In the damp hollows of a Hawaiian mountainside, something stirs in the shadows of cobwebs. It is not a spider. Nor the long-legged centipedes or impaling air-snatchers that usually star in the islands’ arthropod horror show. Instead, it is something stranger — a caterpillar wrapped in the body parts of the dead.

Yes, this thing is for real.

Aptly nicknamed the “bone collector,” the larva haunts a six-square-mile patch of Oahu’s Wai‘anae Mountains, lurking exclusively in spider webs and disguising itself in the corpses of its prey.

“This behavior was utterly unknown,” said Daniel Rubinoff, an entomologist at the University of Hawaii at Manoa and lead author of the study published last week in Science. “It’s not just incredible — it’s unimaginable.”

A House of Corpses

At first glance, it looks like a lump of debris. A mash of insect limbs, cracked carapaces, spider exoskeletons. Then it moves.

When Rubinoff’s team first stumbled upon one of these creatures in 2008, it seemed like a bag of bug parts. Then the caterpillar poked out its head.

He assumed it was a fluke. But field surveys turned up more cases — always among webs, always adorned with the macabre — until the pattern became clear. These larvae weren’t just collecting junk by accident. They did it by design and with purpose. They were dressing to survive.

“It’s a decorate or die situation,” Rubinoff told Ars Technica. “In evolutionary history, the ones that didn’t decorate their cases were probably removed from the gene pool pretty quickly.”

The caterpillar belongs to Hyposmocoma, a wildly diverse genus of Hawaiian moths with over 600 known species. Some spin cases that resemble cigars. Others use wood chips or shells. But none, until now, used corpses.

Using their silk, bone collector caterpillars weave together a grotesque suit of armor. They snip, chew, and shape the remnants of beetles, weevils, flies, even spider molts — always scavenged from the host web.

The tailoring is precise. “These caterpillars are able to discern differences in objects in their environment,” Rubinoff told The New York Times. “Those that are too big are chewed down to a more comfortable size.”

Predator in a Predator’s Lair

But why would a caterpillar — a notoriously vulnerable life stage — choose to live in a spider’s home?

It’s a gamble that seems to pay off. The web offers food, in the form of trapped or leftover prey, and possibly protection from other predators. The disguise, Rubinoff believes, fools the spider landlord.

“A spider detects vibrations in the web, rushes out to grab its prey, smells itself and prey it’s already eaten, and assumes there is nothing new to eat,” he explained. The caterpillar blends into this detritus like a walking trash heap in a silk-strung trap.

David Wagner, an entomologist at the University of Connecticut who was not involved in the study, called it “one of nature’s most improbable connections.” He knows only one other moth species that frequents webs — but it’s a vegetarian, snacking on leaf matter stuck in the silk.

These caterpillars, by contrast, are hunters. In lab tests, bone collectors readily chewed through silk to devour live Drosophila pupae. They even ate each other.

“That just gives you a sense of how they go after food,” Rubinoff told Scientific American. “And recognize that there’s food inside things that maybe don’t look like food.”

An Evolutionary Ghost

Genomic analysis suggests that the bone collector split off from other Hyposmocoma caterpillars more than five million years ago. That’s before Oahu even existed. Its ancestors likely evolved on older Hawaiian islands that have since eroded back into the sea.

Today, it survives only in a narrow sliver of habitat — and even there, it is astonishingly rare. Across more than 150 field surveys, scientists found just 62 specimens.

Its preferred webs belong to at least four nonnative spider species, which have inadvertently helped the caterpillar persist. But the bone collector’s future is far from secure.

Hawaii’s ecosystems are under siege from invasive ants and parasitic wasps. Rubinoff warns that the bone collector “could be one new ant species away from being obliterated.”

These threats have already driven other native insects to extinction. “We’ve lost entire genera of endemic insects [in Hawaii],” he said. “It’s not on the verge of winking out, but in the context, it seems likely.”

A Reminder of What’s Left to Discover

For all its grotesque ingenuity, the bone collector is also a stark reminder of how much remains unknown.

“I’ve been looking at it for over a decade, and it still blows my mind,” Rubinoff said.

Scientists now hope to study the bone collector’s genome more closely to understand how it selects body parts, how it discerns size and material, and what sensory powers guide its scavenging.

But even without those answers, the caterpillar is just awesome to fathom.

“Insects do everything,” said Akito Kawahara, a lepidopterist at the Florida Museum who was not involved in the research. “They’re amazing. In some ways, I was not surprised — because I know insects do some really crazy things.”

For Rubinoff, the discovery is a call to keep looking. “We’re finding stuff that we didn’t even imagine was out there,” he said. “It shows how interesting evolution can be. It really is — I don’t want to say magic — but it’s incredible.”