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Arachnids are arthropods in the class Arachnida (/ ə ˈ r æ k n ɪ d ə /) of the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaroons. [2] Adult arachnids have eight legs attached to the cephalothorax.
Diagram of a spider leg and pedipalp – the pedipalp has one fewer segment. Arachnid legs differ from those of insects by the addition of two segments on either side of the tibia, the patella between the femur and the tibia, and the metatarsus (sometimes called basitarsus) between the tibia and the tarsus (sometimes called telotarsus), making a total of seven segments.
Basic characteristics of arachnids include four pairs of legs (1) and a body divided into two segments: the cephalothorax (2) and the abdomen (3). The ventral side of a brown widow spider . The epigastric plates and furrow are visible, as well as the hourglass marking on the underside of the abdomen, which is a characteristic feature of widow ...
The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma (cephalothorax). [1] It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others).
The word arthropod comes from the Greek ἄρθρον árthron ' joint ', and πούς pous (gen. ποδός podos) ' foot ' or ' leg ', which together mean "jointed leg", [25] with the word "arthropodes" initially used in anatomical descriptions by Barthélemy Charles Joseph Dumortier published in 1832. [1]
Pedipalps (commonly shortened to palps or palpi) are the secondary pair of forward appendages among chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to the chelicerae ("jaws") and anterior to the first pair of walking legs.
Opiliones (commonly known as harvestmen) are an order of arachnids and share many common characteristics with other arachnids. However, several differences separate harvestmen from other arachnid orders such as spiders. The bodies of opiliones are divided into two tagmata (arthropod body regions): the abdomen (opisthosoma) and the cephalothorax ...
Scorpions are predatory arachnids of the order Scorpiones. They have eight legs and are easily recognized by a pair of grasping pincers and a narrow, segmented tail, often carried in a characteristic forward curve over the back and always ending with a stinger. The evolutionary history of scorpions goes back 435 million years.