Butterflies are known for the striking and often colourful patterns on their wings – but they’re not just there to look pretty. According to new research, a butterfly’s flight and wing pattern work together to create powerful visual illusions that help them evade predators.
The study was conducted by scientists from the University of Exeter and the University of Essex, England, with the findings published in the journal Nature.
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The butterfly family is extremely diverse, from species sporting transparent wings to ones that look deceivingly like a dried leaf. Scientists have known for a long time that this unique insect makes use of camouflage, mimicry and other tricks to deter predators.
But camouflage is often lost in flight. This new study provides the first evidence that butterflies are also masters of visual misdirection – using their flight and wing pattern, they are able to scramble a predator’s sense of speed and direction.
The discovery helps answer the question of why butterflies, despite sporting colours that make them stand out, which should attract predators, so often escape with their life intact, with the attacker snapping at thin air. In fact, very few predators specialise in catching butterflies mid-flight, while moths are hunted by many more visually guided aerial predators.
“The stripes and spots on many butterflies’ wings interfere with the way visual systems try to guess direction and speed of moving things, boosting false motion cues while hiding the butterfly’s true heading,” said Dr George Hancock from the University of Exeter’s Centre for Ecology and Conservation.
“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.”
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Studying the wings
The mechanism of this evasion tactic works much like the 'barber-pole illusion' – the red-and-white striped pole common outside barber shops, with stripes that appear to travel upward even though the pole is 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 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.”
To prove this theory, the researchers combined multiple lines of evidence. First, they used a high-speed camera to film butterflies taking off at over 1,000 frames per second, capturing how their patterns behave in flight as seen through the eyes of a bird.
Then, they looked at the patterns of around 400 European butterflies to confirm that motion illusion patterns are widespread across different species. To test how patterns affect real attacks, 100 volunteers acted as 'predators' and tried to catch virtual butterflies on a touchscreen.
“It might seem strange that a butterfly would evolve to be so visible, especially as very few European species are toxic or unpalatable,” said Dr Hancock. “But our study reveals the wide variety of different patterns butterflies can use to create illusions that confuse predators.”
Butterflies are not the only animals with eye-catching patterns – from stripes on zebras and snakes, and the flapping wings of birds, to the flicking tails of fish, the scientists think motion confusion might be far more widespread than previously thought.
Read the full findings here.
Top image: European Swallowtail butterfly. Credit: Helen Briolat
