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The newly described “bone collector” caterpillar species disguises itself with the body parts of dead insects so that it can live among spiders and poach their prey. This is the only caterpillar known to use such grisly camouflage or have spiders as roommates – and it’s a carnivore and a cannibal to boot.

at the University of Hawai‘i at Mānoa and his colleagues discovered the caterpillar while hiking the Waianae mountains in Oahu more than two decades ago. They were searching for other species in the same genus, Hyposmocoma, also known as Hawaiian fancy case caterpillars. “We see this little, tiny sac covered in bug bits, and honestly, we weren’t sure what it was,” says Rubinoff. “And then we take it back [to the lab], and we realise there is a little caterpillar in there.”

The newly described species of Hyposmocoma – which has not yet received a scientific name – lives on cobwebs inside tree trunks, among rocks and other enclosed spaces. It is about the length of a fingernail and feeds on insects trapped in spider webs. “Only 0.13 per cent of all caterpillars on the planet are carnivorous,” says Rubinoff. “So it is incredibly hard for a caterpillar to evolve to eat meat.”

The bone collector avoids becoming prey itself with a macabre method: adorning its silken case with fragments of dead insects and the spider’s moulted exoskeleton. The critter carefully sizes up each body part – which might include ant heads, beetle abdomens or fly wings – before weaving it into its disguise.

“That’s the only way to survive, probably, living with a spider – by covering yourself in bits of the spider’s own shed skin and its past meals,” says Rubinoff. This leaves the caterpillar smelling and tasting more like a bag of trash than a juicy snack to its arachnid housemate. After about two to three months, it then metamorphoses into a moth smaller than a grain of rice.

If the bone collector’s accessorising weren’t gnarly enough, this caterpillar is also a cannibal. The researchers learned this after placing two of the larvae in the same cage, leading to the larger one feasting on its smaller, weaker brethren. This is why you only ever see one bone collector per spider web, says Rubinoff.

The researchers have found just 62 of these critters across more than 150 field surveys conducted over roughly 22 years, all within the same 15 square kilometres of the Waianae mountain range.

Genetic analysis indicates its lineage is about 3 million years older than the island of Oahu, meaning it was once more widespread. “Since the arrivals of humans in a place like this, we’ve lost lots of native species,” says Rubinoff. “It is both a miracle that we were able to find [the bone collector], and really sad that they are so restricted to this one spot.”