Using high-speed synchrotron X-ray imaging, a team of scientists led by Prof Christine Ortiz of Massachusetts Institute of Technology was able to look at how bombardier beetles detonate small explosions in their bodies to produce a scalding defensive spray.

Bombardier beetles are ground beetles of the family Carabidae that possess the ability to fire boiling, irritating liquid toward their attackers.

For as long as scientists have been studying these insects they have been baffled by their ability to produce this noxious spray while avoiding any physical damage. But now that conundrum has been solved, thanks to a new study published in the journal Science.

“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 Prof Ortiz, who is the senior author on the study.

The liquid bombardier 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.

In their study, Prof Ortiz and her colleagues used high-speed synchrotron X-ray imaging to produce detailed images that revealed how the process works in the bombardier beetle Brachinus elongatulus.

The 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 opening and closing of this passageway between a chamber holding the precursor liquid and an explosion chamber seems to take place passively; an increase in pressure during the explosion expands the membrane, closing the valve.

Then, after the pressure is released when the liquid is ejected, the membrane relaxes back to its original state and the passage reopens, allowing the next pulse to form. This all takes place so rapidly that the process had never been directly observed.

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 5 times faster.

“Both the speed and the heat serve to make the spray even more effective against potential predators,” said study leas author Eric Arndt, a graduate student at Massachusetts Institute of Technology.

“The pulsing nature of the spray may help protect the structure of the beetle’s reaction chamber, allowing time for the chamber walls to cool a bit before the next pulse.”

“The study 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,” said Prof R. Jeffrey Dean of Cleveland State University, who was not involved in the current study.

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Eric M. Arndt et al. 2015. Mechanistic origins of bombardier beetle (Brachinini) explosion-induced defensive spray pulsation. Science, vol. 348, no. 6234, pp. 563-567; doi: 10.1126/science.1261166