How some beetles produce scalding defensive spray

How some beetles produce a scalding defensive spray

7:18 AM, 3rd May 2016
How some beetles produce a scalding defensive spray
Bombardier beetles eject a liquid called benzoquinone, which they superheat and expel in an intense, pulsating jet. The explosive mechanism used by the beetle generates a spray that's much hotter than that of other insects that use the liquid, and propels the jet five times faster.

CAMBRIDGE, US: Bombardier beetles, which exist on every continent except Antarctica, have a pretty easy life. Virtually no other animals prey on them, because of one particularly effective defence mechanism: When disturbed or attacked, the beetles produce an internal chemical explosion in their abdomen and then expel a jet of boiling, irritating liquid toward their attackers.

Researchers had been baffled by the half-inch beetles’ ability to produce this noxious spray while avoiding any physical damage. But now that conundrum has been solved, thanks to research by a team at MIT, the University of Arizona, and Brookhaven National Laboratory.  

The findings are published in the journal Science by MIT graduate student Eric Arndt, professor of materials science and engineering; Christine Ortiz of Brookhaven National Laboratory; and Wendy Moore of the University of Arizona.

“Their defensive mechanism is highly effective,” Arndt said, making bombardier beetles “invulnerable to most vertebrates, and invertebrates” — except for a few very specialized predators that have developed countermeasures against the noxious spray.

The liquid these beetles eject is called benzoquinone, and is actually a fairly common defensive agent among insects, Arndt said. But bombardier beetles are unique in their ability to superheat the liquid and expel it in an intense, pulsating jet.

The key is that they synthesize the chemical at the instant of use, mixing two chemical (hydrogen peroxide and hydroquinones) precursors in a protective chamber in their hindquarters. As the materials combine to form the irritant, they also give off intense heat that brings the liquid almost to the boiling point — and, in the process, generates the pressure needed to expel it in a jet.

Seeing inside a living beetle

In the current study, the researchers used high-speed synchrotron X-ray imaging to “see” inside the abdomens of living bombardier beetles during explosions. They used a facility at Argonne National Laboratory to carry out the experiments and produce detailed images that revealed, for the first time, how the process works, with a camera recording the action at a rate of 2,000 frames per second.

The explosive mechanism used by the bombardier beetle generates a spray that is not only much hotter than that emitted by other insects that use the same chemical irritant, but also propels the jet five times faster. Both the speed and the heat serve to make the spray even more effective against potential predators. The pulsing nature of the spray may help protect the structure of the beetle’s reaction chamber, Arndt said, allowing time for the chamber walls to cool a bit before the next pulse.

Jeffrey Dean, a professor of biology at Cleveland State University who studies the defence mechanisms of the bombardier beetle, said the new work is a “wonderful confirmation of the qualitative passive ‘pulse jet’ model” first proposed by his team. “Although the findings are not unexpected, I’m amazed at the progressive advances in techniques.

This research was supported by the Department of Energy, the Department of Defence through the US Army Research Office, the National Security Science and Engineering Faculty Fellowship, and the National Science Foundation.

© MIT News 

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