Touch Not the Cat

The venomous puss caterpillar is no cuddly kitten.

(Illustration by Miriam Martincic)

As we sat stuck at home last fall, a flurry of local news stories seized on the apocalyptic spirit of the season:

“Weird-looking poisonous caterpillars are on the loose in Virginia,” one headline announced. “Alien hairball sends woman to emergency room,” another read. A third sounded like the trailer from a horror flick: “A bizarre little insect that looks like a walking toupée and squirts venomous pus from knifelike spines is terrorizing Virginia this year.”

But were we really coming under assault from hordes of venom-squirting caterpillars?

For the record, those walking toupées are puss (not pus) caterpillars. And they are, in fact, venomous. And, yes, one unlucky New Kent woman did land in the ER after a too-close encounter with a puss caterpillar left her with pain that felt, “like a scorching-hot knife passing through the outside of my calf.”

But extension entomologist Eric Day, director of the insect identification lab at Virginia Tech—and a guy who has seen a bug or two in his life—is here to tell you that, no, the caterpillars are not on the attack. “Sometimes the news cycle gets low, and scary bugs make a great story,” says Day. “In no way are we having an invasion of these things.”

Still, what is the likelihood that you might come in contact with one? “Very rare,” says Day, who spends a lot of time outdoors yet nevertheless has never encountered a puss caterpillar himself, except when it’s been sent to his lab for identification.

The puss caterpillar is the larval stage of the southern flannel moth, a small, innocuous furry yellow creature with a vague resemblance to the Pokémon character Píkachu. But to emerge into mothhood requires first surviving caterpillardom. And that means not getting picked off along the way by the Very Hungry Animal Kingdom.

So as a caterpillar, Megalopyge opercularis bristles—literally—with a defense system packing a punch so painful that the puss has earned a reputation as the most venomous caterpillar in the U.S. But then it holes up in a cocoon and emerges as a perfectly harmless fuzzy moth. It’s kind of like if Chucky the killer doll took a sabbatical and came back as Raggdy Andy.

To look at, the puss caterpillar doesn’t exactly telegraph “danger don’t touch!” It’s a luxuriantly long-haired rounded hump with a tail, an ambling toupée, something like a furry dolphin or a very miniature cat (which is almost certainly how it got landed with the name “puss,” as in “in Boots”).

Concealed within those long hairs, however, are sharp, hollow spines, each equipped at the base with its own reservoir of venom. Brush against the caterpillar, and those spines inject the venom into unfortunate you. And then? “The sting produces an immediate intense burning pain,” details a fact sheet from the University of Florida. Also good to know? The toxicity of the sting increases with the size of the larvae.

In addition to inflicting severe pain, the touch of the puss can also leave a pattern of blistering red welts on the skin and induce a spreading allergic rash. Less commonly, it can bring on nausea, vomiting, rapid heart beat, low blood pressure, seizures, muscle spasms, and convulsions. Also numbness, chest pain, difficulty breathing, and vision problems. “Other than that,” concludes a North Carolina State extension publication cheerily, “the sting is not troublesome.”

The reason the puss caterpillar is sometimes erroneously referred to as a “pus” caterpillar seems to be a case of conflating the unfamiliarly archaic name “puss” either with the caterpillar’s venom or with the blistering rash it causes. It is also sometimes known as the “stinging asp,” which is certainly more on point—an asp being a venomous snake.

Whatever you want to call it, as the caterpillar goes through its larval molts, it also eats its own castoff skin, including the venomous spikes. It further sustains this admirable efficiency by spinning itself a cocoon from its own hair, which doesn’t seem like a structural material that would pass the Big Bad Wolf test. But the cocoon is in fact quite tough and durable, to the point where empty cocoons can persist long enough to grow a layer of lichen.

It seems like a lot of work, because as soon as the adult emerges it expeditiously goes about producing a new generation of venomous offspring and dies soon thereafter.

But for a brief, burning moment of glory, the notorious puss caterpillar commandeered the headlines as the most feared and funny-looking venomous caterpillar in America.

This article originally appeared in the December 2021 issue.

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