Most Metal Caterpillar Ever Bone collector caterpillar adorns itself in insect body parts The caterpillars even tailor the body parts, nibbling away at excess material to ensure a proper fit. Jennifer Ouellette – Apr 24, 2025 2:00 pm | 12 "If you're going to live in Smaug's lair, you'd better look like treasure." Credit: Rubinoff lab/University of Hawaii, Manoa "If you're going to live in Smaug's lair, you'd better look like treasure." Credit: Rubinoff lab/University of Hawaii, Manoa Story text Size Small Standard Large Width * Standard Wide Links Standard Orange * Subscribers only   Learn more This Hawaiian caterpillar raids spiderwebs camouflaged in insect prey’s body parts, and it's not above cannibalism in a pinch. Credit: Rubinoff lab/University of Hawaii, Manoa. We think of moths and butterflies as relatively harmless creatures, but there are certain species with a darker side—for example, carnivorous caterpillars that eat aphids, butterflies that drink alligator tears, or "vampire" moths that feed on livestock blood. Add to that list the newly discovered "bone collector" caterpillar, which conducts daring raids on spider webs for sustenance, camouflaging itself in the body parts of already-consumed insects to avoid being eaten. Not only that, but according to a new paper published in the journal Science, the caterpillars can tailor those insect parts, nibbling away at any excess material to ensure a proper fit. Daniel Rubinoff, an entomologist at the University of Hawaii, Manoa, studies a genus of moths found in Hawaii called Hyposcoma, or as he has dubbed their larval form, "Hawaiian Fancy Case" caterpillars, so named because they spin their own casings, adding to them as they grow, although the materials used can vary widely.  There are now more than 600 species within this genus, many of them not yet officially described, so it was a rich research area to explore. The discovery of the bone collector species was serendipitous. "You never forget your first bone collector," Rubinoff told Ars. His team was on Oa'hu looking for Hyposcoma when they came across a little tree hollow and spotted something at the bottom that at first glance just looked like "a bag of bug bits." The caterpillar then stuck its head out, and the researchers realized it was a new kind of case. Rubinoff assumed that the spider web also found in the tree hollow was a coincidence; the caterpillar just used the materials readily available in the tree hollow to make its fancy case. But then the team started finding more of these caterpillars, all covered in the body parts of other insects and shed spider skins, and all in the vicinity of spider webs. "We started realizing these things are only hanging out where there are spiders," said Rubinoff, who spent several years verifying that this was, indeed, a rare new species. "It's the sort of thing you really want to be sure of because it's not just incredible, it's unimaginable." A genomic analysis confirmed the researchers' suspicions and shed some light on the bone collector's possible evolutionary pathway. The bone caterpillar may have only just been discovered by humans, but it's at least 5 million years old and possibly as old as 12 million years, predating the island of O'ahu on which it now exclusively resides in an area of about 15 square kilometers in the Wai'anae Mountains. No other known member of the same lineage has yet been found, suggesting that the species originated on an early island in a chain that has since subsided. Dressed for success Why do the caterpillars do this? "It's a decorate or die situation," said Rubinoff. "In evolutionary history, the ones that didn't decorate their cases were probably removed from the gene pool pretty quickly. But a few of them started incorporating bug and spider bits in their cases and survived. Selection would drive them toward having the sensory capacity to detect those bits and use them as camouflage. If you're going to live in Smaug's lair, you'd better look like treasure." Bone collector larva in web. Rubinoff lab/University of Hawaii, Manoa Bone collector larva in web. Rubinoff lab/University of Hawaii, Manoa Pinned adult female (left) of the bone collector caterpillar and portable case (right) in which the larva resides decorated with body parts from ants, bark beetles, weevils, and flies. D. Rubinoff et al., 2025 Pinned adult female (left) of the bone collector caterpillar and portable case (right) in which the larva resides decorated with body parts from ants, bark beetles, weevils, and flies. D. Rubinoff et al., 2025 Bone collector cases. Rubinoff lab/University of Hawaii, Manoa Bone collector cases. Rubinoff lab/University of Hawaii, Manoa Pinned adult female (left) of the bone collector caterpillar and portable case (right) in which the larva resides decorated with body parts from ants, bark beetles, weevils, and flies. D. Rubinoff et al., 2025 Bone collector cases. Rubinoff lab/University of Hawaii, Manoa It's a jumbled-up, messy kind of treasure, since arranging the body parts in too orderly a fashion would defeat the purpose of camouflage as they crawl around the three-dimensional cobwebs they favor. "They're not going to do a tightrope walk between two trees; they're hiding in a little hole in a log where there are cobwebs," said Rubinoff. "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." The next step is to take a closer look at the caterpillar genome to find an underlying mechanism for this unusual behavior, as well as details on how the caterpillars can distinguish between bug bits and, say, dirt, and how they are able to perceive size for tailoring purposes. A bone collector can be quite selective, picking up potential body parts among the web detritus and probing them with its mandibles, chewing larger pieces down to the desired size. Nor will the caterpillars accept other materials when they spin their cases: It's the discarded corpses of their enemies or nothing, even in captivity. Rubinoff has already brought several into the lab, where the caterpillars can gorge themselves on Drosophila pupae with no fear of spiders interrupting the feast. This confirmed that bone collectors are no mere scavengers; they are predatory, chewing right through the silk to eat the live pupae. They will even cannibalize each other, "which is why you don't see more than one at each spiderweb," said Rubinoff. The clock is ticking, however, as the bone collector is extremely rare and in danger of extinction, due to the large number of invasive species—especially non-native ants and parasitic wasps—that have found their way to Hawaii. Thus far, the bone collector has been able to adapt and raid the cobwebs of non-native spiders to survive. "I don't want to say it's on the verge of winking out, but in the context, it seems likely," said Rubinoff. "We've lost entire genera of endemic insects [in Hawaii]. It could be one new ant species away from being obliterated." Science, 2025. DOI: 10.1126/science.ads4243  (About DOIs). Jennifer Ouellette Senior Writer Jennifer Ouellette Senior Writer Jennifer is a senior writer at Ars Technica with a particular focus on where science meets culture, covering everything from physics and related interdisciplinary topics to her favorite films and TV series. Jennifer lives in Baltimore with her spouse, physicist Sean M. Carroll, and their two cats, Ariel and Caliban. 12 Comments