X-Rays Reveal Secrets of Bombardier Beetle’s Machine-Gun Bum

A bombardier beetle sprays and releases 'gunsmoke' in response to researchers' stimulus. Charles A. Hedgcock / Charles A. Hedgcock / University of Arizona

Bombardier beetles are famous in the animal world for their defensive mechanism: More than just a sting or acid spray, these creatures release a boiling-hot mixture of noxious chemicals aimed with pinpoint precision. The chemical mechanism of the spray is well-known, but close observation shows that it's not just a spray or stream. Rather, it's a rapidly pulsed series of piping hot bullets, up to 500 per second — and scientists only recently figured out how it's done.

Inside the beetle's abdomen are vessels that keep the reactive elements separated. When tiny valves open up, a special reinforced combustion chamber fills up with both chemicals, and it's from there that they explode outward at attackers. But how is the machine-gun-like pulse created?

Researchers, including the University of Arizona's Wendy Moore, Brookhaven National Lab's Wah-Keat Lee, and Eric Arndt and Christin Ortiz at MIT, trained a special X-ray machine on the reactive chambers of hundreds of beetles, one by one, and goaded them into spraying. The footage of the internal processes shows that the rapid fire happens because the pressure created by the explosive mixture actually flexes a thin part of the chamber wall, closing the valve that let the chemicals in. The pressure goes down as the spray leaves the beetle, and the valves open again. The principle isn't far removed from the combustion chambers in car engines.

But why? Moore thinks it helps make the blast sustainable and accurate. "By having a pulsed delivery, these small beetles produce a relatively large amount of defensive spray, which they can aim precisely and with great force and speed," Moore said in a news release. "This is truly one of the most remarkable and elegant defensive mechanisms documented to date."

The paper is published in the May 1 issue of the journal Science.

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