This camouflaged critter wears severed insect body parts like a coat

It’s Halloween every day for the bone collector caterpillar. The moth larva decorates its drag-along dwelling with scavenged body parts from dead insects. This ghastly getup may allow the creatures to live alongside spiders without being detected.

These pill-sized caterpillars look like piles of parts—an ant head here, fly legs and wings there, a stuck-on weevil head, and jettisoned spider legs jutting out all over. Beneath this constructed case is a “generic white gushy body,” says Daniel Rubinoff, an entomologist at the University of Hawaiʻi at Mānoa. “It’s gross at one level, but it’s also kind of adorable. They’re cruising around like, ‘Oh, this looks delicious. I’ll eat some of this and then put the rest on my back.’”

Rubinoff and his colleagues described the new carnivorous caterpillar and its curious behavior April 24 in Science. They’ve yet to give the species a scientific name. These moths belong to a genus called Hyposmocoma found in Hawai’i known for spinning portable silk cases in which they hide. Some decorate their cases with pebbles, diatoms, or lichen. But no other known Hyposmocoma uses insect remains.

Using carcasses as camouflage

Carnivory itself is unusual for caterpillars. Among butterflies and moths, 99.9 percent eat plants or fungi, Rubinoff says. But bone collector caterpillars slink through cobwebs in rotted logs, tree hollows, or crevices in rock and dine on newly dead or weakened insects they find, even gnawing through silk for a meal. After chowing down, the bone collectors check out what’s left and attach pieces to their cases using silk that they produce.

Rubinoff first spied the bone collector over two decades ago while looking for other case-wearing caterpillars that feast on rotting wood. But these creepy crawlies seem rather rare. In 22 years of scouring Hawaiian forests, his team has collected only 62 individuals.

“It’s a great piece of field work,” says evolutionary biologist David Lohman at the City College of New York who was not part of the study. Small, drab, and holed up in covert cobwebs, the caterpillars would be tricky to spot, he says.  

After finding the critters, Rubinoff’s team brought them back to the lab. There, they glimpsed bits of bone collector behavior. Captive caterpillars go after slow moving prey and can even become cannibalistic in shared quarters.

Bone collectors seem to methodically curate their cases. Lacking insect and arthropod parts, the caterpillars won’t tack on other types of debris. And they pay special attention to size. They probe prospective additions with their mandibles, rotate them and chew big pieces down to size. “It’s very serial killer-esque,” Rubinoff says.

The carcass castles seem crafted to hide the bone collectors’ identity. Rubinoff has noticed the larvae lurking near spiders. A bone collector can’t outrun a spider, but covered in insect slivers and shed spider skins, it may smell to the spider like a mixture of itself and past meals. The researchers suspect that camouflaging themselves as the walking dead likely helps the caterpillars avoid becoming dead meat, and that bone collectors likely evolved to live with and steal from spiders. Other parts of the world host other insects that rob spiders. But “there isn’t anything else in Hawaiʻi that does this,” Rubinoff says.

(Butterflies get all the love—but caterpillars may be even more stunning.)

Bone collectors are a rare breed

Rubinoff’s team has been poking around rotting logs all over the Hawaiian Islands, and yet they’ve only found these creatures on Oʻahu in a 15-square-kilometer area of one mountain range. “You would expect this to be particularly threatened, and the fact they find it only in one tiny spot on one island is pretty sobering,” says Naomi Pierce, an evolutionary biologist at Harvard University who wasn’t part of the work.

Based on where bone collectors fit in their family tree, this species evolved over six million years ago—before their current home island formed around three million years ago. The insects had to have migrated from a different Hawaiian island that formed earlier.

Unlike other Hyposmocoma moths, the bone collector has no sister species across other  islands. That suggests that something has destroyed the bone collector’s close kin, Rubinoff says. “I am sure that prehuman contact, this lineage was widespread.”

Many native Hawaiian butterflies and moths have similarly disappeared. Nearly 40 percent were presumed or possibly extinct, Rubinoff and his team recently estimated. These creatures face threats due to loss of habitat, climate change and predation from introduced species, such as ants, which didn’t evolve in the islands. Part of bone collectors’ resilience may stem from their ability to use invasive spiders’ webs. Still, without action, these oddities—possibly the last of their kind—may not survive. “I would imagine it’s not long for this world,” Pierce says.

Hawaiʻi, and other archipelagos, can evolve such curious species—such as those that belong to Hyposmocoma—because of how isolated they are, says Akito Kawahara, a lepidopterist at the University of Florida in Gainsville who wasn’t part of the work. Rubinoff’s team has previously found Hyposmocoma caterpillars that live underwater. Others decorate their cases with specific colors of lichen. It’s not surprising that the carnivorous, body-part collector fits with this family, he says. “It just makes you think about what’s out there that we really haven’t seen.”

“When it comes to butterflies and moths, most people think of the adult,” Kawahara says. The bone collector caterpillar metamorphoses into a glam-looking moth, with wings decked out in white fringe. But the critters’ macabre lifestyle reveals how fascinating an insect’s teenage phase can be. “We kind of forget about the caterpillars and what they’re doing. And it just shows you how diverse they are.”