Tiger beetles mimic ultrasound to warn off bats

More fascinating behaviour from the insect world has been revealed in a study of tiger beetles and how they try to avoid bats, which see them as a hearty meal.

Most bats rely on echolocation to find their meals. They emit high-frequency sound waves which bounce off prey and return information about its size and distance.

Bats’ selective pressure as the main predator of night-flying insects has led to at least 6 orders of insects evolving ears capable of detecting ultrasound.

But tiger beetles (family Cicindelidae) use an even bolder strategy: they respond to bats with their own ultrasonic signal.

The researchers suspected they use ultrasound to warn bats they are noxious, because tiger beetles produce benzaldehyde and hydrogen cyanide as defensive chemicals.

Turns out, that was wrong.

“Responding to bat echolocation is a much less common ability than just being able to hear echolocation,” says Harlan Gough, invertebrate conservation biologist and lead author of a new study in Biology Letters that finally solves the mystery. 

To investigate the phenomenon researchers collected 20 species of tiger beetle in southern Arizona and played them audio of bat echolocation attack sequences.

Seven species responded by producing the ultrasonic sound by essentially clapping with their wings.

The ultrasound-producing tiger beetles swing their protective shells slightly backwards, causing the beating hind wings to strike them

Tests involving feeding 94 tiger beetles to big brown bats (Eptesicus fuscus) swiftly disproved the hypothesis that they were warning bats off due to their toxicity. Almost all the beetles were completely consumed without issue.

“Even if you identify a chemical, that doesn’t mean it’s a defence against a particular predator,” says Akito Kawahara, director of McGuire Center for Lepidoptera and Biodiversity at the Florida Museum of Natural History in the US.

“You don’t actually know until you do the experiment with the predator.”

Instead, the researchers now believe tiger beetles produce ultrasound to mimic tiger moths (subfamily Arctiinae), which are noxious to bats. Comparing recordings of the 2 groups of insects revealed a clear overlap in the ultrasound produced.

The researchers suspect there may be even more undiscovered examples of ultrasonic mimicry, given how understudied the acoustics of the night sky are.

“We think it’s not just tiger beetles and moths. It appears to be happening with all kinds of different nocturnal insects, and we just don’t know simply because we haven’t been testing in this manner,” says Kawahara.

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