Each toad flung its sticky tongue at its prey, nabbed and swallowed.
The amphibians that swallowed a fully loaded bombardier beetle were in for a surprise, the researchers soon observed.
"An explosion was audible inside each toad, which indicates that (the bombardier beetle) ejected a chemical spray after being swallowed," the authors wrote.
Then they watched and waited.
The common toads vomited their prey 35 percent of the time. The stream toads ejected their meals 57 percent of the time.
That pattern provided clear support for their hypothesis about the amphibians' evolutionary adaptation.
But back to the beetles: All 16 of the vomited insects were "alive and active" 20 minutes later, the researchers reported.
And not only did the beetles survive, but 15 out of 16 lived at least another two weeks. One escapee survived for 562 days after its sojourn in the toad stomach.
Those were the lucky ones. Almost all the beetles that were poked into ejecting their defensive chemicals before meeting the toads were "successfully digested," according to the study.
The fact that the depleted beetles were so readily digested told the researchers that the beetles' boiling chemical spray was indeed their ticket to freedom.