Flies obey unique flight rules

Many insects determine their altitude by measuring how quickly the ground is passing beneath them, but Drosophila adopts a separate technique -- it avoids crashing by flying at the same height as a nearby horizontal edge, according to a new study in Current Biology.

The virtual reality flight arena that Straw and his colleagues used to film flies Image courtesy of Andrew Straw

Lead author Andrew Straw, who studies neuroscience and bioengineering at the California Institute of Technology, found that Drosophila uses horizontal edges created by bushes, tree branches, and the tops of rocks, for example, to fly at a steady altitude.

Understanding how insects fly may help scientists understand how flight evolved and how their brains coped with the challenge, Straw said. The results may also assist in the construction of flying robots, he added.

Straw and his team built a 3-D virtual reality arena to figure out what rules Drosophila used to control its flight altitude. He wrote a computer program that allowed a series of cameras to track the flies in real time, and then used that tracking data to instantaneously modify the flies' environment to see how they'd respond.

"It can't be overstated how challenging this problem was from a technological perspective," said Mark Frye, who studies the physiology of insect flight at the University of California, Los Angeles, but was not involved with the study. "Straw built a multiple camera system where all the cameras talk to each other and figure out where the fly is and how fast it's going. It took a lot of computer programming prowess."

Previous flight studies had shown that other insects determine their altitude by measuring how quickly the ground is passing beneath them. The faster the ground appears to be moving, the closer the insect is to the ground, much like how the ground appears to whiz faster by the window of a plane as it descends. If the ground starts moving too fast or too slow, the insect can adjust its altitude.

The researchers tested whether Drosophila follows this method by projecting black and white horizontal stripes onto the floor. The fruit fly was released and allowed to fly halfway down a tunnel. At the halfway point, Straw used his high-tech tracking system to match the fly's forward motion with the stripe pattern on the floor. To the fly, then, the ground appeared stationary, which should have signaled the fly to descend.

However, the flies did not descend. Their altitude remained constant no matter if Straw manipulated the ground to appear as if it traveled slower or faster in the second half of the tunnel. Straw knew they were using a different system to maintain their altitude.

They then tested the hypothesis that flies might be matching their altitude by projecting a horizontal shadow on the side of the tunnel. As the flies reached the midpoint of the tunnel, Straw raised the edge of the projected shadow by five or ten centimeters while leaving the stripes on the floor unchanged. The flies' altitudes increased a corresponding amount.

Straw believes that fruit flies control their altitude using the same methods used to determine direction while walking -- by orienting themselves using horizontal landmarks. "Using the molecular genetic toolbox in Drosophila," Straw said, "we can eliminate small groups of neurons and get to what neurons are responsible" for both altitude regulation and horizontal steering.

"This article also highlights the growing importance in life science of computational robotic models of biological mechanisms," Dario Floreano of the Swiss Federal Institute of Technology, who wrote a commentary that accompanied Straw's research, said in an email.

A. Straw et al., "Visual Control of Altitude in Flying Drosophila," Current Biology doi:10.1016/j.cub.2010.07.025, 2010.

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