Feb. 7, 2000 Washington, D.C.-- Honeybees rely on visual cues to gauge the distance to a food source, and new information about their "optical odometers" may set the stage for pocket-sized surveillance technologies featuring insect vision, says the author of a 4 February Science article.
"Our study suggests that honeybees use cues based primarily on image motion to monitor flight distances of hundreds of meters in natural outdoor environments," reports Science author Mandyam V. Srinivasan.
Passing many visual landmarks-such as trees or flowers-makes the insects feel they have traveled a long way, just as telephone poles whizzing by a car window may enhance a passenger's sensation of speed, says Srinivasan, a professor with the Australian National University's (ANU) Centre for Visual Science within the Research School of Biological Sciences.
When foraging bees locate a meal more than 50 meters from the hive, it has long been known that they return to the colony and waggle their abdomens in the direction of the food source. The longer the dance, the farther the journey to food. If a meal is located closer than 50 meters, bees simply turn a few circles, performing what's called a "round dance."
In the late 1960s, other researchers had suggested that bees determine the distance to food based on the amount of energy expended during flight, Thomas Collett of the University of Sussex (U.K.) explains in a related Science "Perspectives" essay on Srinivasan's work. Then, in 1996, a different team discovered that bees flying between very tall buildings performed waggle dances suggesting they had flown half the distance signaled by bees traveling the same course near street level, presumably because "as the ground drops away, it doesn't seem to move as quickly by the bees' eyes," Srinivasan says.
Building on this earlier work, Srinivasan and his coauthors-Shaowu W. Zhang of ANU and Monika Altwein and Jüergen Tautz of Germany's Universität Würzburg-have prompted bees to waggle even when food is close to the hive, by bombarding their eyes with optical cues during the journey.
The research, sponsored by the U.S. Defense Advanced Research Project Agency (DARPA) and the Australian Defense, Science and Technology Organization, ultimately may guide new military technologies. "We are interested in exploring the possibility of developing autonomous, flying vehicles that incorporate some principle of insect vision to avoid obstacles, perhaps for surveillance applications," Srinivasan says. Such technologies might include, for example, "microair vehicles," just six inches long or smaller, adds Alan S. Rudolph, a DARPA program manager.
To test bees' optical odometers, Srinivasan's group sent Apis mellifera lingustica Spinola specimens on a search for snacks inside a narrow tunnel, which was 6.4 meters long, 20 centimeters high, and only 11 centimeters wide-roughly 21 feet by 8 inches by 4.3 inches. The tunnel entrance was positioned near a specially designed "bee farm," a hive sandwiched between transparent observation windows.
In a series of experiments, a feeder was then placed near the tunnel entrance, or 6 meters inside the tunnel, so that it was between 35 and 41 meters from the hive, a distance known to prompt only round dances among bees flying outdoors.
Decorating the tunnel's interior with a random, black-and-white pattern prompted most bees (90 percent) to waggle, although they danced in circles when flying similar distances between various outdoor feeders on the ANU campus. When the tunnel was adorned instead with horizontal black and white stripes, bees mainly performed round dances (86.7 percent), apparently because they were flying parallel with the lines, and therefore weren't receiving an exaggerated amount of optical input.
Flying close to tunnel walls-particularly those decorated with overly busy wallpaper-amplified the bees' perception of distance, skewing the insects' optical odometer by as much as a factor of 31, the Science paper concludes. Consequently, 6 meters inside a tunnel feels to a bee like 186 meters outdoors.
In summary, a bee waggles one millisecond for every 17.7 degrees of image-motion it sees, the researchers found. The formula isn't absolute because a bee's perception of distance is "environment dependent," the researchers report. But, the findings should help scientists better understand the optical mechanisms that allows bees to locate a promising buffet.
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