The Rice-Size Fly That Hits the Bull’s-Eye Every Time

http://www.nytimes.com/2017/03/10/science/robber-flies-eyes.html

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Despite being just the size of a rice grain, robber flies, which live all over the world, are champion predators. In field experiments, they can detect targets the size of sand grains from nearly two feet away — 100 times the fly’s body length — and intercept them in under half a second. What’s more, they never miss their mark.

A team led by scientists at the University of Cambridge has started to unveil the secrets to the robber fly’s prowess. In a study published Thursday in Current Biology, the team outlined the mechanics of the fly’s pursuit, from its impressive eye anatomy to how it makes a successful catch every time.

Notably, the researchers observed a behavior never before described in a flying animal: About 30 centimeters from its prey, the insect slows, turns slightly and brings itself in for a close catch.

“This ‘lock-on’ phase and change in behavior during a flight is quite remarkable,” said Sam Fabian, a graduate student at Cambridge and an author of the study. “We would actually expect them to do something very simple — just accelerate and hit the target.”

The scientists surveyed robber flies in the field using a “fly teaser,” which consisted of beads on a rapidly moving fishing line controlled by a motor. As the flies charged at the bait, the researchers captured their movements using high-speed cameras.

At the start of the robber fly’s conquest, it sits on a perch and scans the sky for passing prey. When it glimpses a potential meal, it takes flight, maintaining a steady angle between itself and its target.

This proactive strategy, using a “constant bearing angle,” is also employed by fish, bats and sailors, Mr. Fabian said.

Maintaining a constant bearing angle gets the robber fly in the ballpark of its prey. Then, at the 30-centimeter mark (how the fly judges this distance is still unclear), it subtly changes course so as to approach its target from behind.

“We think this second proactive phase allows the animal a higher probability of catching its prey,” said Paloma Gonzalez-Bellido, a lecturer at Cambridge and another author of the study. In the same way that it’s easier to pass a baton by running to someone from behind versus head-on, looping around makes it easier for the fly to align itself with its kill.

Next, the scientists wanted to understand the robber fly’s startling visual capabilities. Whereas we have a single lens in each eye, the robber fly has several thousand lenses per eye, which range in size. In the center of each eye, the researchers found, is a concentration of large, forward-pointing lenses.

This central area, called the fovea, is extremely high resolution and is what allows the flies to see their prey from a distance equivalent to more than one and a half soccer fields for humans.

“They basically have permanent binoculars,” said Trevor Wardill, a research fellow at Cambridge and an author of the paper.

Rob de Ruyter, a professor of biophysics at Indiana University, who was not involved in the study, said, “The extent to which this fovea is specialized is unusual.”

Other flies, like the house fly, have localized areas of high resolution in their eyes called love spots, but these are not nearly as exaggerated as the robber fly’s fovea. Dragonflies also have impressive foveae, but their bodies are 10 times larger and can more easily accommodate many large lenses in their eyes.

These adaptations that make the tiny robber fly a well-oiled hunting machine are an example of how insects must develop highly specialized strategies to survive in this world, Dr. de Ruyter said.

“Insects die at such high rates, and have such short generation times,” he said. “As a result, they try out new evolutionary solutions much faster than we can.”