Mystery Solved: Why Flies Are So Hard to Swat

Why are buzzing flies so hard to swat?

It’s a question that’s vexed humankind for millennia, but scientists may finally have the answer.

Flies on the wing react to looming threats as if they were fighter jets—by banking away in a fraction of the blink of an eye, according to a study published April 10 in the journal Science.

Too fast for the swat, the airborne insects harness their aerodynamic force within a wingbeat or two to almost instantaneously change their course, the study of fruit flies found. (See “Video: Inside Look at Insect Flight.”)

Indeed, this happens so quickly—in less than one hundredth of a second—that scientists required three high-speed cameras, each able to take 7,500 frames per second, to capture it.

Not only that, but the escape maneuver—which sees fruit flies (Drosophila hydei) fling themselves into rotations of 90 degrees or more, flying almost upside down at times —is very controlled and specific.

A fruit fly flaps its wings 200 times a second during normal flight and even faster when taking evasive action. Photograph by F. Muijres and F. van Breugel, University of Washington.
A fruit fly flaps its wings 200 times a second during normal flight and even more when taking evasive action.
Photograph by F. Muijres and F. van Breugel, University of Washington

That’s according to study co-author Florian Muijres, a researcher at the University of Washington’s Dickinson Lab, Seattle, where the study took place.

Small differences in the beat of the wings generate the force needed for the fly to suddenly pitch and roll. The differences have to be very precise; otherwise, Muijres said, “the animal would constantly spin out of control.

“Based on the direction of the looming threat—whether from the back, the front, or the side—the flies perform a different type of escape maneuver that is very controlled,” he added.

Speedy Flies

That contrasts with the escape tactic of a resting fly, a topic previously investigated at the Dickinson Lab.

From a resting position, the fly performs a wild leap and stumbles through the air using the legs. “Only on the second phase will they gain control and start flapping their wings,” Muijres said.

The new study, which employed winged robots dunked in vats of mineral oil to better understand the flies’ lightning-fast movements, challenges recent studies that instead point to flies turning using a yawing maneuver.

Yawing is what the driver of a car feels when turning the vehicle, which is rather different from the face-flattening motion the pilot of a banking fighter jet experiences. (See videos: “World’s Weirdest: Flies and Maggots.”)

Yawing, or pirouetting around a vertical axis, is a sensible way for hovering insects to change direction, according to Graham Taylor, a professor of mathematical biology at Oxford University in the U.K.

For changing direction while flying, however, insects need to redirect their aerodynamic force to create the rotation needed to curve their flight path, explained Taylor, who wasn’t involved in the new study.

Time lapse images from a high-speed video shows how a fruit fly startled by a looming shadow (off camera at the bottom right) performs a rapid roll to bank away from the threat. Photograph by F. Muijres, University of Washington
Time-lapse images from a high-speed video show how a fruit fly startled by a looming shadow (off camera at bottom right) performs a rapid roll to bank away from the threat. Photograph by F. Muijres, University of Washington

“Airplanes, helicopters, and other insects bank to direct their aerodynamic force vector into a turn, so the fact that fruit flies do so too isn’t really a surprise,” he said.

What is a surprise, Taylor said, is the remarkable rapidity of the escape response, “and the subtlety of changes the flies make to their wingbeat when turning.

“The flies are responding to an approaching threat in half the time it takes to start blinking in response to a camera flash,” he said. “And the time it then takes to accomplish the turn after that is even faster again, so they are throttling up to full power in a 50th of the time it takes you to complete that blink.”

That flies can react so rapidly suggests they’re wired for it—with a neuron dedicated to such in-flight emergencies running down the body to the wings and muscles.

Robot Inspiration 

Understanding the workings of this control mechanism in an animal with a brain as tiny and limited as a fly’s could help in the development of mini airborne robots that can find their own way around obstacles.

Likewise, Taylor said, changes in the fruit flies’ wingbeat, which the study team measured using the winged robots, could provide useful insights into flight control for designers working on flapping micro-vehicles. (See “The Dream of Human-Powered Flight Takes Off.”)

Indeed, study co-author Muijres noted that Delft University of Technology in the Netherlands, to which team member Johan Melis is attached, is involved in such a project, a dragonfly-like autonomous flapper known as the DelFly.

But what about trying to swat the real thing? Should we just give up? It’s difficult, Muijres said, but the study did offer one tip.

When you come at it from the side, the insect flies straight away from you, “so if you keep your hand going in the same direction, you can catch the fly.

“Try it,” he said, “and let me know.”

Changing Planet

James Owen is a journalist and author based in Stockholm, Sweden. After cutting his teeth on the news and features desks of several UK newspapers, he struck out as a freelance writer, specializing in life sciences and natural history. His fish biography 'Trout' (Reaktion Books) was published in 2012.