Fruit fly head. Close-up of the head of a fruit fly (Tephritis postica), showing its small antennae (top left) and one of its large compound eyes. Each compound eye is made up of numerous simple eyes called ommatidia. Each ommatidium sends a signal to the fly's brain, which combines the thousands of images to form a mosaic view of the world. The image is not sharp but is excellent at detecting movement.