Fruit flies, fighter jets use similar nimble tactics when under attack

When startled by predators, tiny fruit flies respond like fighter jets – employing screaming-fast banked turns to evade attacks.

Researchers at the University of Washington 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.

“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 Michael Dickinson, UW professor of biology and co-author of a paper on the findings in the April 11 issue of Science. “We discovered that fruit flies alter course in less than one one-hundredth of a second, 50 times faster than we blink our eyes, and which is faster than we ever imagined.”

In the midst of a banked turn, the flies can roll on their sides 90 degrees or more, almost flying upside down at times, said Florian Muijres, a UW postdoctoral researcher and lead author of the paper.

“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,” he said.

The fruit flies, a species called Drosophila hydei that are 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,” Dickinson said.

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, Muijres said. 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 remarkably fast and accurate evasive maneuvers is the next thing researchers would like to investigate, Dickinson said.

Neural Activity in Bats Measured In Flight

Animals navigate and orient themselves to survive – to find food and shelter or avoid predators, for example. Research conducted by Dr. Nachum Ulanovsky and research student Michael Yartsev of the Weizmann Institute’s Neurobiology Department, published today in Science, reveals for the first time how three-dimensional, volumetric, space is perceived in mammalian brains. The research was conducted using a unique, miniaturized neural-telemetry system developed especially for this task, which enabled the measurement of single brain cells during flight.

The question of how animals orient themselves in space has been extensively studied, but until now experiments were only conducted in two-dimensional settings. These have found, for instance, that orientation relies on “place cells” – neurons located in the hippocampus, a part of the brain involved in memory, especially spatial memory. Each place cell is responsible for a spatial area, and it sends an electrical signal when the animal is located in that area. Together, the place cells produce full representations of whole spatial environments. Unlike the laboratory experiments, however, the navigation of many animals in the real world, including humans, is carried out in three dimensions. But attempts to expand the scope of experiments from two to three dimensions had encountered difficulties.

One of the more famous efforts in this area was conducted by the University of Arizona and NASA, in which they launched rats into space (aboard a space shuttle). However, although the rats moved around in zero gravity, they ran along a set of straight, one-dimensional lines. Other experiments with three-dimensional projections onto two-dimensional surfaces did not manage to produce volumetric data, either. The conclusion was that in order to understand movement in three-dimensional, volumetric space, it is necessary to allow animals to move through all three dimensions – that is, to research animals in flight.

Ulanovsky chose to study the Egyptian fruit bat, a very common bat species in Israel. Because these are relatively large, the researchers were able to attach the wireless measuring system in a manner that did not restrict the bats’ movements. Developing this sophisticated measuring system was a several-year effort. Ulanovsky, in cooperation with a US commercial company, created a wireless, lightweight (12 g, about 7% of the weight of the bat) device containing electrodes that measure the activity of individual neurons in the bat’s brain.

The next challenge the scientists faced was adapting the behavior of their bats to the needs of the experiment. Bats naturally fly toward their destination – for example, a fruit tree – in a straight line. In other words, their normal flight patterns are one-dimensional, while the experiment required their flights to fill a three-dimensional space.

The solution was to be found in a previous study in Ulanovsky’s group, which tracked wild fruit bats using miniature GPS devices. One of the discoveries was that when bats arrive at a fruit tree, they fly around it, utilizing the full volume of space surrounding the tree. To simulate this behavior in the laboratory – an artificial cave equipped with an array of bat-monitoring devices – the team installed an artificial “tree” made of metal bars and cups filled with fruit.

Measuring the activity of hippocampus neurons in the bats’ brains revealed that the representation of three-dimensional space is similar to that in two dimensions: Each place cell is responsible for identifying a particular spatial area in the “cave” and sends an electrical signal when the bat is located in that area. Together, the population of place cells provides full coverage of the cave – left and right, up and down.

A closer examination of the areas for which individual place cells are responsible provided an answer to a highly-debated question: Does the brain perceive the three dimensions of space as “equal,” that is, does it sense the height axis in the same way as that of length or width? The findings suggest that each place cell responds to a spherical volume of space, i.e., the perception of all three dimensions is uniform. The researchers note that for those non-flying animals that essentially move in flat space, the different axes might not be perceived at the same resolution. It may be that such animals are naturally more sensitive to changes along the length and width axes than that of height. This question is of particular interest when it comes to humans because on the one hand, humans evolved from apes that moved in three-dimensional space when swinging from branch to branch, but on the other hand, modern, ground-dwelling humans generally navigate in two-dimensional space.

The findings provide new insights into some basic functions of the brain: navigation, spatial memory and spatial perception. To a large extent, this is due to the development of innovative technology that allowed the first glimpse into the brain of a flying animal. Ulanovsky believes that this trend, in which research is becoming more “natural,” is the future wave of neuroscience.