The Order Squamata

Lizards and snakes

Members of this order, the most successful group of modern reptiles, have bodies covered with small overlapping scales. There may be a single row of broad scales along the belly, and the head often has well-defined scales, sometimes containing bony plates fused to the skull. The tongue is notched or forked, the teeth are fused to the rims of the jaws. Some lizards are legless, like snakes. The females usually lay eggs with parchment-like shells, but some hatch the eggs in their bodies. There are two sub-orders, whose members are not always easily distinguishable.

Sub-Order Lacertilia
Many lizards are snake-like in appearance with small limbs, or no limbs at all, and small eyes and ears. Some families consist entirely of limbless forms, and many others have some limbless members. A few lizards - some skinks, for example - give birth to live young. Some forms can shed the tail and regenerate it, but the new tail lacks the original pattern and has a cartilaginous rod instead of vertebrae. The skin is shed in pieces, and many lizards can change colour, if only slightly. There are 20 families.

Family Gekkonidae
Geckos: 400 species
These lizards are usually nocturnal and have big eyes, generally covered by a transparent membrane which is cleaned with the tongue. Their digits are short, usually with large, backward-curved claws, and most geckos have hair-like filaments on the undersides of their digits which can hook on to irregularities on vertical surfaces as smooth as glass. They are the only lizards with well-developed voices.

Family Pygopodidae
Flap-footed lizards: 13 species
These snake-like lizards from Australasia lack forelimbs and have flap-like hind limbs.
Family Xantusiidae
Night lizards: 11 species
These nocturnal lizards with geck-like eye membranes hide by day in crevices in rocks and logs. They are found in Central America and south-western North America.
Family Iguanidae
Iguanas: about 700 species
Iguanas have teeth on the inner sides of their jaws. There are long-legged, often swift-running forms, and some have a scaly crest on their backs. They occur mainly in the New World, but also in Madagascar, the Galapagos Islands, Fiji and Tonga.
Family Agamidae
Agamid lizards: about 300 species
The agamid lizards are the counterparts in Europe, Africa, Asia and Australia of the iguanas, and closely resemble them, except that their teeth are borne on the rims of the jaws. Like iguanas, some forms have high crests supported by vertebral spines. Some species have throat sacs, some have spiny tails, some have spiny skins, and some have frills and run on their hind legs.
Family Chamaeleontidae
Chameleons: 85 species
The feet of these tree-living lizards are adapted to grasping by having two toes opposed to the other three. Chameleons also have grasping tails. Their eyes move independently and their fused eyelids admit light through a small hole in the middle. The skull is crested, and the sticky tongue, which is half as long as the animal, shoots out to catch insects. Some members of the genus Chamaeleo, which contains 73 of the 85 species, have flamboyant horns.
Family Dibamidae
Burrowing lizards: 4 species
Found in Indo-China and from the Philippines to New Guinea, and in Mexico, these short-tailed lizards are blind and earless, and are either limbless or have small limbs.
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Family Scincidae
Skinks: about 700 species
The bodies of skinks tend to be elongated, and their limbs are small and sometimes absent, as are their ear apertures. They burrow in sand or live in leaf litter. The family includes the genus Eumeces, comprising 59 species, found in North and Central America, Asia and North Africa. These have longer legs than other skinks, and live in steppe and stony desert. About half the species lay eggs; the others bear live young.
Family Feyliniidae
Limbless skinks: 4 species
These termite-eaters from equatorial Africa and Madagascar are entirely without legs.
Family Gerrhosauridae
Plated lizards: 25 species
Found in the dry areas of Africa and Madagascar, these lizards have armour-like scales.
Family Cordylidae
Girdle-tailed lizards: 23 species
These lizards are found in the grasslands of eastern and southern Africa. Members of the genus Platysaurus have flattened bodies, enabling them to shelter in rock crevices.
Family Lacertidae
African and Eurasian lizards: about 150 species
These small, agile lizards have very long fragile tails and head shields which are fused to the bones of the skull.
Family Teiidae
South American lizards or tegus: about 200 species
Tegus are the New World counterparts of the Lacertidae, but their head shields are separate from their skull bones.
Family Anguidae
Slow worms: 40 species
These lizards are found in the New World, Europe and parts of Asia; they have fragile tails and forked tongues. Some species are completely legless.
Family Anniellidae
Legless lizards: 2 species
These Californian lizards are found in sandy ground, in which they catch their insect prey. They are small burrowers with no limbs or external ears, and small eyes.
Photo by John H Tashjian
Family Xenosauridae
Xenosaurid lizards: 4 species
This is a little-known group. All species have strong legs and robust bodies covered in a mixture of large and small scales. There are three species in Central America and one in China.
Family Helodermatidae
Venomous lizards: 2 species
These are the only poisonous lizards. The venom glands are in the lower jaw, not in the upper jaw as in snakes.
Family Lanthanotidae
There is one species, the earless monitor, Lanthanotus borneensis which is found in Borneo. It has short legs and its lower eyelids have a clear window.
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Family Varanidae
Monitors: 24 species
All these large lizards belong to the genus Varanus, found throughout the warmer regions of the Old World. They are swift-moving predators.
Family Amphisbaenidae
Worm-lizards: 120 species
This is a family of burrowing, legless lizards with short, thick tails. The small head is not marked off from the body, so both ends look similar - hence the family's scientific name ('going both ways'). They are found in Florida, Mexico, South America, the Mediterranean and Africa.

Sub-Order Serpentes
Most snakes, unlike lizards, are adapted to swallow prey larger than themselves; flexible ligaments and joints allow the two parts of the lower jaw to move apart during swallowing and give them some independence of movement. Snakes are always legless, and their skin is usually shed whole. The tail does not regenerate if lost; the eyelids are fused to form a transparent covering. The left lung is usually reduced in size, while the right lung is greatly enlarged and elongated. In many forms the forked tongue is protruded and retracted constantly through a notch in the snout, without the mouth being opened. Snakes probably evolved from burrowing lizards, and burrowing snakes, like lizards, appear to have evolved several times. Venomous snakes predominate only in Australia. There are 11 families.

Family Anomalepidae
Anomalepid snakes: about 20 species
This is a small tropical South American group of blind snakes, with large head shields.
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Family Typhlopidae
Blind snakes: 150 species
This is the commonest burrowing family. The skull is rigid and the front of the head has a single plate for pushing through earth. The body is cylindrical, and the very short tail ends in a spine. There are no teeth in the lower jaw; and the eyes are tiny and often do not function. These snakes, which come to the surface after heavy rain, feed on earthworms and millipedes. They are found in sourthern Eurasia, Africa, Madagascar, tropical parts of the Americas and Australia.
Family Leptotyphlopidae
Blind or thread snakes: 40 species
These blind snakes, with large teeth in the lower jaw and none in the upper, are found in Africa and tropical America.
Family Uropeltidae
Shield-tailed snakes: 43 species
These small, primitive burrowers from India and Ceylon have a large shield-like scale at the end of the tail. They do not lay eggs, but give birth to aboaut six live young.
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Family Aniliidae
Pipe snakes: 10 species
Members of this small family of burrowers, from South America and south-east Asia, still have vestiges of hind limbs. They prey on other snakes.
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Family Xenopeltidae
There is one species, the sunbeam snake, Xenopeltis unicolor, found in south-east Asia. A burrower with irridescent brown scales, it preys on other snakes. The teeth of its lower jaw are set in a loosely hinged bone.
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Family Boidae
Constrictors: about 70 species
These large, non-poisonous snakes often have claws, which are vestigial hind limbs, and sometimes they have two fully developed lungs. They kill their prey by constriction; the snake coils its body round its victim and squeezes, causing suffocation. Nearly all constrictors are found in the tropics. The largest species, the reticulated python Python reticulatus occasionally grows to 33 ft.
Family Acrochordidae
Oriental water snakes: 2 species
These snakes are well adapted to life in estuaries and coastal waters, with nostrils on top of the snout, and small eyes. They give birth to about 30 live young at a time.
Family Viperidae
Vipers: 100 species
The viper has tubular fangs at the front of the mouth, folded back when the jaw is closed, but erect when it is open; they are so long that the viper need only strike, and not chew, its victim. The venom runs down a canal in each fang, and is more potent just after the snake sheds its skin. It primarily affects the circulatory system of the victim and causes swelling, inflammation and haemorrhage. There is only a vestige of a reduced lung, and often no trace of it at all. The family includes the rattlesnakes of the genera Crotalus and Sistrurus whose modified tail skin vibrates to produce a warning. Rattlesnakes are members of the group known as pit-vipers because they possess heat-sensitive organs located in pits between the eyes and the nostrils. Pit-vipers are found mainly in America, whereas true vipers, which lack facial pits, live in the Old World.
Family Elapidae
Cobras, mambas, coral snakes and sea-snakes: about 200 species
All these snakes are highly poisonous. The grooved or tubular fangs at the front of the mouth are not very long, so the poison must be injected by chewing. The venom affects mainly the nervous system, and does not usually produce local effects. The sea-snakes usually give birth to live young, but the land forms usually lay eggs.
Family Colubridae
Colubrid snakes: about 1100 species
This family has the most species and its members are found throughout the world. Most are harmless, some have poison, but the small fangs at the back of the jaw are rarely harmful to large mammals. There are no traces of hind limbs, and the left lung is small or absent.

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