Followers of nature lore find appearances fascinating, and even professional biologists can't resist speculating on the possible functions of visually arresting features of animals, especially insects. These musings can be fruitful but are often fanciful, and hard to demonstrate or verify. But imagining is always fun, and for children and novices, provides good exercise in scientific thinking. More experienced biologists sometimes come up with productive ideas for research and experimentation.
It's fair to assume that a nutritious morsel like a caterpillar would have defenses against predators, and that its appearance and behavior might reveal many of these. Take our bold caterpillar, that of the white-marked tussock moth (Orgyia leucostigma). At first glace the thing makes no sense, a seeming hodgepodge collage of clashing visual and structural elements. But might this very "whatsit"-ness throw predators off their game?
Look at the caterpillar from different angles. What comes to mind? Head on it looks like a jumping spider seen face to face, the head plate with red dots for eyes, and the long, black hair bundles resembling the front legs.
The four white tufts could be wooly aphids or scale insects. In this visual conception the caterpillar is split into two objects, spider and aphid colony, with the main body of the caterpillar mere background, perhaps a twig.
The effectiveness of this impression would depend on the predator. Something that eats spiders or aphids would not be repelled or disinterested, and the caterpillar's "defense" might backfire. But once attacked, the caterpillar's tactile attributes would kick in. It would not feel like the spider or wooly aphid that the predator is used to, nor would the taste be right. Still, the caterpillar might be injured, even fatally.
Speculation along such lines leads inevitably to hypotheses that, as Arsenio Hall put it, "make you say 'hmmmm'." Doubt ensues, and may put a damper on the whole mental enterprise. But try a more general approach. Here we have this caterpillar, but what about other caterpillars? Many, like our white-marked tussock, have bumps, bristles, hairs or spines. These project outward from the soft body of the caterpillar, forming a mantle of defenses. I use the plural because these projections are clearly multi-purpose. All these accessories are stiff and unsavory, and many are actually irritating.
Contact may lead a predator to conclude there's nothing edible about the thing.
Bristles and spines may avert contact by looking like burs or other inedible plant parts. Hairs could make a caterpillar look like a bit of feather or fur torn off a passing bird or mammal, or a catkin flower fallen from a poplar or willow tree.
Many caterpillars lacking such deceptive or defensive decor, and are green like leaves, or brown or gray like bark or twigs. Most caterpillars feed on plants, so these colors make perfect sense as camouflage. More refined camouflage resembles bark or mimics damaged or blemished leaves. The shapes of some caterpillars are twiglike or leaflike. Some caterpillars combine cryptic colors with hairs or spines. This gamut of appearances protects caterpillars from many dangers.
I can't leave the subject without mentioning lighting. Spines and hairs illuminated by the sun can be glaringly bright, shimmer in the wind, or produce sharp rays or starbursts exactly like those produced by pinholes in leaves. Large caterpillars are "countershaded." The typically sunlit bottom of the caterpillar (caterpillars cling upside down to twigs) is darker than the shaded top, so that its plumpness is flattened to resemble the leaves around it. Long hairs that lie flat along the shanks of a caterpillar can also produce a visual flattening effect.
Except for a few books published in the early decades of the last century, field guides to butterflies and moths neglected caterpillars, providing one or two pages of illustrations of them compared to 20 or more pages illustrating adults. This situation was finally remedied in 2005 with the publication of "Caterpillars of Eastern North America" by David L. Wagner, a professor at the University of Connecticut. This excellent book provides accounts of nearly 700 species of lepidoptera, with photos of all their caterpillars, and most of the adults. Princeton University Press is the publisher. The writing is intelligent, stylish and entertaining. If you like caterpillars, or think you might if you knew more about them, this is the book.
And now is the time. As summer wanes caterpillars reach full growth and begin looking for places to overwinter. Many leave the trees, shrubs and smaller plants they grew on, and wander far and wide, bringing them to places where they stand out in contrast to their backgrounds. This makes late summer a great time to see caterpillars, and to marvel at their astonishing variety of form, color and pattern.