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; 360?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.