What is the unusual physical characteristic of Iguanas?

Iguanas are a genus of herbivorous lizards that are native to tropical areas of Mexico, Central America, South America, and the Caribbean. The best-known species is the common, or green iguana, which occurs from Mexico southward to Brazil. This common iguana is green with dark bands that form rings on the tail; females are grayish-green and about half the weight of males. Iguanas also possess atrophied venom glands that produce a weak harmless venom.

Iguanas have a so-called “third eye” on the top of their heads. Known as the parietal eye, it looks like a pale scale and can’t discern shapes or color but does sense light and movement, helping iguanas anticipate predatory birds from above. This "third eye" has only a rudimentary retina and lens and cannot form images, but is sensitive to changes in light and dark and can detect movement. This helps the iguana detect predators stalking it from above.