Bombardier beetles, which exist on every continent except Antarctica, have a pretty easy life. Virtually no other animals prey on them, because of one particularly effective defense mechanism: When disturbed or attacked, the beetles produce an internal chemical explosion in their abdomen and then expel a jet of boiling, irritating liquid toward their attackers. Researchers had been baffled by the half-inch beetles' ability to produce this noxious spray while avoiding any physical damage. But now that conundrum has been solved, thanks to research by a team at MIT, the University of Arizona, and Brookhaven National Laboratory. The findings are published this week in the journal Science by MIT graduate student Eric Arndt, professor of materials science and engineering Christine Ortiz, Wah-Keat Lee of Brookhaven National Laboratory, and Wendy Moore of the University of Arizona.

"Their defensive mechanism is highly effective," Arndt says, making bombardier beetles "invulnerable to most vertebrates, and invertebrates" -- except for a few very specialized predators that have developed countermeasures against the noxious spray.

The liquid these beetles eject is called benzoquinone, and is actually a fairly common defensive agent among insects, Arndt says. 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.