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Mimicry is employed for numerous reasons, and in many ways is the most obvious example of convergence. Insects are the masters of mimicry, but considering its benefits it is not surprising that many other organisms have evolved it as well. In most cases, mimicry is visual - one species resembles another in appearance - but it can also be olfactory or vocal.
Defensive mimicry helps prey to escape from potential predators. It is usually Batesian, where a harmless species adopts the warning (aposematic) colouration of a toxic or unpalatable species, which makes it less likely to be attacked by a predator. An octopus living in Indo-Malayan waters goes one step further and impersonates a range of venomous animals by changing its colouration and patterning and adopting a particular body posture. Muellerian mimicry is also known, whereby harmful species gain a mutual advantage by sharing colouration, as a predator learns to avoid both of them. A classic example of this is Heliconius butterflies, where races of H. erato and H. melpomene have convergently evolved the same colourations, and H. numata colouration mimics another species, Melinea. In addition, the developmental genetic changes underlying the colour changes are apparently relatively simple and largely conserved between mimics with repeated use of key genes to create new patterns. Many insects mimic leaves, sticks and vegetation, which, strictly speaking, is not mimicry but mimesis, a form of camouflage where prey takes on properties of an object or organism to which a potential predator is indifferent.
Aggressive mimicry occurs where predators mimic harmless species to avoid being detected by their prey. The predatory sabre-toothed blenny is an adept mimic of a harmless cleaner fish - when approached by a bigger fish in need of cleaning, it bites off little pieces of flesh instead. Some fireflies attracted by bioluminescent flashes seemingly sent by a female that is willing to mate end up being eaten by a predatory firefly mimicking female signals. Another well-known example is hunting spiders that closely resemble the ants on which they prey, but such convergence of mimicry of insects and spiders to an ant morphology has evolved at least 70 times. The parasitic mite Planodiscus, for instance, sits on an ant's leg and mimics its shape and even the arrangement of bristles so well that it normally remains undetected by the ant.
A rather special case of mimicry occurs in the context of obligate brood parasitism (which has evolved several times independently in birds). To circumvent host defences, the parasite often employs egg mimicry, i.e. lays eggs that mimic those of the host, thus reducing the risk of its egg being recognised and rejected by the host. In common cuckoos, the chick furthermore displays vocal mimicry, mimicking the begging calls of an entire brood of host chicks, to secure parental care from the host.
Sexual mimicry, where females mimic males of the same species or vice versa occurs, for example in mammals and most remarkably in cephalopods. The ability of cephalopods to change colour and body patterns by use of their chromatophores allows for female impersonation, which enables a "low rank" male to sneak past a competitor and successfully fertilise the female.
Mimicry does not only occur in animals but, remarkably, also in fungi. One species of fungus produces very smooth, small balls, which are readily mistaken by a termite for eggs and transported into the nest, a safe environment for the fungus to germinate. There are even more complex examples as some fungi induce the formation of pseudoflowers from leaves of their host plant. These pseudoflowers then attract insects, which do not distribute the plant's pollen but actually the spores of the fungus.