The subphylum of the Chelicerata includes 4 classes: Euryptera (extinct), Pycnogonida (sea spiders), Merostomata (horseshoe crabs) and Arachnida. The latter includes animals such as spiders, scorpions, mites and ticks. The name "Chelicerata" refers to the modified mouth parts (chelicerae). These structures are ontogenetically related to the first pair of legs in ancestral arthropods. Spiders form the order of the Araneida (id. Araneae) within the Arachnida. There is no single universally-accepted taxonomy, but three major suborders are recognised: the Mesothela (with still primitive segmentation noticeable in the abdomen), the Orthognata (Mygalomorpha) and the Labidognatha (Araneomorpha). The name Arachnida refers to the young Lydian maiden Arachne who, according to Greek mythology, could weave finer and more beautiful threads than anyone else, thus arousing the jealousy of the Gods. After a competition whose outcome was undecided, and after Arachne had insulted the Gods, Pallas Athena, protectress of the fine arts, robbed her of her reason and transformed her into a spider. She continued weaving after her transformation, and that is how we got spider webs….

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The number of spider species is estimated to be 30,000 to 34,000. There are only a small number of spiders which are potentially harmful for human beings. The reason for this is that most species simply have too little venom, or their fangs are too short to penetrate human skin. The venom of some spiders is only active against their natural prey and has no effect on human beings. Further, for some species the probability of spider-human contact is very low. Mortality as a result of spider bites is very low compared to snakebites, although there is a moderate morbidity (globally > 10,000 each year).

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Some spiders are large. The record is held by Theraphosa leblondi, the Goliath bird spider from Guyana. A giant specimen had a body length of 10 cm, a leg span of 26 cm and fangs of 25 mm. Some species have a limited geographical range (e.g. Atrax robustus in a limited part of Australia, Phoneutria nigriventer in Brazil), yet others are quasi cosmopolitan. Spiders are found on all continents, except Antarctica. They are found in various habitats and some species are found up to a height of 5000 m. There is even a species (Argyreta aquatica) which spends the whole life under water. The name refers to the silvery reflection of the air bubble in which the animal is encased when submerged. There have been repeated instances of spiders, originally endemic in one area, accidentally being introduced into another area where they had not been naturally present. Several years ago there was a notorious incident where Latrodectus mactans hasselti was accidentally introduced in Hasselt, Belgium. The ecological consequences have thus far remained limited. Knowledge about spider bites is limited and incomplete, given that bites are often not noticed immediately (including that of the black widow) or because the spider was not identified.