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 defense 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, which was published in the journal Science.
The liquid these beetles eject is called benzoquinone, and is actually a fairly common defensive agent among insects. 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 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.
“For decades, the complex mechanism of how the bombardier beetle achieves spray pulsation as a chemical defense has not been understood, because only external observations were used previously,” said co-author Christine Ortiz.
However, 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 X-ray images of the explosion reveal the dynamics of vapor inside the beetles’ abdomens. They show that spray pulsation is controlled by the passageway between two internal chambers. Two structures control this process: a flexible membrane and a valve.
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.
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