The Protective Mimicry of Insects.

New Zealand Graphic, Volume XLIX, Issue 4, 22 January 1913, Page 35

The Protective Mimicry of Insects.

By

WALDEMAR B. KAEMPFFERT.

IN the eyes of the naturalist the world is a vast arena, and every creature a gladiator engaged in a fierce combat with a myriad of enemies—a combat in which mercy is unknown, in which f'.umbs are relentlessly turned down, and in which treachery and cunning are qualities as virtuous as courage and strength. Not merely in the instinctive hatred of one animal for another is this combat rooted but chiefly in the lust of life, in the desire to escape starvation. Lack of food means weakness to an animal; and weakness means death. Every hour, every minute, every second this bloody battle is waged.

Darwin called this incessant warfare “ natural selection,” or “ the struggle for existence ” —terms that have taken their

place in the vocabulary of everyday life. Although the weapons and the tactics employed in this struggle are much the same among both the highet and the lower animals, the most striking confirmation of the Darwinian theory is to be found in the insect world. In that world we find that both the hunter and the hunted have unconsciously contrived almost incredibly subtle artifices for outwitting each other, and that each insect is given a disguise by nature, designe I to render its wearer invisible to his foe and sometimes to his prey. We have been told that one method whereby the weak are enabled to escape the strong and the cowards to elude the brave consists in a protective simulation of surrounding objects. For the same reason that a woodsman has elad himself in green ever since the day's of Robin

Hood, many insects have adopted liveries that harmonise closely with the flowers and the trees upon which they habitually repose. So exact is the protective resemblance that even the professional collector is often deceived. Instances of this form of concealment are well-nigh innumerable. The Catocala moth, a widely distributed genus, is conspicuous enough in flight; but once it rests on a treetrunk, flattened against the bark, with its well-defined dark hind wings drawn beneath the mottled gray fore wings, it defies discovery. So accurately has nature painted and spotted the fore wings to imitate -the effect of rough bark that the most vigilant enemy of the moth must pass it by. Often the adaptation is so refined that these moths are tinted

to resemble one tree more closely than another, because that particular kind of tree is usually selected for feeding or for rest. Thus we find a species of Catocala that looks for all the world like a piece of birch bark, even to the blotches of black. A certain South American beetle is found on one kind of tret only, and is so marvellously well assimilated to the bark that it can be discovered only when it stirs. Some caterpillars that live on trees resemble the lichens ami moss of bark, the imitation being so true that the tuft-like appearance of such growths is produced. The numerous species of the tiger beetle all vary in colour to suit their surroundings, some having the sandy colour of the seashore where they are found: sonic simulating the green, wet, slimy stones on which they crawl; and some finding protection

in a dun-coloured disguise that is an accurate reproduction in colour of the leaf-strewn forest soil where they abound. They are all of one family.— these beetles and yet no two species exactly alike in hue. To escape its enemies each has donned a mask best suited for its purpose in its struggle for existence. A moth usually rests with his forewings outspread over the prominent pattern of his hind wings. In any other posture he would inevitable meet a swift death. A butterfly, on the contrary, rests usually with his wings uplifted and pressed together. Otherwise, the gaudy surface would be as conspicuous as the black ink on this white paper —a signal for attack by relentless and voracious foes. In order, to hide himself, the butterfly has, therefore, lavished all the resources of his imitative art on the under surface of his wings. By far the most astonishing instance of this kind is afforded by the East Indian Kallima butterfly, the blue upper surface

of which is richly and ostentatiously adorned with a stripe of orange, but the under surface of which bears a truly staggering likeness to a leaf, when the wings are drawn together. Here we have an insect that apes not merely the approximate shape and colour of a dead leaf but also the midrib with the delicate veining. the sharp point, and the short stem common to many tropical leaves. It might be supposed that this imitation of an ordinary object is sufficiently minute to protect the KolHhhi from its enemies. Self-pre-servation apparently demands touches even more exquisite; for the resemblance has been so craftily carried out that not merely is a dead leaf simulated hut in the lighter-coloured varieties a dead, shrivelled leaf flecked with parasitic growths, stained, and spotted to give th * appearance of holes eaten by caterpillars, is it any wonder that Alfred Russell Wallace. trained naturalist, keen-eyed observer. was unable to find the Kallinui when it sought refuge from his net in a bush of dead leaves? How absolutely impenetrable is this disguise may lx* gath-

ered from the circumstances that Kallima butterflies so successfully elude their enemies that they are among the most <*olllllloll in India. (Tyloii, and the Malay Archipelago. Hardly two specimens are exactly alike. The colours vary within as wide limits as the hues of decaying leaves. Additional examples of the incredible fidelity with which insects have adapted themselves to their environment could Im* given almost without number. There are •'walking-sticks.” familiar probably to every country-bred boy. that conform perfectly to a spray of twigs with all the polished nodes, and must actually be touched Indore one can be persuaded that they are living things: locusts that can scarcely be told from voting. un< til ling leaves; spiders that deceive their prey with pitiful ease by their fatal res •mblanee to a knot on a treebranch; caterpillars that escape the ready beaks of hungry birds by assuming the rigiditv, shape, ami colouration of a twig with its knobs and buds, only to tall a prey to the pruning shears of some gardener on whom they have only too successfully imposed; and some phasmids known to entomology as the genus Phi/Ilium. persecuted by insect-eaters into so excellent a likeness to a fresh green leaf that, when they crawl among foliage, they seem not insects at all but just a moving mass of leaves. The boughs of an oak may often be infested with larvae that pass muster for budding sprigs. Eggs are laid not only upon the exact plant that will constitute the future caterpillar’s food but on the under side of leaves that most nearly resemble him in colour. Never is a mistake made by the mother. If a caterpillar li\«*s on grass he is sure to be ribb >d and veined up ami dow n like grass; if he feeds on broad leaves lie will display a midrib and branching, vein-like streaks. Men who make it their business to capture the larvae of th • Death’s-head moth, because they receive so much for each spe imen. find but one in many days —all because the yellow and green of th ■ potato plant on which it lives and the lavender of tin* Howers arc so wonderfully well copied. Immunity from attack would be only temporarily attain d if insects were not aide to adapt themselves to those chromatic changes in their surroundings caused by the >eas ms. Verdant leaves are. after all. an accompanim nt of spring and summer alone; and a creature dressed in green would be infallibly betrayed by its glaring contrast with the russet hues of autumn. But nature’s disguise conceals its wearers even in these dire straits. Some caterpillars—that of the Privet Hawk moth is a pertinent instance —have the wonderful power of modifying their colouration to suit their environment. When complete growth has been attained, they- creep from their summer abode of green foliage to the dark ground. Because their green-patched livery would be fatally inappropriate when this migration occurs, we find them just before their dcsi nt assuming a jacket ot brown that harmonise admirably with their new tenement. On-the other hand, some of the •’stick caterpillars”—among them that of tin* August Thorn moth -which would meet a speedy death but for their resemblance to the twigs of tin* elm on which they feed, discard their modest dress of brow n for one of green w hen the cocoon stage of development is reached and tlfrMDackgrouml against which they are M*«*n is one of leaves. So marvellously

assimilative are some larvae, and so farreaching is the law of natural selection, that sometimes two different coloured caterpillars of the same moth may exist side by side. The most striking example of this phenomenon is afforded by the Large Emerald moth, the caterpillars of which are sometimes brown, sometimes green—depending upon the surroundings in which they’ live. Not every insect in the world is a facsimile of a leaf, or a twig, or a piece of bark. Every meadow on a summer day’ swarms with a winged host blatantly' heralding its existence by colours that

must seem cordial invitations to its enemies. Why is it that they’ are not attacked? For a long time that question puzzled Darwin. Here an l countless creatures roaming the world, flaunting themselves in the .sunshine, courting death. What is the talisman that saves them? After much futile speculation it was finally ascertaincd that many of these gaily tinted denizens of the air are horribly distasteful to insect-eating epi-

cureans and frequently endowed with the most nauseous qualities. They find their salvation by advertising themselves boldly and flainbiioyaiitly. Their colours

are danger signals not to be disregarded. The light yellow body of the caterpillar

that develops into the magpie moth is gaudily spotted with orange and black. A little experimental tasting has taught every bird, lizard, and frog to avoid the creature that wears these colours. The caterpillar that strips the foliage of our oaks and elms towards the close of summer is likewise a squirming cylinder of black, yellow, and orange. Insect-eat-

ers reject it often with signs of intense disgmst. And thus red and black ladybirds, yellow-striped hornets, wasps and bees, black and red beetles, and a host of insects preserve themselves by brazenly proclaiming their offensive tastes or odours or dangerous stings to all the inimical world. Other -insects that would prove delicious morsels 'to greedy’ foes have not been slow to profit by the immunity

, that is granted by a warning garb. They have actually mimicked obnoxious species protected by garish hues in order to escape death themselves, and this with such amazing accuracy that not only is the enemy but even the collector com-

pletely tricked. Wallace tells us that one of the crickets of the Philippine Islands is so minute a copy of a certain hostile tiger beetle that a famous en-

tomologist placed it in his cabinet of beetles and retained it there for a long time before he discovered his mistake. In tlie jungles of the Amazon species of butterflies are found that mimic the species Heliconidae. Entomologieally, they are all as distinct as horses and cows, and yet the one species is a photographically exact counterfeit of the other. The Ilelieonidae possess an atrocious odour and taste, and accordingly are as brilliantly conspicuous as oxeye daisies in a green field. So free are they from attack that they flap lazily along, utterly indifferent to danger and perfectly secure in their sickening attributes. The mimickers so cleverly copy the markings, form of wings, and heavy flight of the Heliconidae that spiders drop them from their webs, and small monkeys reject them, despite their palatability. In almost every box of butterflies sent to our museums from South America Heliconidae are to be found, placed side by side with imitators under the impression that they are all of the same species. Some Clear-Wing moths are such exact reproductions of hornets that most of us would .shrink from them in fear. The mimicry is consistently carried out, too; for when seized the insect actually moves its body as if it were about to .sting. It should not be supposed that selfdefence is always the primary object of this masquerade. There are insects of prey as well as birds of prey, and these cunningly adopt mimicry as a strategic weapon. Certain tropical spiders that subsist on ants are as like their prey as the proverbial two peas. Some parasitic flies live in the larval stage upon the larvae of bees and wasps. The parent fly boldly enters the nest of a bee, lays the eggs from which the larvae are to develop, and departs unmolested.

Many a defenceless insect resorts to the expedient of terrifying its enemies by its likeness" to a dangerous animal or by suddenly assuming a horrible aspect. The principle is about the same as that of frightening a child by grimacing at it. Like this gentle parental method <jf correction, it fails as often as it succeeds. The most successful terrorinspiring masker is probably the “hickory-horned devil,” a perfectly harmless caterpillar of the Royal Persimmon

jnoth of America, but so fiercely Threatening in appearance that it engoys an enviable reputation for deadlijiess. Its green body, often half a foot in length, is capped by a vivid orange Itrown, which, on the approach of an lenemy, is ominously shaken in a way that makes a rattle-snake seem lamblike in comparison. A certain South •American caterpillar startled its disfey er er by its unpleasant resemblance io a viper. Indeed, snake-like appearances are not unusual. A part with such dramatic possibilities of intimidation presupposes an actor of considerable size; apd accordingly we find that the caterpillars who assume it are often a foot ;or more in length.

These wonderful species of insects, it has been stated, are all of them products of the struggle for existence. In order to grasp the significance of that struggle as well as its necessity, and to show how leaf-like and bark-like insects (developed from older and less adequatelyconcealed forms, we must not forget the {fundamental principle that all organisms (tend to reproduce their kind in geometrical "ratio,, and that offspring, alithough similar to their parents, are yet ■possessed of useful, inheritable differences. If all insects were permitted to Hive the world would be devastated by (them. It happens, however, that their enemies likewise multiply in geometrical ratio, so that a proper balance is maintained. So numerous are these enemies, land so powerful, that sometimes the quest of food is anything but successful. The food of qne beetle is consumed by another; rain and wind, cold and heat, kill many butterflies; in a word, premature death falls upon a creature in a Thousand and one ways. Although the offspring always outnumber their parents, yet the number of living insects, thanks to birds and beasts of prey, rejmains fairly constant. Because (jf this rapid propagation and (because of the struggle that prevents an overwhelming accumulation of any one species, the exquisite adaptations of butterflies to leaves and of moths to bark Slave been produced. A colossal assumption must ibe made, however, before we. can fully understand how protective resemblance and mimicry have played their part in the struggle for life—an assumpJtion that is, indeed, the weakest spot in tlhe Darwin theory. Natural selection presupposes that every marking, every .tint, every peculiarity of habit must have been useful at some time in the history of a species; and that these characteristics are not only inherited but intensified as they are transmitted. Millions of years ago the Kallima butterfly iwas not the beautiful counterfeit leaf of (to-day. Probably it was like many another butterfly. * And like other butterflies it was unsparingly persecuted by (hostile insect-eaters. Of this primeval Block some members were preserved because of some slight marking or colour (which their ancestors did not possess — tharacteristies, in a word, that brought them more in chromatic harmony with their environment. These markings and (colours were transmitted. The offspring intensified whatever resemblance there iwas to a leaf until finally the adaptation reached its present perfection. After Khat the Kallima was fairly secure and increased abundantly. Although many naturalists are inclined to doubt the possibility of intensifying useful characterise Jtieis by heredity, and have advanced' theories that new species are not necessarily the products of age-long evolution but sports of nature, or spasmodic phenomena, it cannot be denied that the teachings of Darwin still hold a dominating place in biology.