THE RAINBOW LIZARD: Agama agama

EXTERNAL FEATURES OF THE RAINBOW LIZARD: Agama agama
The Rainbow lizard is a common reptile in the tropics. It is active only during the day and thus said to be diurnal. It basks in the sun to raise its body temperature. It however, hides in shady places when the sun becomes very hot and the body temperature. becomes dangerously high. Rainbow lizards live in groups or colonies, which is usually made up of one mature and very colourful male called cock lizard and a number of females and young ones. The cock lizard ' shows a territorial behaviour as it fights and wards away all other matured males within an area or territory and mates with all females in that territory.


Structure:
The body of the lizard consists of a head and trunk. The head is joined to the trunk by a short neck. It has a long tapering tail. The entire body is covered with dry, horny and overlapping scales. The scales protect the lizard against mechanical injuries and desiccation, which enables the lizard to survive in its terrestrial habitat. The body is also elongated.


Head: The head is wedge-shaped and bears a wide-gaped mouth which is terminal in position. The wide-gaped mouth enables the lizard to swallow its prey whole. In the mouth are a number of similar, conical and sharp teeth. called homodont teeth, which are used to hold onto the prey to prevent it from escaping. The teeth are not used for chewing food. In the mouth is a long, sticky tongue, used for trapping preys. Above the mouth is a pair of nostrils used for gaseous exchange. Posterior to the nostrils is a pair of slightly protruding or bulging eyes with eyelids, nictitating membrane and a ridge of hard scales as shown above. Posterior to the eyes and slightly at a lower level is a pair of eardrums or tympana. The eardrums are found in a shallow depression on the lateral sides of the head. They are used for hearing. The short neck allows the lizard to turn the head without moving the whole body. Above the neck on the mid-dorsal line is a crest of skin called nuchal crest, which is more conspicuous in the male.
It is aised when the male is in an aggressive state or lighting. Below the chin and neck is a fold of skin called gular fold, which is also more conspicuous in the male. It is lowered during courtship display by the male.


Trunk: The trunk bears two pairs of limbs: a pair of fore limbs and a pair of hind limbs. Both pairs of limbs are laterally attached to the trunk, this arrangement however, does not permit the body to be raised very high above the ground. Each leg terminates in five digits which end in sharp curled claws. Each fore limb is distinguished into an upper arm, lower arm and hand. The fore limbs are short and provide support. The hind limbs are distinguished into a thigh, shank and foot. They are elongated and more mascular and are used to provide the thrust for fast running and jumping. On the ventral surface and slightly behind the hind limbs is a transverse vent called the cloaca, used for defaecation and mating. There is a hemi-penis in the male at each end of the cloaca. The cloaca is boarded by hard scales called pre-anal pads, which ensure that the cloacas of the mating partners are brought in very close or intimate contact. The trunk terminates in a long tapering tail. The tail easily breaks to allow the lizard to escape from predators when it is caught on the tail. The tail is capable of some degree of regeneration after being broken.

The adult male is longer, larger and has more brilliant coloration than the adult female. The cock lizard has an orange head, dark blue trunk and a tail which is dark blue or blue at the base orange in the middle and black at the tip. The gular fold and nuchal crest are also more prominent or conspicuous in the cock lizard. The cloaca is boarded by pre-anal pads and the presence of a hemi-penis in the cock lizard.

The adult female has a grey or greenish brown head and brown or black trunk and tail with yellow spots. The nuchal crest and gular fold are not conspicuous or are less prominent in The Size and colour in both sexes is similar in the young lizards.


LIFE PROCESSES OF THE LIZARD

Locomotion: The lizard can move slowly, run fast and jump. The fore and hind limbs are laterally attached or at right angles to the body. This makes it difficult for the body to be raised above the ground. During movement, the body is slightly raised from the ground while the fore limbs provide support and pull the body forward. The more muscular and elongated hind limbs provide the thrust which pushes the body forward. They also provide the force for take-off during jumping. The inability of the lizard to raise its body above the ground 'over long periods of time, does not permit the lizard to run over long distances contmuously.The opposable digits on the two pairs of limbs enable the lizard to grasp oblects when climbing walls and trees.

When it jumps, the limbs and belly are used to absorb the shock of landing. When it is not moving, the body rests on the belly.

Nutrition: The rainbow lizard, feeds on insects such as termites, ants, grasshoppers, crickets, praying mantis and cockroaches. These are trapped with the long, extensible sticky tongue. The food or preys are swallowed whole with the help of the wide-gaped mouth. Larger preys are however, grasped by the mouth and crushed with the powerful jaws and teeth before swallowing. The homodont teeth are used to prevent the prey from escaping.

Gaseous Exchange: The two lungs are the organs for gaseous exchange. They lie in the rib case at the anterior region of the trunk. During inhalation, the intercostals muscles contract, the ribs are lowered and volume of the abdominal cavity increases. Pressure on the lungs decreases, the lungs expand and air passively rushes into the lungs from the atmosphere through the nostrils and r espir atory tubes. Gaseous exchange occurs in the lungs. Oxygen diffuses into the blood and Combines with haemoglobin to form oxyhaemoglobin. The absorbed oxygen is used in respiration to release energy. Carbon (IV) oxide evolved, diffuses into the lungs from the blood.

During exhalation, the intercostals muscles relax, the ribs are raised and the volume of the abdominal cavity decreases. The pressure on the lungs increases which forces the expired air out of the lungs through the nostrils. ‘

Excretion: The main nitrogenous waste substance is uric acid, which is removed from the blood by the two kidneys. The uric acid is emptied into the alimentary canal, where it mixes with feaces. Water is reabsorbed from the waste materials in the rectum and solid uric acid is eliminated from the body through the cloaca. Excretion of dry or solid uric acid is an adaptation for water conservation for a successful terrestrial life.

Carbon (IV) oxide evolved during respiration. is eliminated by diffusion through the lungs.

Reproduction: The matured male rainbow lizard mates with all the adult females in its territory. Mating is preceded by courtship display by the cock lizard. This involves the lowering of the gular fold, raising of the nuchal crest and nodding of the head. It chases and grips the female by the neck and pushes its tail beneath her to attach its choaca to that of the female. The pre-anal pads bring their choaeas into intimate contact. The hemi-penis is used to introduce sperms into the cloaca of the female. The sperms fertilize the eggs internally.

The female lazard digs a hole in the soil with its fore limbs and lays about six eggs, which are covered by porous leathery shell into the hole.
The eggs are covered by the female with soil and left unattended to by both parents. The eggs absorb moisture or water from the soil and swell to several times their original size. After about six or seven weeks, the eggs hatch into young lizards, which use their “egg-tooth located in the snout, to cut through the leathery shells. They then crawl out of the soil. They feed on insects and grow. The young lizards resemble the adults but are smaller, lack adult colouration and are sexually immature. The young lizard becomes sexually matured in about eighteen months.



ADAPTATION OF LIZARD T0 TERRESTRIAL HABITAT

The body of the lizard is covered with dry overlapping scales. The scales protect the lizard from mechanical injuries and desiccation or loss of water on land. Conservation of water is also aided by the excretion of solid or dry uric acid as its nitrogenous waste product. Internal fertilization and the development of the fertilized egg or embryo on land adapt the lizard to terrestrial life. The above characteristics are adaptations for reproduction and water conservation for successful terrestrial life.


Adaptation to Movement:
The short fore limbs provide support on the ground and pull the lizard forward during movement. The elongated and more muscular hind limbs provide the thrust which pushes the body forward during movement. They also provide the force for take-off during jumping. The opposable clawed digits enable the lizard to grasp objects during climbing of trees and walls.


Adaptation for Feeding:
The wide-gaped mouth is used to swallow their Preys whole. The powerful jaws are used to crush larger preys before swallowing. The homodont teeth are used to hold onto prey to prevent them from escaping during feeding.
The long, extensible sticky tongue is used by the lizard to trap or catch preys.

Adaptation to Gaseous Exchange:
The possession of a pair of nostrils and a pair of lungs enable the lizard to utilize atmospheric or gaseous oxygen on land. The surface of the lungs and the epithelial lining of the respiratory tubes are constantly kept moist with mucus produced by mucus-secreting glands in the epithelial lining. This enables respiratory gases to dissolve before diffusion, ensuring efficient diffusion of the gases. The alveolar pockets in the lungs are thin-walled, highly vascularised and have a large surface area. This makes diffusion and transportion of respiratory gases more efficient.

Possession of accessory respiratory structures, ribs and intercostal muscles, enable inhalation and exhalation to take place. This ensures that the lungs are ventilated with a steady stream of air.


Adaptation for Protection:
The possession of dry, overlapping scales protect the lizard against loss of water and mechanical injuries. The long tail easily breaks to allow the lizard to escape from its predators when caught by it. The tail is however, somewhat capable of regeneration. The possession of a short neck allows the head to turn independently of the rest of the body. This allows the lizard to watch its predators without having to turn the whole body and thus enable it to successfully escape from predators.

The possession of opposable digits with sharp claws is to grasp objects, to climb trees and Walls and to escape from predators. It can also jump from great heights and land on the fore limbs and belly to escape from predators.

Post a Comment

0 Comments