Defensive or protective mimicry takes place when organisms are able to avoid encounters that would be harmful to them by deceiving enemies into treating them as something else. The first three such cases discussed here entail mimicry of organisms protected by warning colouration: Batesian mimicry, where a harmless mimic poses as harmful; Mullerian mimicry, where two or more harmful species mutually advertise themselves as harmful; and Mertensian mimicry, where a deadly mimic resembles a less harmful but lesson-teaching model. The fourth case, Vavilovian mimicry, where weeds resemble crops, is important for several reasons; and humans are the agent of selection. Aposematism is a primary defence mechanism that warns potential predators of the existence of another, secondary defensive mechanism. "By definition, primary defences operate before a predator initiate any prey-catching behavior (Robinson, 1969; Edmunds, 1974) and their function is to prevent pursuit".[3] The organism's secondary means of defence may include: Unpalatability such as from the bitter taste arising from some insects such as the ladybird or tiger moth, or the noxious odour produced by the skunk, or: Other danger such as the poison glands of the poison dart frog, the sting of a velvet ant or neurotoxin in a black widow spider. In these particular examples, the organism advertises its capabilities via either bright colouration in the case of the ladybird, frog and spider; or by conspicuous stripes in the case of the skunk. Various types of tiger moths advertise their unpalatability by either producing ultrasonic noises which warn bats to avoid them [4] or by warning postures which expose brightly coloured body parts (see Unkenreflex), or exposing eyespots. Velvet ants have both bright colours and produce audible noises when grabbed (via stridulation), which serve to reinforce the warning. Aposematic signals are primarily visual and involve bright and contrasting colours. Research indicates that more often than not, warning signals are honest indications of noxious prey because conspicuousness evolves in tandem with noxiousness.[5] Thus, the brighter and more conspicuous the organism, the more toxic it is.[6] The most common and effective colors are red, yellow, and black.[7] These colors provide heavy contrast against green foliage. Red, yellow, and black are also resistant to changes in shadows and luminescence, have luminescence contrast, and are highly chromatic. The final advantage of these colors is to provide distance dependent camouflage. This strategy utilizes the fact that although very conspicuous, these colors are not as visible at very large distances. This is an advantage because predators will not decide to investigate the unknown, conspicuous object and therefore will not give the organism any unnecessary attention to begin with.[8] There is no "perfect" warning coloration though because signals are dependent on the environment. The background, light conditions, and predator vision all play a role in shaping a well adapted warning coloration.[9] They may be accompanied by one or more signals other than colour. These may be specific odours, sounds or behaviour. Together, the predator encounters a multi-modal signal which is more effectively detected.