Butterfly wings create dazzling optical illusions

Rabbits and doves are the animals most associated with human magicians, but butterflies may be the real magicians of the animal kingdom. The insects harness the power of optical illusions to avoid predators, according to a study published today in the journal Nature. Their flight and wing pattern work together to create optical illusions that scramble a bird or other predator’s sense of speed and direction. This confusion makes the predators misdirect their attacks.

According to the team from the University of Essex in England, this helps solve a long-standing butterfly puzzle. Intricate wings make them stand out, yet very few predators specialize in catching butterflies mid-flight. By comparison, a wide range of predators, including mice, rats, bats, and birds, hunt moths. 

“The stripes and spots on many butterflies’ wings interfere with the way visual systems try to guess the direction and speed of moving things, boosting false motion cues while hiding the butterfly’s true heading,” George Hancock, a study co-author and University of Exeter visual ecologist, said in a statement. “The illusions interfere with the predator’s most basic visual targeting system and disrupt the final ‘ballistic attack’ in the tens of milliseconds when it commits to grabbing its prey, with no time to change course. As a result, birds and other predators will often simply miss.”

What do butterflies and barber poles have in common?

The optical illusion inside butterfly wings is similar to the barber-pole illusion. When looking at the red-and-white striped poles outside of barber shops, it appears that the stripes travel upwards. However, the pole is only spinning around 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,” added study co-author and University of Exeter visual ecologist Jolyon Troscianko. “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 Barber Pole Effect

Troscianko used a slow-motion video of a butterfly that was put through a bird-vision computer model. He then noticed that the butterfly was glowing downwards, even though the butterfly itself 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,” Troscianko explained.

Catching digital butterflies

To investigate further, the team used a high-speed camera to film real butterflies taking off at over 1,000 frames per second. The high frame rate helped them capture how their wing patterns behave in flight, and mimic what a bird may see.

Next, they studied the patterns of roughly 400 European butterfly species and found that motion illusion patterns are widespread. To further test how patterns affect real attacks, 100 volunteers acted as “predators,” by trying to catch virtual butterflies on a touchscreen.

Lastly, they used a computer simulation to “evolve” more than 50,000 wing patterns from random starting points, selecting for the patterns that create the most confusion when the virtual butterfly is in motion.

European butterflies

They saw that the patterns converged on designs strikingly similar to those found in nature. This suggests that optical 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,” Hancock said. “But our study reveals the wide variety of different patterns butterflies can use to create illusions that confuse predators.”

Both butterflies and moths are also well known for using camouflage when they are resting, but movement breaks camouflage. This study shows how highly visible butterflies evolved their colors for a completely different line of airborne defense to solve that problem.

Understanding nature’s many patterns also has implications outside of the colorful world of butterflies. 

“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,” Troscianko said. “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.”