The pupil itself is usually a clue to the habits of the lizard, in that vertical pupils are common to nocturnal forms while round pupils are common to diurnal species. As in any pupil, the aperture is widest in darkness and may be nearly as large as the eye, as in many geckos. In bright light the pupil contracts, becoming a small opening.
It is known that many lizards possess the ocular cones needed for color vision. This is useful in determining breeding colors and territorial warnings.
The lower lid of the lizard is usually the most developed lid. It may contain a clear spectacle or window so the lizard can see with its eyes closed. Many geckos, the Pygopodidae, and certain other groups lack movable eyelids. Instead, the eye is covered by a clear rounded scale, keeping the eye permanently open. To clean this scale lidless lizards can utilize the tongue.
The ear of a lizard can be either exposed or concealed. In some species the tympanic cavity is covered by a clear thin membrane. The Chamaeleontidae, Lanthanotidae, Dibamidae, and Anniellidae all lack an external ear opening. Many skinks, some agamids, and a variety of other species also lack the external ear. Hearing, however, is not dependent solely upon airborne sounds, but can be detected by an acute sense of feel, especially in blind, earless forms.
Another feature associated with the head is the tongue. The long forked and protrusible tongue is common to the Varanidae, the teiid Tupinambis, and a few smaller forms. Many lizards have the anterior portion of the tongue nicked, but not all have retractable tongues. The most interesting tongue is that of the chameleon. It is a projectable affair with a sticky tip and may be as long as the body of the lizard itself. When employed it is fired quickly, and accurately, to secure the insect prey. Most lizards use the tongue as an organ to bring particles of air to the Jacobson’s organ, while others use it only mechanically as an aid to swallowing.
As far as limbs go, we can generalize by saying that many burrowing forms are limbless or have reduced legs, and most arboreal lizards have long limbs with long digits. Limbs are of little use to fossorial lizards. One or both pairs of legs may be absent, and the number of claws could be less than five.
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