a single lens that tells the difference between light and dark
                                                            
                                                     
                        
                                                
                        
                        
                            An eye that has a single lens. Simple eyes can be as small as a horseshoe crabís or as large as a whaleís.
                                                            
                                                     
                        
                                                
                        
                        
                            A simple eye detects light and dark; it is called an ocellus. Butterfly and moth larva have simple eyes only; adults have simple eyes plus compound eyes.