This caterpillar wears the body parts of insect prey

A severed ant head. A fly wing. A beetle abdomen. These body parts ripped from devoured insects festoon a newfound caterpillar’s protective coat.

Dubbed the “bone collector,” this caterpillar species sports remains of prey as camouflage while it stalks spider webs for trapped bugs, researchers report in the April 25 Science. The carnivorous caterpillar, found on the Hawaiian island of Oahu, is the first known by scientists to live in spider hunting grounds and fully decorate itself with body parts.

Relatively few caterpillars eat meat, with about 300 carnivorous varieties out of nearly 200,000 documented moth and butterfly species. The bone collector caterpillar belongs to the Hyposmocoma genus, also called Hawaiian fancy case caterpillars, endemic to the islands.

While walking in the Waianae Mountains more than 20 years ago, entomologist Dan Rubinoff and colleagues spied an odd caterpillar next to a spider web in a tree hole. “It [was] covered in little bits of bug,” says Rubinoff, of the University of Hawaii at Mānoa in Honolulu. He initially dismissed it as a curious coincidence but eventually kept crossing paths with more weird larvae. In all, he and others spotted 62 bone collector caterpillars over two decades, but only within a 15-square-kilometer range.

Each caterpillar takes up residence in a spider web enclosed in a tree, log or rock cavity. (Only one caterpillar usually inhabits a web, since they will eat one another.) There, the caterpillar lurks, waiting to prey on insects stuck in the web while masking its scent and texture with leftovers from the spider’s meals and skin shed by its eight-legged landlord.

“This is a decorate or die situation,” Rubinoff says. A plain protective case “might not be enough to stop the spider from trying to puncture the case and eat the caterpillar.” 

When curating its collection, a caterpillar carefully probes and rotates bodily remains. The carnivorous critter nibbles large pieces down to size before weaving them onto its silken case. After a few months of gluttony, the caterpillar seals off the end of its case — now a cocoon — to transform into a moth.

Genetic analysis revealed that the bone collector caterpillar’s lineage is at least 6 million years old, far more ancient than Oahu, so an ancestor probably traveled there from a different island. But while other Hyposmocoma lineages are represented by multiple species on several of the Hawaiian Islands, the bone collector is the only species in its lineage, Rubinoff says.

“I’m really glad we discovered it before it went extinct.”