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Scanning electron micrograph of the compound eye of a blowfly, Calliphora vomitoria. The picture shows a small part of the surface of the eye. The hexagonal units visible are the tips of the ommatidia. Each hexagon is a single lens, about 20 microns across, the light from which is funnelled down to photoreceptors. This picture shows the lenses of about 200 ommatidia; each eye has a total of 4500. Pigment cells lining the funnel of each ommatidium prevent light leakage to its neighbours, producing a mosaic image. Unlike human eyes, compound eyes have no mechanism for focussing the image. But they have properties of particular use to a fly; 360º degree vision, and, due to the large number of edges between each tiny lens, an enhanced ability to detect movement. This allows very fast reactions to threats from predators, including a human being attempting to swat them. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR JEREMY BURGESS/SCIENCE PHOTO LIBRARY)