Silly Caterpillar, You Shouldn’t Be Devouring Snails Alive

A snail-eating caterpillar in its characteristic silk burrito of protection, doing what it does best: tying a snail down before devouring it alive. Damn, now I want a burrito. Not of silk, of course–carnitas, I'm thinking.

That children’s book The Very Hungry Caterpillar is bullcrap. I mean, there’s no way a caterpillar could eat all that food, not to mention those kinds of foods. Ice cream? Give me a break. And chocolate cake? Now I’m kinda worried this little thing is hypoglycemic.

And sausage? Really? Wait, actually, scratch that. Caterpillars are diehard vegetarians, but no, not me, says Hawaii’s Hyposmocoma molluscivora. Incredibly, it’s got an appetite for snails, and a big appetite at that. As a snail slumbers, this creepy-crawly carefully approaches and spins silk over the snail’s shell, pinning it to a leaf. Then it crawls in there and devours the trapped victim alive.

The so-called snail-eating caterpillar joins just .13 percent of caterpillars that are predatory. And while other meat eaters go after insects, this species solely targets snails (even if it’s starving, it won’t touch plants). “That’s just ridiculous,” says entomologist Daniel Rubinoff of the University of Hawaii. “In an evolutionary sense, it’s like a vampire cow, essentially. You have all these other cows running around eating grass like they’re supposed to be, and then suddenly you discover one that is attacking and sucking the blood of fish. That’s how weird it would be. Not even other mammals, but fish.”

This caterpillar, which is only a bit over a quarter inch long, spins itself a little burrito-like case that it slips into and drags around for camouflage. But around 10 years ago, folks on Maui noticed that some of the caterpillars had stuck snail shells to these cases, perhaps as an extra fashion accessory to kick up their camo one more notch.

The assumption went that the caterpillars were just coming across empty shells. But then Rubinoff caught the things on video actually hunting snails. Even with the evidence Rubinoff still had a hard time coming to terms with the whole thing. “Even though I had video of it, I still really deep down couldn’t believe it,” he says. “It was just such a stretch, such a bizarre thing to see.”

A caterpillar pinning down a snail with its silk. Someone should really tell it that it’s not a spider. Break the news gently, though.

Snails may be slow, but a caterpillar burdened with a burrito is downright ungainly. So the hunter will only approach sleeping snails—if the target is active, the caterpillar won’t bother. If the snail is satisfactory, the caterpillar will start spinning silk over its shell, pinning it to the leaf below. Think of it like that scene in Gulliver’s Travels where our hero wakes up to find that the tiny Lilliputians have tied him to the ground, only Gulliver survives to go on other adventures. The snail won’t. The only adventuring it’ll be doing is sliding through a caterpillar’s guts.

And unlike the Very Hungry Caterpillar, Hyposmocoma molluscivora exercises a little thing called restraint. “We’ve actually got videos of snails waking up halfway through and trying to get away,” says Rubinoff. “The caterpillar doesn’t attack it, just waits. The snail gives up and goes back inside, and then the caterpillar finishes the spin, comes around, and goes into the snail shell.” It then proceeds to consume the victim alive in its own home.

The whole saga so dramatically departs from typical caterpillar behavior that it’s no wonder Rubinoff had trouble believing it. Caterpillars can’t be bothered with delayed gratification—they just gnaw at leaves and gnaw at leaves some more, as any gardener can tell you. Hyposmocoma molluscivora is a zen master of self-control, planning out a sophisticated attack and launching it only if it’s sure to succeed.

This is about where the snail’s life ends. It shall be remembered, though, as caterpillar turds.

And think about what’s going on physiologically with this caterpillar. It should have different mouthparts than a vegetarian caterpillar, yeah? Nope, as it turns out. They’re pretty much the same. Rubinoff is looking to do more work here, but it may be that snail-eating caterpillars don’t need all that different mandibles. For a vegetarian slicing through leaves, scissor-like mouthparts work great. And scissor-like mouthparts could work just as fine for meat eaters too. (Vampire cows, on the other hand, would need something other than a cow’s typical grinding molars.)

Another physiological conundrum is how the snail-eating caterpillar’s tummy is handling the switch to meat. “Vegans get sick when they eat a burger, and we’re programmed or able to eat meat pretty easily,” Rubinoff says. “If you’re a species that doesn’t eat protein like that, how do you make that kind of adjustment?” At the moment, it’s still a mystery.

Hawaii: The Land Where Snails Rule and Fish Poop Out Beaches

Then there’s the why. Why would Hyposmocoma molluscivora give up the vegan lifestyle? After all, Hawaii isn’t exactly hurting for lush vegetation.

The answer may be that the snails had it coming. Hawaii has historically been lousy with the things, scientists having described over 1,000 different species (many, though, have gone extinct thanks to humans, while many are in serious trouble). Island ecosystems tend to be a bit goofy like that: Not every kind of animal will make it there from the mainland—keep in mind that Hawaii is wildly isolated, and accordingly only has two native mammals, a bat and a seal. Among the animals that do get there, some will grow more successful than others by assuming niches they normally wouldn’t bother with.

“In other places, there are lots and lots of things that eat snails,” says Rubinoff. “There are beetles that eat snails and a range of other animals that will go and attack snails. And Hawaii happens to have a really high diversity of snails.” Because of this diversity, the snail-eating caterpillar would have done well to start hunting them, thus filling the niche that other predators may not have been around to fill themselves.

And it’s not just this species that got creative on the islands. The group the snail-eating caterpillar belongs to, Hyposmocoma, tallies some 400 species with all manner of lifestyles. “There are Hyposmocoma that are aquatic, that dive underwater, and eat algae and lichens around streams,” says Rubinoff. “So Hyposmocoma molluscivora is almost par for the course for Hyposmocoma, and that seems to be something that Hawaii brings out.”

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