Hawaii is a beautiful tropical paradise and also home to formidable creepy crawly predators. There are spiders that impale prey in midair and venomous centipedes that can stretch nearly 15 inches long.
And then there are the carnivorous caterpillars, an evolutionary rarity. And now scientists have discovered one very hungry caterpillar that doesn’t just eat other insects — it decorates itself in the macabre mishmash of the body parts of its meals.
Nicknamed the bone collector, this caterpillar and its grisly taste in couture are described Thursday in the journal Science. “This behavior was utterly unknown,” said Daniel Rubinoff, an entomologist at the University of Hawaii at Manoa and an author of the study. His team initially compared the caterpillars to crawling crime scenes.
The bone collector caterpillar is found only within a six-square-mile swath of a single mountain range on the island of Oahu. There it resides exclusively on cobwebs spun by spiders in logs and rock cavities. As the caterpillars skulk about the webs, they scavenge dead and dying insects and other arthropods ensnared in the sticky silk.
According to David Wagner, an entomologist at the University of Connecticut who was not involved with the new study, the caterpillar’s reliance on spiders presents one of nature’s most improbable connections. “It is remarkable that a caterpillar would tie its fate to a spider — a clear and present danger for both caterpillar and moth alike,” Dr. Wagner said. He’s aware of only one other moth species that frequents spider webs. But that species is a vegetarian that snacks on plant material caught in the web.
Dr. Rubinoff first encountered the bone collector caterpillar in 2008 when he examined a web inside a tree hollow. The caterpillars are incredibly rare. More than 150 field surveys in the area have yielded only 62 bone collector specimens.
The scientists determined that the bone collector belongs to Hyposmocoma, a diverse genus of small moth species only found in Hawaii. During their larval stage, Hyposmocoma caterpillars spin protective cases of silk that resemble everything from candy wrappers to cigars.
Like hermit crabs, these caterpillars carry their cases with them as they move before emerging from them as full grown moths. Some species decorate their mobile homes with bits of wood, algae and shells to blend into their environments.
The bone collector caterpillar puts a macabre spin on the practice. Using silk, the caterpillar weaves bits of the dead insects it encounters on the spider’s web. The researchers have identified parts from six families of insects attached to the caterpillars, including weevil heads and beetle abdomens. The caterpillar even incorporates pieces of exoskeleton molted by their arachnid neighbors.
Dr. Rubinoff and his colleagues brought several bone collectors back to their lab. They were surprised by how picky the caterpillars were when it came to adorning their cases. “These caterpillars are able to discern differences in objects in their environment,” Dr. Rubinoff said. The larval moths eschewed other available detritus, choosing to harvest exclusively from insect corpses.
But not any remains will do. The caterpillar uses its mandibles to carefully rotate and probe prospective body parts. Those that are too big are chewed down to a more comfortable size.
Bone collecting caterpillars are less picky about their diet. The team found the captive caterpillars would eat any insect prey they could catch — including one another.
However, they do have to contend with their eight-legged landlords. The team observed bone collector caterpillars frequenting the webs of at least four introduced spider species. The team posits that the caterpillar’s ghastly garb helps disguise them among the insects trapped in the web. Dr. Wagner suspects the caterpillars also undermine their culinary appeal by disguising themselves as “trash heaps” of objects the spiders didn’t scarf down.
The bone collector’s approach appears to work — the researchers never observed a spider consuming a bone collector or entangling one in silk.
The team studied the bone collector’s genetics and determined that it had most likely diverged from other carnivorous Hyposmocoma caterpillars more than five million years ago. This was millions of years before Oahu emerged from beneath the ocean, making it likely that the bone collector’s ancestors once lived on other islands.
The bone collector’s current slice of paradise may be at risk. While the caterpillar has adapted to thrive on webs spun by nonnative spider species, its habitat is threatened by invasive ants and parasitic wasps. According to Dr. Rubinoff, conservation attention is desperately needed to save Hawaii’s endemic arthropods, including its body-snatching caterpillars.
