Spiders (order Araneida/Araneae) is a collective term containing more than 46,700 different species of arachnids. Spiders differ from other insects in having eight rather than six legs. They are widely spread on all continents (except Antarctica); however, far more species are found in the tropics compared to temperate or cold regions.
Spiders produce silk used for web-building, packaging prey and eggs, making draglines, or casting nets. Young spiders and some smaller species can produce long silk strands so lightweight they can catch the wind, thus able to carry the spiders over long distances (ballooning), contributing to their extensive distribution. Due to their abundance, spiders are seen as the most relevant predators of insects, and are even used by humans to control insect populations, for example, in rice fields in China or in apple orchards in Israel. There are passive hunters capturing prey in webs but also active hunters using specialized hunting techniques.
The body length of spiders varies from around 0.5 to 90 mm. The largest species of spiders are Tarantulas (mygalomorphs). Tarantulas inhabit warm climates and are most prevalent in the American continent like the goliath bird-eating spider (Theraphosa leblondi) in the Amazon. Irrespective of species, female spiders normally are larger than males, sometimes more than twice as large. This is probably a result of selection: larger female spiders are more fertile, smaller male spiders are more lightweight which allows them to build and traverse silk bridges more easily, leading to more mating opportunities.
Like insects, spiders are protected by an outer skeleton (exoskeleton). Contrary to insects, spiders’ bodies are split into two parts: the cephalothorax and the abdomen. The cephalothorax contains the brain and stomach, while the gut, hearts, reproductive organs and silk glands are located in the abdomen. A narrow stalk, the pedicel, connects the abdomen to the cephalothorax. Important vessels as nerve cord and blood vessels pass through the pedicel. The legs are attached to the cephalothorax and covered in long bristles (setae) which contain several types of sense organs. If necessary, spiders can amputate their own legs and grow new (shorter) legs at the next molt.
Altogether, spiders have six pairs of extremities: four pairs of legs, one pair of pedipalps (for handling food and sensing the environment) and one pair of chelicerae (jaws, containing venom ducts and sometimes venom glands). Most spiders have venom glands, with many species producing venoms so effective that they can help defend the spider against predators. Even though spider bites can be lethal to flying vertebrates, only very few species can be dangerous for humans. For example, the venom of black widows, which are found all over the world but not in central Europe and northern Eurasia, contains a nerve toxin that causes severe pain in humans but is usually not fatal.
Finally, like many other animals spiders have a fascinating variety in outer appearances, serving different needs such as camouflage, mimicry, warning potential predators or attracting mating partners.