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The Arachnids are a class of eight-legged arthropods. They are a highly successful group of mainly terrestrial invertebrates: spiders, scorpions, harvestmen, ticks, and mites, and a number of smaller groups.
Almost all adult arachnids have eight legs, and arachnids may be easily distinguished from insects by this fact, since insects have six legs. However, arachnids also have two further pairs of appendages that have become adapted for feeding, defense, and sensory perception. The first pair, the chelicerae, serve in feeding and defense. The next pair of appendages, the pedipalps have been adapted for feeding, locomotion, and/or reproductive functions.
There are some adaptations for life on land. They have internal respiratory surfaces. These may be trachea (tubes), or a modification of gills into a 'book lung'. This is an internal series of lamellae used for gas exchange with the air.
Diet and digestive system
Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Only the harvestmen and some mites eat solid food particles. Predigestion avoids exposure to internal parasites. Several groups secrete venom from specialized glands to kill prey or enemies. Several mites are external parasites, and some of them are carriers of disease (vectors).
Arachnids pour digestive juices produced in their stomachs over their prey after killing it with their pedipalps and chelicerae. The digestive juices rapidly turn the prey into a broth of nutrients which the arachnid sucks into a pre-buccal cavity located immediately in front of the mouth. Behind the mouth is a muscular, pharynx, which acts as a pump, sucking the food through the mouth and on into the oesophagus and stomach. In some arachnids, the oesophagus also acts as an additional pump.
The word Arachnida comes from the Greek for 'spider'. In legend, a girl called Arachne was turned into a spider by the goddess Athena. Arachne said she'd win a weaving contest against the goddess. Arachne won, but the Athena got mad and turned her into a spider, for her hubris (pride) in challenging a goddess.
- 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
- Acarina mites and ticks (30,000 species)
- Parasitiformes — holothyrans, ticks and mesostigmatic mites
- Opiliones — phalangids, harvestmen or daddy-long-legs (6,300 species)
- Scorpiones — scorpions (2,000 species)
- Pseudoscorpionida — pseudoscorpions (3,000 species)
- Solifugae — solpugids, windscorpions, sun spiders or camel spiders (900 species)
- Amblypygi — "blunt rump" tailless whip scorpions with front legs modified into whip-like sensory structures as long as 25 cm or more (140 species)
- Palpigradi — microwhip scorpions (80 species)
- Ricinulei — ricinuleids, hooded tickspiders (60 species)
- Schizomida — "split middle" whip scorpions with divided exoskeletons (220 species)
- Thelyphonida — vinegarroons or whip scorpions (formerly Uropygida) forelegs modified into sensory appendages and a long tail on abdomen tip (100 species)
|Wikispecies has information on: Arachnida.|
- Ruppert E.E. Fo, R.S. and Barnes R.D. 2004. Invertebrate zoology 7 ed, Brooks/Cole. p520 ISBN 0030259827.
- Foelix, Rainer F. 1996. Biology of spiders. Oxford University Press. ISBN 0-19-509593-6.
- Schultz J.W. 2000. A phylogenetic analysis of the arachnid orders based on morphological characters. Zoological Journal of the Linnean Society 150: 221–265. 
- Pinto-da-Rocha R. Machado G. & Giribet G. 2007. Harvestmen — the biology of Opiliones. Harvard University Press ISBN 0-674-02343-9