Category Science

          Amphibians are a small group of cold-blooded, vertebrate animals. They include two main groups: the salamanders and newts, and the frogs and toads. The word “amphibian” means “double life”. Most amphibians spend the first part of their lives underwater, taking in oxygen from the water through gills, like fish. Their adult lives, however, are spent on land, breathing air through lungs. They return to the water to lay their eggs.

          Most amphibians have four limbs. Only the salamanders and newts have tails. All amphibians have moist skin, through which they can “breathe in” oxygen in the water. They can also take in or lose water from their bodies through their skin. Because they need to keep their skin moist, most amphibians spend much of their time in cool, damp conditions. Many live in or near water, even as adults.

          Amphibians undergo metamorphosis, a change in the body from young to adult. The most obvious change in salamanders and newts is that they lose their feathery gills and develop lungs. Frogs and toads change from plant-eating tadpoles with gills, a tail and no legs, to carnivorous, tail-less adults with lungs and long legs.

SALAMANDERS AND NEWTS

          Salamanders and newts are long-bodied amphibians with long tails. They are often nocturnal, and are all carnivorous. They feed on insects, worms, slugs and snails, and the young also prey on frog tadpoles. Some salamanders live in water all the time, and several even keep their gills into adulthood. Others live on land but return to the water to lay their eggs. Newts will travel several kilometres to return to the breeding ponds where they grew up.

FROGS AND TOADS

          Frogs and toads have short, tail-less bodies and long hind legs. Frogs are usually smooth-skinned. They spend most of their time in water. Their very long legs and streamlined bodies mean that they can make huge leaps and swim quickly. Toads have shorter legs, fatter bodies and lumpier skin. They live mostly on land, in dark, damp places to keep their skins moist and cool. Toads crawl or hop rather than leap.

          Both frogs and toads usually lay their jelly-like eggs (spawn) in water, often returning to the same pond or lake year after year to breed. Some tree frogs lay their eggs inside a foam “nest” on an overhanging branch, so that when the tadpoles hatch, they will fall into the water. Many frogs and toads leave their eggs to hatch on their own. Others carry them on their backs to keep them safe. Mouth-brooding frogs hold their tadpoles in a throat sac, until they jump out as tiny frogs.

          Frog and toad tadpoles mostly feed on water plants or filter algae from the water. As adults, however, they are carnivorous, feeding on insects, snails or worms. The larger frogs and toads will also eat fish, other frogs, and even small mammals.

          In hot climates, toads and frogs burrow underground to avoid the heat of the day. Frogs that live in colder climates often spend winter in hibernation at the bottom of ponds, breathing through their skin.

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          Forming a much larger group than the cartilaginous fish, bony fish have skeletons made of bone. Their bodies are usually covered with overlapping scales. They also have gas-filled swim bladders inside their bodies that allow them to stay afloat even when they are not moving.

          Some bony fish live in fresh water (rivers and lakes), while others live in the seas and oceans. Many feed on plant material, but some are carnivorous. Among the largest predators are the barracudas, which strike at great speed with their powerful jaws. Even larger is the blue marlin, which can be over four metres in length. Probably the most extraordinary of the large bony fish is the ocean sunfish, or mola mola. Its round body can be as large as a small car, and it “rows” itself through the water using its fins.

          Most bony fish have good eyesight, and can see in colour. Their eyes are on the sides of their heads, giving them a wide field of vision. Some kinds of bottom-dwelling fish have eyes that point upwards, to spot predators or prey above them. Fish such as salmon, which are preyed upon by many other animals, swim in large groups for protection in the same way as some animals, for example, cattle, herd together on land.

          Seahorses rely on camouflage to keep themselves safe, while angelfish stay near clumps of seaweed or rocks. Some fish hide in the stinging tentacles of sea anemones.

          Several kinds of fish have protective armour made of fused scales, while others have sharp spines. Bright colours are often a warning signal to predators. The markings of the lionfish warn that its long, sharp spines are poisonous. Other fish inflate their bodies suddenly to startle an attacker.

          Most fish have a familiar streamlined body shape, but several kinds look very different. Eels have long bodies with narrow dorsal fins that do not stick out from their bodies like those of other fish. Seahorses live in shallow waters and swim upright. They grasp on to seaweed with their tails to avoid being swept away by the current.

          Many ocean-living fish are found close to the surface, where there is warmth and light and tiny plants, called plankton, to eat. Small plankton-eating fish may be prey for larger fish.

          Fewer fish live in the deeper, darker waters. The oarfish lives between 300 and 600 metres down. With its ribbon-like body and red “mane”, it may be the sea monster described in old sea legends. Some deep-living fish travel up through the water to hunt, while others feed on scraps of food that drift down from the surface. Some deep-living fish are able to make their own small lights to attract prey towards their gaping mouths.

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          Fish are vertebrates (animals with backbones). They live in water all the time. Most kinds cannot survive out of water, because they do not have lungs to breathe air. Instead, they take in oxygen from the water using gills in their heads.

          Fish have hairless, streamlined bodies with fins and a tail. Many have a protective layer of overlapping scales. Fish are cold-blooded animals, so their body temperature depends on their surroundings. Some species have a special substance in their blood to stop them freezing in cold waters.

          Fish move by flexing the muscles along their bodies in a wave-like motion. The fins help to balance and steer the fish while the tail can be used for propulsion. The streamlined shape of most fish, as well as a coating of slimy mucus on their bodies, helps them to swim easily through the water.

          Most fish lay vast numbers of eggs at once, which they leave to hatch out on their own. Newly-hatched fish are perfect, tiny replicas of their parents. A few kinds of fish, such as sharks, carry their eggs inside their bodies, and give birth to live young.

          Fish were the first vertebrates to evolve, millions of years ago. One of the oldest types alive today is the coelacanth, fossils of which date back 90 million years.

          There are two main groups of fish, the cartilaginous fish and the bony fish. Cartilaginous fish include sharks, rays and chimaeras. Most are found only in the seas and oceans. Cartilaginous fish have skeletons made of soft cartilage. Instead of flat, overlapping scales, they have tiny, pointed, tooth-like scales. Their gills can be opened and closed, but, unlike the bony fish, usually do not have protective flaps covering them. Cartilaginous fish must keep moving all the time, to stay afloat in the water.

          Sharks are mostly predators, although the largest kinds of all, the whale shark and the basking shark, feed only on plankton, which they filter from the water using a part of their gills. Unlike bony fish, most of which have good vision, sharks rely on smell to hunt their prey. Sharks can detect a single drop of blood in a huge volume of water. They also have sensors on their bodies that can pick up tiny electrical signals produced by the movements of their prey.

          Sharks have large, razor-sharp teeth, which arc set in rows. If the shark loses a tooth during feeding, a new one comes forward in the mouth to take its place. Most sharks feed on fish or squid, but the larger sharks, such as the great white, also eat turtles and large mammals such as seals. They slam into their prey, tearing out huge chunks of flesh. Despite their fearsome reputation, only the largest of the predatory sharks, such as the tiger shark or the great white shark, are dangerous to humans.

          Rays, and their relatives the skates, are cartilaginous fish with flat bodies and often long, narrow tails. Their gills and mouths are on their undersides. They have large, wing-like fins that they flap as they swim, making them look as if they are flying through the water. Rays feed on fish and shellfish near the sea bed. Sometimes they hide in the sand to ambush passing prey.

          Like sharks, a few kinds of rays can be dangerous. Torpedo rays can give off a powerful electric shock, while a jab from the venomous spine on a stingray’s tail can kill a human. Unlike sharks, however, rays do not see humans as prey.

          Chimaeras are a group of cartilaginous fish with long bodies and tails. They include the rabbit fish, a bottom-dwelling fish with large eyes and a venomous spine on its dorsal fin.

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          Spiders and scorpions are arachnids. They differ from insects because they have two, rather than three, body sections, eight legs instead of six and no antennae or wings. Most arachnids also have eight simple eyes and no compound eyes. They rely more on sound than sight. Most have bristles on their legs that are sensitive to vibrations in the air or through the ground.

          Arachnids feed by piercing their prey with their sharp fangs, paralyzing or killing it. Their saliva turns the body tissues of the prey into liquid that can then be sucked up.

          Spiders are carnivores, feeding mainly on insects and other spiders. Many spiders use webs to catch their prey. Any insect that flies into a web is caught in the strands, which are often sticky. As the insect struggles to free itself, the spider, sitting at the edge of the web, feels the vibrations and comes to claim its prey. It avoids becoming caught in the threads itself by walking on the claw-like tips of its feet.

          Silk is used by the spider for many other purposes besides forming webs. It can be a safety line when jumping, be woven into a sac by a female to hold her eggs, or used to wrap up prey before feeding.

           All spiders have poisonous venom, which they use to paralyze or kill their prey. Some, such as the black widow spider, are so poisonous that they can kill humans.

SCORPIONS

          Scorpions have a sharp sting on the end of their tail-like abdomen that injects poison into their prey. It is also used for defence. Like other arachnids, scorpions have an extra pair of “limbs” either side of their jaws. These form large pincers for the scorpion to grasp its prey. After hatching, young scorpions climb up their mother’s pincers on to her back, where they are carried until they shed their skins for the first time and become independent.

MYRIAPODS

          Myriapods are a group of arthropods that includes centipedes and millipedes. Myriapods have long bodies made up of segments, and as many as 200 pairs of legs. They must live in dark, damp places because their bodies easily dry out.

          Millipedes have short, strong legs for burrowing through soil or dead leaves. Most feed on plant material, chewing with their strong jaws. Centipedes are carnivorous predators. Most have longer legs than the millipedes, and can scuttle at great speed after their prey. They use the large “poison-claws” on their heads to capture and paralyze their prey before eating it.

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            Like termites, ants live in large colonies with a winged queen and king, wingless workers, and sometimes soldier ants. Their nests have separate chambers for eggs, larvae and sometimes even food stores.

          Bees nest underground or in hollow trees and other small spaces. They feed on pollen and nectar which they collect from flowers using their long tongues.

          Wasps are carnivores. Some solitary-living wasps lay their eggs on or inside hosts, often butterfly or moth caterpillars. The larvae feed on the host as they grow, usually killing it in the process. Other wasps build nests from mud, or paper nests made out of chewed-up wood. These kinds of wasps bring their insect prey to the nest to be fed to the grub-like larvae.

            Unlike a queen bee, which always has workers in attendance, a queen wasp finds a nest site by herself and raises the first brood of workers. The workers then continue the nest-building and other tasks.

          The black and yellow stripes of bees and wasps are a warning sign to predators. Both bees and wasps have a sharp, pointed sting on their abdomen which injects painful venom into an attacker’s skin. Honeybees defending their nests are able to sting only once, as the act of stinging also kills them.

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          The largest and most successful groups of insects are the beetles. All beetles have two pairs of wings, but only the back pairs are used for flying. The front wings, called the elytra, form a hard, protective case. This fold over the fragile back wings when the beetle is on the ground. As the beetle takes off, the elytra open. Some beetles do not fly at all, so their elytra are fused together.

          The hard elytra are good protection against predators. Many beetles also have other methods of defence, such as squirting poisonous chemicals at their attackers, or startling them with loud noises.

          Beetles hatch from their eggs as grub-like larvae. They grow and then pupate, emerging as fully-grown adults. Some plant-eating beetle larvae are legless grubs, while predatory beetle larvae have large jaws and legs which they use to catch their prey.

          Fleas live on or near the bodies of animals, feeding on their blood and so often spreading diseases from one animal to another. Fleas carried on the bodies of rats were responsible for the plague in the 14th century, known as the Black Death. Fleas have a powerful “trigger” in their hind legs that allows them to travel over 100 times their own length in a single leap.

          Some flies also feed on blood, biting animals and humans and spreading disease. However, most flies are very useful insects. Many help to pollinate flowers by feeding on nectar and pollen, while others are scavengers. They actually help to keep our environment clean and healthy, by feeding on dung or rotting material such as dead plants and animals.

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