Mutant fruit fly compound eye. Coloured scanning electron micrograph (SEM) of the abnormal compound eye of a mutant fruit fly (Drosophila melanogaster). The eye is formed from numerous lens structures known as ommatidia, but a genetic mutation has caused these to develop unevenly. Fruit flies are widely used in genetic experiments, particularly in mutation experiments, because they reproduce rapidly and their genetic systems are well understood. Magnification: x35 when printed 10 centimetres wide.