Visual Flight Rules—How Honeybees Judge Altitude

In 1963, a pair of researchers reported (“Windkompensation und Seitenwindkorrektur der Bienen beim Flug über Wasser”) that when honeybees overflew a body of water, they had no difficulty navigating as long as there were waves or ripples on the surface, but if it was completely still, with a mirror-like surface, the bees would rapidly lose altitude and crash into the water.

Now a team of researchers have constructed a tunnel with adjustable mirror configurations to study this phenomenon, “Floor and ceiling mirror configurations to study altitude control in honeybees”. An article in Science Alert, “There’s a Really Weird Effect When Honeybees Fly Over a Mirror”, summarises their findings.

The experiments took place inside a 220-centimeter-long (87 inches) rectangular tunnel placed outdoors, with mirrors on the ceiling and the floor that could be covered to look like plain old walls.

When all the mirrors were covered, the honeybees usually flew from one side of the tunnel to a sweet treat on the other side while maintaining a near-constant altitude.

When the ceiling was pulled back to reveal a mirror, seemingly doubling the height of the tunnel, the bees easily made it across.

But when the floor became a mirror, making the ground look doubly far away, the crashes began. The bees would start out flying normally, but after about 40 centimeters (15 inches) of flight, their altitude would begin to drop until the insects collided with the glass bottom.

When both the ceiling and the floor were mirrors, creating a parallel pair of infinite walls, the bees would start losing altitude after flying for only about eight centimeters (three inches), hitting the ground soon after.

Human pilots have similar difficulty judging altitude when flying over featureless terrain and are trained to rely upon instruments rather than their eyes. Honeybees, however, have neither barometric nor radar altimeters.

Interestingly, another experiment from 2010, “Visual Control of Altitude in Flying Drosophila” indicates that fruit flies do not use flow of terrain beneath them to control altitude.