Science Matters – How moths outwit bats

The oak beauty moth, with hairy thorax

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Have you ever wondered why moths have hairy bodies while butterflies don’t?

Scientists at Bristol University have recently researched this question and come up with an intriguing answer (*).

The bodies and wings of both butterflies and moths are covered in scales, but on a moth’s thorax these scales are elongated to resemble hairs. Why? Because this modification very effectively protects moths from becoming bat-fodder.

Butterflies are active during the day but moths are mostly nocturnal. And under the night sky moths make a tasty meal for insectivorous bats. These tiny mammals detect their prey by echo-location, a method of hunting that involves emitting repeated ultrasonic squeaks, generally with a dominant frequency range of 20-60 kilohertz (kHz). Their prey is then located from the direction of the returning echoes.

Moths have evolved various mechanisms to evade these predators: some have developed ears that detect bat echolocation calls, allowing them to take evasive action, but others, the earless moths, have to rely on passive defences – so-called acoustic camouflage – that is, their hairy thoraces.

Comparing tomographic echo images from two species of earless moths with two species of diurnal butterflies, the Bristol scientists found very little effect on the ultrasound echoes from the thin layer of thoracic scales of butterflies. In contrast, the hair-like scales covering moth thoraces absorbed around 67 per cent of the ultrasonic sound energy in the broadband 20-160 kHz. This provides clear protection from hunting bats and consequently is a very significant survival advantage.

Addressing the mechanisms behind the ultrasound reduction achieved by moth thoraces, the scientists found that the hair-like scales form a connected network of air pockets, which act as porous ultrasound absorbers. Sound waves entering the network cause air molecules in the pockets to vibrate, thereby dissipating energy and reducing echo reflection.

The overall arrangement and microstructure of hair-like moth scales closely resembles man-made fibrous materials designed to absorb sound but, interestingly, the Bristol team showed that the moth scales have a considerably higher absorption capacity. So perhaps we could learn lessons from these small hairy creatures.

* J. R. Soc. Interface 17: 20190692 (2020).