Living Illusions

by Joseph DeSisto

Good camouflage requires more than just color. Millions of years of natural selection have favored birds that can easily identify a brown moth on a brown background, but some species are a little more sophisticated. Lappet moths hide on tree bark – their odd shape, combined with mottled color, helps break up their outline, so visual predators such as birds have a hard time recognizing them as moths.

A lappet moth (Phyllodesma americana) from Arkansas. Photo by Marvin Smith, licensed under CC BY-ND-NC 1.0.

A lappet moth (Phyllodesma americana) from Arkansas. Photo by Marvin Smith, licensed under CC BY-ND-NC 1.0.

These moths lay their eggs on trees including birch, oak, poplar, willow, and many others. The caterpillars munch on leaves by night, hiding on twigs and bark by day. They are also well-hidden, but because they have to be able to live on a variety of different trees, each of which has a differently-colored bark, lappet caterpillars don’t have a color that matches a particular background. Instead they, like their parent moths, have bodies with distorted outlines, specifically a lateral fringe of long hairs.

On bark, this helps a caterpillar “merge” with the bark on which it rests. On a twig, maybe not so much:

A resting lappet caterpillar on one of its favorite hosts, scrub oak. Photo by Joseph DeSisto.

A resting lappet caterpillar on one of its favorite hosts, scrub oak. Photo by Joseph DeSisto.

Animals that depend on camouflage have to stay very still to avoid detection, but if they are spotted, staying still quickly becomes futile. Many animals use color to startle predators as a backup plan, the best-known example being the red-eyed tree frog. At rest, the frogs appear a solid leafy-green, but if disturbed, they quickly open their eyes. The sudden appearance of two giant, bright red eyes can be enough to startle a predator, which might give the frog time enough to make a hasty escape.

The iconic red-eyed tree frog (Agalychnis callidryas). Photo by Carey James Balboa, in public domain.

The iconic red-eyed tree frog (Agalychnis callidryas). Photo by Carey James Balboa, in public domain.

Lappet caterpillars have a similar but more elaborate trick. If a potential predator (or human finger) brushes against a caterpillar, it first flexes its body so the hairs on its back part. This reveals two striking, blood-red bands, a warning this caterpillar might be toxic.

Photo by Joseph DeSisto.

The lappet caterpillar has only to flex its body to reveal these striking bands. Photo by Joseph DeSisto.

Let’s say this doesn’t work, and the bird isn’t intimidated. If a bird’s beak (or my tweezers) pinches the caterpillar, it bends its head back over its body, revealing legs surrounded by pitch-black spots:

Photo by Joseph DeSisto.

This caterpillar doesn’t like me very much. Photo by Joseph DeSisto.

Still not startled? The lappet caterpillar has one last show. In desperation, it falls from its perch in the trees and, on hitting the ground, flops upside-down, revealing a bright yellow-and-black belly:

Photo by Joseph DeSisto.

Yellow and black are universal warning signs — many toxic animals share these colors. Photo by Joseph DeSisto.

If this doesn’t work, the caterpillar is pretty much toast. For all its show, the lappet caterpillar isn’t poisonous – at worst, some of its hairs are mildly irritating to the skin. Like the red-eyed tree frog, it relies entirely on visual illusions to ward off predators. It sounds risky, but it’s worked at least some of the time for millions of years.

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