1902 Encyclopedia > Mimicry

Mimicry




MIMICRY is the name given in biology to the advantageous resemblance (usually protective) which one species of animal or plant often shows to another. The word was first applied in this methaphorical sense by Mr W. H. Bates, and it has since been accurately defined and limited, in its biological application, by Mr A. R. Wallace. Briefly put, the essence of the phenomenon of mimicry consists in the following relation. A certain species of plant or animal possesses some special means of defence from its its enememies, such as a sting, a powerful and disagreeable odour, a nauseous taste, or a hard integument. Some other species inhabiting the same district or it, and not itself provided with the same special means of defence, closely resembles the fist species in all external points of form and colour, though often very different in structure and unrelated in the biological order. For example, a South America family of butterflies, the Heliconidae, are distinguished by their very varied and beautiful colours, and their slow and weakly flight ; they might easily be captured by insectivorous birds, but their remains are never found on the ground amongst the rejected wings of other butterflies which cover the soil in many places. They also possess a strong pungent odour, which clings to the fingers for many days ; and this fact led Mr Wallace to suspect that they have a disagreable taste, and would not therefore be eaten bye birds after a single trial. Mr Belt has since experimentally proved the truth of that belief. But among the totally distinct of the Pieridae, most of which are white, there is a genus of small butterflies, known as Leptalis, edible by birds, some species of which are white like their allies, while the greater number exactly resembles one or other of the Heliconidae in the peculiear shape and colouring of their wings. As regards structure, the two families are widely different ; yet the resemblance of a species of one family to a species to a species of the other is often so close that Mr Bates and Mr Wallace, experienced entomologists, frequently mistook them for one another at the time of capture, and only discovered their mistake upon nearer examination. Mr Bates observed several species or varieties of Leptalis in the Amazons valley, each of which more or less exactly copied one of the Heliconidae in its own district. Accordingly, they seem to be mistaken by birds for the uneatable insects they mimic, and so to be benefited by their resemblance. This, which may perhaps be regarded as the most typical instance of true mimicry is also the first to which the word was applied.

In considering the phenomena under review, it may be well to give first the chief observed facts, which are quite independent of any particular explanation, and then the theory which has been started to account for them by Mr Bates and Mr Wallace. Before doing so, however, true mimicry should be carefully discriminated from one or two superficially similar modes of resemblance among organic beings, whose real implications are very different. It must not be confused with more accidental or adaptive resemblance, due either to simple chance or to similarity of external conditions. As a case of the first sort, we may take certain African Euphorbiaceae, which, growing in dry deserts, have acquired a very close likeness to the cactuses that cover the equally dry deserts of Mexico ; or again the sub-Antarctic gallinaceous bird. Chionis alba, which living, on the sea-shore, has acquired a coloration like that of the gulls, together with the legs of a wader. These resemblances, however, do not as such subserve any function. The species apparently mimicking and the species apparently mimicked either do not inhabit the same district or do not come into any definite relation with one another. The likeness is either accidental, or else it is due to similar adaptation to similar circumstances. In cases of true mimicry, on the other hand, the mimicking species derives a direct advantages from its likeness to the species mimicked ; the resemblance is deceptive ; and this is equally true whether we suppose the mimicry to be produced by creative design or by natural selection. On either hypothesis, however it come by its likeness, the mimicking species escapes certain enemies or obtains certain sorts of food by virtue of its resemblance to some other kind.

It should also be added that the word mimicry, as applied to such cases, in only in a metaphorical sense. It is not intended to imply any conscious or voluntary imitation by one species of the appearance or habits of another. All that is meant is the fact of an advantageous resemblance, a delusive similarity, which gives the mimicking animals or plant some extra protection or some special means of acquiring food which it would not otherwise have possessed but for its likeness to the creature mimicked.

Taking animals first, mimicry does not occur very frequently among the higher classes. In the vertebrates it is comparatively rare, and among mammals probably only one good case has yet been adduced. This is that of Cladobates, an insectivorous genus of the Malayan region, many species of which closely resemble squirrels in size, in colour, and in the bushiness and posture of the tail. It has been suggested by Mr Wallace (from whom most of the following examples have been borrowed) that Cladobates may thus be enabled to approach the insects and small birds which form its prey under the disguise of the harmless fruit-eating squirrel. In this case, as in some others, the resemblance is not protective, but is apparently useful to the animal in the quest for food.

Among birds, Mr Wallace has pointed out that the general likeness of he cuckoo, a week and defenceless group, to the hawks and gallinaceous tribe makes some approach to real mimicry. But besides such vague resemblances there are one or two very distinct cases of true mimicry in this class of vertebrates. In Australia and the Moluccas lives a genus of dull-hued honey-suckers, Tropidorhynchus, consisting of large, strong, active birds, with powerful claws and sharp beaks. They gather together in noisy flocks, and are very pugnacious, driving away crows and even hawks. In the same countries lives a group of orioles, forming the genus Mimeta ; and these, which are much weaker birds, have not the usual brilliant colouring of their allies the golden orioles, but are usually olive-green or brown. In many cases species of Mimeta closely resemble the Tropidorhynchi inhabiting the same island. For example, on the island of Bouru are found the Tropidorhynchus bouruensis and Mimeta bouruensis, the latter of which mimics the former in the particulars thus noted by Mr Wallace:—" The upper and under surfaces of the two birds are exactly of the same tints of dark and light brown. The Tropidorhynchus has a large bare black patch round the eyes ; this is copied in the Mimeta by a patch of black feathers. The top of the head of the Tropidorhynchus has a scaly appearance from the narrow scale-formed feathers, which are imitated by the broader feathers of the Mimeta having a dusky line down each. The Tropidorhynchus has a pale ruff formed of curious recurved feathers on the nape (which has given the whole genus the name of friar-birds) ; this is represented in the Mimeta by a pale band in the same position. Lastly, the bill of the Tropidorhynchus is raised into a protuberant keel at the base, and the Mimeta has the same character, although it is not a common one in the genus. The result is that on a superficial examination the birds are identical, although they have important structural differences, and cannot be placed near each other in any natural arrangement." Allied species of Tropidorhynchus in Ceram and Timor are similarly and Timor are similarly mimicked by the local Mimeta of each island. Mr Osbert Salvin has likewise noticed a of mimicry among the birds of prey near Rio Janeiro. An insect-eating hawk, Harpagus diodon, is closely resembled by a bird-eating hawk, Accipiter pileatus. Here the advantage seems to be that the small birds have learned not to fear the Harpagus, and the Accipiter is able to trade upon the resemblance by catching them unawares, both birds being reddish –brown when seen from beneath. But the Accipiter has the wider range of the two ; and where the insect-eating species is not found it no longer resembles it, but varies in the under wing-coverts to white. Here again the resemblance, though advantageous, is not protective.

Among replies, Mr Wallace has instanced some curious cases where a venomous tropical American genus of snakes, Elaps, with brightly-banded colour, is closely mimicked by several genera of harmless snakes, having no affinity with it, but inhabiting the same region. Thus the poisonous Elaps fulvus of Guatemala has black bands on a coral-red ground ; the harmless Pliocerus aequalis of the same district is coloured and banded precisely like it. The likeness affords t he unarmed snakes a great protection, because other animals probably will not touch them, mistaking them from the venomous kinds.

It is among the invertebrates, however, and especially among, that cases of mimicry are most frequent and were first observed. In the order Lepidoptera, besides the classical instance of Leptalis and the Heliconidae, a genus of another family, the Erycinidae, also mimics the same group. The flocks of one species of Ithomia, an uneatable butterfly, often have flying with them a few individuals of three other widely different genera, quite indistinguishable from them when on the wing. In the tropics of the Old World, the Danaidae and possess a similar protective odour, and are equally abundant in individuals ; they are closely mimicked by various species of Papilio and Diadema. Mr Trimen, in a paper on "Mimetic Analogies among African Butterflies," gives a list of sixteen species or varieties of Diadema or its allies, and ten species of Papilio, each of which mimics a Danais or Acraea of the same region in the minutest particulars of form and colour. The Danais tytia of India has semi-transparent bluish wings, and a border of reddish-brown ; this coloration is exactly reproduced in Papilio agestor and Diadema nama, all there insects frequently coming together in collections from Darjiling. In the Malay Archipelago the common and beautiful Euplaea midamus is so exactly mimicked by two rare species of Papilio that Mr Wallace generally mistook the latter at first for the ordinary insect. An immense number of other instances among the Lepiodoptera have been quoted from other parts of the world.

Occasionally species of Lepidoptera also imitate insects of the other orders. Many of them take on the appears of bees or wasps, which are of courses protected by their stings. Thus the Sesiidae and Aegeriidae, two families of diurnal moths, have species so like hymenopterous insects that they are known by such names as apiformis, vespiforme, ichneumoniforme, sphegiforme, and so forth. The British sesia bombiliformis closely resembles the humble bee ; the Sphecia craboniformis is coloured like a hornet, and carries its wings in the same fashion. Some Indian Lepidoptera have the hind legs broad and densely hairy, so as exactly to imitate the brush-legged bees of the same country. Mr Belt mentions a Nicaraguan moth, Pionia lycoides, which closely mimics a distasteful coleopterous genus, Calopteron ; and Professor Westwood pointed out that the resemblance to the beetle is still increased in the moth by raised lines of scales running lengthwise down the thorax .





Among the Coleoptera, or beetles, and other orders, similar disguises are not uncommon. Mr Belt noticed species of Hemisptera and Coleoptera, as well as spiders, in Nicaragua, which exactly resemble stinging ants, and thus no doubt escape the attacks of birds. The genus Calopteron is mimicked by other beetles, as well as by the moth Pionia. In the same country, one of the Hemiptera, Spiniger luteicornis, has every part coloured like the hornet, Priocnemis, which it mimics; "in its vibrating coloured wing-cases it departs great from the normal character of the Hemiptera, and assumes that of the hornets." Mr Wallace mentions the longicorn beetle, Cyclopeplus batesii, which "differs totally in outward appearance from every one of its allies, having taken upon itself the exact shape and colouring of a globular Corynomalus, a little stinking beetle, with clubbed antennae." Erythroplatis corallifer, another longicorn, almost exactly resembles Cephalodonta spinipes, one of the common South-American Hispidae, which possesses a disagreeable secreation ; and Mr Bates also found a totally different longicorn, Streptolabis hispoides, which resembles the same insect with equal minuteness. Some of the large tropical weevils have the elytra so hard that they cannot be pierced by a bird’s beak ; and these are mimicked by many other comparatively soft and eatable insects. In southern Brazil, Acanthotritus dorsalis closely resembles a Curculio of the hard genus Heiliplus ; and Mr Bates found Gymnocerus cratosomoides, a longicorn, on the same tree with the hard weevil, Cratosomus, which it mimics. Other beetles resemble bees, wasps, and shielded bugs. Hairy caterpillars are well known to be distasteful to birds, and comparatively free from attack ; and Mr Belt found a longicorn, Desmiphora fasciculata, covered with long brown and black hairs, and exactly mimicking some of the short, thick wooly caterpillars common on the bushes around.

Amongst other orders, one of the most interesting cases is that of certain Diptera or two-winged flies which mimic wasps and bees. Sometimes this likeness only serves to protect the insect from attack, by inspiring fear of a sting. But there are also a number of parasitic flies whose larvae feed upon the larvae of bees, as in the British genus Volucella ; and these exactly mimic the bees, so that they can enter the nests or hives to deposit their eggs without being detected even by the bees themselves. In every country where such flies occur they resemble the native bees of the district. Similarly, Mr Bates found a species of Mantis on the Amazons which exactly mimicked the white ants on which it fed. On the other hand, the defenceless species itself may mimic its persecutor, as in the case of several crickets, Scaphura, that exactly resemble various sand-wasps, and so escape the depredations of those cricket-killing enemies. Another cricket from the Philippines Islands, Condylodra tricondyloides, so closely copies a tiger-beetles, Tricondyla, that even Professor Westwood long retained it among that group in his cabinet, and only slowly discovered his mistake. The cases here mentioned form but a small part of all those that have hitherto been observed and described in the insect world. They amount altogether to many hundreds.

Among plants, though included in the above definition for the sake of formal completeness, instance of true mimicry are rare of almost unknown. Perhaps the nearest approach to this phenomenon in the vegetal world is found in the resemblance borne by the dead-nettle, Lamium album, and a few other labiates, to the stinging nettle, Urtica dioica and U. urens. The true nettles are strikingly protected from animal foes by their stinging hairs ; and the general appearance of the dead-nettle is sufficiently like them to prevent human beings from plucking it, and therefore probably to deter herbivorous mammals from eating it down. Mr Mansel Weale mentions another labiate, Ajuga ophrydis, of South Africa, which closely resembles an orchid, and may thus induce insects to fertilize its flowers. Mr Worthington Smith has found three rare British fungi, each accompanying common species which they closely resembled ; and one of the common species possesses a bitter and nauseous taste ; so that this would seem to be a case of true mimicry. Many diverse instances alleged by Mr A. W. Bennett, Dr Cooke, and others cannot be considered as genuine mimetic resemblances in the sense here laid down. They are mere coincidences or similar adaptations to similar needs ; and the word ought to be applied strictly to such likenesses alone as benefit the organism in which they occur by causing it to be mistaken for another possessing some special advantage of its own.

The theoretical explanation of mimicry on evolutionary principles may best the considered in connexion with the general subject of protective coloration and variation in form, of which imitative colouring may benefit a species. It may help the members of the species to escape the notice of enemies, or it may help them to deceive prey. In the first case imitative hues the animal or plant to avoid being itself devoured ; in the second case they enable it to devour others more easily, and so to secure a larger amount of food than less deceptively coloured compeers. In the former instance we must suppose that such individuals as did not possess the deceptive colouring have been discovered and destroyed by enemies with highly developed sight, while which possessed it have survived. In the latter instance we must suppose that the individuals which have no protective colouring have failed to secure sufficient prey, through too readily betraying presence, and that only those which possessed such colouring have become the parents of future generations. It is difficult, however, to separate these two cases, and in many instances the same colouring may aid a species both in escaping its peculiar enemies and in deceiving its peculiar prey. They may therefore most conveniently be considered together.

Colour is always liable to vary from individual to individual, as we see in the case of domesticated fowls, rabbits, dogs, and other animals, as well as in most cultivated flowers, wherever natural selection cannot act to keep the typical specific hues pure and true. But in a wild state certain conspicuous colours are sure to prove disadvantageous by betraying the individual, and these will sooner or later get weeded out, under certain circumstances, either through the action of enemies or by starvation resulting from the inability to escape the notice of prey. On the other hand, certain other colours are sure to benefit the individual by harmonizing with the tints of the environment and these will be spared by natural selection, so that the individuals possessing them will pair with one another, and will hand down their peculiarities to their descendants. In this way many species will acquire and retain a coloration that harmonizes with their environment as a whole or with some special part of it. The degree to which the protective coloration will be carried, however, must depend upon the sharpness of the senses in those other organisms which it is desirable to deceive. Large dominant herbivorous or frugivorous mammals or birds, with relatively few enemies, would not be benefited by protective coloration, and so they seldom exhibit it. The grasses or fruits on which they feed cannot make any attempt to escape them. But carnivores generally require to deceive their prey, and therefore a large number of them exhibit marked deceptive colouring. Still more especially do small defenceless birds or mammals need to escape the notice of the carnivores, and they accordingly very generally possess dull colours, because any variation in the direction of conspicuousness is certain to be promptly cut off. Above all, among insects, which are so largely the prey of birds, of reptiles, and of other animals possessing highly developed vision, protective coloration in one form or other is almost universal, except where a nauseous taste, hairy skin, or hard external, coverings afford a different kind of protection. In every case the weeding out of ill-protected forms must depend upon the relative keenness of vision in the enemies or of the prey, be they mammals, birds reptiles, insects, or spiders. Hence the existence of protective coloring and of mimicry incidentally affords us valuable hints as to the perceptive faculties of the various classes against which each organism is thus unconsciously guarded.

Where the general aspect of the environment is most uniform, and where little but a impression of colour without individual form can be conveyed, the hues of animals, are also usually uniform, to match their surroundings, and no special imitative adaptations of form occur. Thus, among the Arctic snows, a brown or black animals immediately be perceived, and if defenceless at once devoured, while if a carnivore it would seldom or never approach unperceived near enough to its prey to effect a capture. Hence all such variations are at once repressed, and almost all Arctic animals, like the American polar hare, are pure white. Elsewhere bears are black or brown ; in the polar region the native, species is nearly indistinguishable from the snow in which it lives. Where the environment undergoes a regular change from season to season the colour o the fauna varies with it. The Arctic fox, ermine, the alpine hare, the ptarmigan and many other birds, are all more or less brown among the brown hill-sides of autumn, and snow-white among the winter snows. Almost equally general is the sandy colour of deserts, though this, instead of being uniform, is slightly varied from grain ; and nearly all the birds, reptiles, and insects of Sahara exactly copy the sandy grey hue of the desert around them. Soles and other flat-fish (Pleuronictidae) closely imitate the colour ands speckled appearance of the sand on which they lie. The fishes and crustaceans which inhabit the Sargasso weed are coloured the same yellow as the masses of algae to which they cling. Aphides and many small leaf-eating caterpillars are bright green like the neighbouring foliage.

Where the environment is somewhat more diverse, the resemblance begins to show more specialized features. The lion, a large ground-cat of desert or rocky districts, is uniformly brown ; but the tiger and other jungle-cats have perpendicular stripes which harmonize with the bamboos and brown grass of their native haunts ; while the leopards, jaguars, and other tree-cats have ocellated spots which conceal them among the mingled light and shade of the forests. Large marine animals have the back black, because the water looks dark when seen from above, but their bellies are white, so as to harmonize with the colour of the surface when seen from below. Dr Weismann has shown that most unprotected caterpillars imitate the stripes and shades of leaves among which they feed. Those which lives upon grasses are longitudinally like the blades, those which live among small leaves are spotted and varied so as to resemble the distribution of light and shade in the bushes, and those which live upon large veined leaves with oblique ribs have oblique lines to harmonize with them. In some cases even the unripe berries are represented on the caterpillar by small reddish spots. A specialized form of this particular device is found in the clameleon, the chameleon-shrimp, many flat-fish, and some amphibians, all of which can vary their coloration to suit that of the surface on which they rest. The action is reflex, and ceases if the animal is blinded.





Where the environment is very varied, as in tropical forests, we find the greatest variety of colouring as well as actual imitation of particular forms ; and the protective resemblances become at once closer and more common. Birds, reptiles, spiders, monkeys, and other active predaceous creatures are constantly hunting for insects and similar small prey amongst the fallen sticks or leaves ; and among the most powerless classes of insects only those which very closely resemble specific objects in the environment can easily escape them. A gradual passage can be traced from the most general to the most special resemblances under such circumstances. Many forestine birds have a ground-tone of green in their plumage, which occurs nowhere but in the tropics. Some tree-lizards are green like the leaves on which they sit, others are marbled to resemble the bark where they lie in wait for their prey. Arboreal snakes often hang like lianas or other creepers. Insects which cling to the trunks of trees can seldom be distinguished from the bark. A Sumatran butterfly, Kalliman paralecta, always settles on dry among dead leaves, and can then hardly be perceived among the brown foliage, which it imitates even in the apparent blotches and mildew with which its wings are covered. The family of Phasmidae, including the leaf and stick insects, carries such forms of imitation very far indeed. Most of them are large, soft, defenceless creatures ; but some, like Phyllium, closely resemble green leaves, so as to be almost indistinguishable while feeding ; and others exactly imitate short broken twigs of bamboo. Mr Wallace found one such insect. Ceroxylus, in Borneo, apparently overgrown with a creeping moss or jungermannia ; and Mr Belt discovered a larval form in Nicaragua whose body was prolonged into thin green filaments, precisely like the moss in which it lurked. In other instances the insect probably uses its disguise rather to deceive its prey than to escape its enemies. Sir Joseph Hooker believes that an Indian Mantis deludes the little creatures which form its food by its singular likeness to a leaf ; while Sir Charles Dilke found one which had its head and fangs moulded into the deceptive appearance of an orchid, so that small flies were actually attracted in search of honey into its very jaws. Outside the class of insects, similar phenomena sometimes occur. Thus, according to Mr Bates, many showy little tropical spiders double themselves up at the base of leaf stalks so as to resemble flower buds, and thus delude the flies on which they prey. Even among the vertebrates Mr Belt mentions a green Nicaraguan lizard looking like the herbage by which it is surrounded, and decked with leaf-like expansions, which hide its predaceous nature from passing beetles or butterflies.

These last instances are divided from the mimicry by a very narrow line. But they differ in the fact that some vague object only in the general environment is simulated, not a particular protected species, as in genuine mimetic resemblance. If we allow, however, that natural selection can produce the white colour of Arctic animals, and the sandy hue of the sole and the flounder, it is easy enough to extend the same principle to the leaf-insect and the stick-insect, or even to real mimicry, as in the case of the Leptalis and the Heliconidae. Certain Phasmidae may at first have varied in the direction of green coloration, and these would naturally escape the eyes of birds readily than their fellows. After the lapse of many generations, all the Phasmidae of that special group would have become green, and the birds which preyed upon them would have learned in many cases to penetrate the disguise ; for, as Mr Belt has observed, each fresh deceptive resemblance in the prey is to be followed by increased keenness of discrimination in the enemies of the species. At this stage the ordinary green Phasmidae would often be killed, while only those which happened to approximate rudely in the venation of their wings to leaves now escape the sharper and more experienced eyes of the birds. Thus step by step the disguise would become more and more perfect, only the best-protected of each generation escaping on the average, while all the worse-protected would be discovered and devoured. Given the usual luxuriance of tropical life, it is not difficult to understand how favorable variations might continually occur, until at length we get such perfect deceptions as those of the leaf-insects, the stick-insects, and the moss-grown larvae.

The phenomena of true mimicry may be explained by a parallel genesis. Suppose, to begin with, a group of large and brilliant butterflies like the South-American Heliconidae, protected by a nauseous taste and odour, and therefore never eaten by birds. To such insects slow flight and conspicuous hues are a positive protection, because they enable birds readily to discriminate them, and therefore prevent attacks, just as the banded body of the wasp and the hum of the bee prevent us from catching and killing them upon a window pane. Suppose, again, that in the same district there lives a widely different species of edible butterfly presenting some very slight and remote resemblance to protected species. At first, no doubt, the resemblance will be merely an accidental one of general hue ; it may even be so slight as to deceive nobody except upon the most distant and casual glance. No, suppose these edible butterflies to be devoured in large quantities by birds, then a few of them may happen to gain safety by associating with the flocks of inedible butterflies which the birds refuse. After a time, even if the habit of consorting with the protected species becomes fixed in the race, the birds will begin to recognize the edible insects amongst the flocks, especially such as vary most in the opposite direction from the protected species. On the other hand, they will overlook such as vary most in the same direction as the inedible kind ; and thus the least mimetic individuals will be destroyed, while the most mimetic will be left to pair with one another and to produce young, most of whom will present the like peculiarities. From generation to generation the birds will go on picking out every bad copy, and sparing all the best ones, till at last the two species become absolutely indistinguishable upon the wing. But the mimicry will never of course affect any but the most external and noticeable parts of the organism ; it will be to the last a mere matter of colour, shape of wing, visible appearance of legs or antennae, and so forth. The underlying structural differences will remain as great as ever, though externally masked by the deceptive resemblance in form and hue.

In like manner we may explain the genesis of the mimetic resemblance borne by Volucella to the humble bee. Suppose an undisguised fly to enter the bees’ nest, it would be at once attacked and killed. But if it presented some very slight resemblance to the bee it might manage to lay its eggs undisturbed, and its larvae would then be able to feed quietly upon the larvae of the bee. With each new generation the more flimsy disguises would be more and more readily detected, and only those flies which varied most in the direction of resembling the bees would survive or lay there eggs in peace. On the other hand, those which actually succeeded would possess great advantages over their neighbours, because their larvae would thus obtain a safe and certain supply of food, and be guaranteed the protection of the bees’ nest. In this way the flies would at last, by constant survival of the best-adapted, come exactly to imitate the bees amongst which they lived.

The theory of the origin of mimetic forms thus briefly sketched out is due to Mr Bates and Mr Wallace, and it explains all the facts more fully than any other. It shows us, first, why the mimicking organism always imitates a specially protected species ; secondly, why the two always inhabit the same district ; thirdly, why the mimicking species is always much rarer than the species mimicked ; fourthly, why the phenomenon is confined to a few groups only ; and fifthly, why several different mimicking species often imitate the same protected form. It also accounts for the absence of mimicry amongst large or dominant animals, and its absence of mimicry amongst large or dominant animals, and its comparative commonness amongst small and defenceless kinds. And by affiliating the whole of the phenomena upon the general principles of protective colouring it reduces a seemingly strange and marvelous fact to a particular case of a well-known law. Whatever theory be adopted, however, the facts and most of their implications remain the same. For, whether we suppose these imitative resemblances to be due to direct creative design or to survival of favourable variations, it is at least clear that the disguise subserves a function—that it is purposive and not accidental. Hence we may draw from the phenomena of mimicry certain important psychological implication. On the hypothesis of evolution, it is obvious that the mimicry can never go further than the senses of the creatures against whom the disguise is advantageous would naturally carry it ; and even on the hypothesis of special design it is not likely that the imitation would be made more accurate than would be necessary for practical purposes of deception. There is much evidence in favour of this view. Mr B. T. Lowne, for example, who has carefully measured the curvature of the facets in the compound eyes of insects, upon which depends the minimum size of apprehensible objects, finds that the mimicry in the case of the flies parasitic upon bees’ nests has proceeded just so far as the structure of the bee’s eye would lead us to expect, and no further. In other words, so far as measurable of angular distance subtended can guide us, such a fly seems to be absolutely indistinguishable by a bee fromone of his own species, within the limits of ordinary vision. The pictures cast upon the sensorium by the fly and by a brother bee are simply identical. In many other cases it can be shown that the mimicry seems specially intended to deceive the eyes of a particular class of animals ; while there is no case of mimicry where the only enemies or prey consist of plants or eyeless animals. Naturally there can be no mimicry without a creature to deceive ; the very conception implies an external nervous system to be acted upon, and to be acted upon deceptively. Thus mimicry in plants must have reference to the eyes of animals, in animals themselves to the eyes of one another. We may conclude, accordingly, that if a leaf-insect is green with faint violet-brown veins to the wings, exactly like a certain leaf, in order to deceive sundry tropical birds, then those birds are capable of perceiving the forms and colours imitated to that particular degree. So the presence of mimicry in any group may guide us to a rough idea of the perceptive powers of those creatures whom the mimicry serves to deceive. The exact imitation of sand and coloured pebbles in the flat-fish is a fairly safe indication that the predaceous fish by whose selection they have been developed (through the weeding out of ill-protected variations) can pretty accurately distinguish form and colour. The long green pipe fish which cling around green see-weed have probably acquired their existing hues to deceive the eyes of small sharks ; the Phyllopteryx eques, a hippocampus which looks precisely like a piece of tangled and waving focus (see figure, vol. xi. p. 852), has doubtless in the same way taken on its delusive likeness to the algae among which it lives. So the cricket which resembles its foe the sand-wasp must have gained its present shape and hue by deceiving its enemy, and therefore it suggests the probability of highly developed vision on the part of the wasps. There seems every reason to believe that in many instances insects, spiders, and even lizards have developed mimetic or other deceptive resemblances in order to delude the eyes of insects ; while in other cases the disguise has been unconsciously adopted to deceive fish, amphibians, reptiles, birds and mammals. Moreover, we have some grounds for believing that the sense of colour is exceptionally strong in birds and in one or two insect orders ; and the mimicry of colour seems to have proceeded to the greatest length amongst animals which are most exposed to the attacks of these classes, or which would find it advantageous to deceive them. It may be added that these same classes have been most effective in producing the bright hues of flowers and fruits, on Mr Darwin’s hypothesis, or are at least in any case most intimately correlated with such vegetable structures as fertilizers of blossoms and dispersers of seed. Mimicry is thus to some extent a rough gauge of the perceptive faculties of the species deceived by it. The vocal mimicry which occurs among certain birds, such as the mocking-bird, starling, parrot, and bullfinch, must of course be placed in a wholly different category from these biological cases. It is a direct volitional result, and it is mimicry in a literal not in a figurative sense. The faculty seems to be due to the play-instinct alone, and not to subserve any directly useful function. (G. A)



The above article was written by: Grant Allen; author of This Mortal Coil, The Woman Who Did, etc.; The Colour of Flowers, Flowers and their Pedigrees, and other popular works on botany; also of historical guide-books.




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