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countershading : ウィキペディア英語版
countershading

Countershading, or Thayer's Law, is a form of camouflage. Countershading is the pattern of animal coloration in which an animal’s pigmentation is darker on the upper side and lighter on the underside of the body. This pattern is found in many species of mammals, reptiles, birds, and fish, and has existed since at least the Cretaceous period.
When light falls on a uniformly coloured object such as a sphere from above, it makes the upper side appear lighter and the underside darker, grading from one to the other. This pattern of light and shade makes the object appear solid, and therefore acts as a visual cue which makes the object easier to detect. Countershading reduces the ease of detection of predators and prey by counterbalancing the effects of self-shadowing, again typically with grading from dark to light. In theory this could be useful for military camouflage, but in practice it has rarely been applied, despite the best efforts of the artist Abbott Handerson Thayer and zoologists such as Hugh Cott.
The precise function of various patterns of animal coloration has been debated by zoologists such as Hannah Rowland (2009), with the suggestion that there may be multiple functions including flattening and background matching when viewed from the side; background matching when viewed from above or below, implying separate colour schemes for the top and bottom surfaces; outline obliteration from above; and a variety of largely untested non-camouflage theories. A related mechanism, counter-illumination, adds the creation of light by bioluminescence or lamps to match the actual brightness of a background. Counter-illumination camouflage is common in marine organisms such as squid. It has been studied up to the prototype stage for military use in ships and aircraft, but it too has rarely or never been used in warfare.
The reverse of countershading, with the belly pigmented darker than the back, enhances contrast and so makes animals more conspicuous. It is found in animals that can defend themselves, such as skunks. The pattern is used both in startle or deimatic displays and as a signal to warn off experienced predators. However, animals that habitually live upside-down but lack strong defences, like the Nile catfish and the luna moth caterpillar, have upside-down countershading for camouflage.
== Early research ==

The English zoologist Edward Bagnall Poulton, author of ''The Colours of Animals'' (1890) discovered the countershading of various insects, including the pupa or chrysalis of the purple emperor butterfly, ''Apatura iris'',〔Poulton, 1888.〕 the caterpillar larvae of the brimstone moth, ''Opisthograptis luteolata'' and of the peppered moth, ''Biston betularia''.〔Poulton, 1887.〕〔Thayer, 1909. p 22.〕 However he did not use the term countershading, nor did he suggest that the effect occurred widely.〔
The New Hampshire artist Abbott Handerson Thayer was one of the first to study and write about countershading. In his 1909 book ''Concealing-Coloration in the Animal Kingdom'', he correctly described and illustrated countershading with photographs and paintings, but wrongly claimed that almost all animals are countershaded.〔Thayer, 1909.〕 For this reason countershading is sometimes called Thayer’s Law. Thayer wrote:
Thayer observed and painted a number of examples, including the Luna moth caterpillar ''Actias luna'', both in its habitual upside-down feeding position, where its countershading makes it appear flat, and artificially inverted from that position, where sunlight and its inverted countershading combine to make it appear heavily shaded and therefore solid.〔Thayer, 1909. Plate XII.〕 Thayer obtained a patent in 1902 to paint warships, both submarines and surface ships, using countershading, but failed to convince the US Navy to adopt his ideas.〔Goldstein, 2009, pp. 233–235.〕
Hugh Bamford Cott in his 1940 book ''Adaptive Coloration in Animals'' described many instances of countershading, following Thayer in general approach〔Cott, 1940. pp. 35–47.〕 but criticising Thayer's excessive claim ("He says 'All patterns and colors whatsoever of all animals that ever prey or are preyed upon are under certain normal circumstances obliterative.'") that effectively all animals are camouflaged with countershading. Cott calls this "Thayer straining the theory to a fantastic extreme".〔Cott, 1940. pp. 172–173.〕
Both Thayer and Cott include in their books photographs of a non-countershaded white cockerel against a white background, to make the point that in Thayer's words "a monochrome object can not be 'obliterated', no matter what its background"〔Thayer, 1909. Caption to Figure 7.〕 or in Cott's words "Colour resemblance alone is not sufficient to afford concealment".〔Cott, 1940. Caption to Plate 7.〕 Cott explains that

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