You might expect that the miniature brains and eyes of tiny robber flies would limit their ability to launch sophisticated in-flight predatory attacks on their prey. But, according to researchers reporting in Current Biology on March 9 who've captured the rice-sized predators' tactics on film, that's not so.

Their movies show that robber flies sit and wait for a tempting prey item (or a bead) to fly past. Once they do, the flies take off using an interception strategy known as constant bearing angle (CBA), keeping their prey at a constant angle to ensure that they'll eventually meet. That's impressive, but there's more. Once the robber fly reaches a distance of about 30 centimeters from its target, it "locks on," slowing down and curving its flight path to make a successful catch even more likely. Their secret to pulling it off is all in the eyes.

"We knew that these flies likely had an improved vision compared to other true flies, but we never imagined that they would give dragonflies, which are ten times larger, a run for their money with regards to spatial resolution of the retina," says Paloma Gonzalez-Bellido of the University of Cambridge. "Likewise, although we expected these flies to employ CBA, we were surprised by their use of a lock-on phase to ease the capture."

The researchers, including Gonzalez-Bellido and Trevor Wardill, made their discovery by presenting flies in their natural habitats with beads ranging from about 1 to 4 millimeters in diameter on a fishing line. They recorded the flies' reaction to seeing one of those beads zoom past using two high-speed video cameras. The films allowed them to reconstruct the insects' precise flight trajectories in three dimensions.

The films revealed the flies' unexpectedly sophisticated flight paths. The evidence also showed that the flies, with a body only about 6 millimeters long, have eyes capable of detecting prey items smaller than 2 millimeters up to 100 body lengths away.

To further explore the flies' visual abilities, the researchers investigated the internal anatomy of their eyes. Their studies revealed a series of adaptations in essential structures of the eye that optimize the spatial resolution, creating an area of remarkably high acuity, known as a fovea.

"We have shown that when such a miniature nervous system is under strong pressure, the resulting adaptions allow the animal to find solutions that we often think pertaining to much larger animals," Gonzalez-Bellido says. "For example, an acute fovea and the use of CBA are known from large predators, including humans." In other words, even the littlest creatures can have impressive sensory and cognitive abilities.

The researchers say they're still working to understand how the insects know when to lock on and shift their path and speed. They're also exploring how the behavior is driven at the cellular level.

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This work was funded by the Air Force Office of Scientific Research, an Isaac Newton Trust/Wellcome Trust ISSF/University of Cambridge Joint Research Grant, a Biotechnology and Biological Sciences Research Council David Phillips Fellowship, a Royal Society International Exchange Scheme grant, a Swedish Research Council grant, and a Shared Equipment Grant from the School of Biological Sciences (University of Cambridge).

Current Biology, Wardill and Fabian et al.: "A Novel Interception Strategy in a Miniature Robber Fly with Extreme Visual Acuity" http://www.cell.com/current-biology/fulltext/S0960-9822(17)30085-4