A bee takes off from its hive and lands on a flower. This apparently effortless landing, however, is no mean feat. It requires a considerable amount of sensory feedback and fine speed control – miscalculations can be catastrophic. In order to land successfully, bees must slow down at just the right rate, and they require sensory information to control their final descent. When landing on a flat surface, the information comes in the form of optic flow. As a bee flies over the ground, patterns on the ground move past it. The speed of this largely translational movement informs the bee how quickly it approaches its target. But bees don't always land on flat surfaces; flowers and hives can be at any orientation. This raises the question, what information do bees require, and how do they use it, to prepare for landing?
Emily Baird from Lund University, Sweden, and her collaborators from Germany and Australia thought that another visual cue, a more complex version of optic flow, might just answer this puzzle. The group noted that, as a bee flies towards its intended landing site, that site's image will appear to grow in the bee's eyes. This visual expansion could help the bee gauge its approach speed and deceleration.
To test this theory, the team of researchers devised a series of experiments using optical illusions. While visual expansion could provide information about an approach, not all images are equally informative – and some are plain misleading. A checkerboard pattern will appear to expand strongly as you approach it: bees flying towards a checkerboard target slow down and land with very little variation in their approach trajectories. However, a circle of equally sized wedges of alternating color provides very few cues of apparent expansion and the bees that fly toward this target approach with highly variable speeds; some didn't even slow down until they had practically crashed into it.
These results were a vote in favor of Baird and her collaborators' visual expansion hypothesis, but they weren't enough. The team wanted to know whether the bees landed successfully by holding the rate of visual expansion of their target constant. The team returned to their bag of optical illusions and devised spiral-patterned targets with misleading ‘apparent’ visual expansion. These patterns appeared to expand or contract when rotated during the insect's approach. If the researchers were correct and the bees were using visual expansion to guide their landing speeds, the bees would land according to their illusory visual cues.
And that is exactly what the bees did. When approaching a rotating spiral that appeared to be expanding, the bees slowed their flight much earlier than they did without the misleading visual cues. Similarly, when bees approached a spiral that appeared to contract, they approached at higher speeds. The bees were indeed relying on visual expansion to modulate approach speed.
The beauty of Baird and colleagues' findings lies in their simplicity. By modulating approach speed based on visual expansion, bees can slow to safe landing speeds with minimal information. One parameter, visual expansion, is sufficient to guide a landing without additional information, such as distance, orientation or even the bee's own speed. In fact, Baird and colleagues think this system may be guiding flight landings in other species, and could be easily applied to robots. Visual expansion is an elegant solution to the complicated problem of safe landings; it just took a little bit of visual trickery to figure it out.
