Arsenic is toxic to most multicellular life. But caterpillars of one species happily dine on arsenic-loaded leaves, even as their bodies accrue astonishing levels.

Benjamin Jaffe at the University of Wisconsin-Madison first got an inkling of the caterpillars’ unflappable appetite while studying their food – a fern called ladder brake that can accumulate high concentrations of arsenic from the soil.

While studying the plants in Florida, he unexpectedly found caterpillars devouring the ferns. He chemically analyzed these fern moth caterpillars ( Callopistria floridensis ), revealing they had accumulated levels …