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MIMICRY – The art of impersonation
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Paul Smart - The illustrated encyclopedia of the Butterfly world |
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The incidence of mimicry among butterflies has been the subject of considerable study and controversy in the past. It is l certainly one of the most striking and interesting features of tropical butterflies that so many apparently very similar forms in fact belong to quite different species or families . The concept of mimicry is based on the assumption that certain species are unpleasant, unpalatable or inedible to predators such as birds and
other vertebrate enemies of butterflies, and that these species are coloured with conspicuous patterns of 'warning' colours which are recognized and avoided by the predators. It is further assumed that other harmless and quite palatable butterflies have evolved similar colour patterns and so derive protection because predators mistake them for the unpalatable species and leave them alone. The unpalatable butterflies are normally referred to as 'models' while the palatable species that resemble them are called 'mimics' .
Two basic kinds of mimicry have been distinguished : Batesian mimicry in which there is an unpalatable model species and a palatable mimic, and Müllerian mimicry in which two or more unpalatable, and often unrelated, species have evolved the same colour pattern.
Many of the basic assumptions of the theory of mimicry have been inadequately tested under scientifically controlled conditions . But there are many similar cases in the animal world where harmless species have adopted the colour patterns of some more unpleasant or harmful creature as a means of protection against predators. The markings of harmful species are usually simple patterns in striking colour contrasts that form a recognizable 'badge' which is easily learnt and recognized by predators. The black and yellow warning colours of a wasp is a good example of this.
There are several unlelated and harmless creatures that have evolved similar wasp-type colour patterns such as some of the hoverflies and the wasp beetle Clytus arietis. 'Such examples of mimicry may be enhanced by behavioural mimicry such as the production of a wasp-like buzzing sound. The mimic species are bluffing; sheltering as it were behind the wasp's reputation. Among butterflies certain species may be unpalatable to predators because of an unpleasant taste or smell, or because they contain poisonous substances which may be derived in the larval stage from their foodplant. For example, Miriam Rothschild has shown that the widely distributed tiger butterfly Danaus chrysipus contains cardenolides (heart poisons) absorbed in the larval stage from the milkweed plants (Asclepiadaceae) on which they feed. Such protected species are usually conspicuously coloured in combinations of black, red, orange, yellow and white. Furthermore they often fly slowly, do not attempt to escape when disturbed and positively advertize their presence.
It is assumed that predatory species quickly learn to recognize the pattern and general appearance of the inedible species and avoid them. A young inexperienced bird will attack and attempt to eat the tiger butterfly but will rapidly learn that it is inedible and highly unpleasant. It win Subsequently associate its distasteful encounter with the conspicuous appearance of the butterfly and will not attempt to attack another one so long as it can remember its previous experience. There is some .experimental evidence to confirm that this is actually the case when a bird is presented with unpalatable butterflies. The important point is that once a bird has learnt to avoid a particular colour pattern from attempting to eat an inedible butterfly it will subsequently avoid all butterflies with that pattern including perfectly harmless ones. In this way the mimics enjoy the same protection as the models in areas where they occur together.
It is quite easy to imagine the selective process which refines the mimics until in some cases they resemble their models so precisely that they can only be distinguished by a detailed examination of the wing venation. Mimics which resembled their models more closely would be mistaken more often by predators for the model and would therefore be avoided more often. Conversely butterflies which were poor mimics would tend not to be mistaken for the models and would be more likely to be predated. In this way poor mimics would gradually become eliminated, leaving only those forms that closely resembled the model. Mimicry often extends to behaviour as well as colour, so that the mimic may adopt the habits and flight behaviour of the model species. Any trait that tends to 'give away' the mimic will be eliminated over a long period of time by natural selection.
There are many examples of the two types of mimicry defined earlier. In practice it may be difficult to draw a distinction between the types because in some areas in the tropics many species may be involved in what are called 'mimicry rings. These are groups of species including both Batesian and Mullerian mimics which all have a common colour pattern.
Batesian mimicry
Among butterflies certain families of butterflies include large numbers of unpalatable species which act as models for Batesian mimics. For instance the family Danaidae includes many distasteful species and thus supplies the models for many of the classic examples of mimicry by members of other families. The plain tiger Danaus chrysipus is a model for the female of the diadem Hybolimnas misipus, a member of the Nymphalidae
These specimens are from India but the two species occur together throughout Africa, parts of Asia and much of Australasia. Another well known instance is provided by the North American monarch Danaus plexippus, which is the model for the viceroy mimic Limenitis archippus. In this species both sexes are mimics, unlike the previous example. The specimen of the monarch shown is unusually small as it is normally larger than the viceroy. The mimic in this case is very different from the normal Limenitis butterflies, as is demonstrated by comparing the mimic with the closely related Limenilis arthemis. In both these instances a member of the family Nymphalidae is mimicking a species from the family Danaidae.
The situation can become more complex when the mimic is found in areas where the usual model does not occur, or is at least uncommon. In such cases the mimic may resemble another species of model, as in the instance of the viceroy which mimics Danaus glilippus in parts of Florida and the south-western United States where the monarch is less common. This raises some of the basic requirements for a successful Batesian mimicry situation. The model and the mimic must occur together in the same area, and in the same habitats. Furthermore the model should always be more abundant. This is because it is essential for the warning colour pattern to be learnt by predators by actual unpleasant experiences. In other words some distasteful butterflies must be eaten before the others are avoided. If the butterfly population includes a high proportion of edible mimics predators have a high chance of taking the harmless butterflies, and so will not learn to avoid the warning colour pattern so quickly, so much of its protective value is lost. In the wild this situation does not arise.
Usually the mimics are decidedly scarce and difficult to encounter, while the models may be conspicuously abundant. One of the most striking and ingenious of all Batesian mimics is the very local Papilio laglaizei from Papua. This species monics not another butterfly but a day-fiying moth, Alcidis agarthyrsus. The resemblance between their upper surfaces is remarkable enough, as the illustration shows pictures below, but the outstanding feature is only visible from below. A agarthyrsus has a brilliant orange ventral surface to the abdomen - a body colour unlike that of any Papilio.
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The unusual feature of this pair is that (1. Alcidis agartyrsus (Uraniidae) is a day-flying moth and it mimicis a swallowtail butterfly) (2. Papilio laglaizei - (Papilionidae) both are from New Guines |
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There three Central African species are quite unrelated, although all very closely resemble Acraeid Bematiste epaea (3. Elymniopsis bammakoo, 4. Pseudacraea eurytus, 4. Papilio cynorta "female" |
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This colour is matched exactly in a patch on the anal fold of each hind wing of P. laglaizei - just where it covers the abdomen when the butterfly is at rest.
The African mocker swallowtail Papilio dardanus has polymorphic females which provide an interesting example of multiple Batesian mimicry. The male is a rather distinctive insect with a creamy yellow ground colour and black markings, with a tail on the hind wing. In certain parts of its range (Ethiopia and Madagascar) the females are very similar to the males in colour, pattern and shape. The majority of female forms elsewhere are tailless, quite different in appearance and mimic a great variety of distasteful species of other families. More than one hundred forms have been named and only four of the most familiar examples are illustrated here. The form hippocoon is modelled on the friar Amauris niavius (family Danaidae) while the orange form planemoides mimics Bematistes poggei, a member of another distasteful family (Acraeidae). Batesian mimicry on a large scale is evident within the family Papilionidae. In both the old and new world the swallowtails which feed on Aristolochia 'advertize' the presence of body poisons with their aposematic or warning colours, which are generally red spotting of the wings or body. Their mimics, resemble them closely enough to deceive the inexperienced lepidopterist.
Müllerian mimicry
This is the term used to describe the close physical resemblance between different species which have similar unpleasant properties. It is an advantage to both species if two unpalatable butterflies which occur in the same area have a common badge or warning colour pattern. This will be apparent when it is remembered that predators will only learn to avoid a particular warning colour pattern after they have experienced the unpleasant taste by trying to eat the insects. In a given area a certain number of butterflies will have to be sacrificed before all the local predators have learnt to avoid their particular colour pattern. If two species are involved and they share the same pattern the losses to each species will be approximately halved. Evolutionary pressure caused by predator selection would tend to favour the convergent evolution of similar colour patterns until they become very similar. This process would of course have taken place over a very long period of time. In some cases mimetic forms may have arisen initially as random mutations which then survive because they quite fortuitously resembled another species which was unpalatable. Without fossils or other evidence of ancestral forms it is not possible to come to firm conclusions on these points. The appearance of closely related but non mimetic species may provide pointers to possible lines of development but no more.
A better understanding of the selective processes that have produced these mimetic patterns could be obtained by carefully investigating how predators recognize warning colour patterns and what particular features in the pattern are significant. Research is needed to establish whether it is the generalized wing pattern, or only particular features such as spotting or banding ; or whether it is the juxtaposition of certain colours that are learnt and recognized by the predators. This is particularly relevant to the evolutionary puzzles posed by same of those mimetic forms which reproduce every fine detail of their models, such as members of the Nymphalid genus Pseudacraea. There are many examples of Müllerian mimics among the family Heliconiidae which share similar unpalatable characteristics and appearance. The mimetic pair Heliconius telesiphe and Podotricha telesiphe is particularly interesting. These are Peruvian butterflies, while the forms of the same species that fly together in Ecuador have a yellow hind wing band in place of white, demonstrating the long-standing interdependence of these species.
A pioneer in this field of study, the late R. C. Punnett, pointed out that even if one member of a Müllerian group were more abundant than the others it would still benefit from a common colour pattern, while the rarer species would gain considerable advantage, as in the Batesian situation.
This brief account is a very simplified survey of a fascinating and complex subject which needs considerably more intensive study and research. There are many controversies which have not been resolved and even some of the really basic assumptions, such as the idea that vertebrate predators are the selective agents in the evolutionary process, have been questioned. Some very complex mimetic, associations exist where apparently both Batesian and Müllerian mimicry are at work. Furthermore there are many butterfly species that exhibit striking parallels in pattern and coloration, such as the Neotropical genera Agrias, Callithea, Callicore, and Siderone. There is no apparent explanation for these instances of parallel development which remain a puzzle.
Mimicry rings
The most complex associations of mimetic species occur in the so called mimicry rings. These are groups of species, often quite unrelated, which all share a common mimetic pattern and appearance. The rich Neotropical fauna include some striking examples of such groups, the most interesting of which revolves around the orange and black patterned butterflies with long narrow wings. Possibly more than a dozen species, from several different families, are involved in this group, including Heliconius numata and Melinaea mothone. as well as at least two moths, Castnia strandi and Pericopis hydra. The two butterfly species are so similar, despite being unrelated, that only from a detailed examination of their structure is it possible to tell them apart. Three members of a similar group are shown above pictures. These two butterflies and one moth fly together in Peru, where they form a Müllerian association as they all contain unpalatable substances. An interesting North American mimetic complex is developed around the pipe-vine swallowtail Battus philenor. This species is the model for at least one other swallowtail (Papilio trozlus) , the red-spotted purple (Limemris astyanax ) and possibly also the dark females of both Papilio glaucus and Speyeria diana.
Examples of mimetic groups from the old world are rather less frequent, but the most conspicuous occur in the Ethiopian region: The pictures show above for example are three Central African species of different families which share a very similar appearance based on the Acraeid species Bematistes epaea.
False mimicry
Further examples of similarity between butterflies may be noticed among the family sections of other book, but it may be easy to jump to the wrong conclusions solely on the basis of the appearance. Unless two butterflies occur in the same part of the world, they cannot be mimetically associated.
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