If you’re a moth, you don’t want to get in the way of a hungry bat. Well, as long as they’re not the nectar-sipping kind. Bats are swift, agile predators. They soar quietly through the air, nearly invisible in the darkness, hunting not by sight but by sound. Moths need to be creative to avoid these skilled predators… otherwise they’ll be moth à la mode.
How does a moth evolve to outsmart these flying bundles of rabies? The modern structure of a trait evolves as long as there is some functional and beneficial use of a previously existing feature on which natural selection can work.
Noctuid moths take evasive flight maneuvers when they hear the ultrasonic pulses emitted by bats. Ears—or, rather, specialized tympanic membranes—are not unheard of for an insect. Over thousands of years, the auditory range of noctuid moths was refined to be able to detect the incredibly high-pitched ultrasonic frequencies used by bats. Believe it or not, moths able to listen in on “batspeak” are 60% less likely to be a midnight snack.
Okay, so now that moths can eavesdrop in on ultrasonic frequencies, what happens next? Taking defenses one step further, African arctiid moths evolved to respond to bat sonar with an ultrasonic pulse of their own. This signal acts as an aposematic warning of their unpalatability, just as the bright color of monarch butterflies warns birds they’re toxic.
Wait!! Back up a second. Moths can “talk” now, too? This is unbelievable!
Not really. Natural selection would have favored moths that could emit the whatever frequencies would startle bats and jam their echolocation reception, since this helps the moth to survive. A vocal apparatus previously used for attracting mates or communicating with other moths could have been repurposed to create an ultrasonic clicking sound, used by arctiid moths in response to bat echolocation. This way, the arctiids trick bats into thinking the echo is reflected from prey located somewhere else, so when bats dive in for the kill, all they find is empty air.
Use it or lose it: High Arctic moths living in the absence of bats have diminished reaction to ultrasonic frequencies. Without natural selection pressures, their tympanic membranes have effectively atrophied. The structures and neural pathways are not as developed because they simply aren’t needed. Rather, these moths have evolved to be better at staying warm in cold environments and have a higher reproductive output.
To wrap things up, I want to stress the lack of direction in evolution. Arctiid moths able to jam bat sonar are no more evolved than noctuid moths, nor are they superior to the High Arctic moths with reduced defensives. All three kinds of moths are equally well adapted to their environmental niches, and the structures they have evolved all come from the functional usage of preexisting ancestral characteristics. The variability within a population coupled with selection pressures has shaped the modern form of an organism based on the functions they needed to accomplish within their unique ecological niche.