The Beetle That Mixes a Boiling Spray Inside Itself

A bombardier beetle keeps two pairs of glands inside its abdomen. The reservoir holds an aqueous solution of hydroquinones and hydrogen peroxide — sitting together harmlessly, slowly browning over weeks. The reaction chamber is muscle-walled, much smaller, and stocked with two enzymes: catalase and peroxidase. The two compartments are separated by a one-way valve.

When the beetle is attacked, it forces a slug of the reservoir liquid through the valve. The catalase tears apart the peroxide into water and oxygen; the peroxidase oxidizes the hydroquinones into 1,4-benzoquinone. The reaction is exothermic enough to bring the mixture to about 100 °C, vaporizing roughly a fifth of it. The pressure spike snaps the valve shut, protecting the beetle from its own chemistry.

Then the spray jets out of a steerable nozzle on the abdomen. High-speed photography by Eric Arndt and Wendy Moore at MIT and Arizona showed it isn't a steady stream. It's a pulsed jet, around 500 pulses per second, each pulse triggered by a fresh micro-injection from the reservoir. The beetle effectively runs a tiny pulse-jet engine for as long as it needs to.

A single beetle has enough reserve for around 20 discharges. The jet aim is good enough to hit a finger from any angle around the body. Creationist tracts in the 1980s seized on the chemistry as something evolution couldn't have built — until biologists pointed out that benzoquinone defenses are common across beetles, and the bombardier's pulse-jet is just an unusually well-tuned version of the same trick.