A reptile is a vertebrate which, like amphibians, is ectothermic, its body temperature influenced by the temperature of its surroundings. Its body is covered with dry skin, from which grow scales.
Body wastes, eggs and sperm all leave the body of a reptile through the final section of the gut, which is called the cloaca. A reptile's kidneys can change body wastes from liquid to solid form. Land reptiles' urine forms part of the limy waste material passed out of their bodies, but aquatic reptiles usually pass fluid urine as well as solid wastes.
The sperm produced by a male reptile fertilizes a female's eggs inside her body (in amphibians, fertilization takes place outside the body). The fertilized eggs of most reptile species are enclosed in shells and are laid on land. However, some species retain their eggs within their bodies until the young hatch.
Most reptiles eat other animals, though a few eat plant material.
Most reptiles have four limbs. Snakes have no limbs (though pythons have remnants), and some lizards have reduced limbs, only two hind limbs, or scaly flaps instead of hind limbs.
Lungs and hearts
When it breathes, a reptile expands its ribs, drawing air into its lung. After oxygen and carbon dioxide have been exchanged in the lungs, the reptile breathes out by contracting its ribs. In a few water-living species, some exchange of gases takes place through the skin or even the cloaca.
Turtles, lizards and snakes have three-chambered hearts. In the single lower chamber, oxygenated blood coming from the lungs may mix with deoxygenated blood from the body.
Crocodiles have four-chambered hearts, like those of birds and mammals. Normally oxygenated and deoxygenated blood do not mix, but during a long dive, when fresh air is not available, a special valve allows deoxygenated blood to pass back into the crocodile's body tissues.
Desert survivors
Australia's many dry areas are full of reptiles which are adapted to arid conditions in a number of was:
·Their body wastes have very little water in them.
·They obtain water from their food, and from licking dew. The scales of some lizards channel water to the mouth.
·Because of their low energy requirements, reptiles can survive food scarcities during drought.
·Small reptiles feed on ants and termites, which are plentiful in the desert. The small reptiles are then eaten by larger reptiles.
·A reptile becomes active when the temperature is right for its species. In the desert, it may forage at night, and shelter in a burrow during the day, or spend daytime shuttling from sunshine to shade and back again. Some lizards climb some distance off the ground into vegetation to avoid ground heat.
Wetlands predators
The floodplains of northern Australia support a greater weight of predators in a given area than do Africa's Serengeti Plains.
The African predators are mammals such as lions and hyenas. The Australian ones are reptiles, such as water-living pythons, file snakes, freshwater turtles, crocodiles and a variety of lizards, including the Frilled Lizard. These creatures eat each other, as well as insects and other invertebrates, fish, frogs and small mammals.
2008年4月25日星期五
2008年4月7日星期一
How spiders live and behave?
Silk, the secret of success
One major reason for the success of spiders is the silk they produce and the ways in which they use it.
Spider silk is mainly made up of proteins. It leaves the spider's body as a liquid which hardens rapidly in the air, then does not dissolve in water. It is very strong, and can stretch to add one-third more to its length, then snap back to its original length without change in shape.
Silk is produced in glands which make up a large part of a spider's abdomen. Different glands make different sorts of silk to be used for purposes including the manufacture of egg sacs, lining shelters and constructing webs. Silk is also used for the safety lines many spiders drag behind them, which save them if they fall. Young spiders spin silken threads on which they "balloon" through the air.
Many tiny tubes connect each silk gland with a spinneret. The sticky fluid which coats the strands in orb webs and catching lines is applied in these tubes.
Mate, not dinner
A female spider is often much larger than a male of the same species, and is sometimes quite different in appearance. Courtship is lengthy, for the male must convince the female he is a mate and not prey.
To introduce himself, a male may posture, dance, pluck a female's web or present gift-wrapped prey. He spins a special web, expels sperm onto it, then sucks the sperm into bulbs on the ends of his pedipalps. When he judges it is safe to do so, he inserts the end of a pedipalp into the female's reproductive opening and discharges the sperm it carries.
The female stores the sperm until she is ready to lay eggs. She may lay 100 at a time, protecting them in a silken egg sac.
Sacs and tiny spiders
Silken egg sacs may be hidden in vegetation, or in crevices, or buried in the ground. Most are guarded. The sacs may be carried around on the spinnerets, between the legs or in the jaws. Female wolf spiders may allow young to climb on their backs and transport them.
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Camouflage
Spiders may rely on camouflage to protect them from predators such as birds. They may blend in with the colour or texture of their background in order to lie in ambush waiting for their own prey. A wraparound spider has a broad abdomen which is so flattened that the spider can wrap itself around a twig. Bird-dropping spiders and crab spiders are also well camouflaged by colour and texture. Some spiders mimic insects such as ants.
One major reason for the success of spiders is the silk they produce and the ways in which they use it.
Spider silk is mainly made up of proteins. It leaves the spider's body as a liquid which hardens rapidly in the air, then does not dissolve in water. It is very strong, and can stretch to add one-third more to its length, then snap back to its original length without change in shape.
Silk is produced in glands which make up a large part of a spider's abdomen. Different glands make different sorts of silk to be used for purposes including the manufacture of egg sacs, lining shelters and constructing webs. Silk is also used for the safety lines many spiders drag behind them, which save them if they fall. Young spiders spin silken threads on which they "balloon" through the air.
Many tiny tubes connect each silk gland with a spinneret. The sticky fluid which coats the strands in orb webs and catching lines is applied in these tubes.
Mate, not dinner
A female spider is often much larger than a male of the same species, and is sometimes quite different in appearance. Courtship is lengthy, for the male must convince the female he is a mate and not prey.
To introduce himself, a male may posture, dance, pluck a female's web or present gift-wrapped prey. He spins a special web, expels sperm onto it, then sucks the sperm into bulbs on the ends of his pedipalps. When he judges it is safe to do so, he inserts the end of a pedipalp into the female's reproductive opening and discharges the sperm it carries.
The female stores the sperm until she is ready to lay eggs. She may lay 100 at a time, protecting them in a silken egg sac.
Sacs and tiny spiders
Silken egg sacs may be hidden in vegetation, or in crevices, or buried in the ground. Most are guarded. The sacs may be carried around on the spinnerets, between the legs or in the jaws. Female wolf spiders may allow young to climb on their backs and transport them.
。
Camouflage
Spiders may rely on camouflage to protect them from predators such as birds. They may blend in with the colour or texture of their background in order to lie in ambush waiting for their own prey. A wraparound spider has a broad abdomen which is so flattened that the spider can wrap itself around a twig. Bird-dropping spiders and crab spiders are also well camouflaged by colour and texture. Some spiders mimic insects such as ants.
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