From
http://www.anapsid.org/parietal.html
The Parietal "Eye"
The parietal eye is a photosensory organ connected to the pineal body, active in triggering hormone production (including reproduction) and thermoregulation. It is sensitive to changes in light and dark, it does not form images, having only a rudimentary retina and lens. It is visible as an opalescent gray spot on the top of some lizard's heads; also referred to as "pineal eye" or "third eye."
Despite its common name, the parietal 'eye' cannot see (discern actual images) as their eyes see, but this is organ is one very good reason why you cannot use white light for heat at night, and why untamed igs (and even some tamer ones) flee from your hand when you swoop down from overhead to pluck them out of their enclosure, or why, when in the car, they duck when you drive under freeway overpasses, and why it takes off like a berserker when a teeny tiny speck of a bird or plane almost too small to be seen with unaided human vision goes flying by waaaaaay overhead.
Many skinks and agamids, such as blue-tongue skinks and water dragons, have this "eye" as well. Humans have one, too, but ours has no outlet at the top of our skull (see Circadian Rhythms and Your Biological Clock).
I can't get the picture to copy into my post, but the website has a good picture of where the "third eye" is on a beardie. Go figure . . .
