“Adaptive Gravity” Keeps Insect Swarms from Collapsing in on Themselves

July 22, 2016 | Erica Tennenhouse

flying insects
Photo credit: USFWS Mountain-Prairie/flickr (CC BY 2.0)

Flying insects move in sync by listening to each other.

New research published in the New Journal of Physics is shedding light on the mass movements of flying insects.

How swarms of insects coordinate their motion is a great mystery. While the collective movements of schools of fish and flocks of birds are built up from a series of interactions in which individuals mimic their neighbors’ movements, swarming insects don’t align with each other when in motion.

Researchers focused on the movements of swarming flies known as midges. Like fruit flies, moths, and bees, midges have a sound-sensing organ that forms part of their antennae — the Johnston's organ. When midges fly, their beating wings produce sound that the researchers believe might help direct their collective movement.

"We started from the assumption that when one midge hears another, it reacts by accelerating towards the source with a strength that is proportional to the received sound intensity," said study lead author Dan Gorbonos, from the Weizmann Institute of Science in Israel, in a press release.

They developed a model of the midges’ swarming behavior. Oddly enough, they began to notice similarities between the structure of midge swarms and that of astrophysical objects, such as globular clusters consisting of thousands of stars.

SEE ALSO: Insects Fly in the Face of Airplane Aerodynamic Laws

"The decay rate of the sound intensity falls off according to an inverse-square law — just as the gravitational attraction between two masses does," says Gorbonos.

With their gravity-like interactions, swarms would theoretically be expected to collapse in on themselves. However, the researcher identified a key difference between the real gravity that acts between masses and the acoustic interactions in midge swarms.

They call it “adaptive gravity,” referring to the fact that an animal’s perception of sound is not fixed, but rather adapts to the total sound intensity. For example, we find it easy to hear a whisper in a quiet environment, but people have to shout to be heard in a noisy room.

Adaptive gravity might explain why the swarm doesn't collapse in on itself, like the gradual gravitational collapse of stellar clusters. When the swarm becomes dense and the buzzing gets too loud, midges become less sensitive to the noise and each individual midge reacts less to the acoustic pull of its neighbors.

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