Is it a bird? Is it a plane? No it’s a fruit fly.

Scientists have discovered the manoeuvres the common fruit fly (Drosophila hydei) utilizes when it needs to escape a predator.

When startled by predators, tiny fruit flies respond like fighter jets; they turn by rolling their bodies sideways. This allows them to turn five times faster than they normally do. This was one of the results of a study undertaken at the University of Washington, which was published in Science on 11 April. Among the American scientists, the list of authors also includes the name of Johan Melis, a student from Delft. This research was his final project and was awarded a score of 9.

The researchers used an array of high-speed video cameras operating at 7,500 frames a second to capture the wing and body motion of flies after they encountered a looming image of an approaching predator. Their findings are of interest to the military. The work was funded amongst others by the US Air Force Office of Scientific Research and the US Army Research Office.

“Although they have been described as swimming through the air, tiny flies actually roll their bodies just like aircraft in a banked turn to maneuver away from impending threats”, said co-author of the paper, Michael Dickinson of the University of Washington in a press release. “We discovered that fruit flies alter course in less than one one-hundredth of a second, fifty times faster than we blink our eyes.”

Florian Muijres is the lead author of the paper. He supervised Johan Melis’ graduation work, and once studied at the Faculty Aerospace Engineering in Delft himself. “In the midst of a banked turn, the flies can roll on their sides 90 degrees or more, almost flying upside down at times. These flies normally flap their wings 200 times a second and, in almost a single wing beat, the animal can reorient its body to generate a force away from the threatening stimulus and then continues to accelerate.”

Johan Melis’ contribution involved translating the camera recordings into the exact behaviour of the flies. He used video of the fleeing flies to construct a 3D computer model of their body orientation and wing flaps.

The fruit flies, about the size of a sesame seed, rely on a fast visual system to detect approaching predators. “The brain of the fly performs a very sophisticated calculation, in a very short amount of time, to determine where the danger lies and exactly how to bank for the best escape, doing something different if the threat is to the side, straight ahead or behind,” Dickinson said.

“How can such a small brain generate so many remarkable behaviors? A fly with a brain the size of a salt grain has the behavioral repertoire nearly as complex as a much larger animal such as a mouse. That’s a super interesting problem from an engineering perspective.”

The researchers synchronized three high-speed cameras each able to capture 7,500 frames per second, or 40 frames per wing beat. The cameras were focused on a small region in the middle of a cylindrical flight arena where 40 to 50 fruit flies flitted about. When a fly passed through the intersection of two laser beams at the exact center of the arena, it triggered an expanding shadow that caused the fly to take evasive action to avoid a collision or being eaten.

With the camera shutters opening and closing every one thirty-thousandth of a second, the researchers needed to flood the space with very bright light. Because flies rely on their vision and would be blinded by regular light, the arena was ringed with very bright infrared lights to overcome the problem. Neither humans nor fruit flies register infrared light.

How the fly’s brain and muscles control these fast and accurate evasive maneuvers is the next thing researchers would like to investigate.

Florian T. Muijres, et. al., Flies evade looming targets by executing rapid visually directed banked turns, Science 11 April 2014. DOI: 10.1126/science.1248955

Watch the fly’s body roll as it executes a banked turn after seeing a looming shadow (off camera). The video has been slowed down 300 times. The maneuver is also depicted in two animated sequences; the final segment can be viewed in 3-D with red/cyan glasses.

Credit: F. Muijres, University of Washington

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