Arachnids are a subphylum of arthropods with chelicerae, including spiders, scorpions, harvestmen, ticks, and mites. They have eight legs, no antennae or wings, and breathe with book lungs. Arachnids are primarily terrestrial, carnivorous, and use venom to disable prey. They are divided into 11 orders.
Arachnids are an old subphylum of arthropods characterized by their chelicerae (food-handling appendages located near the mouth). They take their name from the Greek aracne, which means spider. Common arachnids include spiders, scorpions, harvestmen, ticks, and mites, making up over 100,000 named species and over one million estimated species. Since most mites are microscopic and tropical, their diversity has been poorly characterized.
These were among the first animals to live on land, along with nematodes and myriapods (millipedes, etc.) they reached land during the Devonian, about 410 million years ago, represented by the trigonotarbids, a now extinct group. For a while, the oldest known fossil of a land animal was an arachnid.
Unlike insects, to which they are related, arachnids have eight legs instead of six. Only a few mites have six or four legs. In most species, the front two pairs of legs are adapted for functions beyond walking, such as feeding, sensing, and defense. Also distinguishing them from insects, arachnids have no antennae or wings.
Unlike related arthropods such as crustaceans, arachnids are primarily terrestrial. They breathe with book lungs, air exchange systems with a high internal surface area. These book lungs evolved from book gills, a type of gill found today only on horseshoe crabs, an evolutionary relic.
Most species are liquid-eating carnivores, although some species (forage harvesters and some mites) eat solid food particles. Arachnids prey primarily 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 mode of feeding much more common among aquatic arthropods such as krill.
Some species have highly specialized eyes, giving them vision among the best of terrestrial invertebrates. The jumping spider, for example, has eight large eyes which it uses to observe its surroundings and plan long leaps.
Arachnids are divided into 11 orders: the Amblypygi (tailless scorpions), Araneae (spiders), Palpigradi (palpigrades), Opiliones (father long legs), Pseudoscorpions (false scorpions), Ricinulei (ricinuleids), Scorpionida (true scorpions), Schizomida (micro scorpions), Uropygi (scorpions), Acari or Acarina (mites and ticks) and Solpugida (wind scorpions).
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