Going to Hawaii? Watch out for the flesh-eating caterpillars

Flesh-eating caterpillars lurk in Hawaii’s rainforests

Islands can produce some of the strangest evolutionary novelties on the planet. Island-living elephants shrink to tiny sizes, while tortoises grow gigantic. The fate of species on islands is its own specialized study because the only way species can arrive on an island is over the water. Scientists, in the study of island biogeography, focus on how plants, animals, and microbiota end up on the islands where they occur.

What happens after they arrive is apparently anybody’s guess. Islands are unusual because they can lack the stiff competition of mainland ecosystems. Common factors in our daily lives, like ants, can be completely lacking. Because so many pieces of an ecological puzzle are missing on an island, niches remain open for the organisms that do arrive and get a foothold. Animals and plants end up doing things on islands that their kindred are not known to do anywhere else in the world. A recently discovered example is a caterpillar that has broken all the rules of caterpillardom. It eats meat. It hunts its prey. It uses its silk as a weapon. It deliberately camouflages itself with non-caterpillar components. And it’s a brutal killer.

Like a wolf that dives for clams

This particular capterpillar and its four just-discovered relatives reside on one of the most isolated island chains in the world, the Hawaiian archipelago. These islands are well known for evolutionary novelties, and these new species of the genus Hyposmocoma are no different. Well, actually, they’re very different. One scientist has said that discovering the behavior of these larval moths is like discovering a wolf species that dives for clams.

This caterpillar, a tiny, brutal, sneaky killer, creeps up on its prey, an unsuspecting snail resting on a leaf in the Hawaiian rainforest. The caterpillar itself is bound in silk, and it proceeds to spend almost a half hour anchoring the hapless snail to the leaf with more silk. The silk, made of gelatinous proteins, pins the snail by its shell as tightly as a spider wraps its threads around prey.

Once the caterpillar has immobilized its target, preventing the snail from escaping through a fall off of the leaf, the nascent moth emerges from its own silk casing. The snail retreats into its shell, and the caterpillar follows, beginning to feed on the trapped snail, starting with the head. It literally eats the snail alive.

This behavior is extraordinarily unusual for a caterpillar, the juvenile form of moths and butterflies. The vast majority of caterpillar species are vegetarian; of the 150,000 known species, only 200 have been identified as flesh eaters and predators. These few do not use their silks to trap their food, and they don’t eat snails, which are mollusks, targeting instead soft-bodied insects.

Caterpillar divers and adaptive radiation

But the genus Hyposmocoma is known for its diversity. Some of its members dive underwater for food. The interesting thing about the snail-eating caterpillars is that they seem to have radiated through almost all of the Hawaiian islands. The first species was identified on Maui, but since its discovery, researchers have found species on most of the other islands. Evolutionary biologists are intrigued by the many novel aspects of this caterpillar’s life history because it is so unusual for this many unique factors—novel food source, novel hunting technique, novel eating technique—to have evolved in the same species.

Wearing the spoils of capture as camouflage

One other unique thing about this caterpillar’s approach to dinner is its use of decoration. Once the mollusk-eating caterpillar has spent the day dining on escargot, it will attach the snail’s empty shell to its silken casing, along with bits of lichen and other materials, in an apparent attempt to camouflage itself.

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