Qtyr flies are real flying acrobats, able to spot their prey, dodge obstacles and catch smaller insects in flight at high speed. Scientists have long wondered how they cope with this, despite the fact that their brain is the size of a grain of sand.

According to a new paper published in the Journal of Experimental Biology, kytyr flies combine two different feedback-based navigation strategies: one involves intercepting prey when the view is clear, and the other allows the flies to bypass any obstacles in flight.

One of the challenges of robotics is to create robots that can move around in an environment with many obstacles - something that people and animals instinctively do every day. According to the authors, many robotic systems rely on path planning techniques: using sound (sonar) or lasers, they send signals and then capture reflections. Then, based on this data, an obstacle map is built.

However, this approach is extremely energy-intensive. Humans and animals do not need complex maps or special techniques to understand the location of an object, its size, speed, and other details. We simply respond to any relevant stimuli in our environment in real time. Thus, the development of navigational behavioral algorithms based on biological systems is of great interest to roboticists.

Past research has focused on the ability of various species, including fruit flies and pigeons, to navigate cluttered areas. “However, in these cases, the only goal was to avoid obstacles,” the authors write. "Navigating around obstacles is more difficult when a specific location is the target, because obstacle avoidance must be balanced against the navigation target."

That's why bioengineer Samuel Fabian of Imperial College London and three collaborators at the University of Minnesota decided to conduct their own experiments using a species of predatory fly, Holocephala fusca, as a test subject. This species was chosen because of the very predictable prey interception trajectory. In addition, its small size and fast movement (most flights last less than a second) clearly require fast response with minimal computational overhead.

The hunting behavior of the ktyr fly is similar to the behavior of falcons, hawks and the tactics of using modern guided missiles. The ktyrs usually hunt by sitting down somewhere where they have a good view of the sky. Once the fly spots potential prey and starts chasing, the fly must navigate to intercept the prey and avoid any obstacles in its path, such as branches.

The flies were given a moving target in the form of a small reflective silver bead, which was pulled along a transparent fishing line. “The flies didn’t really realize it wasn’t real prey, even when they were very close,” Fabian said. "If it was something small enough, they usually mistake it for food."

There was also an obstacle on the frame: a strip of acetate, painted with black acrylic paint, thin (2.5 cm) or thick (5 cm), located just below the path of the target. The precise placement of the strip and the initial trajectory of the fly determined whether the object became an obstacle in the flight path and obscured the target.

The researchers recorded all the flies' flights in the field to get the most natural behavior possible. Then they digitally reproduced 26 flights of gypsy flies chasing a moving bead in the presence of an obstacle. The results were as follows.

In the absence of obstacles, the flies maintained the same trajectory throughout their flight in order to intercept and catch the bead. When a thin or thick black bar partially blocked their view for short periods (less than 0.1 seconds), the flies took evasive maneuvers to get around the obstacle before returning to the interception course. Sometimes the fly would veer off to the side, reacting to the black stripe, even if the stripe didn't block their line of sight. And when the researchers closed the line of sight of the flies for more than 0.1 seconds, the flies completely refused to intercept.

Fabian et al. concluded that the qtyr flies used a simple obstacle avoidance strategy in combination with their standard interception strategy, which they called combined guidance. The faster the obstacle got bigger in their line of sight, the more quickly they dodged it. The flies returned to the interception trajectory as soon as the indicated obstacle disappeared from the field of view. They paid attention to their surroundings even when focused on a goal.

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