The toad’s reaction to the explosion deep in its stomach is not instantaneous. But in time the body shakes, the mouth opens, and the culprit is expelled: a mucus-covered beetle that will live to fight another day.

Japanese scientists captured footage of the great escape during lab tests that pitted the walking powder kegs that are bombardier beetles against hungry toads of different species and sizes. So effective were the beetle’s defences against being eaten alive that even the researchers were taken aback.



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“The escape behaviour surprised us,” said Shinji Sugiura, an agricultural scientist who performed the studies with Takuya Sato at Kobe University. “An explosion was audible inside several toads just after they swallowed the beetles.”



From a chemical standpoint, bombardier beetles are among the most unstable animals on the planet. When threatened, they mix chemicals in their hindquarters, hydrogen peroxide and hydroquinones, to produce an explosion of searing benzoquinone irritant. The boiling spray repels most predators the beetles encounter.



Play Video 0:15 Beetle's toxic spray in slow motion

Sugiura and Sato wanted to know if the bombardier’s defences might help them survive being swallowed by the toads they encountered in the forests of central Japan. To find out, they collected beetles, Japanese common toads, and Japanese stream toads, and filmed what happened when predator met prey.



The footage captured heroic feats of survival. The toads caught the bombardier beetles with lightning fast flicks of the tongue. But once ingested, the beetles detonated their toxic bombs. At times, Sugiura said, the explosions made an audible “bu” sound inside the amphibians. Vomiting ensued.

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The defence was not always effective though. Only 34.8% of common toads and 57.1% of stream toads vomited up the beetles, which all survived their encounter with the predator’s stomach juices. The odds of survival favoured large beetles being gobbled by small toads, probably because the bigger beetles unleashed more devastating toxic explosions.

While some beetles were thrown up within 15 minutes, others remained in the toads’ stomachs for nearly two hours, the equivalent of a Jonah-esque three-day ordeal in human terms. To check that the explosions were key to survival, the scientists disarmed a batch of beetles by triggering their sprays until the chemicals ran out, and then left them alone with toads. Only 5% of those eaten were vomited up, according to a report in Biology Letters.

How some bombardiers lived for so long in the toads’ stomachs remains a mystery. In tests, Sugiura found bombardier beetles had a better chance than other ground beetles of surviving for 20 minutes inside toads’ stomachs, perhaps because they are better protected against stomach acid. “The bombardier beetle species may have evolved a high tolerance for toads’ digestive juices,” he said.