Iguanas have a third eye
Also known as a parietal eye, this organ is used to alert iguanas (as well as other reptiles) of aerial threats.
Unlike the lizard's other two eyes, the parietal eye is quite simple in its physiology and can only detect changes in lightness and darkness. But that's more than enough to evade predators.
So where exactly is it? In this photo, the parietal eye is that little light-colored scale on the top of the lizard's head between the two eyes.