Eight-legged spiders are the most numerous members of the arachnids with over over 40,000 identified species.
Spiders are found on every continent, bar Antarctica, and inhabit most habitats with the exception of the sea. Some spiders can be found 5,000 metres in the air, using a behaviour called "ballooning" to transport them over vast distances. They are carried by a fine trail of silk, which acts like a kite.
Being invertebrates, spiders have an exoskeleton comprised of a cuticle made of chitin. Their bodies are made up of two main parts; the cephalothorax and abdomen. Most spiders have eight eyes and their special mouthparts, called chelicerae, that are specialised with venom-injecting fangs.
Spiders are almost exclusively carnivorous, with most digesting their prey before consuming by injecting them with enzymes. All that is left is an empty shell.
Spiders hunt by passively using sticky silk webs or actively catch by ambushing using silk lassos and nets, jumping on or running down their prey. Their main prey consists of an array of insects, however some of the larger spiders will predate on lizards and birds.
There is one exception to the carnivorous spiders rule, with one herbivorous species called Bagheera kiplingi. A type of jumping spider from Central America, Bagheera kiplingi consumes a protein and fat-laden liquid from the numerous nubs on a certain kind of tree, although they do also supplement their diet by capturing various prey.
Light, elastic and strong, silk is one of the most incredible materials found in nature. Silk is famously stronger by weight than a corresponding steel thread.
Silk is utilised by spiders for a variety of functions, including the building of sticky webs, safety lines, webbed dens for protection from the environment, wrapping up prey or offspring, use as a net or lasso for catching prey or as a kite to take the spider airborne.
Silk is produced by the spinnerets located at the rear of the spiders abdomen. Initially, silk is a liquid mix of protein that solidifies in contact with air.
Relationship With Humans
Spider venom may be medically useful as chemicals found in the toxin may prove vital for treating a number of maladies. As most spider venom specifically targets invertebrates it has also proved to be a useful non-polluting pesticide.
Arachnophobia, the fear of spiders, is a common phobia among humans. Although there is a case to suggest that the fear is instinctive, with spider-avoidance passed down by our ancestors as an advantageous behaviour, it is more likely that the fear has propagated in the west as a culturally learned behaviour.
Tarantulas are cooked and eaten as delicacies in Cambodia and Venezuela, which was shown in Human Planet. The taste was compared to that of crab.
- Using a process called "ballooning" spiders have been found 5 kilometres in the air.
- The goliath bird-eating spider can reach the size of a dinner plate.
- The bolas spider lassos its prey with a sticky lure at the end of a silk thread.
- Silk is stronger by weight than steel.