This Beetle Fires Boiling Chemicals Out of Its Bum

A bombardier firing its boiling chemicals from two views: side and temporary blindness.

In his epic encyclopedia Natural History, the great Roman naturalist Pliny the Elder wrote of the bonnacon, a sort of bull whose defensive strategy was to hit its foes with blasts of dung “so strong and hot, that it burneth them that follow after him in chase, like fire, if haply they touch it.” Natural history in Pliny’s time, you see, consisted of a good amount of hearsay. From 12-year-old boys, apparently.

There is, though, a remarkable real-life version of the bonnacon: the bombardier beetle. While it doesn’t weaponize its dung per se, it has evolved a cannon in its caboose, where chemicals mixed in a special chamber violently burst out of the critter in a boiling, noxious, pungent spray that can repel even the most daring of predators.

There are hundreds of species of bombardier beetles all over the world, with various defensive mechanisms. Some have non-explosive, foamy excretions of chemicals, while others like the African bombardier beetle can actually aim their explosive spray in virtually any direction like an angry lawn sprinkler. We’ll be talking about the latter here. The spraying bombardier beetles, not lawn sprinklers.

Does This Cannon Make My Butt Look Big?

In the bombardier’s abdomen is a chamber that holds a mixture of hydrogen peroxide–the stuff you put on cuts and no, you shouldn’t try disinfecting your wounds with bombardier beetle explosions–and chemicals called hydroquinones. When the beetle feels threatened, this chamber empties into another reaction chamber that contains catalysts to kick off the explosion.

The bombardier’s crazy coloration scares away most predators, save for the vicious Australian wild blurry pushpin.

Here the hydrogen peroxide rapidly decomposes into oxygen and boiling water, while the hydroquinones oxidize into benzoquinones–highly irritating chemicals that have been known to stain the skin of human handlers a yellowish brown for up to three weeks. This mix explodes out of the beetle, not as a single stream, but as a volley of rapid-fire blasts, in what scientists have likened to the pulsing propulsion system of Germany’s V-1 “buzz bomb” in WWII. The consequent chemical burn (free idea for a metal band name: The Consequent Chemical Burn) incapacitates smaller attackers like ants, and spooks the hell out of much, much larger foes as well, such as unfortunate amphibians.

“You’ve got 100 degrees centigrade temperature, you’ve got a chemical burn, the steam comes off like a smoke, and then also the reaction kind of hisses,” said entomologist Terry Erwin of the Smithsonian Institute. That adds up to a bad situation for any hungry frog that pokes its tongue in the wrong place. “There might be 200 of these beetles under one rock, and they all fire at the same time, and you’ve got a smokescreen, or vaporscreen, as it were,” Erwin said.

An incredible defense to have evolved, for sure, but the chemicals here are actually quite uncomplicated. Hydrogen peroxide is a natural byproduct of metabolism in almost all living creatures. And insects use quinones to harden their shells. The bombardiers have just figured out how to store these chemicals instead of breaking them down or using them up.

The bombardier can fire these compounds more than 20 consecutive times before running out of ammo. But how can the beetle flash-boil chemicals to over 200 degrees Fahrenheit inside itself without melting? I mean, I’ve been on this planet for 29 years and I can’t even boil water in a pot without somehow injuring myself.

“The insect cuticle is pretty tough stuff, and this reaction chamber where it all happens is very, very dense-walled,” said Erwin. “And when they open the turret, then all of this stuff goes directly out of the beetle.”

The ant nest beetle, a cousin of the bombardier beetle, aims its spray by bouncing it off its wing covers. This one looks a bit like Arthur from The Tick, though it probably isn’t also a former accountant. But who am I to judge.

“They can actually swing that turret around and shoot over their head, shoot to the right, shoot to the left, underneath them,” added Erwin. “The only place they can actually not shoot is where their mouthparts are.” You wouldn’t want to get a taste of your own medicine if you were one of these beetles, now would you?

According to a study by the legendary late ecological chemist Thomas Eisner, such an incredibly evolved trait was likely driven by one of the world’s feistiest selection pressures: ants. You see, to escape swarming ants, ground beetles like the bombardiers have to unfurl their wings from covers and are unable to take flight as rapidly as, say, a bee. Having such a dexterous turret allows the bombardier to hold its ground against the ants to buy time, deftly dispatching the attackers clambering over its body. Indeed, some bombardier species’ bum cannons are so effective that their wings have even become vestigial and useless.

Now, I’d be remiss in fully profiling the bombardier beetle without mentioning that it has historically been a favorite of creationists, who argue that such a complicated mechanism couldn’t have evolved on its own. While we don’t have good fossils charting its evolution, we don’t need no stinking fossils to demonstrate the gradual development of the bombardier beetle’s cannon.

Read more at Wired Science