Scorpions are members of the phylum Arthropoda, and more specifically of a subphylum of that taxon known as Cheliceriformes. Scorpions are members of the subclass Arachnida, those arthropods having eight legs and chelicerae mouthparts, and lacking antennae. Scorpions are the oldest arthropod terrestrial group, whose aquatic ancestry dates back to Silurian times over 400 million years ago. Their terrestrial invasion occurred in the Devonian period. Scorpions are cosmopolitan in distribution, occurring on all continents except Antarctica. Although most people envision scorpions as desert creatures, scorpions are found in the tropical jungles, temperate forests, and savannahs and in high elevations on mountains. They are mostly nocturnal creatures as they have little defense against the ultraviolet radiation of the sun. They are quite variable in size: Some are as small as thirteen millimeters, while others (the South African Hadogenes troglodytes) range up to eighteen centimeters in length.
Scorpions are segmental in form, the body being divided into an anterior prosoma and a segmented abdomen. A one-piece carapace covers the prosoma. The abdomen is divided into a preabdomen of seven segments and a postabdomen of five segments, ending in a stinging apparatus. The pedipalps are pincerlike and are used to capture and hold prey. The chelicerae are pincerlike as well and are used to macerate the prey. There are four pairs of walking legs, all ending in a pair of claws. All scorpions are carnivorous and are essentially liquid feeders. Copious amounts of digestive enzymes are poured over macerated areas of the prey and the liquid is then pumped into the stomach. Scorpions are well equipped with sensory structures. They have a pair of simple eyes located in the center of the carapace. Additionally, there are fromtwo to five pairs of eyes located along the anterior and lateral margins of the carapace. Scorpions have many setae or sensory hairs located over the dorsal surfaces of the body. These hairs function to pick up vibrations and air movement and are used to detect prey. The hairs are large on the pedipalps and are called trichobothria. Unique to scorpions is a pair of ventrally located, comblike appendages called pectines. These structures are mainly chemoreceptors and are used to pick up pheromone trails of insects. Pectines are also used to dig burrows, although the legs mainly perform this function. Scorpions breathe by means of book lungs that are ventrally located and open to the outside via a pair of spiracles. In this way, the book lungs are kept moist for oxygen diffusion.Acirculatory system is present, with a dorsally situated heart that opens via ostia or pores into the hemocoels and book lungs. Scorpions are also well equipped to deal with excretory wastes, using Malpighian tubules. These tubules filter nitrogenous wastes fromthe hemocoels and deposit the waste into the gut tube for elimination.
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