Fossils reveal that many animals once had a third eye on top of their heads. It has disappeared in mammals and birds, but some reptiles and amphibians still have this residual ‘parietal’ eye.
In iguanas, it is often visible as a grey spot. You can also make it out (albeit less clearly) in some other lizards, such as skinks, anoles and young tuataras, as well as in frogs and salamanders.
Mostly, it is not functional and fades under skin and scales as animals mature. However, a few lizards use their third eye to detect blue and green light and ‘read’ the time of day, like a clock.
By sensing shadows and light changes, the eye can detect danger and may also act as a navigational compass. It uses chemicals to detect light, unlike other vertebrate eyes, which use rod cells or cone cells.
