Chapter 6. Behavioral Communications

6.3. Distance measurement in bees

Because honey bee dancers indicate to their nestmates how far they have just flown, it follows that they must have some ways to measure the distance between the hive and the food source. Potentially bees can measure this in several ways:

1. Time spent travelling

2. Energy spent travelling

3. Optic flow

The famous Austrian ethologist Karl von Frisch (1886-1982), who was awarded the Nobel Prize (together with Konrad Lorenz, Nikolaas Tinbergen, both of whom studied mostly birds) in 1973 did many ingeneous experiments to tackle this problem. von Frish and other had overwhelming evidence that bees do not use time spent, but rather energy spent on their out-going flight (from hive to food) to gauge the distance. For example, a headwind from hive to food source would presumably take the bees greater energy and they would dance for a further distance than when there is no wind. Food on top of a hill would also be indicated a further distance than an equadistant food source at the same level. Putting weights on workers also caused them to indicate a greater distance. Finally, bees were forced to walk through a tunnel in front of the hive, then able to fly to the food, bees would come back indicating a much greater distance, presumably because walking is energetically more costly than flying.

However, more recent evidence has now proven the theory wrong and further the new theory can explain all the above experiments. The new and accepted theory is that bees use "optic flow" to measure the distance travelled. Optic flow is a measure of the image movement cross the visual field in the bee's compound eyes. This is similar to how we humans judge how fast a train or car is moving by looking at how fast the trees are flying by outside the window. A bee, however, seems able to quantify and remember how much optic flow occurred during her outward flight and use that information in her dance. Dr. Herald Esch (Notra Dame University in Indiana), a student of von Frisch, did an elegant study to prove that this was the case. Bees were trained to fly to a food station tethered to a helium baloon at a certain distance, then their dances were recorded. When the baloon was raised in the air, thereby increasing the distance sligthly and also increasing the energy expenditure greatly, but the bees came home and indicated a shorter distance than then the food was on the ground! This occured because as the bees tried to fly high in the air, even though spending more time and more energy in reaching the food, their optic flow was actually less because optic flow was a function of both speed, and also the distance between the perceiver and the background. More recent experiments used small tunnels with black and white patterns on both sides (photo on left) to study whether optic flow is really used as an "odometer" by bees. It was found that bees will dance as if the distance is 200 meters away, while the tunnel is only 6 meters long. Further by changing the black and white patterns, one can precisely manipulate the "amount" of optic flow, and it was discovered that bees will change their dances accordingly. Changing the direction of strength of airflow in the tunnel did not affect their dances, again confirming that bees use predominently, if not solely, optic flow to measure distance flown.

Optic flow is also used for judging the distance to landing. If a bee keeps its perceived image speed constant then the actual speed would decrease to zero as it approaches touchdown. This has enlightened engineers to construct helicopters that can land automatically.

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