A cross-section below ground In every house, garden, field or forest on Earth, there is a world of activity that we seldom notice. On the ground, in the soil, in the air and in the water live many small creatures such as insects, spiders, millipedes, worms and snails. They are all invertebrates, animals without backbones. Some are large, predatory creatures, big enough to feed on rodents and small lizards. Others are far too tiny for us to see without a microscope. Invertebrates are an extremely wide-ranging group of animals—far larger than vertebrates. Land invertebrates include insects, spiders, scorpions, worms, gastropods, millipedes and centipedes. There are even more marine invertebrates, animals such as crabs, sponges and molluscs, which live in the world's oceans.
Arthropods are invertebrates that have a hard outer skeleton, called an exoskeleton, instead of an internal skeleton. This is made of a light, strong material, called chitin, that supports and protects their soft inner parts. They have legs and antennae made up of jointed segments (the word arthropod actually means "jointed legs").
Insects, centipedes, spiders, scorpions and crustaceans (including woodlice) are all types of arthropod. As arthropods grow, they moult their outer skeleton—which cannot grow any bigger—and grow a new one.
Common earwigInsects are a group of arthropods. All insects have six legs and a body divided into three sections: the head, thorax and abdomen. Insects have a pair of long sense receptors on their heads, called antennae. Most insects have one or two pairs of wings. Many insects are plant-eaters, but others are carnivorous, feeding on other insects or small animals. The mouth shape of an insect depends on its diet: some have tube-like mouths for sucking up fluids; others have jaws for chewing up their food.
Arachnids are a group of arthropods that have eight legs. They include spiders and scorpions. Arachnids feed by piercing their prey with sharp fangs, paralyzing or killing them with their venom. Their saliva turns the victim's body tissues into liquid that can then be sucked up.
Myriapods are a group of arthropods that includes centipedes and millipedes. They have long bodies made up of segments, and as many as 300 pairs of legs or more. Myriapods must live in dark, damp places because their bodies, lacking waterproofing in their exoskeletons (outer skin covering), easily dry out. The name myriapod means “many legs”.
Earthworm Worms are long, thin, soft-bodied invertebrates that have no legs, eyes or ears. Some worms burrow in the ground, while others live in water or as parasites inside other animals or plants. Earthworms are a kind of segmented worm, worms whose bodies are divided into ring-like segments. The group also includes leeches and ragworms.
Slugs and snails are gastropods, a group of molluscs with soft bodies that move about on one large foot. They move by alternately shortening then stretching their bodies. Slugs and snails have two pairs of feelers, used for sight, touch and smell. Most species feed on leaves and fungi. Unlike slugs, snails have hard shells, which give them some protection from predators.
Tardigrades (also known as water bears) are tiny water-dwelling, eight-legged animals with clawed feet, usually no more than 0.5 mm when fully grown. First discovered in 1773, they live in many environments, including high mountaintops, polar regions, deep oceans and tropical rainforests—although they are commonly found on mosses and lichens where they feed on algae and small invertebrates. Tardigrade fossils date back 530 million years to the Cambrian Period.
Tardigrades are the hardiest of all known organisms, able to survive extreme conditions that would instantly kill nearly all other life forms. They can withstand crushing pressure, temperature ranges from −272°C (−458 °F ) to 150°C (300 °F) and radiation hundreds of times higher than the lethal dose for a human. They can go without food or water for more than 10 years, and live in the vacuum of outer space.
Consultant: Chris Jarvis
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