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Locusts are the swarming phase of certain species of short-horned grasshoppers in the family Acrididae. These are species that can breed rapidly under suitable conditions and subsequently become gregarious and migratory when their populations become dense enough. They form bands as nymphs and swarms as adults. Both the bands and the swarms are nomadic and rapidly strip fields and greatly damage crops. The adults are powerful fliers; they can travel great distances, consuming practically all green material wherever the swarm settles.
Locusts are edible insects and are considered a delicacy in some countries. There have been references to their consumption as food throughout history. On swarming they are known to produce a toxin that renders them inedible and causes a skin reaction in sunlight. 
Though the female and the male look alike, they can be distinguished by looking at the end of their abdomens. The male has a boat-shaped tip, while the female has two serrated valves that can be either apart or kept together. These valves aid in the digging of the hole in which an egg pod is deposited. Desert locusts can measure roughly 75 millimetres (3.0 in) in length.
In addition, a number of "grasshopper" species such as the Senegalese grasshopper Oedaleus senegalensis, and the rice grasshopper Hieroglyphus daganensis (both from the Sahel), often display locust-like behavior and change morphologically on crowding.
There is no consistent taxonomic distinction between locust and grasshopper species; the basis for the definition is whether a species forms swarms under intermittently suitable conditions. In English the term "locust" is used for grasshopper species that change morphologically and behaviorally on crowding, forming swarms that develop from bands of immature stages called hoppers. These changes are examples of phase polymorphism; they were first analysed and described by Sir Boris Petrovich Uvarov. He discovered them in his research on the desert locust, whose solitary and gregarious phases had previously been thought to be separate species. They also are referred to as statary and migratory morphs (though strictly speaking their swarms are nomadic rather than migratory). Charles Valentine Riley and Norman Criddle were also involved in achieving the understanding and control of locusts.
Research at Oxford University has identified that swarming behavior is a response to overcrowding. Increased tactile stimulation of the hind legs causes an increase in levels of serotonin. This causes the locust to change colour, eat much more, and breed much more easily. The transformation of the locust to the swarming variety is induced by several contacts per minute over a four-hour period. It is estimated that the largest swarms have covered hundreds of square miles and consisted of many billions of locusts. Plagues of locusts appear in both the Bible and the Quran, including one of the biblical Plagues of Egypt, where locusts ate all the crops of Egypt.
In a paper in the 30 January 2009 edition of the AAAS magazine Science, Anstey, Rogers, et al. showed when desert locusts meet, their nervous systems release serotonin, which causes them to become mutually attracted, a prerequisite for swarming.
Swarming grasshoppers have short feelers, or antennae, and hearing organs on the abdomen (rear segment of the body). As winged adults, flying in swarms, locusts may be carried by the wind hundreds of miles from their breeding grounds; on landing they devour all vegetation. Locusts occur on nearly every continent.
Several organisations around the world monitor the threat from locust plagues. They provide locust forecasts detailing regions likely to suffer from locust plagues in the near future. In Australia this service is provided by the Australian Plague Locust Commission, in Northern and Sahelian Africa the service is provided by the United Nations Food and Agricultural Organisation.
The migratory locust (Locusta migratoria) ranges from Europe to China, and even small swarms may cover several square miles, and weigh thousands of tons. Control by spreading poisoned food among the bands is very effective, but it is cheaper to spray concentrated insecticide solutions from aircraft over the insects or the vegetation on which they feed. They eat the equivalent of their own weight in a day, and, flying at night with the wind, may cover some 500 kilometres (310 mi). The largest known swarm covered 513,000 km², comprising approximately 12.5 trillion insects and weighing 27.5 million tons.
A biological pesticide to control locusts was being tested across Africa by a multinational team in 1997. Dried fungal spores of a Metarhizium species sprayed in breeding areas pierce the locust exoskeleton on germination and invade the body cavity, causing death. The fungus is passed from insect to insect and persists in the area, making repeated treatments unnecessary.
The extinction of the Rocky Mountain locust has been a source of puzzlement. Recent research suggests the breeding grounds of this insect in the valleys of the Rocky Mountains came under sustained agricultural development during the large influx of gold miners, destroying the underground eggs of the locust.
Locusts are used as models in many fields of biology, especially in the field of olfactory, visual and locomotor neurophysiology. It is one of the organisms for which scientists have obtained detailed data on information processing in the olfactory pathway of organisms. It is suitable for the above purposes because of the robustness of the preparation for electrophysiological experiments and ease of growing them.
The International LUBILOSA Program was set up to find methods of non-chemical control of locusts. The program successfully developed the mycoinsecticide 'Green Muscle' and over its 12-year period program staff contributed a large number of scientific papers on subjects as diverse as fungal production, (bio)pesticide application, socio-economics and thermal ecology. Locusts thus provided a valuable "test bed" for better biological understanding and developing new technologies for microbial pesticides.
The word "locust" is derived from the Vulgar Latin locusta, which was originally used to refer to various types of crustaceans and insects; English "lobster" is derived from Anglo-Saxon loppestre, which may come from Latin locusta. Spanish has mostly preserved the original Latin usage, since the cognate term langosta can be used to refer both to a variety of lobster-like crustaceans and to the swarming grasshopper, while semantic confusion is avoided by employing qualifiers such as de la tierra (of the land) when referring to grasshoppers, del mar and del rio (of the sea/of the river) when referring to lobsters and crayfish, respectively. French presents an inverse case; during the 16th century, the word sauterelle (literally "little hopper") could mean either grasshopper or lobster (sauterelle de mer). In contemporary French usage, langouste is used almost exclusively to refer to the crustacean (two insect exceptions being the langouste de désert and the langouste de Provence).
The use of "locust" in English as a synonym for "lobster" has no grounding in anglophone tradition, and most modern instances of its use are usually calques of foreign expressions (e.g. "sea locust" as mistranslation of langouste de mer). There are, however, various species of crustaceans whose regional names include the word "locust". Thenus orientalis, for example, is sometimes referred to as the flathead locust lobster (its French name, cigale raquette, literally "raquet cicada," is yet another instance of the locust-cicada-lobster nomenclatural connection). Similarly, certain types of amphibians and birds are sometimes called "false locusts" in imitation of the Greek pseud(o)acris, a scientific name sometimes given to a species because of its perceived cricket-like chirping. Often, the linguistic non-differentiation of animals not only regarded by science as different species, but that also often exist in radically different environments, is the result of culturally perceived similarities between organisms, as well as of abstract associations formed within a particular group's mythology and folklore (see Cicada mythology). On a linguistic level, these cases also exemplify an extensively documented tendency, in many languages, towards conservatism and economy in neologization, with some languages historically only allowing for the expansion of meaning within already existing word-forms. Also of note is the fact that all three so-called locusts (the grasshopper, the cicada, and the lobster) have been a traditional source of food for various peoples around the world (see entomophagy).
The word "locust" has, at times, been employed controversially in English translations of Ancient Greek and Latin natural histories, as well as of Hebrew and Greek Bibles; such ambiguous renderings prompted the 17th-century polymath Thomas Browne to include in the Fifth Book of his Pseudodoxia Epidemica an essay entitled Of the Picture of a Grasshopper, it begins:
|“||There is also among us a common description and picture of a Grasshopper, as may be observed in the pictures of Emblematists, in the coats of several families, and as the word Cicada is usually translated in Dictionaries. Wherein to speak strictly, if by this word Grasshopper, we understand that animal which is implied by τέτιξ with the Greeks, and by Cicada with the Latines; we may with safety affirm the picture is widely mistaken, and that for ought inquiry can inform, there is no such insect in England.||”|
Browne revisited the controversy in his Miscellany Tracts (1684), wherein he takes pains (even citing Aristotle's Animalia) to both indicate the relationship of locusts to grasshoppers and to affirm their like disparateness from cicadas:
|“||That which we commonly call a Grasshopper, and the French Saulterelle being one kind of Locust, so rendered in the plague of Ægypt, and in old Saxon named Gersthop.||”|
In general the word "locust", being of origins older than the modern discipline of entomology, does not have a precise meaning as a common name. Some regional English usages of the word are non-standard, but the trend is increasingly towards the uniform application of the word to swarm-forming grasshoppers. As remarked by Thomas Browne, "Locust" can refer to the large species of swarming short-horned grasshoppers (in the family Acrididae), and rarely to the praying mantis ("praying locust").
In English in the last century or two, the major use of the word locust is in referring to the swarming species of grasshoppers and also, sometimes interchangeably, to the so-called seventeen-year locust, more correctly called the periodical cicada, several species in the North American genus Magicicada. This confusion apparently arose because of the perceived swarming habit of Magicicada, which in fact is a misunderstanding; Magicicada does not swarm in the sense that polyphenic locusts do. They simply exist in large numbers at all times in a very long life cycle, spending typically 13 or 17 years underground, then emerging just long enough to mate and lay eggs, before dying and apparently disappearing until the emergence of the next generation.
In some perceptions, whether regional or simply mistaken, the cicadas are the true locusts, according to which the modern interpretation of the biblical eighth plague is a wrong translation, and the insect in the bible should be a "grasshopper".
Compound words involving "locust" have also been used by anglophone translators as calques of archaic Arabic, Greek, Hebrew, or other language names for animals; the resulting formations have, just as in the case of the Brownian grasshopper/cicada controversy, been, at times, a cause of lexical ambiguity and false polysemy in English. An instance of this appears in a translation of Pliny included in J.W. McCrindle's book Ancient India as Described in Classical Literature, where an Indian gem is said by the Roman historian to have a "surface [that] is even redder than the shells of the sea-locust."
Several cultures throughout the world consume insects. Even Islamic and Jewish dietary laws, which prohibit the consumption of other insects, allow locusts to be eaten, particularly as famine foods for the poor.
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