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These Beetles Use Butt-Mounted Chemical Weapons To Survive Toad Attacks

Sequential images of emetic (vomiting) behavior in a juvenile Bufo japonicus. The bombardier beetle Pheropsophus jessoensis escaped from the toad's mouth 88 min after being swallowed. Sugiura et al./Biology Letters, 2018

Many insects store bitter chemicals in their tissues to dissuade predators, but bombardier beetles see unpalatable taste and raise the self-defense game to a new level.

When threatened, special reservoirs in the beetle’s abdomen open up to mix the compounds produced within: Hydrogen peroxide in one and a mix of hydroquinones and water in another. When they meet, a potent exothermic reaction causes the resulting liquid mixture to boil before it pops violently out through a valve on the tip of the abdomen—and onto the unlucky predator trying to eat the beetle.


“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 to WIRED.

All of the estimated 649 species within the bombardier family, called Carabidae and found on every continent except Antarctica, eject noxious toxins in one fashion or another, typically with impressive aim. Some even do so in rapid bursts of up to 20 excretions before running out of ammo, so to speak. Small attackers can be killed by the discharge, and larger ones are often dazed, injured, and convinced to hunt elsewhere.

The beetle’s unique adaptation has intrigued naturalists for some time, particularly after it was observed that the beetles could apply their butt rockets to escape the mouths of predatory toads. After all, the chance of survival after the jaws close is normally pretty low in the animal kingdom. 

Shinji Sugiura and Takuya Sato of Kobe University in Japan decided to test whether bombardiers could recover from the even more treacherous situation of actually being swallowed.


Their experiment, published in Biology Letters, involved trapping individual beetles from 15 carabid species inside containers with a toad that eats carabids in the wild. Half of the beetles were rendered defenseless beforehand by forcing them (with repeated pokes) to eject their complete store of chemicals.

Unsurprisingly, nearly all the treated beetles were successfully eaten after being licked up by the toads. But 43 percent of those packing chemical weapons were able to induce the toad to vomit them back up within 12 to 107 minutes after swallowing. Larger beetles were observed to have better rates of survival, likely because they carry larger volumes of the caustic spray.

Though the researchers were unable to witness exactly what transpired within, it seems pretty certain that the beetles deployed their unsubtle defense mechanism.

“An explosion was audible inside each toad, which indicates that P. jessoensis [one of the carabid species] ejected a chemical spray after being swallowed,” they wrote. 


Moreover, the newly freed beetles were active and seemed relatively unscathed considering some had just spent the runtime of a film inside the stomach of another organism.

“The vomited beetles were covered with large amounts of mucus, which suggests that they survived the toad digestive system (esophagus or stomach). Fifteen of these beetles (93.8%) survived for at least 2 weeks.”

In case you’re wondering, Sugiura and Sato reported that the toads survived the tests and none were “seriously harmed”. A little indigestion, however, sounds probable.

[H/T: National Geographic]


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