A guide to arachnidae
Emperor scorpion by 'kevinzim' (flickr)
Arachnids are highly recognisable 8-legged arthropods belonging to the subphylum Chelicerata. Arachnids are a very large diverse group with 75000 species of arachnids worldwide, they occur in abundant numbers and diversities in very warm to hot, arid and tropical/ subtropical regions.
Arachnids are divided into 11 distinct orders. The first order is Scorpionida (the scorpions) which comprises of 1260 species. Opitophthalmus and Lisposoma are some of the notable less venomous scorpions whilst Parabuthus, Uroplectes and Lychas represent some of the highly venomous scorpions. The second order, Pseudoscorpiones consists of about 2000 species of false scorpions, resembling the scorpions but having two pairs of large, pincers instead of a sting. Chelifer cancroides is often found in rooms with dusty books while Garypus titanius is the largest known pseudoscorpion. The third order, Uropygi is made up of about 105 species of whip scorpions owing the name to their whip-like tails. Mastigoproctus, Hypoctomes and Chajnus are some of the most widely found whip scorpions. About 70 species of whip spiders or tail-less whip scorpions make up the fourth order Amblypygi, within which Damon diadema and Phrynus marginemaculatus are thought to be one of the few arachnids that exhibit social behaviour. The next two orders, Palpigradi and Schizomida are made up of 21 species of micro-scorpions and short-tailed scorpions respectively. The seventh arachnid order Opiliones constitutes of about 2400 species of harvestmen, more commonly known as daddy long legs due to them having exceptionally long and thin walking legs. Hadrobunus grandis, Phalangium opilio and Trogulus torosus are some of the widely known harvestmen, the latter being also the largest species. Closely related to Opiliones are the 48200 species of mites and ticks, members of order Acari. Most of them are parasites and infectious; spider mites, gall mites, demodex mites being few of them. About 1075 species of sun-spiders belong to the order Solifugae; Galeodus and Gluvia being among the few noteworthy species. 60 extant species of hooded tickspiders, belonging to the order Ricinulei are found in Africa and warmer parts of America; Ricinoides, Cryptocellus and Pseudocellus being some of them. The last order, Araneae comprise of about 35000 species of true spiders. They are diverse group of arachnids, ranging from species such as jumping spiders to ones such as water spiders, tarantulas, trapdoor spiders etc. Only three spider families, namely, Uloboridae, Holarchaeidae and Liphistilidae are non venomous. Venom of sand spiders, recluse spiders, black widow spiders and tarantulas can cause even death while that of Hobo spider and yellow sock spider cause less severe symptoms.
Arachnids were among the first animals to make the transition from water to land. The oldest known arachnid fossil, Palaeotarbus jerami, dates from the Silurian period, more than 417 million years ago (mya). The order Trigonotarbida is an extinct group of arachnids whose fossil record extends from the late Silurian to the early Permian period, some 419-290 million years. Trigonotarbids looked like spiders but lacked silk spinnerets and were probably non- venomous. The Devonian Attercopus (386 mya) shows the earliest silk spigots known in the fossil record, but not the spinnerets of true spiders. The oldest true spiders are found in the Carboniferous period, about 300 mya with their spinnerets placed underneath the middle of the abdomen, rather than at the ends as in the modern spiders. The ancestors of recent ones, namely, Mygalomorphae and Araneomorphae, appeared more than 250 mya. The ancestors of camel spiders and hooded tickspiders were also found in the Carboniferous period. The early scorpion-like creatures found during the late Silurian period were amphibious. The late Silurian Proscorpius has been classified as a scorpion but differs from the contemporary ones in that it seems entirely aquatic and had gills rather than book lungs. Fossils of the terrestrial scorpions with book lungs have been discovered in early Devonian rock forms about 402 mya. The harvestmen, whip scorpions and pseudoscorpions appeared during the Devonian period. The oldest known fossil pseudoscorpion dates back 380 mya.
Acanthoscurria geniculata by Frosted Peppercorn
Arachnids share certain common traits. The body is always separated into two parts: the cephalothorax or prosoma and the abdomen or opisthosoma. The prosoma is covered by a protective shield or carapace. It contains the mouth area, the claw-like chelicerae, pedipalpi and eight legs, four on either side. The abdomen may be segmented (in scorpions) or unsegmented (in ticks and spiders). Abdominal appendages are either lacking or modified into spinnerets of spiders, used for spinning webs and pectines of scorpions. They breathe by means of book lungs, modified from the book gill. It is an assemblage of hollow flat plates through which air circulates. Haemolymph flow among these plates and carbon dioxide and oxygen are exchanged passively via diffusion along a concentration gradient. Ricinuleids, pseudoscorpions and some spiders possess a bundle of tracheal tubes connected directly to the spiracle and had evolve from book lungs. The open circulatory system distributes blood from the heart to an enlarged blood space through arteries. They have a dorsal, tubular heart with various openings or ostia so that blood can be returned back to it. Arachnids lack jaws and use their chelicerae or pedipalps to break the prey into small fragments and pour digestive juice, ejected from midgut (in spiders) or the salivary gland (in ticks and mites), over their prey. The prey is turned into a soup of nutrients which is sucked into the mouth through pharyngeal pumping and finally reaches the stomach where it is further digested. Two kinds of excretory organs are found in arachnids: four pairs of coxal glands along the side of prosoma and one or two pairs of Malpighian tubules emptying into the gut. The primary excretory product is guanine although uric acid and other compounds are also excreted. They have two kinds of eyes: the lateral and median ocelli. Most of them possess hair-like setae, located throughout the body surface, that are sensitive to various stimuli. Many of them also own more complex structures called trichobothria. The reproductive organs are abdominal in location. Males have one or a pair of diffuse or compact testes along with one or two vasa efferentia. Females have single or paired, compact or diffuse ovaries and one or two oviducts. Insemination into the female may come from the male gonopore in a liquid medium (in spiders) or may be contained in spermatophores (in scorpions & ticks). The lay yolky eggs though scorpions may be either ovo-viviparous or viviparous.
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