Fly eye facets. Coloured scanning electron micrograph (SEM) of facets on the eye of a fly. Each of the two compound eyes comprises several thousand image-forming facets called ommatidia. Each ommatidium is a simple eye, which sends a signal to the fly's brain. The brain combines the thousands of images to form a mosaic view of the world. The image is not sharp but is excellent at detecting movement. In between the ommatidia are hair-like structures called interommatidial bristles. These bristles are frequently modified, duplicated or absent in mutant Drosophila fruit flies.