The Parents' Review

A Monthly Magazine of Home-Training and Culture

Insect Mimicry.

[Alfred Thornley, 1855-1947, "studied at Oxford, took holy orders and worked as deacon and priest at parishes in Nottinghamshire. He had a wide interest in natural history, but specialized in entomology and the study of Diptera. He was relieved of most of his parochial duties, and worked with the County Education Department in training school teachers in natural history. Thornley was Chaplain and Professor of Economic Entomology at Cirencester Agricultural College for three years, before retiring to Cornwall. Thornley gave his collections, journals and notebooks to The Natural History Museum in 1946." Source]

An old proverb says, that "Imitation is the sincerest form of flattery," but Nature employs this art for a more utilitarian purpose. It has been humourously remarked that, in working out her economy, she uses a vast system of "fraud and hypocrisy," as one of her most approved principles of progress. You see what your ordinary powers of seeing tell you is a twig; behold it rises up and walks: it is a caterpillar! a seed, and lo, it is a beetle! Again, what for all the world looks like a dead and decaying leaf lies on the ground; you touch it, and away it flies. It is a moth or a butterfly. In this singular way Nature preserves the lives of myriads of her creatures. And see what an amount of energy she saves by this device of mimicry! An insect instead of running or flying away, simply remains still,--quite passive, a state which does not imply any particular intelligence on its part,--and the fierce hungry eyes of its devourer pass it by. It is only in danger whilst it is moving. We might call this the "principle of least energy." In some of her processes, nature appears rather as a reckless spendthrift, as when out of fifty seeds, but ten mature; but in the principle of mimicry she appears as a great economist, and warns us that under much, if not all, of her apparent wastefulness, there is really a true economy. Nature is not penny wise and pound foolish. This great principle of mimicry is best studied under some simple heads, and perhaps the following will suffice for this short paper.

Imitation by living creatures of (a) dead objects; (b) other living creatures. And each of these kinds of mimicry may be helpfully considered again under the heads of resemblance (1) in colour, (2) in form, (3) in attitudes and habits. No doubt a better and more elaborate classification might be suggested, but the above will help to draw out the observing powers along profitable lines of study. It is needless to say the most perfect form of mimicry is that in which colour, forms, and attitudes conduce to perfect the resemblance to some object. This principle of mimicry obtains throughout the whole animal kingdom: the stripes of the tiger and zebra, the white of the ptarmigan [grouse, a type of bird], the intricate patterns of many snakes, as well as the colours of insects, all receive some plausible explanation from this principle.

The most casual observer must have met with numerous examples of that kind of mimicry in which a creature imitates a dead object. The present season of the year is not unfavourable to the study of this species of mimicry. Let any one take an old umbrella, and holding it under the boughs of an oak tree or hazel bush, beat or shake the branches gently into it. He is almost sure to obtain some objects for his purpose. He will see what he takes to be small pieces of twig fix themselves at one end and quickly assume a rigid attitude, in which the resemblance to brown and black twigs, or young and tender green ones, is so exact, that even the little snags and lumps, and the tiny lichen covering them are reproduced. In this instance colour, shape, and attitude, all conduce to perfect the resemblance; and it might be incidentally mentioned, that with respect to colour, the investigations of Professor Poulton, F.R.S., and others, have shown that young caterpillars of several species of moths have the power, up to a certain age, of assimilating their colours to the prevailing tint of their environment. A batch of eggs of a very common moth,--the "Brimstone" (Rumia Crataegata)*,--was divided into two equal portions. The one part hatched and reared under the influence of a dark environment, the other in a light one. It was shortly observed that the two broods of caterpillars differed considerably. In the first, the caterpillars were dark brown with a purplish bloom in patches upon them; in the other, the prevailing tint was a light brown with greenish shades. It might be objected that the artificial conditions under which such experiments are carried out, preclude us from arguing that such effects would take place in nature. But the result of the experiment was simply to prove that caterpillars are sensitive to their surroundings, and that this change of colour can be quite independent of the nature and quality of the food, and is entirely the result of environment, and this the experiments seem completely to have established.

* [Wikipedia says "The English zoologist Edward Bagnall Poulton, author of The Colours of Animals (1890) described countershading in insects including the caterpillar larvae of the brimstone moth" (it's on page 151), but Wikipedia classifies the moth as "Opisthograptis luteolata."]

These interesting experiments give us a great clue to the origin of the colours of caterpillars. We are not surprised to find that the prevailing colour is green--the tint of growing stuff, that is, if the colour has been acquired for the purpose of protection: and it is a remarkable fact that if a caterpillar or an insect be coloured in any way which contrasts violently with its environment, we may be almost certain that it is able to defend itself by some special contrivance. Some kinds of caterpillars are hairy, or covered with numerous spines or bristles, or are able to protrude curious looking instruments, or have strong scent glands. Others, apparently devoid of all these means of defence, and yet decorated with the most gaudy colours, have been found to be, by experiment, most distasteful to birds and other enemies. Anyone can put this to the test who will. There is a well-known caterpillar found in our hedgerows, the larva of a pretty moth, appearing in October and November, the figure-of-eight moth (Diloba Caeruleocephala). It is an almost smooth gaudy creature--blotched with yellow and slaty grey and black--but anyhow very conspicuous. If this be given to birds, say fowls, it is astonishing to see how it is rejected. A young chicken might make an attempt once but it would refuse it afterwards. There are many other very beautiful caterpillars, in which the gay coloration acts as a danger signal, and says as plainly as possible "nemo me impune lacessit" ["no one attacks me with impunity;" the motto of Scotland]. It must have been a long process by which their enemies came to know that these colours were of a warning nature, but that it is so, experiment seems abundantly to prove. This distastefulness need not be absolute; it would be sufficient to secure a positive advantage to the species, if others were only preferred before it. But it is a wonderful thought that these creatures do go about in comparative safety, protected as it were by their own beauty, and every increase of that beauty, would only make them more conspicuous, and so safer.

Whilst upon this subject of colour and markings, I might say some very plausible explanations have been offered of some of the characteristic markings of caterpillars. For instance, it will be found that most young caterpillars are green, thus being hidden by the foliage of the plant upon  which they feed, but as they increase in size, fine longitudinal lines appear, these are particularly plain and beautiful in those species which feed habitually upon grass, or on herbage with long ribbon-like leaves and numerous longitudinal veins. In many species these lines are permanent, but in some they gradually disappear and are replaced by oblique lines and spots. But I think I am right in saying that no species is known with longitudinal and oblique lines at the same time. The oblique lines certainly imitate the veining of leaves, and even when there are delicate shades of pink or red as in some species of Hank moth caterpillars, the resemblance of them to the veining of leaves in faint shadow is easily seen. The change from longitudinal markings to oblique is most interesting, as it usually indicates a change in the habits of feeding of the species or is a survival of a very ancient pattern, when the larva fed upon longitudinally-veined plants. To those who have time and leisure, there is here an opportunity for much interesting experiment and observation. Any one with a little care can rear caterpillars, when suitable food is at hand. At first the eggs of some common and easily reared species should be chosen. These should be hatched under such conditions that all that takes place can be easily observed, and at least once a day a careful sketch of the caterpillar, coloured if possible, should be made. Sometimes it would be scarcely necessary to draw the whole of the larva, a single segment with its evolving lines and spots would be sufficient. The point would be to obtain a continuous record of each marking, and the time of the appearance of each new character.

Perhaps the most remarkable case of mimicry of the kind we are considering, occurs in the case of an Indian and Malayan species, most appropriately called the leaf butterfly [Kallima inachus]. So wonderful is the mimicry in this case, that I am tempted to copy a few extracts from the very graphic account of it given by Mr. A. [Alfred] R. Wallace, F.R.S., in his interesting work, "The Malay Archipelago:"--

"This species is 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 discover 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 close before my eyes, but that in its position of repose it so closely resembled a dead leaf attached to a twig, as almost certain to deceive the eye, even when gazing full upon it. I captured several specimens on the wing, and was able fully to understand the way in which this wonderful resemblance is produced.
"The ends of the upper wings terminate in a fine point, just as the leaves of many tropical shrubs and trees are pointed, while the lower wings are somewhat more obtuse, and are lengthened out into a short thick tail. Between these two points there runs a dark curved line, exactly representing the mid-rib of a leaf, and from this radiate on each side a few oblique marks, which well imitate the lateral veins. These marks are more clearly seen on the outer portion of the base of the wings, and on the inner side towards the middle and apex, and they are produced by striae and markings which are very common in allied species, but which are here modified and strengthened so as to imitate more exactly the venation of a leaf. The tint of the under surface varies much, but it is always some brown or ashy colour, which matches with those of dead leaves. The habit of the species is always to rest on a dead twig and among the dead or dried leaves, and in this position with the wings closely pressed together, their outline is exactly that of a moderately-sized leaf, slightly curved or shriveled. The tail of the hind wings forms a perfect stalk, and touches the stick while the insect is supported by the middle pair of legs, which are not noticed among the twigs and fibres which surround it. The head and antennae are drawn back between the wings, so as to be quite concealed, and there is a little notch hollowed out at the very base of the wings, which allows the head to be retracted sufficiently."

In an allied Indian species, small patches of black dots occur on the under wings, simulating the appearance of a minute fungus which grows upon old and dead leaves.

We pass on to consider a species of mimicry which consists in the more or less perfect imitation of living objects, the imitated creature being either feared on account of its ample means of offence or defence, or belonging to a class which, as it were, do not come within the experience of the imitating species' enemies, as for example where a large moth imitates a humming-bird. I think some of my readers must have seen sometime or other the singular caterpillar of the Elephant Hawk moth [Deilephila elpenor]; and if so I need hardly remind them of its wonderful resemblance to a small snake. Its purplish brown colour, its swollen segments near the head, with their two great eye-like spots, giving it a most evil appearance. I am quite sure a person seeing it for the first time would be afraid to take it up. Nor can there be much reasonable doubt that it is also feared by our small insectivorous birds. Such extraordinary and conspicuous caterpillars as those of the Puss moth [Cerura vinula] and the Lobster moth [Stauropus fagi], no doubt present some repulsive appearance to their enemies, by which they pass their lives in comparative security. As far as I am aware, in these three species mentioned, they possess no disagreeable flavour, which would lead to their being rejected by their enemies, so that "ugliness" in their case is security. It will be quite obvious that if we are ever to discover the purpose and meaning of the fantastic shapes of some caterpillars and insects, they must be studied in close contact with their natural environment, and not under artificial conditions. We must know something of the nature of their enemies, and of other insects frequenting the same places. We should have, as it were, a personal acquaintance with them during their whole life history. Nature contains innumerable secrets, and the finding out of these is the great reward of patient and intelligent observation. It is so easy to think that others have already done the work, and there is nothing left for us to do. Yet upon this subject of mimicry, with its associated subject of the meaning and purpose of the forms and markings of insects, there is yet a vast deal to be done, nay, the very alphabet of it remains still to be made out. And I will venture to say more, he who enters upon such studies as these, has entered into a very enchanted palace of beauty; he sees infinite purpose and design everywhere, he realizes what the true poet alone knows--

                  This world's no blot for us,
      Nor blank; it means intensely, and means good.
      To find its meaning is my meat and drink.

Lastly we approach the consideration of a form of mimicry, which is really more wonderful than any we have hitherto discussed. Not many years ago some entomologists pointed out a remarkable resemblance in colouring, between the butterflies of one species and those of another, generally of a totally different genus. One being the imitated, the others the imitators. For instance a well-known foreign butterfly, Danäis Chrysippus, is mimicked by at least six other species, one strangely enough being a moth. Nor yet have we arrived at the full extent of the wonder, for it is frequently, and in the case before us particularly so, that the female alone is the imitator; the males being often exceedingly different in both colouring, and even form, from the females; so that at first the males and females of the same species were taken for different insects and placed in separate genera. Sometimes the females of one species only, out of a whole genus will be mimics: the males preserving the usual pattern and coloration of the genus. It was next discovered that the imitated species was always rendered for some reason or other, distasteful to its usual enemies; so that the purpose of the mimicry, becomes perfectly plain to us, namely that it is of great protective value, the imitating species being passed over by their enemies in mistake for the imitated. It is impossible to realize the perfection of such mimicry as this without seeing the insects together, or some well drawn and coloured plates. If any possess that exceedingly useful series of volumes, "Cassell's Popular Natural History," in Vol. VI are some good figures (uncoloured) of species of butterflies and their mimics. The problem which Nature here presents to us is not so easy to solve, as might seem to be the case from the facts disclosed. But certainly the best contribution towards a solution of it have come from the "Evolutionist." The data in these cases are now well known, and a writer need not hesitate to use the well-worn phraseology of Darwinism. The struggle for existence; the survival of the fittest, the power of indefinite variation conferred on each living thing; these are the elements which go to solve the problem. The outlines of that solution must be something like this: two species, A, the imitated, B, the imitator, were probably at one time more like to each other than the greatest present sexual difference would suggest. Owing to some constitutional toughness or distastefulness, A possesses a greater immunity from the attacks of enemies than B. But individuals of B which most resembled A would be more likely to be preserved than others which differed more from it, and would tend to impart this likeness to their offspring, so that there would be a gradually accumulating racial likeness to A, and as A itself varied in colour, it would, as it were, draw B along the same lines of variation. This is not difficult to see. But the problem is greatly complicated by the fact that it is, say, only the females of B which imitate A; the males of B being very distinct in pattern and coloration. In this short paper I can only throw out a suggestion, which must be stated broadly, and not in detail. In its simplest form it is the problem why the males of so many living things differ from the females. To answer it properly would be to require an exposition of the bold Darwinian theory of "sexual selection," and it must here be sufficient to say that the males of numerous living creatures have acquired their exclusive sexual characters, whether colours or weapons of offence or defence, most probably through rivalry with one another in their struggle for the female. The conflict then between the principles of sexual selection and mimicry have produced those remarkable phenomena amongst insects which we have been considering . To say much more than this would lead us too far astray into the field of speculative zoology. Many more facts are needed before we can satisfactorily explain some of these wonderful and interesting problems. Facts, which it lies in the power of many to collect, though perhaps few to interpret. Yet there must be facts, that there may be an interpretation, and a truer knowledge of those wonderful laws which are "the expression of the mind of God." With deep insight has Nature been represented as the old, old woman with the young face; always old and always new. There is something soothing and calming in the study of Nature. After the interminable worry and strain of social problems, we seem to stand in the presence of the real, as contrasted with the artificial, the jumble which has arisen out of men's sins and blunders. That the hints thrown out in this paper towards a proper study of Nature may bring a new occupation and a refreshing study to many, is the highest reward that the writer can hope for.

Proofread by LNL, June, 2023