By Dr Mike Dickison

In a drawer in the Whanganui Regional Museum's collection storeroom are several curious objects. They resemble large caterpillars, with all their legs and wrinkles depicted, made out of very light wood, quite solid to the touch. From the head of each protrudes a long spike, like a shoot, but also woody. Known as vegetable caterpillars, these curios were once highly prized by Māori and Pākehā alike, and represent the last remains of a moth consumed by a fungus.

The vegetable caterpillar, or āwheto, begins its life as the larva of one of the species of native ghost moths or porina moths. Although related to the porina moth that plague farmers' pastures, these species only live in native bush, and their large caterpillars inhabit silk-lined burrows in the forest floor, emerging at night to eat fallen leaves.

From time to time a few unlucky caterpillars become infected by spores of the fungus Ophiocordyceps robertsii, either by eating them or inhaling them, and are thereafter doomed. The fungus gradually fills and consumes the caterpillar's entire body, leaving only the skin. By this time the dead caterpillar is underground in its burrow, so the fungus sends up a thin stem, or stroma, up to 30 centimetres long, which eventually pokes above the leaf litter.

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The end of the stroma is coated in spores, which rain down over the forest floor to infect more caterpillars, and thus the cycle continues. In some spots in the bush you can see half a dozen fruiting spikes protruding from the ground, each a caterpillar's grave marker.

There are numerous species of caterpillar fungi around the world, and they're often seen as mysterious or even magical objects, surrounded by folklore. Early accounts by Pākehā claimed they were the remains of a caterpillar that burrowed from the top of a tree down to the roots, finally growing a shoot out of its head when it reached the soil - they may have been confused with the related pūriri moth caterpillar, which does tunnel into tree trunks. One Māori story was they were the result of caterpillars swallowing fern seeds, which grew out of their mouths and killed them. Even the name "vegetable caterpillar" is a misunderstanding, as fungi aren't plants.

There are many other kinds of caterpillar fungi, and probably because of their uncanny nature, they're often used in traditional medicine. The Asian species Ophiocordyceps sinensis, which also parasitises ghost moths, is so valued in Chinese and Tibetan traditional medicine, and commands such high prices, that it is now endangered in China, suffering the same fate as rhinos killed for their "medicinal" horns. Curiously, the English name of this vegetable caterpillar in China is aweto, taken from its Māori name, āwheto.

Māori also valued āwheto, for different reasons. Many North Island iwi collected the fungi to use in tattooing. The āwheto were dried, burnt, powdered and mixed with fat into a deep black paste that was then pushed into the cuts made by tattooing tools. When we see the black lines of moko in 19th century photos and Lindauer portraits, we should spare a thought for a lowly fungus and the unlucky parasitised caterpillars.

Dr Mike Dickison is Curator of Natural History at the Whanganui Regional Museum.

In the wild:

The only sign of a vegetable caterpillar is the fruiting spike or stroma emerging from the forest floor.

Curious caterpillars:

Many New Zealand museums have dried vegetable caterpillars (Ophiocordyceps robertsii), as this fungus was once widely collected and sold as a curio.

As an adult:

The forest ghost moth (Aoraia enysii) is one of several species of litter-dwelling native moths preyed upon by the caterpillar fungus.