Researchers in Hawaiʻi have described an unusual species of carnivorous caterpillar that scavenges in spiderwebs while wearing cast-off bits of the spider’s prey.
Nicknamed the “bone collector,” the caterpillar belongs to the genus Hyposmocoma, commonly known as “fancy case” caterpillars because they make variously ornamented protective cases to live in. Endemic to Hawaiʻi, they decorate their cases with bits of moss, lichen or sand to blend in with their environment. The bone collector is the only one known to encase itself in inedible insect parts, the researchers say in a recent study.
Unlike most caterpillars that try to camouflage to avoid being eaten, the bone collector “flips the script,” Daniel Rubinoff, the study’s lead author and entomologist with the University of Hawaiʻi, told Mongabay in a video call. “I want to look like your last week’s meal so that … you notice me for sure. But you think I’m garbage.” This leaves the caterpillar safe from being eaten by the spider but with easy access to a free meal caught in the spider’s web.
The bone collector is one of roughly 600 species of fancy case caterpillars, which fall within one of 18 different lineages. Each lineage is named for its distinctive case shape, including upward of 70 species with burrito-shaped cases, 25 species with smooth cases and 20 species of oyster-shaped cases. But there is only one known bone collector species.
Using DNA analysis, Rubinoff’s team learned that the bone collector’s lineage split off from its closest relatives — other Hawaiʻian carnivorous caterpillars, including one that hunts snails — roughly 6 million years ago. That makes the bone collector older than the Hawaiʻian Islands themselves. Rubinoff says the species likely evolved on older islands that have since eroded back to the sea but not before some caterpillars migrated to nearby younger islands.
Much like Darwin’s finches in the Galapagos, the Hawaiʻian caterpillars adapted differently on each island, based on local conditions.
Part of that adaptation involved avoiding birds. “Before people got to Hawaiʻi, it was the kingdom of birds,” Rubioff said, which put enormous pressure on caterpillars to diversify their appearance to avoid being eaten. There were also no ants, leaving an ecological niche available, and the bone collectors moved in — under rocks and in rotting logs.
The newly described caterpillar is already in danger. It’s only been found on the island of Oʻahu in a 15-square-kilometer (6-square-mile) stand of forest, making it extremely vulnerable to habitat loss and invasive species.
Naomi Pierce, an evolutionary biologist at Harvard University, not part of the study, told National Geographic that the fact the bone collector was found “only in one tiny spot on one island is pretty sobering.”
Rubinoff says there’s more to the bone collectors than the “gee-whiz” factor. Studying caterpillar genetics and behavior could lead to a less toxic method of controlling caterpillars on crops. “Don’t we all want that?”
Banner image: Bone collector caterpillars, courtesy of Daniel Rubinoff.
