Like many insects, bees have more than two eyes - they actually have five. The two largest are compound eyes that are set on either side of the head, each containing 4,500 individual hexagonal facets, which are light sensitive units that work together to produce an integrated visual image, although what they see is different from what we see. According to Lars Chittka and Nigel Raine at the University of London, the clarity of their vision is approximately one hundred times worse than normal human vision. This is because the number of ommatidia is relatively small compared to the 1.5 million photoreceptors in the human retina or the millions of light-sensitive elements in a digital camera.
Susanne Williams and Adrian Dyer at Monash University in Australia created an optical device that simulates the way multiple lenses create an image by using 4,500 parallel-mounted black drinking straws. Using this device, they concluded that in order to see fine details, bees would have to be very close to an object. Color plate D shows how, using their device, they illustrated what a flower might look like to a honey bee. Later work from the same lab applied this imaging system to the understanding of how bees navigate and recognize complex natural landmarks with incredible accuracy.
The bee's other three eyes are simple structures, called ocelli, that are located on the top of its head. These are light-detecting organs that do not produce visual images. They are common in some other insects; for example, some butterflies have ocelli on their genitalia. It is thought that they help the bee sense direction and low levels of light, and they may play a role in enabling bees to follow a streaking scout bee that flies overhead to lead a swarm to its new home.
Bees are also able to see fastmoving objects much better than we can, and the ocelli may play a role in this facility. Recent experiments by Gerald Kastberger at the University of Graz in Austria explored how bees with occluded ocelli react to changes in the light environment during flight. He found that the ocelli seem to help control phototactic behavior in flight course control in honey bees.
Adrian Dyer has done a series of interesting experiments to explore the limits of bee vision. After much trial and error, he trained honey bees, Apis mellifera, to recognize an image of a human face by associating that face with a sugar reward, and they consistently flew to the familiar face when it was placed with other images that were unfamiliar. They even flew to that face when the sugar reward was removed. But when the face was rotated 180 degrees, they were significantly less able to identify it, raising interesting questions about their visual processing. Bees are partially colorblind.
Each ommatidum or facet in the eye contains nine light-sensitive cells that are receptive to different colors. They contain six green receptor cells, according to Motohiro Wakakuwa at Yokahama City University, which are responsible for detecting motion and seeing small targets. The other color receptors vary depending on their position in the eye, and there are now understood to be three types of ommatidia. So, for example, if the bee is looking down, certain receptors are sensitive to green light, but if she is looking up, they are sensitive to ultraviolet. The brain apparently compares complex sets of signals from different sensors to identify color.
Experiments have demonstrated that honey bees can see a wide range of colors, but the spectrum visible to them is shifted into the ultraviolet range, so they can tell the difference between yellow, blue, green, and ultraviolet but cannot distinguish between red and black. They can also see a color, known as "bee's purple," that is a mixture of yellow and ultraviolet, and they can see patterns of polarized light that help them navigate. Rudiger Wehner and Gary Bernard demonstrated that most photoreceptors in a bee's eye are "twisted like a corkscrew," and they found that the amount of the twist corrects for the potentially false perception of colors as a result of polarized glare from reflecting surfaces on plants.
The interior of a bee colony is quite dark and yet the bees inside do all sorts of detailed work, so clearly bees can "see" in the dark; touch and scent play a large part in organizing their activities inside the colony. Because they are red/black colorblind, meaning they cannot distinguish between these two colors, observation hives and bee labs are commonly lit with red lights so that researchers can watch them while the bees carry on in what seems like normal darkness to them.
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08152010
1. What is special about bees living in colonies
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