Iguanas can range from 5 to 6 feet (1.5 to 1.8 m) including their tail. The two species of lizard within the genus Iguana possess a dewlap, a row of spines running down their backs to their tails, and a tiny "third eye" on their heads. This light-sensing organ is known as the parietal eye, visible as a pale scale on the top of the head, and cannot make out details, just brightness. Behind their necks are small scales which resemble spokes, known as tuberculate scales. These scales may be a variety of colors and are not always visible from close distances. They have a large round scale on their cheeks known as a subtympanic shield.
Iguanas have great vision and can see shapes, shadows, colors, and movement at long distances. Iguanas use their eyes to navigate through crowded forests, as well as for finding food. They use visual signals to communicate with other members of the same species.
The tympanum, the iguana's ear drum, is located above the subtympanic shield and behind the eye. Iguanas are often hard to spot, as they tend to blend into their surroundings. Their scale colors are a mode of hiding from larger predators.
Male iguanas, as well as other male members of the order Squamata, have two hemipenes.