Gum leaf skeletonizer: The venomous 'Mad Hatterpillar' that wears its old heads like a crown

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Rather than throw its old head cases away, the gum leaf skeletonizer wears them like a hat to protect itself from predators.

Mad Hatterpillar, larva wearing head capsules from each previous moult. Gregarious and destructive larval stage, a leaf-skeletoniser on eucalypts.
The larvae of the gum leaf skeletonizer wears its old head shells as a hat. (Image credit: Auscape/Universal Images Group via Getty Images)

Name: Gum leaf Skeletoniser (Uraba lugens)

Where it lives: Australia and New Zealand

What it eats: Eucalyptus leaves

Why it's awesome: The caterpillar of the gum leaf skeletonizer moth is a hairy little creature that keeps its old head capsules and stacks them on its head. This macabre adornment has earned it the nickname of the "Mad Hatterpillar," inspired by the character from "Alice's Adventures in Wonderland."

But why undertake this elaborate millinery? One study published in 2016 investigated this question by holding trials involving the caterpillars and their natural predators — spiders and stink bugs. The researchers found that attacks on larvae with a stack of headcases took more than 10 times longer than attacks on larvae that had had their stack removed. They found that the empty head capsules acted as a false target for predators and could also be used to deflect the piercing rostrum of a predator.

Related: Caterpillars evolved their weird chubby little 'prolegs' from ancient crustaceans

Gum leaf skeletonizers are also covered with hollow venomous spines — or hairs — containing histamines that can cause erucism, or caterpillar dermatitis. If these spines pierce human skin, they cause a stinging sensation, itching and a rash. This venom can remain within the spines even after a caterpillar sheds its skin.

Its common English name is derived from the caterpillar's eating habits. The caterpillars eat eucalyptus leaves gregariously, with multiple individuals on the same leaf. They eat through both the under and upper sides of the leaf but avoid the veins — leaving just the skeletal-looking remains of the munched leaf. The presence of these caterpillars can slow the growth of eucalyptus trees and even kill them.

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Endemic to Australia, these caterpillars have also spread to New Zealand where they are considered a pest. To combat them, ecologists released a species of small parasitoid wasp (Cotesia urabae) to act as a biological control agent.

"It was a big project." Toni Withers, senior entomologist at the New Zealand research institute Scion, told Live Science in an email. "New Zealand has no native insects with this stinging hair, and human health was one of the reasons we wanted to avoid outbreaks." 

Megan Shersby
Megan Shersby
Freelance science writer

Megan Shersby is a naturalist, wildlife writer and content creator. After graduating from Aberystwyth University with a BSc (Hons) degree in Animal Science, she has worked in nature communications and the conservation sector for a variety of organisations and charities, including BBC Wildlife magazine, the National Trust, two of the Wildlife Trusts and the Field Studies Council. She has bylines in the Seasons anthologies published by the Wildlife Trusts, Into The Red published by the BTO, and has written for the BBC Countryfile magazine and website, and produced podcast episodes for its award-winning podcast, The Plodcast.