President’s Message: Magic Wands By George B. Schramm, LIBC - TopicsExpress

President’s Message: Magic Wands By George B. Schramm, LIBC President Recently, researchers at the University of Illinois have combined thin silicon sheets of photodiodes and 180 elastic micro lenses and stretched them into a hemisphere. Each of these photodiodes is essentially a camera, and together these cameras can capture a 160-degree-wide field of view. Sound familiar? What these scientists are attempting to duplicate is the compound eye of an insect, like a honeybee. (The bee has other types of “eyes” on its head, the dorsal ocelli, but we’re going to discuss the functions of only the large compound eyes.) A traditional camera, like a digital or film camera, imitates the functionality of the human eye: a single lens that projects light onto a photoreceptive surface, the retina in an eye or the sensor plate in a digital camera. For humans, two eyes and a head that swivels enables us to explore our surroundings without even moving from our seat. But for the honeybee, the movement of the head is restricted, so a pair of single-lens eyes wouldn’t be as useful. Instead, the bee has a compound eye made up of multiple lenses that effectively spreads the retina all over the surface of its head. Unlike the man-made version mentioned above, that only has a 180 micro lenses, the worker honeybee has about 5,000 lenses (the queen has about 3,500 and the drone about 10,000). Each of the six-sided lenses visible on the surface of a compound eye is actually the cap on the end of a long crystalline cone, or wand; it is these wands, the ommatidia, bundled in a hemispherical shape that makes up each compound eye. The tip of each ommatidium is clear to let in light, but the sides of the tube are surrounded by a layer of cells with black pigment to prevent light from passing between adjacent tubes; each ommatidium looks sort of like a conjurer’s magic wand with only a white tip on one end. Inside the ommatidia are nine photoreceptors that relay to the bee’s brain the intensity and color of the incoming light. Unlike a single-lens camera or the human eye, each ommatidium does not project a tiny inverted image of the outside world, but instead the ommatidia act together to create a mosaic of light and dark spots. The “image” created in the bee’s brain is very coarse-grained and nowhere near the resolution of the human eye. While we would consider this lack of spatial resolution to be a disadvantage, the ommatidia of the compound eye have an attribute that is lacking in the human eye and highly valuable to a honeybee: temporal resolution. When we look at a slowly flashing light we can distinctly see each flash, but if we increase the rate to 35 or 40 flashes per second, then the light appears to us to be constantly “on.” The photoreceptors in the human eye cannot respond quickly enough to distinguish the flashes of light above a certain rate. (We actually make use of this “shortcoming” when we watch a movie or a television screen; what is actually a flickering image appears to be a continuous image to our eyes.) By contrast, the ommatidia in the compound eyes of a honey bee can detect flashes of light as high as 200 times per second. So, objects moving quickly past our eyes (or as our eyes move quickly past objects if we were flying like a bee) would look blurry and indistinct, but would appear very clear and obvious in the eyes of a honeybee. We can simulate this difference ourselves at an electronics store. Low-quality flat screen televisions will have a low refresh rate, so scenes from a high action movie will appear “smeared” because the action is happening quicker than the images can appear on the screen, while the same scenes on a higher quality flat screen television appear clear and more “realistic.” The bee’s eyes are like the high-quality flat screen, which is perfect for navigating around flowers at high speed, or in the case of a predator like a dragonfly, for capturing prey. The photoreceptors of the ommatidia are also sensitive to a different range of the electromagnetic spectrum than human eyes. The electromagnetic radiation that pervades our universe ranges from very long wavelengths, such as gamma waves, X-rays, and ultraviolet, to very short wavelengths like infrared, microwave, and radio waves. Our eyes have evolved to be sensitive to a very small range of this spectrum; which is what our eyes and brain interpret as being from blue to red. For honeybees, that range is shifted toward the blue end, so bees see from ultraviolet, which is invisible to us, to orange; what we see as red is invisible to honey bees. If we use a special camera that is sensitive to ultraviolet light to photograph flowers, we can see patterns on the petals that are visible to honey bees. These patterns create radial targets that help guide bees toward the center of the flower. They also help bees to distinguish between flower types; so to us the dandelion and the ragwort both appear as yellow flowers, but the honeybee can see two completely different flowers in ultraviolet light. Although the large majority of the surface of a compound eye is devoted to discerning low-resolution patterns, there is a narrow band on the upper edge of each eye that specifically aids in navigation. The ommatidia in this region are oriented up toward the sky and are sensitive to polarized light. If our eyes were sensitive to polarized light then when we looked up we wouldn’t see the sun’s light diffused across the sky, instead we would see concentric rings of light and dark. The rings would be very thin and light immediately around the sun, and would get increasingly darker and wider as you looked away from the sun. So even if the sun was hidden behind some trees, or even just below the horizon, or the day was cloudy, you could always tell in which direction the sun was just by looking at the width and direction of the polarization rings in the sky. This is what the polarization region of the compound eye does for the bees. After witnessing a foraging worker perform a waggle dance, a bee can then emerge from the hive, determine the direction of the sun from the polarization rings, and fly at the appropriate angle to the sun to find that patch of dandelions. So, despite what you may see in the movies, a bee’s, or a fly’s, compound eyes do not create a mosaic image composed of hundreds of pictures of the same screaming woman, instead they create a single low-resolution, sort of blotchy, but not “smeared,” blue-green-yellow image of a large pale oval with a small dark screaming oval; probably not as frightening an image, but far more useful to the honeybee.