Mimicry and Eyespot Patterns

Eyes that aren't eyes

Open the wings of many butterflies and moths and, instead of camouflage, you find the opposite: pairs of bold rings, usually a dark centre surrounded by lighter bands, placed well away from the insect's real head. The peacock butterfly is a famous example. Some caterpillars carry markings that make the front of their body look like the head of a small snake, and the four-eye butterflyfish has a dark spot near its tail while a dark stripe hides its real eye.

These are eyespots. They don't make an animal disappear. They manipulate where a predator looks and how it reacts.

Why eye shapes grab attention

Many animals are quick to notice eyes. For a predator, finding another animal's eyes tells you where its head is; for prey, eyes facing you may mean you've been spotted by something dangerous. A visual system tuned to find real eyes fast will also respond to anything that shares their basic geometry, such as concentric rings of contrasting colour, even when there's no animal behind them.

Eyespots seem to exploit that. Their effect on predators is well established. In one experiment, blue tits were given peacock butterflies, some with their eyespots painted over. Only 1 of 34 butterflies with intact eyespots was killed, compared with 13 of 20 without them. The butterflies' hissing sound, by contrast, made no difference (Vallin et al., 2005).

Eye-like, or just eye-catching?

Why eyespots work is still debated. Martin Stevens and colleagues gave wild birds artificial prey with different markings and found that large, numerous markings deterred them best, whatever their shape: circles did no better than bars, and making the arrangement look more like a pair of eyes made no difference. They concluded that eyespots work by being conspicuous, not by mimicking eyes (Stevens, Hardman & Stubbins, 2008). Sebastiano De Bona and colleagues, testing birds with real butterfly wing patterns, reached the opposite conclusion: birds reacted to butterfly eyespots much as they did to real owl eyes, which suggests that resembling a predator's eyes is what counts (De Bona et al., 2015). Both effects may play a part, depending on the predator and the pattern.

Two strategies, one bias

Eyespots are used in at least two ways.

Startling. Some moths and butterflies keep large eyespots hidden while resting and flash them when disturbed. The sudden appearance of bold, eye-like rings can make a small bird hesitate or retreat, giving the insect a moment to escape. That's the defence the peacock butterfly experiment tested.

Misdirection. Smaller eyespots near a wingtip or tail may draw attacks away from the real head, towards a part the animal can lose and survive, such as a scrap of wing or a bit of fin. Butterflies are often found with beak-shaped bites taken out of the wing near an eyespot, and similar tail spots have evolved separately in several unrelated fish, which suggests the same solution has been found more than once.

Who's being fooled

As with camouflage, it's the predator's perception that's being exploited, not the eyespot owner's. The predator's fast, largely automatic response to eyes and eye-like patterns is doing exactly what it evolved to do; it just can't tell a convincing stand-in from the real thing quickly enough to matter. Camouflage hides prey from a predator's visual system; eyespots hijack it.