The Mad Hatterpillar, Uraba lugens, larval stage of an Australian moth. Image © Nuytsia@Tas via WIRED Magazine.

The image above may seem a little gruesome if you hear that the pictured caterpillar has a collection of old skulls attached to the top of its own its spiky, irritating-to-the-touch hairs. But the larval insect isn’t a true headhunter, since the eery tower above the caterpillar’s crown is in fact made of its own exoskeleton pieces from previous moltings of its skin. Ed Yong reports on new research regarding a caterpillar that has been known for many decades but is still being studied for explanations on the strange cranium-collecting behavior:

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.

…

To find out [why the caterpillar behaves this way], 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.

Read the rest of the article in NatGeo’s Phenomena blog here.