Some caterpillars defend themselves from predators using toxic chemicals, repugnant smells, or stinging hairs. Some camouflage themselves. Some mimic snakes. Some recruit ant bodyguards. Some create protective fortresses, or make warning clicks, or vomit up their guts.
And then there’s Uraba lugens, an Australian moth colloquially known as the gum-leaf skeletoniser, and even more colloquially known as the Mad Hatterpillar. Like all caterpillars, it grows by shedding its hard outer shell before expanding the soft body beneath. But every time it does, it keeps the part of the shell that once enclosed its head. With every moult, the stack of head capsules grows, eventually becoming a tall, tapering tower.
Yes, this caterpillar wears a hat built from its old heads.
A hat. Of old heads. I do that and everyone complains. The caterpillar does it and it get years of research attention. When Australian entomologist Noel McFarland described the caterpillar in 1980, he wrote:
“It is hard to imagine what (if any) purpose the retention of a stack of head capsules might serve. Perhaps it might attract the first one or two investigative pecks from a bird or lizard… However, the dense, hairy coat alone… would probably serve as an adequate repellent for most birds.”
But birds aren’t the caterpillars’ only concern. They are also menaced by parasitic wasps and flies that lay eggs in their bodies, and by predatory bugs that suck their bodily fluids. Would the tower of heads help the caterpillars against these adversaries?
To find out, Petah Low from the University of Sydney raised gum-leaf skeletonisers and delicately removed the head capsules from some of them with some forceps and a paintbrush. She then pitted these individuals against predatory bugs in Petri dish arenas. The bugs readily attacked, trying to drive their stabbing mouthparts into the caterpillars’ heads. In response, the caterpillars thrashed, curled up, reared up, and vomited. Those with hats used their stacks to deflect or absorb the bugs’ attacks, leading to more protracted struggles. But in the confines of the Petri dishes, the bugs always won.
What about in the field? Low returned several of her caterpillars, both with and without hats, to the wild. Low checked back on them after eight days, and found that a group of hatless caterpillars was no more likely to survive than a group of hat-covered ones. But when they lived in mixed groups, the hats doubled their chances of survival. The hatless ones were also more likely to have been implanted with the eggs of parasitic flies and wasps.
A mixed bag, then. The stacks of skullcaps could clearly act as false targets for predators, and they did, in some cases, protect the caterpillars. Then again, as Low writes, “While providing some level of protection, head capsules may not be equally effective against all predators and are not sufficient to prevent predation by highly motivated predators.” The effectiveness of one’s hat of heads also seems to depend on whether everyone else has hats of heads on their heads or not.
But even if the hats aren’t a perfect defence, they are a very cost-effective one. The caterpillars don’t need to invest in making toxic chemicals or building stinging hairs—they just need to keep a bit of their body that they would otherwise have discarded. Even if that gives them a small advantage, that might be enough. This might also explain why the gum-leaf skeletoniser is just one of several caterpillars that creates a stack of chapeau shields. Several other related species use the same strategy—one in Japan, one in India, and two in Europe.