A little fly has five eyes, two of which contain several thousand separate seeing units Ha depends upon this array of seeing equipment to escape birds, frogs, bats and countless other animals that dine on fly meat, These amazing eyes also explain why the pesky fly always seems to take off dust ahead of the fly swatter
Seen under a magnifying glass, the face of the fly looks like a visitor from outer space, There are two bulging domes on either side of his head reaching down over what should be his upper cheeks These are has amazing compound eyes Between them on top of his head, there are three small simple eyes called ocalli, which it plural fob ocallus, Lower down is his forehead is a pair of feathery antennae which are used as feeling organs and maybe for smelling His mouth is a sucking tube hanging down below his chin
With his five eyes, you would expect that the fly can sae 3 n great detail But this is not so To him, the scenery probably resembles the blurred dots of light and dark which we see when we put a magnifying glass over a newspaper picture However, the fly can see the tiniest movement in the blurred scenery and take off in the fraction of a second
Each compound aye is a dome of transparent skin called the cornea, It is honeycombed with thousands of small, separate lenses Some face up, some face the aides of the dome Each lens or facet captures a separate image and together they give the fly a total picture of the scenery from all directions The lens is the base of a pyramid which tapers back into the eye where it registers on a screen called the retina,
There are light‑sensitive calls inside each seeing unit and also a little dark pigment,
The pigment, like a dark wall, separates a seeing unit from its neighbors and it can be shifted around to adjust the eye to bright and dim light, The total picture coming from all sides of the domed eye falls upon the retina screen It is built up from thousands of small individual pictures arranged like the tiles of a bathroom floor, For this reason it is called mosaic vision It is believed that some of the separate pictures cover the same scenery and produce overlapping images in the total mosaic
Compared with the bulging, complex compound eye, the three ocelli are very small and very simple, Each is a fixed lens, shaped like a little dome The ocellus is pointed in one direction and can capture only a small section of the scenery This is not much use to the fly, who has to keep an eye out in every direction
The main purpose of the ocelli seams to be in aiding the compound eyes Perhaps their lenses capture rays of light from, certain angles and directions and reflect this information to the big compound eyes If this is so, they can keep a lookout for the flickers of light and shade which come from moving objects, The flyts compound eyes give a blurred mosaic picture of the total scenery It is thought that the ocelli may sharpen the picture with flickering signs of movement.