Robber flies avoid obstacles while they hunt moving prey

Robber flies (Asilidae) are predatory insects that feed by hunting down flying prey and snatching them out of the air. In open environments, the flies maintain the line of sight to the potential prey, adjusting their position as the target twists and turns on its path. Such maneuvers are done with great speed and agility.

However, things become much more complex for these hunters when the environment is cluttered with obstacles. So how do they adjust their hunting patterns when there are distractions in the environment?

Researchers from Imperial College London set out to investigate how the predatory robber flies adjust their interception strategy to deal with obstacles in the environment. 

“We used Holcocephala fusca because of its predictable interception path,” said study co-author Samuel Fabian, who teamed up with Mary Sumner from the University of Minnesota for four weeks to film robber flies in 3D while they hunted a tiny bead that was pulled along a transparent fishing line. This species of robber fly is commonly known as a gnat ogre due to its prodigious hunting skill.

“Field experiments are a joy, because you get the most naturalistic behavior from free animals,” said Fabian. Even though the flies were chasing a small, artificial bead on an invisible line, they were still very keen to intercept it. “If something is small enough, they generally seem to assume it’s food, the flies really didn’t know it’s not real prey, even when very close.”

When he and Sumner analyzed the approach strategy of insects intercepting the bead, they found that the flies maintained the same line of sight to the target throughout their approach, in order to capture it successfully. However, when the researchers introduced a wide (5 cm) or narrow (2.5 cm) bar of black acrylic into the insects’ interception path, they took evasive action.

When the black bar blocked the interception path for more than 1 s, the insects often aborted the chase entirely. If their view was blocked for less than 1 s, they swerved away dramatically until they had passed the obstacle, then moved back to their interception course. Interestingly, when the flies could see the black bar but it did not obscure their view, they still veered away from it as they approached the bead, even though the bead remained in sight all the time.

The researchers wondered what strategy the robber flies were using to make these decisions as they approached the bead. The experts compared the insects’ flight paths and the paths they would have taken if there was no obstacle, and realized that the flies were using a very simple obstacle avoidance strategy; 

“The faster the obstacle is getting larger in their field of view the more they turn away from it,” explained Fabian. However, after the fly has passed the obstacle and it begins to grow smaller, the fly is then drawn back toward the bead, resulting in the veering flight paths that Fabian and Sumner recorded, even when the fly’s view wasn’t obstructed.

The researchers concluded that the flies, whose brains are no larger than a grain of sand, are combining two different strategies when hunting prey in a cluttered environment. They use a conventional approach of flying in a direct line to the target when their view is unobstructed, but apply avoidance tactics when an obstacle is present in their view. This results in a simple, hybrid approach that allows them to intercept their prey while avoiding distractions and obstacles in their paths. 

“They are paying attention to their surroundings even when focused on the target,” concluded Fabian. He hopes to inspire robotic designs that use simpler and more computationally lightweight solutions for complex navigation problems in future.

The study is published today in the Journal of Experimental Biology.

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By Alison Bosman, Earth.com Staff Writer