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Spiders are arachnids not insects, but both spiders and insects belong to the largest group of animals on Earth, the arthropods – animals with hard external skeletons and jointed limbs (greek arthro = joint, podos = footed). Arachnids have the head and thorax combined (cephalothorax) with simple eyes, jaws adapted for tearing or piercing prey, a pair of pedipalps and eight walking legs.
Worldwide there are about 70,000 species of arachnids of which 36,000 are spiders. Approximately 2,900 species of them are found in Australia. Only a few species, including the Redback Spider and some funnel-web and mouse spider species, can inflict bites that are potentially fatal to humans. One of the most dangerous of all is the Sydney Funnel-web Spider. There have been no fatalities from Sydney Funnel-web bites since the development of effective anti venom.
Spiders are the only arachnids that have special glands in their abdomen which produce silk.
Spiders were among the earliest animals to live on land. The first definite spiders lived 380 million years ago during the Devonian Period, more than 150 million years before the dinosaurs.
Spiders live in almost every habitat on earth. The only places where there are no spiders are the Polar Regions, the highest mountains and the oceans. A few spider species have invaded the ocean’s edge, living in the rock and coral crevices of the intertidal zone.
Spiders are an important and fascinating part of our natural environment. Their webs are wonders of natural architecture. They have major ecological and agricultural roles as killers of insects. Both their venom and silk are being used in medical research (stroke treatment), pest management (insect specific pesticides) and fibre technology (transgenic biosilk production).
In many parts of Australia spider populations are threatened because their habitats are being destroyed by the clearing and degrading of bush land. Conserving spider habitat not only saves the spiders but also the whole ecosystem of which they are a part. Habitat conservation is an essential element of maintaining sustainable ecosystems.
Many different spiders live alongside humans exploiting the nooks and crannies of houses, sheds and gardens. They are good to have around because they eat insect pests. Very few are harmful.
Some of the smallest spiders in the world are never seen by most humans because they only have pinhead-sized bodies. Small spiders are usually found in damp, cool habitats like forest leaf litter and moss because their small bodies can lose water rapidly in dryer conditions. The largest spiders in the world include the South American Goliath Tarantula, some so big their legs can span a dinner plate. Such spiders may take decades to reach such a size. However, spider size is limited, partly because their respiratory physiology becomes less efficient at very large sizes.
Many spiders have unusual body shapes and colours. This is helpful to spiders in various ways – to deceive and ambush prey, to capture particular sorts of prey, to avoid being eaten and to attract mates.