There are few defences more extreme than that of the bombardier beetles. These insects deliberately engineer explosive chemical reactions inside their own bodies, so they can spray burning, caustic liquid from their backsides. The liquid can reach up to 22 miles per hour, at temperatures of around 100 degrees Celsius. It’s painful to humans—watch Ross Piper take one to the face—and potentially lethal to smaller predators like ants.
The beetle mixes its chemical weapons within glands in its abdomen, each of which consist of two chambers. The reservoir chamber contains a solution of hydrogen peroxide and hydroquinones—that’s the fuel, inert on its own but always on the cusp of extreme violence. The adjacent reaction chamber contains enzymes like peroxidise and catalase—that’s the match.
The two chambers are separated by a valve that stops their contents from mixing. When the beetle is threatened, it opens the valve and contracts the muscular walls of the reservoir, emptying its contents into the reaction chamber. Match meets fuel. The enzymes react with the incoming chemicals in a violently explosive reaction that produces oxygen, water vapour, a lot of heat, and irritating chemicals called p-benzoquinones. This searing, noxious, steaming cocktail then forces its way out through an exit channel as a spray, which the beetle can aim using reflector plates on its abdomen.
The spray isn’t continuous. Instead, the beetle fires between 368 and 735 pulses every second. This extends the range of the chemicals and also potentially saves the beetle’s life. The reaction chamber may be reinforced with layers of sturdy materials like chitin and waxes, but still, lest we forget, it’s creating explosions inside its own body. By pulsing its spray, “it gives the chamber some time to cool down,” says Christine Ortiz from MIT. “A continuous spray would heat the beetle up a lot more.”
Her team, including graduate student Eric Arndt, have taken high-speed X-ray videos of living beetles doing their thing, to work out how they produce such a rapid train of pulses. “We cool the beetle down so it goes to sleep, and then fix it in [the path of] the X-ray beam,” she says. “As it warms up, it realises that it’s fixed and undergoes an explosion.”
The videos revealed that during a spray, the beetle squeezes its reservoir chamber with a constant pressure, and keeps its exit channel continuously open. Neither is responsible for the pulses. Those come from the valve that separates the two chambers.
When the valve opens, a droplet of reservoir chemicals enters the reaction chamber. Boom! The pressure created by the explosion forces a pulse out through the exit channel. It also pushes against a membrane that closes the valve, cutting off the supply of fuel. As the pressure in the reaction chamber drops, the membrane relaxes and the valve re-opens, letting another droplet through. Boom! This continues until the reservoir muscles finally relax and the beetle stops spraying.
The wonderful thing about this set-up is that the beetle pulses its spray passively. It doesn’t need to evolve any special muscles for rapidly opening and closing the valve, and it doesn’t waste energy on doing so. Instead, it staggers its explosions using their own force.
Ortiz thinks that these discoveries may be useful in designing armour that protects against explosions. That’s what her team does: they’ve studied the defences of oysters and fish, and even created prototype body armour based on the scales of an ancient fish called a bichir.
Reference: Arndt, Moore, Lee & Ortiz. 2015. Mechanistic origins of bombardier beetle (Brachinini) explosion-induced defensive spray pulsation. Science http://dx.doi.org/10.1126/science.1261166