An ocellus is a simple eye found in some invertebrates, such as insects and certain species of mollusks. Unlike the complex eyes of vertebrates, ocelli consist of a single lens and are primarily used to detect light and dark, helping organisms navigate their environment. They are often located on the top of the head and can provide important information about the surrounding light conditions. In addition to insects, ocelli can be found in other animals, including some species of spiders and fish. These simple eyes play a crucial role in behaviors like foraging and predation, allowing animals to respond quickly to changes in their surroundings.