OAHU — In a remote and lushly forested area of a single mountain range on the Hawaiian island of Oahu, scientists have discovered a carnivorous caterpillar species that makes a living in such a macabre way that they have nicknamed it the “bone collector.”
The caterpillar prowls spider webs to scavenge trapped and helpless victims such as ants, beetles, weevils and flies, the researchers said. The crafty caterpillar camouflages itself from the spider, which would happily eat it, by hiding its body inside a case it fashions from its own silk and adorns it with inedible body parts that it collected from the dead insects.
Through metamorphosis, this caterpillar eventually turns into its adult form, a moth with a brown and white coloration. Caterpillars are the moth’s larval stage, with a segmented and worm-like body.
This is the world’s only known caterpillar to live with and benefit from spiders, according to Daniel Rubinoff, a professor of entomology at the University of Hawaii at Manoa and lead author of the study published this week in the journal Science.
Its grisly behavior would seem well suited for a crime novel. But it represents an example of the creative paths that our planet’s living organisms take to survive and thrive.
“They need to hide in a tapestry of bug parts to stay alive in the spider’s lair,” Rubinoff said.
“I think it’s actually a hero,” Rubinoff said. “It truly lives ‘in the lion’s den,’ hiding out with a spider and using the spider’s web to provide it with food and probably shelter. The caterpillar will attack prey that can’t get away but is itself very slow and bumbling, trailing a large (silk) case behind it.”
The caterpillars consume weakened or dead insects they encounter in webs spun by spiders in tree hollows and rock crevices.
“So it’s probably getting the leftovers after the spider has fed,” Rubinoff said.
They even resort to cannibalism, attacking other caterpillars of the same species.
The “Bone Collector” was the nickname of a serial killer in author Jeffery Deaver’s 1997 novel “The Bone Collector” and subsequent 1999 film of the same name.
So how did this caterpillar come to share this notorious nickname?
“I think the term is out there in the ether, and just fit with what these caterpillars are doing. It’s a bit tongue in cheek because arthropods don’t actually have bones,” Rubinoff said.
Arthropods are a massive assemblage of invertebrates that includes insects and spiders, as well as crustaceans.
The researchers said the “bone collector” inhabits a patch of mountain forest spanning just 5.8 square miles in the Waianae mountain range. Rubinoff said this caterpillar has a very precarious existence. Only 62 individuals have been observed in two decades of fieldwork.
“Invasive species are the main threat now. Even in protected areas, Hawaii is losing native species due to invasive species taking over habitats and turning them into biological deserts that look like forests but are largely unavailable to native species,” Rubinoff said.
The caterpillar, a previously unknown species, is a member of a group of moths called Hyposmocoma native to Hawaii that includes hundreds of species and arose about 12 million years ago. The researchers believe the “bone collector” comes from a lineage more than 5 million years old.
The overwhelming majority of caterpillars eat vegetation. Predatory caterpillars globally comprise less than 0.13% of the planet’s nearly 200,000 moth and butterfly species. And among those, the “bone collector” is the only one known to find food the way it does, making it unique among the world’s animals.
“The more we can understand how the world around us works, the better off we will be,” Rubinoff said.
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