|
COLORATION - For concealment and advertisement
 |
Paul Smart - The illustrated encyclopedia of the Butterfly world |
|
|
It is the presence of patterned and coloured wings that immediately distinguishes the Lepidoptera from most other groups of insects which tend to have rather uniform, transparent wings. Among the Lepidoptera appear some of the most exquisitely coloured of all living things. This undoubtedly contributes to their popularity, and the fact that they can also easily be preserved has made them a favourite quarry for collectors. Enthusiasm for butterflies especially is not confined to scientists; the same colours and and ease of preservation has made them a favourite material for the manufacture of ornamental goods. Regrettably butterfly wings, in glass or plastic, have been extensively used to decorate table mats and tea trays. Groups of the larger species, arranged in geometrical patterns to form wall decorations were especially popular in the late 19th century. Some of the most brilliant species, particularly those with a metallic lustre, have been incorporated into items of jewellery for personal adornment.
How colour is produced.
Colours may be produced by pigments, by structure or by a combination of these two. The colour is usually present in the layer of microscopic scales which clothes the butterfly’s wings, but it may also be present in the underlying epidermal tissue which remains if the scales are removed, as in the case of the codrus group of swallowtails (Papilionidae) (see here) Pigmentary colours are due to the presence of chemical substances derived from metabolic processes, often as by products of excretion. Butterfly pigments are inadequately known but some have been scientifically investigated and details of their chemical structure are fairly well understood.
Pigments
Melanin is a particularly common pigment and is responsible for blackish-brown colours. Pterins are pigments derived from uric acid and are found only in the family Pieridae among butterflies. Leucopterin (white, xanthopterin (yellow), chrysopterin (orange) and erythropterin (red) in a variety of combinations produce most of the attractive colours found in this family. Flavones produce cream or yellow colours in several butterfly families. These pigments are interesting because they are apparently derived directly from plant leaves during the butterfly's larval stage. At one time it was thought that the green colours of many larvae and pupae could be plant chlorophyll directly obtained in the same way. It now appears that chlorophyll is not found in Lepidoptera but that green pigments are biochemically derived from it. Reds and reddish-browns are also produced by pigments. The unstable nature of many butterfly pigments causes them to fade if exposed to sunlight and many can also be altered artificially by chemical action For example the brick-red of the small tortoiseshell (Aglais urticae) (Nymphalidae) can be converted to a dull purple by hydrochloric acid fumes (forms of similar colouration are occasionally found in wild populations).
Structural colours
Structural colours are produced by the physical properties of butterfly wing scales. When examined under a microscope the individual scales usually reveal longitudinal ridges or ribs. In some species these ribs consist of thin translucent layers of material set at a constant angle to the plane of the scale itself and separated by air spaces. This structure refracts light to produce an overall colour, which may vary somewhat depending on the angle at which the light strikes the scale but is otherwise fairly constant for the species. An an¹logy may be drawn with a thin layer of oil floating on water. Both are transparent and colourless, yet light passing through the oil and reflected back from the water surface suffers an interference which causes the appearance of rainbow colours.
Most animals which have a metallic sheen or iridescent colouring, including humming birds as well as butterflies, owe their unusual splendour to structural factors rather than pigments. The brilliant iridescent blue colour of many of the S. American Morpho butterflies is probable the most familiar example of a structural colour and as a rule green and blue colours in butterf1ies are produced by structure rather than pigment. There is a simple test to distinguish between these two kinds of colouring. If water is put on the butterfly wing, pigment colours are unaffected, but structural colours are dulled owing to the water spoiling the effect of the scale surface.
Another type of physical factor that produces colour has been reported by the physicist E. R. Laithwaite, who has examined a structure akin to a diffraction. grating in some members of the Neotropical genus Pierella (Satyridae). In essence this is a series of fine structures which break up white light into its visible coloured components. The males of Pierella appear relatively dull until viewed from an acute angle, when a small dark patch on the forewing 'lights up' in an extraordinary manner. The specimen of Pierella hyceta (shown on page 56) used to demonstrate this feature is naturally much distorted by the perspective. At its brightest the patch appears green but variation in the angle at which light impinges on the wing produces every colour in the visible spectrum.
|
|
|
|
The Fluffy tit Zeltus amasa - Picture: Phùng Mỹ Trung |
|
White colours in butterflies may also be produced structurally. The scattering of light by microscope transparent particles produces a white effect in the same way that snow appears white, but no actual pigment is present. White hair in humans is similarly an effect created by tiny air bubbles within the hair rather than by a white colouring matter of some sort.
Combination colours
Combinations of structural colour and underlying pigment are much commoner than structural colour alone. In the European purple emperor Apatura iris (Nymphalidae) (shown here) the scales contain the dark brown pigment melanin, but this is overlaid by a structural violet blue colour which is visible only when viewed from certain angles. Butterflies of the genera Colotis and Hebomoia (Pieridae) (shown here) possess a red pigment in the scale walls which is combined with a structural violet to produce orangepink or shades of magenta. The true appearance of these species is; not easily reproduced as photography often fails to reveal the underlying pigment and records only the colours due to structural elements.
Two species of birdwing butterflies Troides (Papilionidae) exhibit the most beautiful colours caused by the combined effect of structure and the translucent gold pigment present in this genus. In a specimen of Troides magellanus (shown on pages 58/59) when viewed from above only the gold pigment is evident, but when illuminated from the rear and viewed acutely from this direction beautiful iridescent pinks and greens are visible on the hindwings.
Transparency
While the wings of most butterflies are coloured and opaque there are a few in which they are wholly or partially transparent. The best examples occur in the Apollo butterflies, genus Parnassius (Papilionidae). Though cases are generally uncommon in temperate latitudes, an extremely diaphanous example of the clouded Apollo Parnassius mnemosyne from Switzerland. Further semi-transparent Parnassius species. In these mountain butterflies the transparent areas are due to scale deficiency. Partial transparency, where patches of wing may be devoid of scales, is not uncommon in butterflies. It may be seen on the wing tips of the protesilaus group of swallowtails (Papilionidae) (pages here) or in the small 'windows' found in the Nymphalid genera Kallima and Anaea .
The predominantly Ethiopian family Acraeidae includes many examples which are thinly scaled and have a largely transparent appearance. The most extreme forms bear more resemblance to insects of other orders such as dragonflies (Odonata). In the Neotropical region transparent species occur in a number of families, such as Danaidae, Ithomiidae, Satyridae and Pieridae. The Satyrid genus Cithaenas and the glasswings Ithomiidae are good examples of butterflies which have largely transparent wings.
The same overall transparent appearance in butterflies can be produced in a number of different ways. The Pierid Dismorhia orise has scales which are reduced in size. In the Ithomiids the scales are further reduced to fine hairs. The Danaid Iruna has full-sized pigmented scales but they are very sparsely scattered. Some Neotropical moths have achieved the same effect as in the butterflies by having transparent scales, while one genus has its normal sized scales set on edge so that the light passes between them.
The adaptive significance of colour patterns
All the colour patterns found in butterflies, ranging from the most dark and drab to the most brilliantly colourful, have some survival value or adaptive significance. This applies not only to the adult insect but also to every stage in the life cycle. The intricate patterns that we see on butterfly wings have been moulded, modified and perfected by a long process of natural selection. The purpose of coloration may vary throughout the life of a butterfly, with the young stages being cryptically coloured or camouflaged while the adults may be brightly coloured for courtship display. Different types of coloration may be found on one insect. It is quite common for the underside of butterflies to be cryptically coloured so that they are camouflaged at rest while the upper wing surfaces have bold markings which are only visible when the wings are opened. Coloration may be very closely related to the behaviour of the butterfly and the adaptive significance of a colour pattern may not become apparent until the insect has been studied in its natural surroundings.
Cryptic colours
Many examples of cryptic coloration are found among the larval and pupal stages of butterflies. During this phase of its life the developing insect is most vulnerable to predation. Coloration is usually combined with protective behaviour - for example many butterfly larvae feed at night, crawling up the foodplant at dusk and returning to the comparative safety of the bases of the leaves during daylight hours.
The European purple emperor Aatura iris affords a good example of camouflage colours in its young stages. The larvae feed on the leaves of sallow (Salix caraea). The eggs are laid on the leaves in July and the young larvae hatch and feed on the leaves till they reach their third instar. At this stage the tiny green slug-like larva crawls down and rests in the fork of a twig, where it will spend the winter. The over-wintering larva changes to a brownish colour when it matches the bark perfectly. In the spring the young lanra resumes feeding as the leaf buds burst open. It quickly changes back to a green colour which exactly matches the shade of the leaves. When resting the larva characteristically sits head upwards on a leaf with its tail attached to the very point of the leaf. It adopts a pose with the anterior part of the body raised up in a curve so that it matches the curving outline of the sallow leaf. In addition to the colour and textural resemblance to the leaf the larva is also counter-shaded with the anterior end darker green than the posterior end. This counteracts the natural shadowing effect so that the larva seems to be flat like the leaf. The pupa is another masterpiece of camouflage. The shape and colour exactly matches a dead or wilted sallow leaf. It is again counter-shaded except that now the tail end is darker.
Many other kinds of protective resemblance may be found in the early stages of butterflies. In addition to mimicking leaves they may resemble other parts of a plant such as buds or twigs, or they may resemble other objects that would be unattractive to a potential predator. There are a number of examples of young stages which resemble bird droppings, for example the larvae of some swallowtails (Pailio) and the pupae of certain Lycaenidae.
Among adult butterflies cryptic patterns are the most obvious forms of protective coloration. Many species are at least partially cryptically coloured, usually on those parts which are visible when the butterfly is at rest. In fact the majority of butterflies are less brightly coloured on the under surface than the upper. Even if a specimen seems strikingly patterned when it is seen isolated in a collection it may still blend totally with its surroundings in the wild. Leaf-like patterns and shapes are particularly common examples of camouflage colouring. Butterflies of the Nymphalid genus Polygonia have a ragged appearance and a dappled cryptic colour pattern. These insects are well concealed when resting among the withered leaves in which they hibemate.
The amazing leaf-butterflies, of the genus Kallima (Nymphalidae), which occur in the Indo-Malayan region are probably the most frequently quoted examples of natural camouflage in the Animal Kingdom. When at rest the fore and hind wings reproduce a large elliptical leaf shape complete with a stalk. The colour is basically that of a dry dead leaf but the resemblance is complete down to the last detail. The underside pattern has a 'midrib' and even 'blemishes' such as 'worm holes' and patches of 'mould'. With these embellishments the butterfly so exactly duplicates the natural appearance of a dry decaying leaf that naturalists have never ceased to marvel at it. The species shown is Kallima paralekta, the subject of an amusing account by the great naturalist and explorer Alfred Russel Wallace who wrote in The Malay Archielago:
“This species was not uncommon in dry woods and thickets, and I often endeavoured to capture it without success; for after flying a short distance, it would enter a bush among dry or dead leaves, and however carefully I crept up to the spot, I could never see it till it would suddenly start out again, and then disappear in a similar place. At length I was fortunate enough to see the exact spot where the butterfly settled, and though I lost sight of it for some time, I at length discovered that it was right before my eyes, but that in its position of repose it so closely resembled a dead leaf attached to a twig as almost certainly to deceive the eye even when gazing full upon it”.
|
|
|
|
The Gaudy baron Euthalia lubentina - Picture: Phùng Mỹ Trung |
|
Such cryptic patterns are usually found on the underside of the wing but in some cases they may 'appear' on the upper surface. Members of the Neotropical genus Hamadryas (Nymphalidae) frequently rest head-downwards on tree trunks with the wings spread, an unusual butterfly posture. The dappled blue and grey colours of the upper surface of the wings blend perfectly with the bark. The wings of the butterfly are held fiat against the tree so that they appear to be part of the surface and there is no shadow to accentuate the outline of the butterfly. The effectiveness of cryptic colours may be considerably reinforced by behaviour such as this. A perfect colour resemblance to the surroundings can easily be spoilt if the resting butterfly casts a shadow which gives way its presence. This danger is greatest in the case of species that habitually rest on the ground or some other flat surface. When resting many Satyrids orientate themselves towards the sun to eliminate their shadow, or at least to minimize it. Some butterflies, such as the grayling Hipparchia semele, tilt their bodies and wings towards the sun
to reduce their shadow size. They behave in a similar manner towards an approaching observer so that little of the wings is visible. Butterflies of the genus Oeneis, which inhabit the flat Arctic tundra, adopt the most extreme poses by lying on their sides to eliminate their shadow which would be very noticeable in their natural habitat.
Some of the transparent butterflies referred to earlier represent another type of probable cyptic coloration. The Satyrids and some Ithomiids inhabit forest situations where there is dappled light and shade. They fly close to the ground and settle frequently. Since they are almost entirely transparent the natural background is visible through the wings and the butterfly itself is difficult to distinguish. When in flight the combination of transparency and dappled light makes these insects very difficult to follow with the eye
Disruptive patterns
Cryptic coloration may be combined with patterns, wing shapes and resting attitudes that enhance the camouflage effect. The most recognisable and obtrusive feature of a resting butterfly is often its triangular outline. It is common to find patterns superimposed on the overall cryptic colours that tend to disguise the shape of the butterfly either by oblitelating the outline or by disrupting the overall shape. The underside of the European small tottoiseshell Aglais urticae affords a good example of both these devices. The brown colour of the underside resembles a dead leaf. The wing margin is chequered in dark and light brown to break up visually the otherwise clear-cut edge. In addition the wing bases are dark brown changing abruptly to light yellowish-brown in the areas furthest from the body. When the insect is resting the contrasting patterns cutting across the wings break up their characteristic outline. The pattern itself is also particularly obvious and the
attention of an observer (or a predator) is drawn to an abstract pattern of colours rather than an instantly recognisable butterfly of a clearly defined shape.
This disruptive type of pattern may be part of the uniform in cryptically coloured species, or it may take over completely as the main form of protection. The Neotropical zebra butterfly colobura dirce has the underside strongly marked in cream and brown to form a bizarre pattern of stripes, bands and zig-zags that visually fragment the wing shape. Similarly the Neotropical genus Diaethna includes species with spectacular patterns in black and white on the under-wing surfaces. These patterns are striking and conspicuous in isolation, and in no way match the surroundings as in cryptically coloured forms. Instead they present a bizarre jumble which is visually meaningless rather than easily recognisable as a butterfly. With cryptic coloration, protection is gained by making the butterfly match its surroundings so that it is difficult to see; with disruptive coloration, the animal can be seen but not recognised for what it is. Both strategems are effective in keeping predators from eating the butterflies concerned.
Eye-spots or ocelli
Another protective device is the use of patterns which draw the attention of a predator away from the vital parts of its potential prey. Eye-spots or ocelli, which occur in the wing patterns of most families of butterflies, are probably the best example of this type of' coloration. Ocelli are most frequent among members of the Satyridae, Brassolidae, Morphidae and Amathusiidae. The resemblance of these spots to real eyes may be very superficial, for example many Satyrid 'eyes' consist of a round black spot with only a tiny white 'highlight'. Sometimes the eye-spot may bear a startling resemblance to a vertebrate eye as in the case of the Neotropical genus Caligo the so-called owl butterflies. When a specimen of one of these butterflies is seen with its lower wing surfaces displayed and the butterfly is viewed upside down, it is strangely reminiscent of an owl's face with two large staring eyes, but it is difficult to imagine any natural situation when the butterfly could display both eyes in this way. Statements that the butterfly habitually rests up-side down are quite erroneous.
The survival value of ocelli has been the subject of much discussion. The chains of eye-spots that may be seen on the underside of most species of Morphidae are probably disruptive patterns helping to break up the shape of the underside. However in most cases ocelli are believed to act as deflecting patterns acting as 'target areas' for potential predators, such as birds. The idea being that a bird, confronted with a butterfly marked in this way, will take a swift peck at a prominent eye spot rather than some more vulnerable part of the butterfly’s body. The resulting damage to the wing would be less harmful than if it had been done to the body; certainly the loss of quite major portions of this wing area in Taenaris butterflies would not adversely affect them to any great extent. In many Lycaenids the ocelli are combined with structural features that reproduce a ‘false head’ at the rear inner angle of the hind wings. This part of the wing is drawn out into fine tails, which may be quite long and elaborate in some tropical species, for example in the Ethiopian genus Hyolycaena. The attention of a
predator is further drawn to the rear end of the insect by special wingmovements. These butterflies habitually rub their wings together, causing the tails to twist about and creating the illusion of a head with actively moving antennae. The supposed function of these patterns requires more careful field observation and verification but certainly it is quite common to find tropical Lycaenids with triangular sections of the rear wings missing where a bird has presumably taken a peck at what it thought was the head of a large insect.
Startling colours
Butterflies with eye-spot markings or other striking colour patterns may also gain protection by startling their enemies with sudden movements that reveal their colours. This is similar to the flash coloration displayed by some moths. The resting moth may be quite inconspicuous with drab coloured fore-wings but when it is disturbed it suddenly raises its forewings to reveal hind-wings with vivid markings such as the bright red under wings of catocala species or the huge eye-spots of the bull's eye moth Automeris io. This shock tactic may startle a predator long enough to enable the insect to
escape unharmed. Although moths show this combination of colour and behaviour particularly well, it is also seen in some butterflies. For example when the grayling butterfly Hiarchia semele is at rest only the cryptic colours of the underside are visible. If the butterfly is approached and disturbed it responds by suddenly raising the fore wings to reveal a large black eye-spot on the underside of the fore wing which is just concealed by the hind wing when at rest. The peacock (Nymhals io) similarly shows only cryptic colours when at rest, but it can dramatically reveal its four big eye-spots by suddenly flashing its wings open if disturbed. Ocelli are also employed by larvae to startle their enemies. The mature larva of the American spicebush swallowtail (Pailio troilus) has two spectacular eye-spots on its thorax. When disturbed it can cause that part of its body to swell up, thus stretching the skin and making the eye-spots even bigger.
Conspicuous colour patterns
There is a great variety of butterfly colour patterns whose survival value is not so obvious. There are some species for instance which are brilliantly coloured and seemingly very conspicuous indeed. Such colour patterns would appear to advertize the butterflies and place them at risk from predators rather than offering them protection but such coloration may have its purpose in fulfilling some role other than a protective one. In particular, brilliant colours and striking patterns may play their part in recognition between individual butterflies for the purpose of territorial behaviour and courtship, just as they do in many birds. Certainly it is possible to attract
the blue male Morho butterflies by waving a piece of bright blue card in the sunlight in the localities which they inhabit.
In these particular insects, the brilliant iridescent blue colour probably also serves another function. They inhabit tropical rain forest and are usually found near water, especially waterfalls. The butterflies follow stream courses with a fast erratic flight dashing through the dappled light and shade. As they pass through a sunny patch the wings flash electric blue, but in the shadows they appear only as dark silhouettes, making it difficult for predators to follow them in night because one moment a brilliant blue object is being chased but the next instant it has disappeared, to be replaced by a
dark shape. This in turn suddenly erupts into brilliant blue on reaching a sunny patch. The momentary confusion and indecision induced in a potential predator may be sufficient to help the butterfly escape unharmed.
Butterflies which are protected by poisonous or distasteful body fluids usually adopt a very striking colour pattern to advertize their presence and identity. It is a positive advantage for such butterflies if they are recognized by their enemies as inedible before they are attacked. For this reason they use bright warning colours such as red, orange or yellow in contrasting patterns, often with black or dark brown stripes. A bird or other predator which catches and tries to eat one of these butterflies learns to associate the pattern and colour with its unpleasant qualities, and avoids catching similar butterflies thereafter. Probably the most universally known butterfly which
has warning colours of this type is the monarch, Danaus lexius (Danaidae). Other particularly good examples of this type of coloration may be found amongst the families Heliconiidae and Acraeidae. There is a tendency for unrelated species which are protected by their distasteful nature to adopt a common colour pattern if they inhabit the same locality, and for other unprotected species to mimic the warning colour pattern. This fascinating phenomenon of mimicry is dealt with in Mimicry.
Hybrids and transitional colour forms
Naturally occurring hybrids of butterflies are very rare, although such forms have been produced in breeding experiments, by hand-pairing, principally among Papilionidae. These hybrid forms may show colours (or perhaps other anatomical features) derived from both parents; and a wild-caught hybrid between Callicore cynosura and C. astazza (Nymphalidae) from Peru is shown here. The upper surface is principally cynosura although the inner fore wing band (not present in Pastazza) is much reduced, and so is the hind wing band. The outer margins of the hind wings also show clearly the reflective purple-blue of Pasrazza and the under surface of this remarkable insect is entirely as in this species. Transitional colour forms are not hybrids but simply examples which show features intermediate between two forms of the same species. Two specimens of the Neotropical Nymphalid Anaea (Siderone) marthesia. This species exists in a number of
distinct forms which have sometimes been considered as constituting quite separate species. However, even the most dissimilar forms are linked by transitional types, each showing a different combination of the features which distinguish the particular forms. Similar transitional examples are found between colour forms of many other butterflies |
