Butterflies aren’t the strongest fliers. Their large wings cause them to flutter about, rather than generating more direct flight paths. But that fluttery flight makes it hard for predators to catch them. And if you have ever tried to catch a resting butterfly, you know they are surprisingly difficult to nab.

A new study helps explain why butterflies clap their wings above their bodies during take-off. The move creates jet propulsion for a quick getaway.

Phys Org ran an interesting article on the flight of butterflies to the title ‘Butterflies create jet propulsion with a clap of their wings’:

“The whimsical, wafting flight of butterflies may not give the impression of top aerodynamic performance, but research published on Wednesday suggests their large flexible wings could be perfectly designed to give them a burst of jet propulsion.

Scientists at Lund University in Sweden set out to verify a decades-old theory that insects “clap” their wings together, squeezing out the air between with such force that it thrusts them forward. In their aerodynamic analysis of free-flying butterflies published in the journal, Interface, they showed that the clap function does generate a jet of air propulsion.

But they also found that the butterflies perform this move “in a far more advanced way than we ever realised,” said co-author Per Henningsson, a professor in the department of biology at Lund University. At the time the wings beat together they “were not just two flat surfaces slamming together,” he told AFP.

Instead, they form a “pocket” shape, believed to trap more air.

When the researchers recreated this using mechanical wings, they found those with butterfly-like flexibility that form this pocket at the time of impact were 22 percent more effective in the amount of force created and 28 percent more efficient in the amount of energy used, compared with rigid wings.

The team suggested that their findings could have uses for drones that use clapping wing propulsion.

Predator evasion

Henningsson said the “dramatic improvement” in performance came as a surprise.

“This is the type of finding that is the most exciting for a scientist—the ones you didn’t really expect,” he said.

“To minimise the risk of capture, butterflies typically take off very fast and suddenly and many of them fly in an erratic and unpredictable manner,” he said.

“If indeed the clap is improved dramatically by the cupped shape of the wings, this would allow a butterfly to take off faster and avoid being captured better, and hence you can imagine a strong selective pressure on this feature.”

Henningsson said, while the theory of the wing clap has been around since the 1970s, studies on butterfly flight had often relied on tethered butterflies or used simulations.

But improvements in technology to measure flow meant the authors were able to observe the creatures in natural flight.”

Wind tunnels

How did they measure this clapping? Johansson and Henningsson caught six butterflies in a meadow near their lab. Each of these silver-washed fritillaries was housed in a net enclosure and fed honey water. To analyse their flight, the scientists placed the butterflies one at a time inside a wind tunnel.

Wind tunnels use fans to move air at specific speeds. When an object is placed in the tunnel, the air needs to flow around it. This lets researchers test how objects move through air. “Smoke” made of tiny harmless droplets of oil is often added so researchers can observe precisely how air flows around something. This shows how aerodynamic the object is.

For this study, fans moved the air just enough to keep the smoke evenly distributed in the tunnel. The researchers also used a laser to light up a layer of smoke in the tunnel just behind the butterfly. Four high-speed cameras placed around the feeding station captured the movement of the butterfly and the smoke as the butterfly was taking off. This allowed the researchers to create a 3D picture of that air movement as the insect flapped its wings. For each trial, the researchers placed a butterfly at a feeding station in the middle of the tunnel. They began recording when the butterfly took flight on its own.

They analysed a total of 25 take-offs by the six butterflies. Each included up to three wingbeats after take-off. The butterflies proved more likely to clap their wings together during the first few wingbeats than later in flight.

A vortex occurs when air or water spins around a central point, like a whirlpool. And the images of the butterfly paths showed air swirling in a vortex as their wings moved. The photos also revealed that the wings’ downstroke pushed air down. This created a force that pushed the butterfly up. When the wings moved up to clap, they made an air pocket. This pocket created a strong jet of air that shot out between the wings behind the butterfly. That jet propelled the insect forward.

Both forces created by the wings give rise to a fluttery flight path. The butterflies rise as their wings move down and shoot forward as their wings move up. A wing clap on take-off, paired with a quick turn, allowed the butterflies to flit away quickly.

Flexibility is the key

Henningsson and Johansson noticed the wings formed the puffed-out pocket just before clapping. They wondered if the wings’ flexibility and this pocket improved the jet propulsion created by the clap. To find out, they made two simple models of butterfly wings. One set was made from balsa wood and was rigid. The other was made from a sheet of latex, a flexible plastic. They attached each set to hinges and rods that moved to mimic butterfly flight.

The pair then studied the air flow created by the two models as they were clapped in the same manner. The rigid wings did not form an air pocket. They instead created two vortex rings. This would make flight “less efficient,” Henningsson explains, “since more energy is wasted forming two rings.” The flexible wing model, however, created a stronger jet of air and just one vortex ring. It behaved more like true butterfly wings.

Unfortunately, the researchers attributed this amazing flight scenario to evolution. But when you examine the intricate equations the scientists had to use to measure the forces and motions involved in their flight it becomes apparent that a Master Designer is behind the whole arrangement.