Nature

There’s a really weird effect when honeybees fly over a mirror

In 1963, an Austrian entomologist named Herbert Heran and the German behavioral scientist, Martin Lindauer, noticed something peculiar in the way honeybees zoom through the air.

When a selection of bees was trained to fly over a lake, they could only make it to the other side if there were waves and ripples on the surface of the water.

If the lake was mirror-smooth, on the other hand, the insects would suddenly lose altitude until they crashed headlong into the liquid-looking glass.

At the time, the findings supported the idea that honeybees use visual cues to navigate during flight, and a follow-up study has now added a fascinating insight into the flying strategies of these talented little aeronauts.

Replicating the 1963 experiment, albeit in a more ethical way, researchers have shown that honeybees watch the ground speeding below them to regulate their altitude in flight.

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.

The findings are very similar to the spatial disorientation that sometimes strikes human aviators. When pilots are unable to see their ground speed, they struggle to maintain their altitude.

Even during a ‘graveyard spiral‘, human senses can deceive us into thinking we are still in level flight. That’s why airplane instruments are so important; they help us overcome spatial illusions and keep our aircraft aloft even when there is no texture or shadow on the ground or water below.

Unfortunately, honeybees don’t have that backup system to help them out. Even when a mirror floor only existed in the second half of the tunnel, their steady flight from the first half was suddenly interrupted by a dramatic plunge.

“Interestingly our double mirror condition allowed us to get closer to the flight conditions of an open sky flight above a calm water surface as used by [Heran & Lindauer]”, the authors of the new research write.

“Our results agree with theirs insofar as the honeybees lose altitude in the absence of ventral optic flow.”

In short, it seems as though bees use visual cues on the ground to maintain their altitude, as opposed to visual cues from above them in the sky.

When the ground is no longer giving the insects a proper baseline, researchers think they drop lower in altitude to see if they can regain that ‘ventral optic flow’.

Thinking it is farther away than it is, they ultimately crash into the ground.

If the bees in the experiment had been given a wider visual field, they could probably have used other cues around them to help maintain altitude. But when flying across a large, still lake or a closed-in tunnel, there are few alternatives the insects can use to gauge their altitude.

Interestingly enough, a similar experiment found fruit flies do not use ventral optic flow to control their altitude. Different species may, therefore, use different techniques to maintain their flight.

At high altitudes, humans are often told to not look down, for fear we will fall. But if a honeybee were to follow those same instructions, its crash would be inevitable.

The study was published in Biology Letters.

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