The 'Skeletonizer' Caterpillar Wears a Hat of Old Skulls to Ward off Predators

Skulls have been used by humans for centuries in spooky scenarios and as terrifying symbols to signify danger. It turns out one species of caterpillar, the gum-leaf skeletonizer, does the same thing in the insect world.

This macabre little critter stacks skulls on its head to fend off predators.

These gothic, 25mm-long creatures "really do capture your imagination," says Dieter Hochuli, from the School of Life and Environmental Sciences at the University of Sydney, Australia. They live in the eucalyptus forests of Australia and New Zealand and have likely been around for millions of years. "Insects do a lot of weird stuff, but these guys look really weird," Hochuli told Newsweek.

Like a lot of creatures, when gum-leaf skeletonizers want to grow, they have to molt or shed their skin. For this species of caterpillar, their head is part of their skin. Normally, creatures would just discard it all, "but these guys create a tower of five, six or seven heads up there," says Hochuli "and they use them to deter things that are trying to eat them."

Hochuli's university department first started working with the fascinating creatures in 2012, before behavioural ecologist Petah Low published a study on the skeletonizer in 2014.

Having previously been dubbed everything from Unicorn Caterpillars to Mad Hatterpillars, the gum leaf skeletonizers are scientifically known as Uraba lugens. Each caterpillar molts around a dozen times before cocooning and turning into a moth—but before that they have to avoid attack.

Not only do they look menacing, these clever critters use their Halloween-like headwear as a decoy. Predators will ideally take a chip off the old block instead of the caterpillar.

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One such predator is the stinkbug, an insect with a giant needle attached to its mouth filled with toxins. "They feed by injecting the needle straight into their prey. We've got videos of them trying to attack these caterpillars, and the first thing the caterpillar does is waves its head around and sort of foists it away," Hochuli said.

"We've also seen the bug try and drive the needle into the caterpillar and what it does is hit the empty bits of head so the caterpillar is able to get away."

By the time the predator has figured out what's going on and that it has actually lanced a skull instead of a juicy caterpillar, hopefully, the caterpillar has had a chance to escape, leaving its old skulls at the scene.

"Wearing heads is a real benefit to the caterpillar in that way," says Hochuli. "Lots of people think it's a pretty metal way to go about life."

If the caterpillar does escape, chances are one of the other caterpillars in the group will be picked off instead.

"These caterpillars are gregarious, they tend to forage in groups lined up together in clusters, we're pretty confident that wearing skulls is not only to avoid being eaten, but to get the predator to focus on the guy next to you. It's a distraction for long enough so the predator turns around and says 'I'll eat this one instead,'" says Hochuli.

"They're an animal that really responds quickly to a shift in the environmental conditions, they always figure out a way to survive.

"With evolution, the selection pressures to avoid being eaten are massive – so if you can get an advantage that allows you to go through your life you take it. This is certainly up there with one of the best ways to avoid being eaten, along with super toxic brightly coloured animals – those things are extraordinary too."

Hochuli believes these little critters are remarkable and "one of the beauties of the natural world".