Lizards are part of a very large and successful evolutionary group. There are around 6,000 species of reptiles, split into four different orders.
Lizards and snakes belong to the same order — Squamata. There are around 5,000 species of lizards and 2,500 species of snakes.
Chelonia is the second-largest order and includes all turtles, tortoises, and terrapins.
Alligators, caimans, gavials and crocodiles make up the order Crocodilia.
The tuatara is the last living member of its order, the rest having died out 100 million years ago.
Lizards, being reptiles, are cold-blooded, meaning they have a body temperature varying with that of their environment. Lizards spend the day basking in the hot sun in order to warm themselves up. Lizards are therefore able to recharge their batteries during the day and can hunt with success at night. Lizards for this reason tend to be more active during the night hours. They rather function like living solar panels.
Both for locating prey and for communication, sight is very important for most lizards, as such, many lizards have highly acute color vision. Almost all lizards rely heavily on body language. They use specific movements, gestures, poses and postures, to resolve disputes, define territorial boundaries, and entice mates. The particular innovation in this respect is the dewlap, a brightly colored patch of skin on the throat. The dewlap is usually hidden between scales. When a display is needed, a lizard can erect the hyoid bone of its throat. This results in a large vertical flap of brightly colored skin beneath the head which can be then used for communication. Some species of lizards also use bright colors in other areas of the body, such as the iridescent patches on the belly of Sceloporus. These colors would be highly visible to predators and make the lizard a target to be easily preyed upon. Therefore the colors are often hidden on the underside or between scales and only revealed when necessary.
Lizards are oviparous animals, that being animals that lay eggs, with little or no other embryonic development within the mother. Lizards lay eggs in which baby lizards form a few months later. Some species of lizard, such as the slow worm, appears to give birth to live young. However this is not technically the case as the female lizard incubates the eggs in her body until they hatch rather than incubating them outside of the body like other species of lizard.
Some lizards change colors very quickly to match their environment, like the chameleon. Their colors are also affected by the sun or the warmth of rocks on which they are basking. Anoles change colors for many reasons. They get darker in order to attract more radiated heat from light sources. They tend to "green up" when they are in a comfortable state. For camouflage they change color in order to blend in with their surroundings. Their mood is widely thought of as affecting their current colors. During mating/breeding season dominant males tend to be the brightest of greens while the defeated or intimidated males transition back into the drab brown coloration. Often, when basking in the sun, the colors of a lizard's skin can become bright and vibrant like the collared Lizard. A cold lizard may be dark and colorless. Lizards shed layers of old skin. The skin or scales of a lizard is made up of keratin. This substance is also what human fingernails are made of.
Most species of lizard are harmless to humans. One very notable exception is the komodo dragon, which is the largest species of lizard in the world. Komodo dragons have been known to stalk, attack and kill humans that get in their way. Some species of lizard have a venomous bite but none of these venomous species of lizard is toxic enough to really harm a human in most cases. The venoms of the Gila monster and beaded lizard are not usually deadly, but they can inflict extremely painful bites due to powerful jaws. Generally, if bitten by a venomous lizard, humans will receive a nasty and distressing bite, which is normally caused by the strong jaws and bite of the lizard rather than the small amount of venom contained in it.
Most lizards are carnivores (meat eaters), though some may be classed as omnivores (eaters of food of both plant and animal origin). The carnivores feed primarily on insects and other small bits of meat they find, such as worms. Some larger species are capable of tackling even rodents or small snakes, but for our backyards they help us out by consuming many of the insects we prefer not to deal with. Without a doubt lizards fit any description of a beneficial species.