How Exploding Beetles Can Survive Being Eaten Alive

Bombardier beetles can escape the stomachs of toads by setting off a powerful chemical reaction

Toad regurgitates bombardier beetle

Bombardier beetles are the chemical weapons experts of the insect world.

When under threat, the beetles, which include 649 different species, create a cocktail of chemicals from glands in their backside. The reaction creates a caustic spray to ward off would-be predators. But as Ian Sample at The Guardian reports, the beetle’s defense system is just as effective on the outside as it is on the inside of their enemies.

Researchers from Kobe University were curious why toads often vomited up these beetles. So they collected Japanese common toads and stream toads, placing them with Asian bombardier beetles, Pheropsophus jessoensis. They then videoed the interactions between the insects and the amphibians, publishing their findings in the journal Biology Letters.

As Sample reports, the beetles and toads were well matched. The toad tongues were so fast, the beetles didn’t have time to react before being slurped up. But, the researchers found, the beetles were still able to detonate their payloads inside.

“The escape behavior surprised us,” Shinji Sugiura, agricultural scientist and co-author of the paper, tells Sample. “An explosion was audible inside several toads just after they swallowed the beetles.”

To create their spray, the beetles combine hydrogen peroxide with hydroquinones, which explodes into an irritating spray of benzoquinone. While the blast can kill an attacking insect, it merely causes the toads to throw up the beetles. As Susan Milius Science News explains, because toads don’t have the same type of gag reflexes as humans, they sort of turn their stomachs inside out to expel the caustic beetles.  

The beetle’s trick, however, is not foolproof.  The researchers found that about 43 percent of toads threw up the beetles. All were alive and just one of the 16 beetles thrown up survived at least two weeks. To make sure it was actually the chemicals that made the toads toss their cookies, the researchers prodded another group of beetles till they exhausted their supply of chemicals. Then they fed them to the toads. Nearly all of those beetles perished after being eaten.

As Douglas Quenqua at The New York Times reports, not all toad-beetle match ups were equal. The toads that shared a habitat with the beetles only tossed them up 35 percent of the time. But 57 percent of the toads from outside of the neighborhood lost their lunch. As Queneua writes, this suggests that toads that live in the same ecosystem may have developed some resistance to the toxin. Large toads also fared better than small toads, who were probably impacted more severely by the explosions.

Perhaps more impressive than escaping the toads is the fact that on average the beetles survived for 40 minutes stewing in the toads’ toxic stomach juices. One heroic little beetle was trapped inside for 107 minutes before it was coughed up. The researchers believe that the beetles must have evolved the ability to survive these gastric juices, Sample reports.

Asian bombardier beetles aren’t the only creatures in nature who can survive being swallowed. As Ed Yong at The Atlantic reports, horsehair worms can survive being swallowed by insects and can even escape the digestive tracts of larger animals. There are also some species of snails that can survive being gulped down by birds, using their feathered predators as a way to disperse to new areas.

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