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Deaf Moths May Use Their ‘Fur’ To Avoid Hungry Bats

Fur-like scales on the insects’ thoraxes absorb the echoes of bat calls, according to new research

A new study has found that moths like the Antherina suraka, pictured here, may use their scales to avoid detection by bats. (Philstone/Wikimedia Commons)
smithsonian.com

Hungry bats love chowing down on tasty, nutrient-dense moths. To better detect when danger is approaching, some moth species have evolved ears that let them hear bats’ echolocation. Others have remained completely deaf, but that doesn’t mean they are defenseless against their batty predators. New research has found that some moth species are equipped with a fuzzy coating that reduces the echoes of bat calls, thereby preventing the fluttery insects from becoming late-night snacks.

A team of researchers led by Thomas Neil of the University of Bristol in the United Kingdom looked at two species of deaf moths (Antherina suraka and Callosamia promethean) along with two species of butterfly (Graphium agammenon and Papilio troilus). While the study is still under review, the team’s findings were presented recently at a conference of the Acoustical Society of America, reports Matthew Taub of Atlas Obscura. And the results of the investigation suggest that “furry” moth thoraxes are very good at absorbing sound, trapping up to 85 percent of the sound energy that was pinged in their direction.

What looks like “fur” on the bodies of certain moths are actually modified scales, Neil, who specializes in bioacoustics, tells Smithsonian.com. He and his team were specifically interested in the downy scales on moths’ thorax and wing joints, which are highly elongated. To find out what moths look like from a “bats’ eye view,” as Neil puts it, the researchers relied on a technique called acoustic tomography: they fired ultrasound pulses at the moths through a loudspeaker, and measured the strength of the echoes that bounced back using a microphone located next to the speaker.

“We do this from hundreds of angles,” Neil explains, “after which we can combine all echoes and use the information to create and image of the moth.”

The secret to moths’ echo-reducing thoraxes lies in the structure and layout of their scales, which look like tiny versions of natural fibers that are used for sound insulation, such as hemp and kenaf. Like these materials, thorax scales are porous; they dissipate “sound energy through thermal and viscous effects as the sound wave enters into the air filled cavities of the material,” Neil says.

Butterfly fur, by comparison, absorbed much less sound energy—a maximum of 20 percent—than the moth thoraxes. The researchers think this is because butterflies, as diurnal creatures, don’t need to worry about bat predation, unlike the nocturnal moths analyzed in the study.

To emphasize just how important thorax fur is to moths, Neil and his team used their measurements to calculate changes in “distance volume,” which let them determine the distance at which bats would be able to detect both a moth with thorax fur and one without. They found that the distance volume went up when the thorax fur was removed, meaning that the moth would be easier to find. In fact, the team calculated that a furless moth faces a 38 percent greater risk of being scouted out by a ravenous bat.

There are other questions that the researchers hope to explore in the future: Moths bodies, for instance, are covered in modified scales, and it would be interesting to know if other parts of the insect also reduce bat echoes. But for now, the study reveals yet another fascinating way that moths have evolved to protect themselves from echolocating predators.

About Brigit Katz

Brigit Katz is a freelance writer based in Toronto. Her work has appeared in a number of publications, including NYmag.com, Flavorwire and Tina Brown Media's Women in the World.

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