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Palaeophonus
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 prosoma (anterior portion) bearing the four pairs of legs, the eyes, the pedipalps, and the chelicerae; and the opisthosoma (posterior portion) containing most of the internal organs and glands. The two portions may be broadly jointed, or connected by a pedicel. The prosoma has a dorsal shield (carapace), and the opisthosoma is segmented in most orders, but not in spiders and mites and only very weakly in harvestmen. 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 more than 60 000 species. Members of five orders occur in northern Europe, the remainder being tropical in distribution.
© A Dictionary of Zoology 1999, originally published by Oxford University Press 1999.
Scorpions enjoy widespread public appeal (much of it based on fear). Their toxicity, relatively large size, and fearsome appearance, notwithstanding the great age of their lineage, contribute to a fascination that has always and continues to surround them (Cloudsley-Thompson 1990). Among the most ancient arthropods, derived from amphibious ancestors that lived in the Silurian, more than 400 million years ago, scorpions have earned the title of ‘living fossils’ (Jeram 1990). The scorpion groundplan, developed so long ago, is highly successful. Paleozoic scorpions closely resemble their modern descendants in basic anatomical details, except that some were considerably larger (Jeram 1994, 1998). Brontoscorpio anglicus measured approximately 1 meter in length (Kjellesvig-Waering 1986) — an order of magnitude greater than the largest extant members of Pandinus from tropical Africa, which average about 20 cm in length (Sissom 1990). Scorpions were formerly considered to be the sister group of all other arachnids because they closely resemble extinct marine eurypterids (the sister group of arachnids). Recent data, however, suggests that scorpions are embedded in the arachnid lineage, and merely retain primitive features.
Cloudsley-Thompson, J.L. 1990. Scorpions in mythology, folklore, and history. In: Polis, G.A. (Ed.) The Biology of Scorpions. Stanford University Press, Stanford, CA, 462–485.
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