How a beetle packs a machine gun into its rear end

Scientists have discovered how the bombardier beetle packs a machine gun into its rear end to fire rapid bursts of a searing hot chemical mix at predators.
When threatened, bombardier beetles combine chemicals in an explosive chemical reaction chamber in their abdomen to simultaneously synthesise, heat and propel their defensive load as a boiling hot spray, complete with "gun smoke."
Wendy Moore, an assistant professor in the Department of Entomology at the University of Arizona College of Agriculture and Life Sciences, and colleagues have unveiled the beetle's firing apparatus in never-before-seen detail, and solved a long-standing mystery of how the animals achieve their rapid-fire capabilities.

"Understanding how these beetles produce - and survive - repetitive explosions could provide new design principles for technologies such as blast mitigation and propulsion," researchers said.
Previous research discovered that each blast from the bombardier beetle is a series of extraordinarily fast micro-pulses, Moore said.
"What wasn't known is what causes each discharge to be pulsed, like a machine gun," she said.

Using enzymes that digest away muscle and fat, Moore could clean the reaction chamber and examine it with optical microscopy in detail as never before.
While studying the shape of the reaction chamber, she noticed that some regions of the chamber wall were thin and not as strong as others, and that one region near the mixing valve looked particularly thin.

She predicted that the thin cuticle would be displaced by the force of the reactions in a way that it would impinge upon the valve, temporarily shutting off the flow of reactants.
The team decided to directly observe the internal dynamics of spray pulsation in live beetles of the species Brachinus elongatulus, a species found in riparian habitats throughout southern Arizona.
The team took hundreds of Brachinus beetles to Wah-Keat Lee, a synchrotron scientist who at the time was with the Advanced Photon Source at Argonne National Laboratory in suburban Chicago, to study them with ultrafast X-ray imaging.
Of 500 beetles, the researchers were able to record 30 discharges from 14 individuals.

Moore and her collaborators observed how the beetles regulate the ultrafast micro-pulses. It turns out the pulses are generated in a passive way, not through an active process involving muscle contraction, as had been hypothesised before.
As the chemicals pass through a valve into the reaction chamber, they mix with enzymes and explosively liberate oxygen gas, water vapour and heat, propelling a hot, noxious spray down the nozzle and out the exit pore.
Each explosion inside the chamber forces a highly elastic region of the chamber wall to expand and impinge on the valve that separates the two chambers, temporarily cutting off the flow of reactants and resulting in a pulsed delivery.