It only has to happen once, and you do not realize your potential 10-20 year life span. Natural selection has hard-wired you to instantly flee when you see that eye. Perhaps you move only a meter or two. However, in the shadowy and confusing world of a tiny bird among thousands of rustling leaves and branches, this movement takes you away from the caterpillar or pupa. The last thing you are going to do is go back into that mass of leaves to high-risk explore to see if you were fooled into leaving your lunch behind.
Visual mechanics are on the side of the caterpillar or pupa.
This way it is almost impossible to determine if an eye is a 1 mm diameter harmless eye at a distance of 3 cm, or a 3 mm diameter dangerous eye at a lethal distance of 9 cm. Further, with one visual field it will be hard to see the 3-D traits that could help distinguish a predator's face from an eye spot on a caterpillar. The color and pattern antecedents for false eyes lie in the camouflage and warning colors found throughout insects, caterpillars and pupae included, and in serendipituously eye-shaped structures such as spiracles. Startle mimicy functions not through learning by the predator, but rather through expression of genetic programming somewhat analogous to the genetic programming that leads a bird to avoid a bright-colored ringed coral snake and its models. Viewing the false eyes on caterpillars and pupae as startle mimicry suggests the possibility that the eye spots on many adult butterfly and moth wings may well also function in the same manner.