Bombardier beetles’s explosion-induced defence mechanism explained

Scientists at the Massachusetts Institute of Technology (MIT), the University of Arizona and Bookhaven National Laboratory finally explain the chemistry and physics behind the defensive mechanism of bombardier beetles.

Bombardier beetle caught with a high-speed camera bursting its irritating liquid as a defensive mechanism (credit: Charles Hedgcock)

Bombardier beetle caught with a high-speed camera bursting its irritating liquid as a defensive mechanism (credit: Charles Hedgcock)

Bombardier beetles defend against predators by expelling a jet boiling, irritating liquid against their attackers. The precise mechanism behind this peculiar defensive strategy has been baffling researchers for a long time, but the riddle has finally been solved and published this week in the journal Science.

The liquid ejected by bombardier beetles is benzoquinone, which is used by several other species of insects. What’s unique about bombardier beetles is their extraordinary ability to superheat the liquid and expel it in an intense, pulsating jet. “Their defensive mechanism is highly effective,” says Eric M. Arndt, co-author of the study. It makes them “invulnerable to most vertebrates, and invertebrates” – except for a few very specialized predators that have developed ad-hoc countermeasures.

Bombardier beetles synthesize the defensive spray at the very instant of use, mixing two chemical precursors in a specialized chamber in their abdomen. As the two substances combine, they form an irritant liquid which also gives off intense heat and almost reach the boiling point. The subsequent increase in pressure completes the job as the liquid itself can now be expelled 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,” says co-author of the paper Christine Ortiz. In the current study, the researchers used high-speed synchrotron X-ray radiation at the Argonne National Laboratory to image the inside of living bombardier beetles as the explosion occurred. The action was recorded with a high-speed camera (2,000 frames/s) which revealed the details of how the process actually works.

The characteristics spray pulsation is controlled by the rapid opening and closing of a passageway between two internal chambers – one holding the precursor liquid and one where the explosion takes place. The mechanism is passive: the increase in pressure in the explosion chamber causes a valve in the passageway to close, the liquid is then ejected which causes the pressure to drop and the valve to reopen for the next burst, creating the unique pulsating jet. The researchers speculate that the burst nature of the spray might also be for the beetles’ own protection as the walls of the chamber have time to cool down in between pulses.

The research solves a long-standing problem in biology and at the same time could also help designing blast-protection systems.

Here is the link to the video explaining the mechanism.

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Carlo Bradac

Carlo Bradac

Dr Carlo Bradac is a Research Fellow at the University of Technology, Sydney (UTS). He studied physics and engineering at the Polytechnic of Milan (Italy) where he achieved his Bachelor of Science (2004) and Master of Science (2006) in Engineering for Physics and Mathematics. During his employment experience, he worked as Application Engineer and Process Automation & Control Engineer. In 2012 he completed his PhD in Physics at Macquarie University, Sydney (Australia). He worked as a Postdoctoral Research Fellow at Sydney University and Macquarie University, before moving to UTS upon receiving the Chancellor Postdoctoral Research and DECRA Fellowships.

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