Mark Francis and Andreas Reiman, The California Landscape Garden: Ecology, Culture, and Design.

One of the most fascinating things about lizards is their so-called third eye, a small spot centered on top of the head. This tiny organ--actually the pineal gland, in a uniquely adapted form--like the other two larger eyes, has a pupil, cornea, lens, and retina, but it lacks an iris mechanism that would allow it to register images. This third eye, it turns out, senses light and temperature, somehow keeping track of how much heat and light a lizard is exposed to and cueing the pituitary and thyroid glands accordingly. By this means, potentially deceptive environmental cues, such as atypically warm weather, are interpreted correctly so that the lizard's reproductive cycle does not accelerate. p. 187