Arachnida

Arachnid
"Arachnida" from Ernst Haeckel's Kunstformen der Natur, 1904
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Subphylum: Chelicerata Class: Arachnida Cuvier, 1812
Orders
See text.

An arachnid is any member of the arthropod class Arachnida, a largely terrestrial group that includes spiders, mites, ticks, scorpions, and harvestmen (daddy longlegs). Arachnida is one of the classes of the subphylum Chelicerata (including horseshoe crabs, sea scorpions, and sea spiders) of the phylum Arthropoda. Arachnids are characterized by four pairs of segmented walking legs and a body divided into two regions, the cephalothorax and the abdomen.

Overview

As arthropods (phylum Arthropoda), the arachnids are characterized by the possession of a segmented body, a pair of jointed appendages on each segment, and an exoskeleton. The hard exoskeleton is made out of protein and a tough polysaccharide called chitin. Periodically, an arachnid must shed this covering when it molts.

As arthropods, arachnids are also characterized by a dorsal heart, a ventral nervous system, an open circulatory system, and a digestive system.

The subphylum Chelicerata, to which arachnids belong, is one of the major subdivisions of the phylum Arthropoda. Most of the marine chelicerates are now extinct. In the Chelicerata, the body is divided into an anterior prosoma (or cephalothorax) composed of eight segments plus a presegmental acron and a posterior opisthoma (or abdomen), composed of twelve segments plus a postsegmental telson. In some celicerates, the segments are considered to have been lost, and daddy longlegs, ticks, and mites do not have segmented bodies.

The chelicerae, which give the group its name, are pointed appendages that grasp the food in place of the chewing mandibles most other arthropods have. The chelicerae serve to macerate food particles. In some arachnids, the first post-oral pair of appendages—pedipalps (leg-like mouthparts)—of some species have been adapted for sensory, prey capture, or reproductive functions. In Solifugae, the palpi are quite leg-like and make Solifugae appear to have ten legs instead of eight.

Yellow mite
Lorryia formosa

Arachnids may be easily distinguished from insects by the fact that arachnids have eight legs whereas insects have six. The larvae of mites have only six legs; the fourth pair appears when they molt into nymphs. Arachnids are further distinguished from insects by the fact they have no antennae and no wings. They have a two-segmented body, made up of a cephalothorax and an abdomen, the cephalothorax being derived from the fusion of the cephalon (head) and the thorax.

Arachnids are chiefly terrestrial invertebrates, comprising some 65,000 to 73,000 named species, including spiders, scorpions, harvestmen, ticks, and mites.

Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Many are venomous—they secrete poison from specialized glands to kill prey or enemies. Others are parasites, some of which are carriers of disease.

Arachnids usually lay eggs, which hatch into immature adults.

Arachnids are named after the mythological figure Arachne.

Major groups

Spiders

Spiders are predatory invertebrate animals that comprise the order Araneae, one of several orders within the larger class of arachnids. They have two body segments, eight legs, no chewing mouth parts, and no wings. All spiders produce silk, a thin, strong protein strand extruded by the spider from spinnerets most commonly found on the end of the abdomen. Many species use it to trap insects in webs, although there are many species that hunt freely. Silk can be used to aid in climbing, form smooth walls for burrows, build egg sacs, wrap prey, and temporarily hold sperm, among other applications.

All spiders except those in the families Uloboridae and Holarchaeidae, and in the suborder Mesothelae (together about 350 species) can inject venom to protect themselves or to kill and liquefy prey. Only about 200 species, however, have bites that can pose health problems to humans (Diaz 2004). Many larger species' bites may be painful, but will not produce lasting health concerns.

Classification

• Trigonotarbida - extinct
• Amblypygi - "blunt rump" tailless whip scorpions with front legs modified into whip-like sensory structures as long as 25 cm or more
• Araneae - spiders (40,000 species)
  • Mesothelae - very rare, basal spiders, with abdomen segmented and spinnerets median
  • Opisthothelae - spiders with abdomen unsegmented and spinnerets located posteriorly
    • Araneomorphae - most common spiders
    • Mygalomorphae - tarantulas and tarantula-like spiders
• Phalangiotarbida - extinct
• Opiliones - phalangids, harvestmen or daddy-long-legs (6,300 species)
• Palpigradi - microwhip scorpions
• Pseudoscorpionida - pseudoscorpions
• Ricinulei - ricinuleids, hooded tickspiders
• Schizomida - "split middle" whip scorpions with divided exoskeletons
• Scorpiones - scorpions (2,000 species)
• Solifugae - solpugids, windscorpions, sun spiders or camel spiders (900 species)
• Haptopoda - extinct
• Uropygi - whip scorpions, with first legs modified as whip-like sensory organs and with a long thin tail at end of abdomen (100 species)
• Acarina - mites and ticks (30,000 species)
  • Acariformes
    • Sarcoptiformes
    • Trombidiformes
  • Opilioacariformes
  • Parasitiformes - holothyrans, ticks and mesostigmatic mites

Arachnida in human culture

Arachnids since ancient times have captured the human imagination, for example as objects of superstitious fear. They make their appearance in the Western zodiac, in the constellation of Scorpius. They are also present in the Egyptian hieroglyphs and diverse mythological characters such as the Greek Arachné and the west African Anansi. There are a number of gods based on arachnids, such as the Egyptian Serket. Images of scorpion-men are present in Assyrian reliefs.

See also

• Tarantulas.us - Forums - Tarantula Discussion Boards and Caresheets.
• Tarantulas.us - Gallery - The largest photo gallery of all tarantula's species.
• Tarantula Tour
• List of scorpion common names
• Arachnophobia
• List of spider common names

References

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Credits

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