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Bees use 'biological autopilot' to land

Bee landings Bees get a perfect touchdown by detecting how fast their landing site 'zooms in' as they approach, new research has found.

The findings, reported today in the Proceedings of the National Academies of Science, are being used to develop landing systems for robotic aircraft that don't depend on expensive radar and sonar.

"Landing is the most difficult part of any aerial manoeuvre - that's probably why they pay pilots so much," says co-author Professor Mandyam Srinivasan, a visual neuroscience expert at the University of Queensland.

"As you approach the ground you've got to slow down appropriately so that when you're close to the ground you are moving with almost zero speed. Timing-wise it has to be orchestrated very well."

Srinivasan and colleagues wondered how flying insects manage landing with such precision and grace given their brain is only "the size of a sesame seed" and they don't have stereovision like humans to calculate the distance from a target.

To investigate this, the researchers used high-speed video cameras to film honeybees as they landed on a target. From the recorded flight path, they were able to calculate the insects' speed at different points in time and in three dimensions.

Once they crunched the numbers they found evidence the bees were sensing the speed at which their target was getting closer and changing their flight speed to make sure they got the perfect touchdown.

'Biological autopilot'

When we move towards an object, it appears bigger. And if we're moving at a constant rate, the object gets bigger faster, at an exponential rate, as we get closer.

"The bee does not allow that to happen," says Srinivasan. "At every instance in time it adjusts its approach speed so that the rate of expansion of the target always stays constant."

Srinivasan says the closer the bee gets to the target, the slower it gets, with the speed being proportional to its distance from the target.

"So if it's twice as far away, it's approaching twice as fast," he says.

"That automatically ensures that it slows down as it approaches its target," says Srinivasan. "It works like a beautiful biological autopilot."

The target used in the experiment was vertical, but Srinivasan says the rule applies to any type of surface and does not require the knowledge of how far away the target is or how rapidly it's being approached.

As part of their study, the researchers used a landing board with a spiral on it, which could be made to appear larger or smaller, depending on the direction the spiral was rotated.

"When we spun the spiral to make it appear to expand, the bees 'hit the brakes' because they thought they were approaching the board much faster than they really were," says Srinivasan.

"When we spun the spiral the other way to make it appear to contract, the bees sped up, sometimes crashing into the disc. This shows that landing bees keep track of how rapidly the image 'zooms in', and they adjust their flight speed to keep this 'zooming rate' constant."

Robotic aircraft

Srinivasan and colleagues are now applying this knowledge to developing a landing system for autonomous aircraft.

Their system weighs around 500 grams and uses off-the-shelf cameras and a computer the size of a smart phone with special software developed by the team that measures the rate an image expands.

The cameras involved don't need to be high resolution or even colour, says Srinivasan -- after all insects see things in very poor resolution.

The landing system could provide an alternative to land-based infrastructure including radar, which is not only expensive but it involves putting out signals that can be detected. In the defence context this could give away an aircraft to the enemy, says Srinivasan.

He says the system could also be used by commercial aircraft in emergencies when there is no airport around.

Srinivasan's research is supported by the ARC Centre for Excellence in Vision Science, the Queensland Government and Boeing Research.