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United States The United States of America known as the United States or America, is a country composed of 50 states, a federal district, five major self-governing territories, various possessions. With a population of over million people, the U. The capital is Washington, D. Forty-eight states and the capital's federal district are contiguous in North America between Canada and Mexico ; the State of Alaska is in the northwest corner of North America, bordered by Canada to the east and across the Bering Strait from Russia to the west.

The diverse geography and wildlife of the United States make it one of the world's 17 megadiverse countries. Paleo-Indians migrated from Siberia to the North American mainland at least 12, years ago. European colonization began in the 16th century; the United States emerged from the thirteen British colonies established along the East Coast. Numerous disputes between Great Britain and the colonies following the French and Indian War led to the American Revolution , which began in , the subsequent Declaration of Independence in ; the war ended in with the United States becoming the first country to gain independence from a European power.

The current constitution was adopted in , with the first ten amendments, collectively named the Bill of Rights , being ratified in to guarantee many fundamental civil liberties; the United States embarked on a vigorous expansion across North America throughout the 19th century, acquiring new territories, displacing Native American tribes, admitting new states until it spanned the continent by During the second half of the 19th century, the Civil War led to the abolition of slavery.

By the end of the century, the United States had extended into the Pacific Ocean, its economy, driven in large part by the Industrial Revolution , began to soar. The Spanish—American War and World War I confirmed the country's status as a global military power; the United States emerged from World War II as a global superpower , the first country to develop nuclear weapons, the only country to use them in warfare, a permanent member of the United Nations Security Council. Sweeping civil rights legislation, notably the Civil Rights Act of , the Voting Rights Act of and the Fair Housing Act of , outlawed discrimination based on race or color.

Moon landing; the end of the Cold War and the collapse of the Soviet Union in left the United States as the world's sole superpower. The United States is the world's oldest surviving federation, it is a representative democracy. The United States is the world's largest importer and the second largest exporter of goods, by value. Although its population is only 4. Despite wide income and wealth disparities, the United States continues to rank high in measures of socioeconomic performance, including average wage, human development, per capita GDP, worker productivity.

The United States is the foremost military power in the world, making up a third of global military spending, is a leading political and scientific force internationally. Joseph Reed. Moylan expressed his wish to go "with full and ample powers from the United States of America to Spain " to seek assistance in the revolutionary war effort; the first known publication of the phrase "United States of America" was in an anonymous essay in The Virginia Gazette newspaper in Williamsburg, Virginia , on April 6, The second draft of the Articles of Confederation , prepared by John Dickinson and completed by June 17, , at the latest, declared "The name of this Confederation shall be the'United States of America'".

The final version of the Articles sent to the states for ratification in late contains the sentence "The Stile of this Confederacy shall be'The United States of America'".

Snow camouflage Snow camouflage is the use of a coloration or pattern for effective camouflage in winter combined with a different summer camouflage. Summer patterns are disruptively patterned combinations of shades of browns and greys, up to black, while winter patterns are dominated by white to match snowy landscapes.

Among animals, variable snow camouflage is a type of seasonal polyphenism with a distinct winter plumage or pelage , it is found in birds such as the rock ptarmigan , lagomorphs such as the Arctic hare , mustelids such as the stoat , one canid , the Arctic fox. Since these have evolved separately, the similar appearance is due to convergent evolution; this was used as early evidence for natural selection.

Some high Arctic species like the snowy owl and polar bear however remain white all year round. In military usage, soldiers either exchange their disruptively-patterned summer uniforms for thicker snow camouflage uniforms printed with mainly-white versions of camouflage patterns in winter, or they wear white overalls over their uniforms.

Some armies have made use of reversible uniforms, printed in different seasonal patterns on their two sides. Vehicles and guns are simply repainted in white. Aircraft too are repainted in snow camouflage patterns. Charles Darwin mentioned the white winter coloration of the ptarmigan in his Origin of Species : When we see The white protective coloration of arctic animals was noted by an early student of camouflage, the naturalist Alfred Russel Wallace , in his book Darwinism.

Zoologists such as Hugh B. Cott have echoed his observations, adding that other animals of the Arctic such as musk ox, reindeer and raven never become white "even in the coldest parts of their range". Cott noted that both animals that hunt, like polar bear and stoat, prey animals like ptarmigan and mountain hare , require camouflage to hide from prey or from predators respectively.

There is little experimental evidence for the adaptiveness of white as camouflage, though the ornithologist W. Tickell, reviewing proposed explanations of white plumage in birds, writes that in the ptarmigan "it is difficult to escape the conclusion that cryptic brown summer plumage becomes a liability in snow, white plumage is therefore another cryptic adaptation. However, Arctic hares which live in the far north of Canada , where summer is short, remain white year-round.

Since these animals in separated groups have evolved separately, the similarity of coloration is due to convergent evolution, on the presumption that natural selection favours a particular coloration in a particular environment.

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The seasonal polyphenism in willow grouse differs between Scandinavian populations. In Scotland , grouse have two plumages, while in Scandinavia there is a third plumage, a white winter morph ; the genetic basis for this is not in the melanin pigment system, is due to regulatory changes. The behaviour of moulting females in springtime depends on their plumage state: they tend to sit on snow while they are white, but choose the border between bare ground and snow when they have more dark feathers, they seem to be choosing the best compromise between food quality.

Climate change from global warming is leading to a mismatch between the seasonal coat coloration of arctic animals such as snowshoe hares with the snow-free landscape; the principle of varying coloration with the changing seasons has military applications. In the First World War and observation positions were hand-painted in disruptive patterns by artists known as camoufleurs , they sometimes varied their patterns seasonally.

Uniforms were of a single colour, such as the British khaki. For example, Austro-Hungarian troops on the Italian front used skis and wore snow camouflage smocks and overtrousers over their uniforms, improvised white cloths over their uniform caps. Several armies in the Second World War in Northern European countries preferred separate winter uniforms rather than oversuits; the Waffen-SS went a step further, developing reversible uniforms with separate schemes for summer and autumn, as well as white winter oversuits.

Other German units fighting in Eastern Europe were at first poorly equipped for winter, having to make do with ordinary summer uniforms, but in the winter of to new white two-piece hooded oversuits with long mitten gauntlets started to arrive. American troops in Europe in the winter of to improvised snow capes and helmet covers from white cloth such as bed linen ; the Red Army issued a report, "Tactical and Technical trends, No.

It advised either all-white using zinc white or titanium white paint for level, open country, or disruptive two-c. Mimicry In evolutionary biology, mimicry is an evolved resemblance between an organism and another object an organism of another species. Mimicry may evolve between individuals of the same species. Mimicry functions to protect a species from predators, making it an antipredator adaptation. Mimicry evolves if a receiver perceives the similarity between a mimic and a model and as a result changes its behaviour in a way that provides a selective advantage to the mimic; the resemblances that evolve in mimicry can be visual, chemical, tactile, or electric, or combinations of these sensory modalities.

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Mimicry may be to the advantage of both organisms that share a resemblance, in which case it is a form of mutualism ; the evolutionary convergence between groups is driven by the selective action of a signal-receiver or dupe. Birds, for example, use whilst avoiding the noxious ones. Over time, palatable insects may evolve to resemble noxious ones, making them mimics and the noxious ones models.

In the case of mutualism, sometimes both groups are referred to as "co-mimics". It is thought that models must be more abundant than mimics, but this is not so. Mimicry may involve numerous species. Mimicry between prey species and their predators involves three or more species. In its broadest definition, mimicry can include non-living models; the specific terms masquerade and mimesis are sometimes used. For example, animals such as flower mantises , planthoppers and geometer moth caterpillars resemble twigs, leaves, bird droppings or flowers.

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Many animals bear eyespots , they may not resemble any specific organism's eyes, whether or not animals respond to them as eyes is unclear. Nonetheless, eyespots are the subject of a rich contemporary literature; the model is another species, except in automimicry , where members of the species mimic other members, or other parts of their own bodies, in inter-sexual mimicry, where members of one sex mimic members of the other. Mimicry can result in an evolutionary arms race if mimicry negatively affects the model, the model can evolve a different appearance from the mimic.

Mimics may have different models for different life cycle stages, or they may be polymorphic , with different individuals imitating different models, such as in Heliconius butterflies. Models themselves may have more than one mimic, though frequency dependent selection favours mimicry where models outnumber mimics.

1. Introduction

Models tend to be closely related organisms, but mimicry of vastly different species is known. Most known mimics are insects, though many other examples including vertebrates are known. Plants and fungi may be mimics, though less research has been carried out in this area. Use of the word mimicry dates to , it derives from the Greek term mimetikos, "imitative", in turn from mimetos, the verbal adjective of mimeisthai, "to imitate". Used to describe people, "mimetic" was used in zoology from , "mimicry" from Many types of mimicry have been described. An overview of each follows, highlighting the similarities and differences between the various forms.

Classification is based on function with respect to the mimic. Some cases may belong to more than one class, e. The terminology used is not without debate and attempts to clarify have led to new terms being included; the term "masquerade" is sometimes used when the model is inanimate but it is differentiated from " crypsis " in its strict sense by the potential response of the signal receiver. In crypsis the receiver is assumed to not respond while a masquerader confuses the recognition system of the receiver that would otherwise seek the signaller.

In the other forms of mimicry , the signal is not filtered out by the sensory system of the receiver; these are not mutually exclusive and in the evolution of wasp-like appearance, it has been argued that insects evolve to masquerade wasps since predatory wasps do not attack each other but this mimetic resemblance deters vertebrate predators. Defensive or protective mimicry takes place when organisms are able to avoid harmful encounters by deceiving enemies into treating them as something else.

The first three such cases discussed here entail mimicry of animals protected by warning coloration: Batesian mimicry, where a harmless mimic poses as harmful. Mertensian mimicry, where a deadly mimic resembles a less harmful but lesson-teaching model; the fourth case, Vavilovian mimicry, where weeds resemble crops, involves humans as the agent of selection. In Batesian mimicry the mimic shares signals similar to the model, but does not have the attribute that makes it unprofitable to predators.

In other words, a Batesian mimic is a sheep in wolf's clothing, it is named after Henry Walter Bates , an English naturalist whose. Camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else.

Examples include the leopard's spotted coat, the battledress of a modern soldier, the leaf-mimic katydid's wings. A third approach, motion dazzle, confuses the observer with a conspicuous pattern, making the object visible but momentarily harder to locate; the majority of camouflage methods aim for crypsis through a general resemblance to the background, high contrast disruptive coloration, eliminating shadow, countershading.

In the open ocean, where there is no background, the principal methods of camouflage are transparency and countershading, while the ability to produce light is among other things used for counter-illumination on the undersides of cephalopods such as squid ; some animals, such as chameleons and octopuses, are capable of changing their skin pattern and colours, whether for camouflage or for signalling. It is possible. Military camouflage was spurred by the increasing range and accuracy of firearms in the 19th century.

In particular the replacement of the inaccurate musket with the rifle made personal concealment in battle a survival skill. In the 20th century, military camouflage developed especially during the First World War. At sea, merchant ships and troop carriers were painted in dazzle patterns that were visible, but designed to confuse enemy submarines as to the target's speed and heading.

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During and after the Second World War , a variety of camouflage schemes were used for aircraft and for ground vehicles in different theatres of war; the use of radar since the midth century has made camouflage for fixed-wing military aircraft obsolete. Non-military use of camouflage includes making cell telephone towers less obtrusive and helping hunters to approach wary game animals. Patterns derived from military camouflage are used in fashion clothing, exploiting their strong designs and sometimes their symbolism.

Camouflage themes recur in modern art, both figuratively and in science fiction and works of literature. In ancient Greece , Aristotle commented on the colour-changing abilities, both for camouflage and for signalling, of cephalopods including the octopus , in his Historia animalium: The octopus Camouflage has been a topic of research in zoology for well over a century. According to Charles Darwin's theory of natural selection, features such as camouflage evolved by providing individual animals with a reproductive advantage, enabling them to leave more offspring, on average, than other members of the same species.

In his Origin of Species , Darwin wrote: When we see leaf-eating insects green, bark-feeders mottled-grey. Grouse , if not destroyed at some period of their lives, would increase in countless numbers.