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Defiance and Disguise
Creativity Among Caterpillars (Well ... Evolution, Actually)
Charles Darwin himself was once confused about caterpillars — the gaudy ones didn’t quite make sense.
According to his own theory of sexual selection, dazzling colors and armaments on animals evolved in part to give males an aesthetic or competitive edge in the ruthless arena of courtship and mating. Yet because caterpillars, as the immature form of moths and butterflies, neither court nor breed, Darwin sought an explanation for the ornamentation and weaponry many of them display.
For an answer, he turned to his colleague and collaborator on evolutionary thought and theory, Alfred Russel Wallace, who did indeed have a hypothesis: “warning coloration.” It was already known at the time that many caterpillars were unpleasant, unpalatable or toxic to birds and other predators. Wallace suggested that a toxic caterpillar would do well to be distinctive, memorable, a bad culinary experience easily learned by a predator. Being a recognizable threat would work well for the predator and prey alike (and even the prey’s mimics).
“Large numbers of animals, more especially insects, are gaudily coloured, either with vivid hues or with striking patterns, so as to be very easily seen,” Wallace wrote in 1889. “Now it has been found, that in almost all these cases … the brilliant or conspicuous colours or markings serve as a warning or signal flag against attack.”
Everything about this caterpillar says, “Don’t mess with me.”
I got my own signal flag when I encountered that Smeared Dagger Moth (Acronita oblinita) above while photographing butterflies in Maine on Tuesday. Everything about this caterpillar says, “Don’t mess with me.” So much so that these and other “aposematic” caterpillars often rest without apparent concern in plain sight.
But before I reveal a crazy thing about the Smeared Dagger Moth, consider another caterpillar now out in force here in Vermont: Maple Leafcutter Moth (Paraclemensia acerifoliella). Find some shrubby Sugar Maples — you might not at first see the beast that lies within, but rather its dinner plate and hideout. The Maple Leafcutter cuts a disc of leaf and hides beneath it to feed on maple tissue largely undetected, except for telltale brown patches where it dined or actual holes where it had cut out its discs (like a hole-punch).
Because caterpillars grow as they eat, Maple Leafcutters outgrow their leafy refuges (even though they don’t grow much longer than 5 millimeters or so). In my image above, note on the right the tiny caterpillar barely emerging from its hideout to cut a larger disc in preparation for another round of binge-eating. Maple Leafcutters actually reside most of the time sandwiched between two discs — like a pita bread — or, as a biologist call it, a “habitaculum.” By any name, it’s not a bad hideout from warblers and other songbirds hunting the trees for a caterpillar meal. After all, those birds aren’t eating ornamental, spiky caterpillars like my Smeared Dagger Moth and its ilk.
Over the years, I’ve conducted various desktop experiments with Maple Leafcutters. Once, for example, I flipped a leaf disc to find the caterpillar with only half its pita in place. I wondered whether it could right itself like an overturned turtle. It did indeed, and soon began to cut out what would become the other half of its shelter. Or bring a bunch of those maple leaves indoors, and soon enough you’ll see little green discs patrolling your walls.
Soon the Maple Leafcutters now in our woods will descend along tree trunks, still in their protective casings, or flutter in free-fall to the forest floor. Within its habitaculum, the caterpillar will spin a silken cocoon, pupate and spend the winter. In May, as the Sugar Maple leaves break from their buds, adult Maple Leafcutters emerge to fly free as tiny moths with steel-blue wings and an orange head. They’ll mate, and the females will lay eggs on fresh maple leaves to complete the cycle.
The adult Maple Leafcutter is more flashy than its caterpillar. But that’s not always the case among moths. To the contrary, many of our most ornate and celebrated caterpillars are rather dull as adult moths. After all, if you’re a moth, it makes sense to broadcast your toxicity when you are most vulnerable to predators — as a caterpillar on a plant rather than an adult flying free at night when songbirds and other predators (except for bats) aren’t as active.
In addition to commandeering a plant’s toxins as defense, moth caterpillars display a range of survival skills: they can resemble twigs or snakes or bird droppings, be covered in unpalatable hairs or spines, or even evert a forked “osmeterium” (not unlike a snake’s tongue) that wafts odd odors toward a predator.
So here you have it — the other crazy thing about the Smeared Dagger Moth I found in Maine is how utterly plain it becomes as an adult. See it below, along with a few other examples of two amazing transformations — not only from caterpillar to flying moth, but from the ornate to the ordinary.
Oh, by the way, Darwin really liked Wallace’s hypothesis on those showy caterpillars: “You are the man to apply to in a difficulty,” he reportedly wrote back to his colleague. “I never heard anything more ingenious than your suggestion, and I hope you may be able to prove it true.”
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