A bird attacking a butterfly will often find its beak snapping at thin air.
The underlying mechanism works much like the 'barber-pole illusion' – the red-and-white striped pole outside barber shops, where the stripes appear to travel upward even though the pole is only spinning in place.
"Butterfly wings deform as they fly – clapping together on the upstroke and peeling apart on the downstroke – so their stripes shift angle and point in different directions as they flap," said Dr Jolyon Troscianko, also from the University of Exeter.
"Combined with their unpredictable flight paths, this patterning creates misleading motion illusions just like the barber-pole, causing a predator to miss entirely, or to strike a less vulnerable part of the wing, such as the hindwing or tail.
“The first slow-motion video of a butterfly I put through our bird-vision computer model was glowing with downwards motion even though the butterfly was moving up.
“First I checked this wasn’t a coding error, that I’d swapped up and down somehow, but then it dawned on me that this would be a perfect way to confuse attacking predators.”
The team combined multiple lines of evidence. They used a high-speed camera to film real butterflies taking off at over 1,000 frames per second to capture how their patterns behave in flight, as seen through the eyes of a bird.
They then looked at the patterns of around 400 European butterflies and found that motion illusion patterns are widespread.
To test how patterns affected real attacks, 100 volunteers acted as “predators”, trying to catch virtual butterflies on a touchscreen.
Finally, they used a computer simulation to “evolve” over 50,000 wing patterns from random starting points, selecting for patterns that created the most motion confusion.
Patterns converged on designs strikingly similar to those found in nature, suggesting that this illusion is a major force shaping butterfly wing evolution.
"It might seem strange that a butterfly would evolve to be so visible, especially as very few European species are toxic or unpalatable," Dr Hancock said.
"But our study reveals the wide variety of different patterns butterflies can use to create illusions that confuse predators."
Butterflies and moths are also well known for using camouflage while resting, but movement breaks camouflage.
This study shows how, once airborne, highly visible butterflies evolved their colours for a completely different line of defence to solve that problem.
"The dazzling stripes on zebras and snakes have long been suspected of confusing predators' motion perception, but firm evidence has been hard to come by," said Dr Troscianko.
"Our study gives us a genuinely new way to analyse motion vision, and we think motion confusion is likely to be far more widespread in nature than previously realised, from the flapping wings of birds to the flicking tails of lizards and fish. It opens up an exciting new research avenue."
The study was funded by the Biotechnology and Biological Sciences Research Council.