different times. This ability to detect a potential predator can greatly increase the moth’s chances of escaping.

There are several reactions of moths to the detection of a bat, such as changing their flight path if the bat is some distance away. If however, the bat has been detected at very close range, moths have been seen to suddenly fall out of the sky, sometimes doing loops and spirals, either through strong powered flight or simply by using the forces of gravity. The escape attempts of moths with hearing abilities can significantly reduce the predatory success rate of bats, with the result that their catches of hearing moths may be less than half those of moths without ears. It may be the sheer variety of the moths’ escape tactics that is the secret of their success: if bats are unable to predict the moths’ next move, they are less likely to be able to catch them.

Even though the use of escape tactics by moths may reduce their chances of being caught by bats, their safety is by no means guaranteed. Bats are very capable fliers and are sometimes able to track the moths in their aerial dive and catch them in their mouth or even on their wing, from which they are scooped into the bat’s waiting mouth!

Moths that do not possess hearing organs, and therefore cannot detect the presence of bats in the same ways, exhibit avoidance tactics of their own. Some species restrict their flying time to a short period at dusk, before bats are numerous, or they may fly more irregularly and unpredictably to make catching them much harder and to discourage bats from attempting to pursue them. Some moths may also fly close to other objects or the ground to make them harder to detect through echolocation. All of these tactics serve to reduce the risk of predation by bats.

Some species of moth possess even more impressive defences against their bat predators. It is known that some moths from the Arctiid family (to which familiar species such as the Garden Tiger belong) are able to produce their own sounds (clicks), which may in turn be heard by the bats. The Dogbane Tiger Moth from North America (Cycnia tenera), is able to produce the same sounds that are made by a predatory bat species as it advances on its moth prey. Located on the surface of the moth near the ear are tymbal organs made up of a series of grooves in the chitin (the hard material that forms the exoskeleton of insects). This area can be flexed or buckled by muscular tension causing a series of high frequency clicks. These closely resemble echolocation calls and therefore interfere with the bat's own echolocation system and confuse the bat into thinking it is hearing its own echo bouncing off a solid object. This tactic may cause the bat to give up the chase, leaving the moth with the chance to live to see another day (or night).

This struggle for survival between bats and moths as predator and prey is the subject of ongoing consideration and debate. It has been suggested that bats and moths are locked in an evolutionary ‘arms race’, in which moths evolve new tactics in response to the bats’ highly developed prey detection abilities, and bats in turn evolve new tactics to overcome moths’ defences. There is some support for this theory, as it seems that some bats have evolved low-frequency calls specifically to prey upon moths that have evolved hearing organs. Because these moths have limited sensitivity to low frequency sounds, bats that use these frequencies are less likely to be detected and are subsequently more likely to be successful in catching the moths.