Monster-eyed bugs

Silkworm moth showing eye spots

Eyespots probably are eyespots after all.  Moths really do try to fool birds into thinking they are staring at an angry cat or owl using fake “eyes” on their wings, a new paper shows.

The researchers cleverly manipulated the position of the white “sparkle” in the moths’ fake eyes.  If the “sparkle” appeared in an unrealistic position, the moth was eaten.

Moths and caterpillars often display pairs of large dark circles.  Tradition has it that these represent eyes – they are even called “eye spots”.  But are these really meant to be eyes?  Research shows that the patches cause predators to “startle”.  However, there has been a recent blazing row minor academic debate over whether these really are images of eyes.

On the one hand, many other moths achieve a “startle” response by flashing bright patches of colour, and the spots may be nothing more than brightly coloured patches.  For example, one recent paper showed that birds avoid moth-like dummies with square or triangular patches just as much as they avoid dummies with circular patches, and that stripes are just as good protection as spots.  Thus, it may simply be their conspicuousness that protects the moth, not their resemblance to eyes.

Classic eye from anime, showing highlight.

But an ingenious new study out last week has demonstrated that most moths with “eyespots” in fact use a clever illusion that makes it very hard to believe these are not meant to be images of eyes.  Most “eyespots”, the authors found, have a “sparkle” – an off-centre white patch that looks like the highlight reflected by the surface of an eye.  All cartoon eyes have a “sparkle”, for example – a simple way to enhance their realism.

Selection of eyespots from moths

Graph showing number of moths eaten with sparkles in different places.

Not only did the authors discover that almost all eyespots carried by moths have this sparkle, but they demonstrated that the position of the sparkle has a clear effect upon the moths’ survival.

What makes this experiment doubly clever is that, by manipulating only the position of the “sparkle”, the researchers cleverly kept the spots at exactly the same brightness, but changed how realistically they resembled a real vertebrate eye.  The fact that this still affected how quickly the moth was eaten shows that the eyespots really are meant to resemble eyes.

All of us at some point have jumped sky-high at what we think is a tarantula crawling up our leg, which in fact turned out to be a piece of fluff.  What we experience here is a startle response, triggered by something resembling a large, hairy spider.  This kind of reaction to potentially dangerous animals makes a lot of evolutionary sense.  Until we realise it’s a piece of fluff, the only thing we want to do is get away from it.  If this were a piece of food – however delicious it might actually be – the last thing on our minds would be pecking at it.

This is what some moths achieve by carrying around the image of two large eyes on their wings.  For a moment, the bird believes it is staring an angry cat in the face, or an owl.  Superb!