Butterfly Wing Patterns May Create Illusions That Foil Predatory Birds, Nature Paper Says
A Nature paper finds butterfly wing patterns and flight paths create false motion cues that stop birds from judging speed and direction.
The proposed mechanism resembles an illusion familiar from old-fashioned striped barbershop poles. Those poles turn in place on a vertical axis, yet people typically perceive the stripes as moving upward rather than rotating with the pole, a well-known effect that arises from processing biases in human vision.
"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," said co-author George Hancock of the University of Exeter in Cornwall.
Hancock said the illusions interfere with a predator's most basic visual targeting system and disrupt the final "ballistic attack" in the tens of milliseconds when the predator commits to grabbing its prey, leaving no time to change course. Birds and other predators, he said, will as a result often simply miss.
Butterfly wings have drawn scientific attention since the growth of the wings was first documented in 1938. In 2021, researchers at MIT captured the structural growth of butterfly wings on video continuously, as a butterfly develops inside its chrysalis, for the first time.
The wings are also an example of structural color in nature: their bright iridescent colors come not from pigment molecules but from what physicists call photonic crystals. The scales of chitin, a polysaccharide common to insects, are arranged like roof tiles, forming what is essentially a diffraction grating, except that photonic crystals produce only certain colors, or wavelengths, of light, while a diffraction grating produces the entire spectrum.