This Mysterious Caterpillar Builds a Toxic Fortress of a Cocoon



Moth caterpillars build their cocoons out of silk. Most of them will leave it at that, but others upgrade their temporary homes by camouflaging them or adding defensive measures. The caterpillars of bagworm moths, for example, bind leaves and twigs together with their silk to make the cocoon blend in with vegetation. Brown-tail moth caterpillars integrate their bristly, stinging body hairs into their cocoons to deter animals that might want to eat them while they’re in there. If you were adapting the story of the three little pigs with insect characters, these plain cocoons would be the caterpillars’ take on a straw house, and the upgraded versions, a stick house. 

Now scientists have found the caterpillar version of a brick house—a hard, multi-walled and chemically-defended cocoon made with tree resin. It’s the first of its kind that scientists know of, and its builder may be new to science, too.

The cocoon was discovered by William Symondson, a British entomologist who does field work in Borneo. One day few years ago, he was leading a group of students through the forest when he spotted a hairy orange and white caterpillar on the trunk of a Vatica rassak tree. The tree, known locally as the rasak, plugs wounds in its bark with a resin that dries in hard sheets. As Symondson watched, the caterpillar broke small flakes of resin from these sheets by biting them and pushing on them with its head, and then bound the pieces together with silk to build two parallel walls. When the walls were complete, the caterpillar strung threads of silk from the top of one wall to the other and crawled between them, where it pulled on the threads until the walls came together and the caterpillar was enclosed. 

When Symondson took a closer look at the walls, he saw that the internal faces were pretty smooth, while the outer faces had angular shards of resin jutting out from them. The hard walls and array of spikes put a protective barrier between the caterpillar and anything that might come huffing and puffing, but the cocoon also turned out to have some chemical weapons. When Symondson analyzed the resin, he found a complex mixture of 250-plus components, including some that are antifungal, antimicrobial, or toxic to animals. Built on a large patch of the same resin, the cocoon is camouflaged both visually and chemically, Symondson says, and the toxic compounds keep parasites and predators away or injure those that try to break in.

Symondson didn’t recognize the caterpillar and wasn’t aware of any that built their cocoons with resin, but assumed that it was a known behavior that he simply hadn’t heard of. When he searched through field guides and scientific papers and consulted an expert in Bornean moths, though, he found that no one else had seen anything like it, either. This orange and white caterpillar is the only one in the world that seems to do this. While he was able to document all the stages of the cocoon building, Symondson wasn’t there to see the moth that later emerged and wasn’t able to find another example of the caterpillar. He hopes to find another specimen and keep an eye on it through its metamorphosis to see what comes out of the resin bunker so the species can either be identified or, if it’s new to science, described and named.