When a toad swallows a bombardier beetle, it thinks it's getting a tasty snack. Most of the time, the toad is correct, but sometimes the beetle fights back from inside the toad's stomach, releasing a boiling hot chemical spray that makes the toad spit up the beetle.

Now that's a spicy beetle!

The use of the spray is an inventive adaptation on behalf of the beetle, especially when the spray is typically used to scare off predators before the beetle gets eaten.

An explosive snack

The different species of bombardier beetles are cleverly disguised chemical weapons. When they feel threatened, two different chemicals — hydroquinone and hydrogen peroxide, which are held in two different glands — mix together in a spray that the beetles shoot out their rears. The spray, which is about 212 degrees Fahrenheit (100 degrees Celsius) and is incredibly stinky, is the result of some mysterious evolution that scientists are still trying to understand.

The beetles use this spray to attack would-be predators and to discourage them from gobbling them up for a meal. Obviously a spray like the one described above would scare off plenty of animals, but could it also help the beetles escape if they were eaten?

Japanese researchers collected a number bombardier beetles specimens, mostly Pheropsophus jessoensis, and different age specimens of two different toad species, the Japanese common toad (Bufo japonicas) and the Japanese stream toad (Bufo terrenticola). The common toad is a regular predator of the bombardier beetle, while the stream toad's habitat doesn't often overlap with the beetle's.

Given that the common toad regularly interacts with the beetle, the researchers hypothesized that it had perhaps built up a tolerance to the bug's boiling spray. The stream toad was used to test that idea. Additionally, the beetles were divided into two different groups. One group was poked at with forceps until they had expelled all their spray while another group was fully "loaded" as it were. The beetles were then fed to the toads.

As you might expect, the beetles that had used up all the spray before being dropped into containers with the toads were devoured by the toads without fail. The beetles that were "loaded" with spray, however, occasionally had a different fate, thanks to their spray.

"An explosion was audible inside each toad, which indicates that P. jessoensis ejected a chemical spray after being swallowed," the researchers wrote.

Anywhere between 12 and 107 minutes after the explosion, 34 percent of the common toads and 57 of the stream toads vomited up their beetles, alive and well, albeit coated in stomach juices. Toads don't have a gag reflex like humans do, so they have to basically flip their stomachs inside out to expel the beetle. The length of time between explosions and vomiting hinged on the toads' ages and sizes. Most of the thrown-up beetles went on to live for another two weeks, while one managed to live for 562 days after escaping the toad's stomach. (The toads, unharmed by the spray overall, were returned to the wild following the experiment.)

The common toads demonstrated a higher tolerance for the beetles' hot spray than the stream toads did, indicating that they may have developed a tolerance for it.The beetles may have, in turn, developed a tolerance for being in the toads' stomach. Either their exteriors somehow resist the digestive acids, or their spray neutralizes the acids. If it's the latter, their spray is a multipurpose defense mechanism.

The study was published in the journal Biology Letters.