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Arachnids are an old subphylum of arthropods chracterized by their chelicerae (food-manipulating appendages located near the mouth). They are named after the Greek arachne, meaning spider. Common arachnids include spiders, scorpions, harvestmen, ticks, and mites, making up over 100,000 named species and over a million estimated species. Because most mites are microscopic and tropical, their diversity has been poorly characterized.
These were among the first animals to live on land, alongside nematodes and myriapods (millipedes, etc.) They reached land during the Devonian, about 410 million years ago, represented by trigonotarbids, a now-extinct group. For a while, the oldest known fossil of a terrestrial animal was an arachnid.
Unlike insects, to which they are related, arachnids have eight legs rather than six. Only a few mites have six or four legs. In most species, the front two pairs of legs are adapted for functions besides walking, such as feeding, sensing, and defense. Also distinguishing them from insects, arachnids have no antennae and no wings.
Unlike related arthropods such as crustaceans, arachnids are primarily terrestrial. They breath using book lungs, air-exchange systems with a high internal surface area. These book lungs evolved from book gills, a type of gill which today are only found on horseshoe crabs, an evolutionary relict.
Most species are carnivorous fluid feeders, although some species (harvestmen and some mites) eat solid food particles. Arachnids mostly prey on insects and smaller arachnids. Many groups use venom to disable their prey, then suck out its internal fluids with a specialized feeding apparatus. Spiders are famous for their elaborate webs, which serve as the only terrestrial example of filter feeding, a feeding mode much more common among aquatic arthropods such as krill.
Some species have highly specialized eyes, which gives them among the best vision of terrestrial invertebrates. The jumping spider, for instance, has eight large eyes that it uses to observe its surroundings and plan long jumps.
Arachnids are divided into 11 orders: the Amblypygi (tailless whipscorpions), Araneae (spiders), Palpigradi (palpigrades), Opiliones (daddy longlegs), Pseudoscorpiones (false scorpions), Ricinulei (ricinuleids), Scorpionida (true scorpions), Schizomida (micro whipscorpions), Uropygi (whipscorpions), Acari or Acarina (mites and ticks), and Solpugida (windscorpions).
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