Arachnida(harvestmen, mites, palpigrades, pseudoscorpions, scorpions, spiders, whip scorpions, etc.) A class of Arthropoda (subclass Chelicerata) which have book lungs or tracheae derived from gills, indicating their aquatic derivation. They have invaded most terrestrial habitats and have secondarily invaded aquatic habitats, although to a very much smaller extent (there is only one species of aquatic spider, and only one species of mite in ten is aquatic). Except for many plant and animal parasites found among the mites, and some scavenging harvestmen (order Opiliones or Phalangida), most arachnids are predatory. Scorpions have been recorded from the Silurian Period and a Silurian scorpion, Palaeophonus nuncius, was perhaps the first terrestrial animal. The first fossil spiders are known from the Devonian. The class is extremely diverse, but apart from the mites the body is in two portions. The number of eyes varies up to 12 in some scorpions, but generally vision is poor, many species being nocturnal and equipped with sensory hairs to detect prey. Pedipalps function as hands, and the chelicerae as jaws or teeth. In all arachnids the mouth is small, and food is generally predigested by enzymes from the mid-gut. Reproductive organs are placed on the ventral surface of the abdomen, and courtship may be complex and prolonged, with parental care of the young common to all. All arachnids are dioecious. The production of silk and poison is characteristic of some orders, but the methods of production and their origins are varied. Silk is produced from abdominal glands in spiders, from the mouth region in mites, and from the chelicerae in pseudoscorpions. Poison is produced from the chelicerae of spiders, the tails of scorpions, and the pedipalps of pseudoscorpions. There are 11 orders, with 60 000 species. Members of 5 orders occur in northern Europe, the remainder being tropical in distribution.