A light-sensitive cell on the surface of an animal's body. This simple eye form cannot gather enough information to see an image; it only detects changes in light and dark. Eyespots can be found on simple organisms such as earthworms and leeches. When the eyespot is stimulated by sunlight, the organisim reacts by moving out of the light. Eyespots can also be found on more complex animals that have well-developed eyes. A dragonfly, for example, has an eyespot on each side of its head in addition to its compound eyes. Since eyespots only detect changes in light and dark, the dragonfly may be using them as navigation aids -- by tracking the horizon where the light sky meets the dark ground.