Of course, over millions of years, evolution has “discovered” and taken advantage of the principles of shading that researchers have explored only lately. Gazelles have white bellies and dark backs—countershading—that neutralize the effect of sunshine from above. The result reduces pop-out so that gazelles are not as conspicuous; they also appear skinnier and less appetizing to a predator. Caterpillars have countershading, too, so they more closely resemble the flat leaves on which they munch. One caterpillar species has “reverse” countershading—which did not make sense until scientists realized that the insect habitually hangs upside down from twigs. One type of octopus can even invert its countershading: if you suspend the octopus upside down, it uses pigment-producing cells called chromatophores in the skin, which are controlled by its vestibular input, to reverse its darker and lighter areas.
Charles Darwin noticed a striking example of nature’s use of shading in the prominent eyelike spots on the long tails of argus pheasants. With the tail feathers at horizontal rest, the orbs are tinged from left to right. During the birds’ courtship display, however, the tail feathers become erect. In this position, the spots are paler on top and duskier at bottom, so the disks seem to bulge out like shiny metallic spheres—the avian equivalent of jewelry.
That a few simple shaded circles can unveil the underlying assumptions of our visual systems—and even how such principles have played a role in shaping evolutionary adaptations—shows the power of visual illusions in helping us to understand the nature of perception.
Editor's Note: This story was originally published with the title "Seeing is Believing"