Weird Creatures: Leaf-Rolling Caterpillars & Their Anal Cannons

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A silver-spotted skipper caterpillar at rest. Photo by Tom Pawlesh.

If this is indeed the “best of all possible worlds,” as the philosopher Gottfried Leibniz proposed in his argument for a benevolent and omnipotent God, then it’s logical to assume that the Creator endowed mankind with the best of all possible defecation methods. However, the definition of “best” is problematic in this case, particularly in light of the extraordinary ability that is possessed by a number of leaf-rolling caterpillars—the ability of fecal firing.

A leaf-rolling caterpillar, for those drug-addled deviants among you whose minds instantly conjured the intoxicated caterpillar in Alice in Wonderland, is rolling the leaf not because it lacks access to a hookah, but because it needs a hiding place (though the title of a 1955 Ecology paper by James Needham, “Notes on a Leaf-Rolling Caterpillar and on Some of Its Associates,” does sound like an undercover DEA operation).

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“Who are you?”

These feeding larvae are indeed hiding from a sting operation of sorts: the predatory presence of wasps. The Polistes fuscatus, or paper wasp, kills caterpillars in order to feed them to its own developing larvae. To avoid this fate the leaf-rolling caterpillar creates a shelter, folding over a leaf and holding it in place by spinning silk fibers. It can then feed on the ends of its leafy home or cautiously venture out and feed on adjacent leaf surfaces.

There’s only one problem with this cloistered and edible home: poop, or to use the more precise scientific term, frass, i.e. insect excrement. The presence of frass is an obvious tip to wasps—and human gardeners who wish to keep herbivorous pests away from their plants—that a caterpillar is near.

The solution that a number of shelter-dwelling caterpillars—at least 17 moth and butterfly families, according to Dr. Martha Weiss of Georgetown University—have evolved to cope with this threat is both martial and comic: “they fire their droppings like howitzers,” as Carl Zimmer puts it in his book Parasite Rex: Inside the Bizarre World of Nature’s Most Dangerous Creatures. “As a bit of frass starts to emerge from the caterpillar,” Zimmer writes, “it pushes a hinged plate back against a ring of blood vessels surrounding its anus. The blood pressure builds up behind the plate, which the caterpillar then releases. The pressure of the blood slams against the droppings so suddenly that it blasts them three feet a second, in a soaring arc that carries them up to two feet away.”

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A banana skipper caterpillar and its frass: “…most caterpillars do not have a defecation sequence in which they…fire off pellets.”

In their paper “Faecal Firing In A Skipper Caterpillar Is Pressure-Driven,” Stanley Caveney, Heather McLean, and David Surry state that “most caterpillars do not have a defecation sequence in which they extrude, retract, tilt and fire off pellets of frass.” However, because the anal haemocoel compartment in non-firing caterpillars becomes pressurized during defecation, the authors find it plausible that the periodic elevation of blood pressure “became adapted to power a projectile mechanism to discharge frass pellets.”

The benefits of such an adaptation have been well confirmed by Dr. Weiss’ research. Her article “Good housekeeping: why do shelter-dwelling caterpillars fling their frass?”, published in Ecology Letters, outlines a series of experiments that she performed to test whether frass ejection in the silver-spotted skipper caterpillar (Epargyreus clarus) is related to hygiene, crowding, or natural enemies. (As Dr. Weiss states, this research is rather unusual: “Despite the taxonomically widespread occurrence of frass ejection behaviour, and the use of associated anal structures as larval key characters, the phenomenon has been little studied.”)

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Polistes fuscatus wasp

By introducing the chemical signal of frass into some caterpillar shelters and visually similar black glass beads into others, Dr. Weiss found that P. fuscatus wasps visited frass-containing shelters far more frequently. Of the 17 E. clarus larvae in frass-bearing shelters, only three survived. Of the 17 E. clarus larvae in bead-associated shelters, 14 survived. No significant differences were found between the frass and bead shelters in the hygiene and crowding tests.

While the skipper caterpillar and its anal cannon go happily on to the next stage of metamorphosis, a curious reader might wonder what happens to all that ejected frass. Though frass may not be as “noble” a substance as the maggot-feeding corpses of Hamlet’s fat kings and lean beggars, it is put to an equally good use. In regions where they can be found, leafcutter ants—the fungus-growing ants that are among the world’s smallest farmers—will use caterpillar frass in their agricultural operations.

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A Cyphomyrmex rimosus ant, a relative of the leafcutters that’s found in the Southern U.S. and South America, carrying a piece of caterpillar frass to its nest. Photo © Alex Wild.

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A Cyphomyrmex rimosus fungus garden. The fungi are the yellow globs clustered around the pieces of frass. Photo © Alex Wild.

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