Susan Thomas – Page 6

Jeepers, Creepers! Where’d You Get those Peepers? Bees have amazing eyes--all FIVE of them! Two are big compound eyes, and the other three are small and simple. The trio of round, single-lensed eyes called ocelli are found in the center of the forehead. They do not provide any detailed images—rather, they detect intensity of light, including ultraviolet (UV) frequencies. Input from these simple dome-shaped lenses is important for maintaining stability during flight and for navigating, also to note the approach of a predator overhead, and to locate flowers via their UV markings. Honey bee’s simple and compound eyes (image: Honeybee HQ) In contrast, a honey bee's compound eyes are a prominent feature and are “designed” through evolution to provide detailed imagery. Each one is made up of thousands of tiny individual lenses (facets): about 7000 in a worker’s eye; 4000 in a queen’s; and 8500 in each drone eye. Why so many lenses for drones? Sharper vision helps in their competition to spot virgin queens approaching the drone congregation area---and mating is accomplished in flight. At high magnification, facets appear to fit together like tiles in a floor, each set at a slightly different angle due to the curvature of the whole structure. The hairs set among the facets are thought to report wind direction to assist in navigation. Honey bee worker’s compound eye, upper portion, at 280 X magnification. Behind it, notice a single ocellus that appears here as a smooth black dome (Scanning electron micrograph by D. Scharf) Each lens sits on top of a tube containing a cone to focus light onto sensitive pigments embedded in nerve cells--which together form the actual photoreceptors. Each of these complete units is called an ommatidium. Each ommatidium of a bee’s compound eye is set at a slightly different angle on the curved surface and consists of a facet (lens) atop a tube containing a light-gathering cone and light-sensitive pigments. (Diagram: S. Riddle) The image captured by each lens is only a small part of the visual field that is within its particular angle of view. However, these separate bits are fused to a single, clear picture – not a mosaic, as used to be fancifully thought, but rather, a clear and extremely detailed, instantaneous view. This is what a bee sees--something like the view from a "fish-eye" convex lens. Because the bee's eye has as many photoreceptors per square millimeter as has the human eye, its close-up vision should be of a quality comparable to our own. These compound eyes are of fixed focal-length; they don't accommodate for near and far vision, so the bee has to move closer to see an object more clearly. But its world is small and intimate; it has no need to see clearly at a great distance, but it can distinguish fine detail of flowers and other objects in its universe. It can see clearly enough the features that serve as landmarks. The bee's eye can also notice movement, as a shape or shadow traverses a pathway across a series of facets. They also see color! Unlike humans, who see a spectrum of red to purple based on a set of red, blue, and yellow photoreceptors, honey bees perceive a range from yellow through ultraviolet (UV), using their three photoreceptors for yellow-green, blue, and UV. With no specific receptor for red, they do not see that color at all, although they can pick up reddish wavelengths in the orange-yellow range. They also see a special hue called “bee purple” composed of yellow and UV light combined. Red flowers appear as black; however, many red flowers also have some yellow or UV markings that honey bees can follow. Indeed, most flowers offer a system of dots, arrows, or lines in the visible or the UV portion of the spectrum indicating the pathway to nectaries and pollen. As bees come close to open blooms, the floral fragrances attract and inform them about the content and quality of nectar each one offers. Foxglove has visual and UV nectar-guide dots; rudbeckia shows no visible marks, but has arrow-like UV markings; alstroemeria’s bold lines point to its nectar and pollen. Images: Pinterest, and Paolo (“Macro-roni”) on Flickr. There are other remarkable features of bee vision that make foraging and navigation efficient. For one thing, with all five eyes they can see polarized light, and accordingly, can determine the sun’s position even when it is obscured by clouds. They use that information along with wind direction, landmarks, and other data to travel their foraging routes and return home again. In flight, bees see in three-dimensional view, and can judge depth and distance. While flying at their fastest travel-speed, they actually see in black and white, but color comes into focus as they slow down a bit to approach a target. Nonetheless, they perceive color five times faster than we do—1/300th of a second versus our 1/50th — and as a result, a bee in flight can distinguish one individual blossom from another, whereas we might see only blur (imagine glancing at a flower-filled meadow as you whiz past it in a car…..whooooosh!). Further, the honey bee’s “flicker threshold” (the rapidity with which its visual system can recover from stimulation to respond again) is also more rapid than ours, allowing bees to more readily identify objects in motion than we can. This increases their ability to identify preferred blooms while flying at a good clip, and to easily land on and pollinate flowers that are nodding in the breeze. Back in the hive, the forager communicates to its nestmates, providing a road-map to find rich food sources by performing a waggle dance using all the information gathered through visual, chemical, and other senses concerning direction, distance, and even landmarks and floral fragrances. Five eyes, two antennae, and a multitude of sensory hairs on the bee’s body from head to toes are fine-tuned to the world through which the forager travels, and which it enriches by its task of pollination along the way.

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Posts by Susan Thomas