Popular Science Monthly/Volume 25/July 1884/Our Debt to Insects
|OUR DEBT TO INSECTS.
By GRANT ALLEN.
IT has often occurred to me as a curious fact, when I have been watching the bees and butterflies in an English meadow of a summer morning, that no one should ever yet have adequately realized (so far as I know) the full amount of human indebtedness to those bright and joyous little winged creatures. I do not mean our practical indebtedness to insects for honey and bees'-wax, silk and satin, cochineal and lacquer, or a hundred other such-like useful products: these, indeed, are many and valuable in their own way, though far less so than the tribute we draw from most of the other great classes of animal life. But there is one debt we owe them so out of all proportion to their size and relative importance in the world that it is strange it should so seldom meet with due recognition. Odd as it may sound to say so, I believe we owe almost entirely to insects the whole presence of color in nature, otherwise than green; without them our world would be wanting in more than half the beautiful objects which give it its greatest æsthetic charm in the appreciative eyes of cultivated humanity. Of course, if insects had never been, the great external features of the world would still remain essentially the same. The earth-sculpture that gives rise to mountains and valleys, downs and plains, glens and gorges, is wholly unconnected with these minute living agents; but all the smaller beauties of detail which add so much zest to our enjoyment of life and nature would be almost wholly absent, I believe, but for the long-continued aesthetic selection of the insect tribes for innumerable generations. We have all heard over and over again that the petals of flowers have been developed mainly by the action of bees and butterflies; and as a botanical truth this principle is now pretty generally accepted; but it may be worth while to reconsider the matter once more from the picturesque and artistic point of view by definitely asking ourselves, How much of beauty in the outer world do we owe to the perceptions and especially to the color-sense of the various insects?
If we could suddenly transplant ourselves from the gardens and groves of the nineteenth century into the midst of a carboniferous jungle on the delta of some forgotten Amazon or some primeval Nile, we should find ourselves surrounded by strange and somewhat monotonous scenery, very different from that of the varied and beautiful world in which we ourselves now live. The huge foliage of gigantic tree-ferns and titanic club-mosses would wave over our heads, while a green carpet of petty trailing creepers would spread luxuriantly over the damp soil beneath our feet. Great swampy flats would stretch around us on every side; and, instead of the rocky or undulating hills of our familiar Europe, we should probably see the interior country composed only of low ridges, unlifted as yet by the slow upheaval of ages into the Alps or Pyrenees of the modern continent. But the most striking peculiarity of the scene would doubtless be the wearisome uniformity of its prevailing colors. Earth beneath and primitive trees overhead would all alike present a single field of unbroken and unvarying green. No scarlet flower, golden fruit, or gay butterfly would give a gleam of brighter and warmer coloring to the continuous verdure of that more than tropical forest. Green, and green, and green, again; wherever the eye fell it would rest alike upon one monotonous and unrelieved mass of harsh and angular verdure.
On the other hand, if we turn to a modern English meadow, we find it bright with yellow buttercups and purple clover, pink-tipped daisies and pale-faced primroses. We see the hedges white with may or glowing with dog-roses. "We find the trees overhead covered with apple-blossom or scented with horse-chestnut. While in and out among the beautiful flowers flit equally beautiful butterflies—emperors, admirals, peacocks, orange-tips, and painted ladies. The green of the grassy meadow and the blue of the open sky serve only as backgrounds to show off the brighter hues of the beautiful blossoms and the insects that pay court to them incessantly.
To what is this great change in the general aspect of nature due? Almost entirely, we may now confidently conclude, to the color-sense in the insects themselves. The lovely tints of the summer flowers and the exquisite patterns on the butterfly's wings have alike been developed through the taste and the selective action of these humble little creatures. To trace up the gradual evolution of the insect color-sense and its subsequent reactions upon the outer world, we must go back to a time when neither flower nor butterfly yet existed.
In the carboniferous earth we have reason to believe that almost all the vegetation belonged to the flowerless type—the type now represented among us by ferns and horse-tails. These plants, as everybody knows, have no flowers, but only spores or naked frondlets. There were a few flowering plants it is true, in the carboniferous world, but they belonged entirely to the group of conifers, trees like the pines and cycads, which bear their seeds in cones, and whose flowers would only be recognized as such by a technical botanist. Even if some stray archaic members of the true flowering groups already existed, it is, at any rate, almost certain that they must have been devoid of those gay petals which distinguish the beautiful modern blossoms in our fields and gardens.
A flower, of course, consists essentially of a pistil or seed-producing organ, and a certain number of stamens or fertilizers. No seed can come to maturity unless fertilized by pollen from a stamen. But experience, and more especially the experiments of Mr. Darwin, have shown that plants produced from the pollen of one flower applied to the pistil of another are stronger and more vigorous than plants produced from the stamens and ovules of a single blossom. It was to obtain the benefit of this cross-fertilization in a simple form that flowers first began to exist; their subsequent development depends upon the further extension of the same principle.
The pines and other conifers, the grasses and sedges, and the forest-trees, for the most part depend upon the wind to waft the pollen of one blossom to the pistil of the next. Hence their flowers generally protrude in great hanging masses, so that the breeze may easily carry off the pollen, and that the pistils may stand a fair chance of catching any passing grain. Flowers of some such types as these were doubtless the earliest of all to be evolved, and their colors are always either green or plain brown.
But wind-fertilization is very wasteful. Pollen is an expensive product to the plant, requiring much useful material for its manufacture; and yet it has to be turned loose in immense quantities on the chance that a stray grain here and there may light upon a pistil ready for its reception. It is almost as though the American farmers were to throw their corn into the Atlantic in hopes that a bushel or two might happen to be washed ashore in England by the waves and the Gulf Stream. Under such circumstances, a ship becomes of immense importance; and Nature has provided just such ships, ready-made for the very work that was crying out to them. These ships were the yet undifferentiated insects, whose descendants were to grow into bees, rose-beetles, and butterflies.
Already, in the carboniferous world, winged insects had begun to exist. Some of these must soon have taken to feeding among the hanging blossoms of the first flowering plants. Insects are fond of the soft and nutritious pollen; and it would seem at first sight as though they could therefore be only enemies to the plants which they visited. But, as they went from flower to flower in search of food, they would carry pollen from one to the other, clinging to their heads, feet, or legs; and so would unconsciously aid in fertilizing the blossoms. Though some of the pollen would thus be eaten up, yet the saving effected by the substitution of the insect as a ship, for the old wasteful mode of dispersal by the wind, would more than compensate for the loss thus brought about. Accordingly, it would naturally happen that those flowers which most specialized themselves for fertilization by means of insects, would gain a considerable advantage over their neighbors in the struggle for existence. For this purpose, their outer leaves ought to assume a cup-like shape, instead of the open clusters of the wind-fertilized type; and their form should be directed rather to saving the pollen than to exposing it; while their efforts must chiefly be expended in attracting the insects whose visits would benefit them, and repelling all others. Those flowers which chanced to vary most in these directions would best succeed from generation to generation; and their descendants would finally become so modified as to be fitted for fertilization by insects only.
It would be needless here to allude once more to the changes in shape and arrangement thus brought about by the action of the insects. The attraction of perfume and honey, the devices of adaptation and modification, by which plants allure or detain their insect visitors, must be taken for granted, and we must pass on to our proper subject of color.
If, when insects were first beginning to visit flowers, there was any special difference by which the pollen-bearing parts could be easily distinguished from the other organs of the plant, we may be sure that it would be seized upon by the insects as a guide to the existence of food, and would so be further strengthened and developed in all future plants of the same species. Now, we have reason to believe that just such a primitive difference does exist between flowers, and leaves or stems; and that difference is one of color. Even if we look at the catkins and grass-blossoms of our own day, we see that they differ slightly in hue from the foliage of their respective plants. But it seems not improbable that color may have appeared much more frequently and abundantly in primitive wind-fertilized flowers than in those of our own epoch; because wind-fertilized flowers are only injured by the visits of insects, which would be attracted by bright color; and hence natural selection would tend to keep down the development of brilliant tints in them, as soon as these had become the recognized guides of the insect eye. In other words, as flowers have now split up, functionally speaking, into two great groups, the wind-fertilized and the insect-fertilized, any primitive tendency toward the production of bright leaves around the floral organs will have been steadily repressed in the one group and steadily encouraged in the other.
Did such a primitive tendency ever exist? In all probability, yes. The green parts of plants contain the special coloring-matter known as chlorophyl, which is essential to their action in deoxidizing the carbonic acid of the atmosphere. But, wherever fresh energies are being put forth, the reverse process of oxidation is going on; and in this reverse process the most brilliant and beautiful colors make their appearance. We are all familiar with these colors in autumn leaves; and we may also observe them very conspicuously in all young shoots or growing branches, especially in the opening buds of spring, the blanched heads of rhubarb or seakale, and the long sprays of a sprouting potato, grown in a dark cellar. Now, the neighborhood of the floral organs is just such a place where energies are being used up and where color is therefore likely to appear. Mr. Sorby has shown that the pigment in petals is often exactly the same as that in the very young red and yellow leaves of early spring, and the crimson foliage of autumn, in the same plant. It would be impossible to go fully here into the evidence which might be offered on this head; an immense mass of facts shows us that color is always tending to appear in the leaves which immediately surround the floral organs; and that this tendency has been strengthened by insect selection of the most conspicuous blossoms, until it has finally resulted in the brilliant corollas of such flowers as those which we now cultivate in our modern gardens.
But all this takes for granted the very fact with which we are now concerned, the existence and growth of an insect color-sense. How do we know that insects can distinguish colors at all? For otherwise all this argument must be fallacious, and the presence of bright corollas must be due to some other cause.
Of all insects, bees are the most confirmed flower-haunters, and they have undergone the greatest modification in relation to their visits in search of honey. We might expect, therefore, that bees would exhibit a distinct color-sense; and this is actually the case. Sir John Lubbock's experiments clearly prove that bees possess the power of distinguishing between red, blue, green, and yellow. Being anxious to see whether insects were really attracted by the hues of flowers, he placed slips of glass, smeared with honey, on paper of various colors; and the bees upon which he experimented soon learned to return to one particular color only, even though both the paper and the honey were occasionally transposed. Thus we have direct evidence of the clearest sort that the higher insects do actually perceive the difference between various colors. Nay, more, their perception in this respect appears to be closely analogous to our own; for while the bees had no difficulty in discriminating between red, orange, or yellow, and green, they did not seem to perceive so marked a distinction between green and blue. Now, this fact is very like that which we perceive to hold good with the human eye, for all of us are much more likely to confuse green and blue than any two other hues.
If, then, bees and wasps, as Sir John Lubbock has shown, and butterflies, as we may infer from other observations, do possess this developed color-sense, we may ask, how did they obtain it? In all probability it grew up side by side with the growth of bright-hued flowers. Just as those blossoms which exhibited the greatest tendency to display a brilliant whorl of tinted leaves, in the neighborhood of their stamens and pistils, would best succeed in attracting insects, so, in return, those insects whose eyes were most adapted for distinguishing the pink and yellow blossoms from the green foliage would best succeed in procuring food, and would thus live down their less gifted competitors.
It may reasonably be asked. How could an animal without a color-sense develop such a faculty by the aid of natural selection alone? At first sight the question seems indeed a difficult one; but it is possible, I think, to suggest a way in which it may have happened. Colors, viewed objectively, consist of ether-waves having different rates of vibration. In an eye devoid of the color-sense, all these ether-waves would doubtless set up the same sort of action in all the ends of the nerves, and would therefore produce exactly the same general sensations. But if in certain eyes there was the slightest tendency for some of the nerve-terminals to respond specially to the oscillations of one particular order, while others of the nerve-terminals responded rather to oscillations of a different order, there would be the first groundwork for the evolution of a color-sense. If this diversity of action in the nerve-ends proved of no service to the animal, it would go no further, because those individuals who possessed it would not be favored beyond those who did not. But if it proved useful, as it undoubtedly would do to flower-haunting insects, natural selection would insure its survival and its constant increase from generation to generation. Even color-blind people among ourselves can be taught by care and attention to discriminate slightly between the hues which they at first confuse; and if we were to choose out, time after time, from a color-blind race, all those individuals who were best able to see these distinctions, we should, no doubt, at last succeed in producing a perfect color-sense. This is just what natural selection seems to have done in the case of bees and butterflies.
Yet it may be urged that insects perhaps had a color-sense before they began to haunt flowers, and that this sense enabled them to pick out the brighter blossoms from the very beginning. Such an hypothesis would make the origin of beautiful flowers a much more simple matter; but we can hardly accept it, for a very good reason. Before the existence of flowers there was probably nothing upon which insects could exert a color-sense. Now, we know that no faculty ever comes into existence until it is practically of use to its possessors. Thus, animals which always live fixed and immovable in one place never develop eyes, because eyes would be quite useless to them; and even those creatures which possess organs of vision in their young and free state lose them as soon as they settle down for life in a permanent and unchangeable home. So, unless insects had something to gain by possessing a color-sense, they could never get one, prophetically, so to speak, against the contingency of flowers at some time or other appearing. Of course, no creature would develop such a sense merely for the sake of admiring the rainbow and the sunset, or of observing gems and shells or other such bright-hued but useless bodies. It is in the insect's practical world of food-hunting and flower-seeking that we must look for the original impulse of the color-sense.
Again, throughout the whole animal world, we see good reasons for concluding that, as a matter of fact, and apart from such deductive reasoning, only those species exhibit evident signs of a color-sense to whom its possession would be an undoubted advantage. Thus, in this very class of insects, bees, as Sir John Lubbock's experiments show us, do undoubtedly distinguish between red, orange, yellow, and green. Butterflies also are attracted by colors, and will, in particular, fly down to objects of the same hue as their own mates. Of course, bees and butterflies, always living among flowers, especially require a good sense of color; and so they quite accord with our expectation. Wasps, again, are omnivorous creatures, living partly upon animal and partly upon vegetable food. Everybody knows that they will quite impartially feast upon a piece of raw meat or upon the sunny side of a peach. Now, wasps, as Sir John Lubbock proved, can also distinguish colors; but they are somewhat less guided by them, apparently, than are bees; and this again bears out the same generalization. Ants are much more miscellaneous in their diet, they have no wings (roughly speaking), and they do not visit flowers except by the casual process of walking up the stems. Hence a color-sense would be of little or no use to them: and Sir John Lubbock's experiments seem to show that they scarcely possess one, or only possess it in a rudimentary form. Once more, moths fly about in the dusk, or quite at night, and the flowers which lay themselves out to attract them are white or pale yellow, since no others are visible in the evening. Thus a perception of red, blue, or orange would probably be useless to them: and Mr. Lowne has shown that the eyes of nocturnal insects differ from those of diurnal insects in a way closely analogous to that in which the eyes of bats and owls differ from those of monkeys and humming-birds. These differences are probably connected in both cases with an absence of special organs for discriminating colors; and we shall see a little later on that, while the day-flying butterflies are decked in crimson and orange to please the eyes of their fastidious mates, the night-flying moths are mostly dull and dingy in hue, or reflect the light only in the same manner as the night-flowering blossoms among which they seek their food. Ascending to the vertebrates, the birds are the class which live most in a world of fruits or flowers; and Mr. A. R. Wallace has pointed out that birds on the whole need to perceive color more than any other animals, because their habits require that they should recognize their food at a considerable distance. But birds possess a very large proportion of certain nerve-terminals called the cones, which are three times as numerous in their eyes as the other kind, called rods. These cones are almost universally believed to be the special organs of color-perception, and in mammals they are actually less numerous than the rods, which are supposed to be merely cognizant of light and shade. Nocturnal birds, such as owls, have very few cones, while nocturnal mammals have none. Again, the yellow spot in the retina, consisting almost entirely of cones, is found in all diurnal birds; but among mammals it occurs only in the fruit-eating class of monkeys, and in man. So that on the whole we may say the positive evidence justifies us in believing that a highly-developed color-sense exists only in those animals which would be decidedly benefited by its possession. And for these reasons it seems improbable that insects ever developed such a faculty until the need for it arose among the beautiful flowers.
Now that we have arrived at this theoretical conclusion, let us hark back again for a while to the reactions which the color-sense, thus aroused, produced upon the flowers which gave it birth.
We may take, as a capital example of an insect-fertilized flower, an English dog-rose. Compare this mentally with the wind-fertilized blossoms, such as grasses and catkins, and it is at once obvious that the great difference between them consists in the presence of a colored corolla. No wind-fertilized plant ever has a whorl of gay petals; and though the converse is not quite true, yet almost all insect-fertilized plants are noticeable for their brilliant tints of red, white, blue, or yellow. The structures in which these pigments reside have no function whatsoever, except that of attracting the insect eye. They are produced by the plant at an enormous physiological expense; and, if their object were not to secure the visits of insects, they would be just so much dead loss to the species. Nor is it only once that these colored corollas have been developed. They occur, quite independently, in both great divisions of flowering plants, the monocotyledons and the dicotyledons. This coincidence could hardly have happened had it not been for that original tendency which we already noticed for pink, scarlet, or orange pigments to appear in the neighborhood of the floral organs. Nor is it twice only, in all probability, that flowers have acquired bright petals through insect visits, but a thousand times over. In almost every family, insect-fertilized, self-fertilized, and wind-fertilized species are found side by side, the one with brilliant petals, the others with small, green, and inconspicuous flowers.
For comparison with the dog-rose, one could not find a better type than that common little early spring blossom, the dog's mercury. It is a wind-fertilized flower, and it does not wish to be seen of insects. Now, this mercury is a very instructive example of a degenerate green flower. For, apparently, it is descended from an insect-fertilized ancestor with bright petals; but, owing to some special cause, it has taken once more to the old wasteful habit of tossing its pollen to the wandering winds. As a consequence it has lost the bright corolla, and now retains only three green and unnoticeable perianth-pieces, no doubt the representatives of its original calyx. Almost equally instructive is the case of the groundsel, though in this case the process of degradation has not gone quite so far. Groundsel is a degenerate composite, far gone on the way of self-fertilization. No class of flowers have been more highly modified to suit the visits of insects than the composites. Hundreds of their tubular bells have been crowded on to a single head, so as to make the greatest possible attractive display; and in many cases the outer blossoms of the head, as in the common yellow ragwort, or in the daisy and the sunflower, have been flattened out into long rays, which serve as pennants or banners to catch the insect eye. They are very successful flowers, perhaps the most successful family on the whole earth. But the groundsel, for some reason of its own, has reversed the general family policy. It is rarely visited by insects, and has, therefore, apparently taken once more to self-fertilization; and a complete alteration has thus been effected in its appearance, when compared with its sister composites. Though it has not yet quite lost its yellow center blossoms, it has no rays, and its bells are almost concealed by its large and ugly green involucre. Altogether, we may say that groundsel is a composite far advanced on its way to a complete loss of the characteristic composite habits. It still receives the visits of a very few stray insects; but it does not lay itself out to court them, and it is, probably, gradually losing more and more of its winged clients from day to day. Thus we see that any flower which will benefit by insect-fertilization, whether it be a monocotyledon or a dicotyledon, high up or low down in either series, is almost sure to acquire brilliant petals; while, on the other hand, any flower which gives up the habit of relying upon insects is almost sure to lose or minimize its petals once more, and return to a state resembling in general type the catkins and grasses or the still lowlier self-fertilized types.
The same sort of conclusion is forced upon us if we look at the various organs in each flower which display the brilliant pigments. The petals are most commonly the seat of the attractive coloration, as in the dog-rose and the marsh-mallow. But in many other flowers, like the fuchsia, the calyx is also beautifully colored, so as to aid in the general display. In the tulips and other lilies, the crocus, the iris, and the daffodil, sepals and petals are all colored alike. In marvel-of-Peru and purple clematis, the petals are wholly wanting. In the common meadow-rue, it is the essential floral organs themselves which act as allurements; while, in the mesembryanthemums, the outer stamens become flattened and petal-like, so as to resemble the corolla of other flowers. In the composites, like daisies, where many blossoms are crowded on one head, the outer row of blossoms is often similarly flattened into rays which only serve the purpose of attracting insects toward the fertile flowers of the center. Nor does the coloring process stop at the regular parts of the flower alone: the neighboring bracts and leaves are often even more beautifully tinted than the flowers themselves. In the great white arums, grown in windows as Ethiopian lilies, the actual blossoms lie right inside the big sheath or spathe, and cluster round the tall yellow spike or spadix in the center: and this sheath acts the part of petals in the more ordinary flowers. Many euphorbias have very inconspicuous little blossoms, but each small colony is surrounded by a scarlet involucre which makes them some of the gayest among our hot-house plants. The poinsettia, which is so familiar a fashionable dinner-table plant, bears little yellow flowers which would not of themselves attract the eyes of insects; but it makes up for this deficiency by a large surrounding bunch of the richest crimson leaves, whose gorgeous coloring makes the tree a universal favorite with tropical bees and butterflies. The lovely bougainvillea carries the same idea one step further, for its small flowers are inclosed by three regularly arranged bracts of a delicate mauve or pink; and, when one sees a tree covered with this magnificent creeper in full blossom, it forms one of the most glorious masses of color to be found in the whole of external nature. Many tropical plants, and especially those of parasitical habit, are much given to developing these extra allurements of colored leaves, and their general effect is usually one of extreme brilliancy. From all these examples, we can draw the conclusion that color does not belong by original nature to one part of the plant rather than another; but that wherever the colored juices which result from oxidation of chlorophyl and its analogues began to show themselves, in the neighborhood of the stamens and pistil, they would attract the attention of insects, and so grow more and more prominent, through natural selection, from generation to generation, till they finally attained the present beauty of the tulip, the rose, the poinsettia, and the bougainvillea.
From this marvelous reaction of the color-sense in insects upon the vegetal world we must next pass on to its reaction upon the hues of insects themselves. For we probably owe the exquisite wings of the butterfly and the gorgeous burnished bronze of the rose-beetle to the very same sense and the very same selective action which have produced the hues of the lily and the hyacinth. What proofs can be shown that the colors of insects are thus due to sexual selection? In the first place, we have the certain fact that bees at least, and probably other insects, do distinguish and remember colors. Not only so, but their tendency to follow color has been strong enough to produce all the beautiful blossoms of our fields and gardens. Moreover, we have seen that while bees, which are flower-haunters, are guided greatly by color, wasps, which are omnivorous, are guided to a less extent, and ants, which are very miscellaneous feeders, not at all. It may be objected that insects do not care for the color apart from the amount of honey; but Mr. Anderson noticed that, when the corollas of certain flowers had been cut away, the insects never discovered or visited the flowers; and Mr. Darwin lopped off the big lower petals of several lobelia-blossoms, and found that the bees never noticed them, though they constantly visited the neighboring flowers. On the other hand, many bright-colored bells have no honey, but merely make a great show for nothing, and so deceive insects into paying them a call on the delusive expectation that they will be asked to stay to dinner. Some very unprincipled flowers, like the huge Sumatran rafflesia, thus take in the carrion-flies, by resembling in smell and appearance a piece of decaying meat. Moreover, certain insects show a preference for certain special flowers over others. One may watch for hours the visits paid by a bee or a butterfly to several dozens of one flower, say a purple lamium, in succession, passing by unnoticed the white or yellow blossoms which intervene between them. Fritz Müller mentions an interesting case of a lantana, which is yellow on the first day, orange on the second, and purple on the third. "This plant," he says, "is visited by various butterflies. As far as I have seen, the purple blossoms are never touched. Some species inserted their probosces both into yellow and into orange flowers; others, as far as I have observed, exclusively into the yellow flowers of the first day." Mr. T. D. Lilly, an American naturalist, observed that the colored petunias and morning-glories in his garden were torn to pieces by bees and butterflies in getting at the honey, while the white or pale ones were never visited. These are only a few sample cases out of hundreds, in which various observers have noted the preference shown by insects for blossoms of a special color.
Again, we may ask. Do different species of insects show different degrees of æsthetic taste? The late Dr. Hermann Müller, who specially devoted himself to the relations between insects and flowers, showed most conclusively that they do. The butterflies, which are at once the most locomotive and most beautiful of their class, appear to require larger masses of color for their attraction than any other group; and the flowers which depend upon them for fertilization are, in consequence, exceptionally large and brilliant. Müller attributes to this cause the well-known beauty of Alpine flowers, because bees and flies are comparatively rare among the higher Alps, while butterflies, which rise to greater elevations in the air, are comparatively common; and he has shown that, in many cases, where a lowland flower is adapted for fertilization by bees, and has a small or inconspicuous blossom, its Alpine congener has been modified so as to be suited for fertilization by butterflies, and has, therefore, brilliant bunches of crimson or purple blossoms. In his last work, he shows that, while bees form as many as seventy-five per cent of the insects visiting the beautiful and attractive composites, they form only fourteen per cent of those which visit the plain green and white umbellates, like the wild-carrot and fool's-parsley. Butterflies frequently visit the composites, but almost never the umbellates, which last depend mainly upon the smaller flies and other like insects. Of two small hedge-flowers (Galium mollugo and G. verum), Müller notes that they agree closely in other points, but the first is white, while the second is yellow, which, he says, renders it more attractive to small beetles. Of certain other flowers, which lay themselves out to attract wasps, Müller quaintly observes that they are obviously adapted "to a less æsthetically cultivated circle of acquaintances." So that the close studies of this accurate and painstaking naturalist led him to the conclusion that insects differ greatly from one another in their taste for color. Probably we shall be right if we say that the most æsthetic among them all are the butterflies, and next the bees—these two classes having undergone the most profound modifications in adaptation to their flower-haunting life—and that the carrion-flies and wasps bring up the rear.
Is there any evidence, however, that insects ever notice color in anything else but flowers? Do they notice it in their own mates, and use it as a means of recognition? Apparently they do, for Mr. Doubleday informed Mr. Darwin that white butterflies often fly down to pieces of white paper on the ground, mistaking them doubtless for others of their species. So, too, Mr. Collingwood notes that a red butterfly, let us say, nailed to a twig, will attract other red butterflies of the same kind, or a yellow one its yellow congeners. When many butterflies of allied species inhabit the same district, it often happens that the various kinds undergo remarkable variation in their coloring so as to be readily recognizable by their own mates. Again, Mr. Patterson noticed that certain blue dragon-flies settled in numbers on the blue float of a fishing-line, while two other species were attracted by shining white colors. On the whole, it seems probable that all insects possessing the color-sense possess also a certain æsthetic taste for colors.
Indeed, it is difficult to see how it could be otherwise. Whenever an animal exercises a faculty much, the exercise comes to have pleasant feelings attached to it; and this is especially the case with all sense-organs. Creatures which live on honey love sweet things: carnivores delight in the taste of blood. Singing birds listen with interest to musical notes: and even insects will chirp in response to a chirp like their own. So, creatures which pass all their lives in the search for bright flowers must almost inevitably come to feel pleasure in the perception of brilliant colors. This is not, as so many people seem to think, a question of relative intellectual organization: it is a mere question of the presence or absence of certain sense-centers.
But it may finally be urged that, even though insects recognize and admire colors in the mass, they would not notice such minute and delicate patterns as those on their own wings. Let us see what evidence we can collect on this head. First of all, insects have not only produced the petals of flowers, but also the special markings on those petals. Now, these markings, as Sprengel pointed out a century since, bear a constant reference to the position of the honey, and are in fact regular honey-guides. If one examines any flower with such marks upon the petals, it will be found that they converge in the direction of the nectaries, and show the bee or butterfly whereabout he may look for his dinner. Accordingly, they must have been developed by the gradual action of insects in fertilizing most frequently those flowers which offered them the easiest indication of where to go for food. Unless insects noticed them, nay more, noticed them closely and accurately, they could never have grown to their present definite correlation with the nectary, a correlation which, Mr. Darwin says, first convinced him of the reality of their function. "I did not realize the importance of these guiding marks," says Sir John Lubbock, "until, by experiments on bees, I saw how much time they lose if honey which is put out for them is moved even slightly from its usual place." In short, insects, like men, are creatures of habit. How complicated these marks sometimes become, we can see in most orchids.
Again, the attention insects pay to comparatively small details of color and form is clear enough from the mimicry which sometimes occurs among them. In some instances, the mimicry is intended to deceive the eyes of higher animals, such as birds or lizards, and can therefore prove nothing with regard to the senses of the insects themselves. But, in a few cases, the disguise is adopted for the sake of deceiving other insects; and the closeness of the resemblance may be accepted as good evidence of acute vision in the class so mimicked. Thus, several species of flies live as social parasites among the hives or nests of bees. These flies have acquired belts of color and patches of hair, closely imitating the hosts whose honey they steal; while their larvæ have even the ingratitude to devour the larvæ of the bees themselves. Of course, any fly who entered a bee-hive could only escape detection and condign punishment at the hands—or rather at the stings—of its inhabitants, provided it looked so like the householders as to be mistaken by them for one of the community. So any fly which showed at first any resemblance to a bee would for a while be enabled to rob with impunity: but, as time went on, the bees would begin to perceive the true nature of the intruders, and would kill all those which could be readily distinguished. Thus, only the most bee-like flies would finally survive; and the extent to which the mimicry was carried would be a rough test of the perceptive powers of the bees. Now, in these particular cases, the resemblance is so close that it would take in, not only an unpracticed human observer, but even for a moment the entomologist himself. Similar instances occur among Mantidæ and crickets.
And now let us apply these facts to the consideration of the problem before us. If those insects which especially haunt flowers are likely to have so acquired a color-sense and a taste for colors, and if they are capable of observing minute markings, bands, or eye-like spots, then we might naturally infer that they would exhibit a preference for the most beautifully colored and variously ornamented of their own mates. Such a preference, long continued and handed down to after-generations, would finally result in the development of very beautiful and varied colors among the flower-haunting species. We might expect, therefore, to find the most exquisite insects among those races which are most fully adapted to a diet of honey and pollen; and such I believe to be actually the case.
Before proceeding further, precautions should be taken against a misconception which has already occurred in this connection. It is not meant that bright colors will be found only among flower-haunters; for it may easily happen that in a few instances other causes may conspire to produce brilliant hues. Nor is it meant that all flower-haunters are necessarily brilliant; for it may also happen that some special need of protection will occasionally keep down the production of conspicuous tints. But what is meant is that brilliant colors are found with very exceptional frequency among the specially flower-haunting animals.
Butterflies are the order of insects which require the largest mass of color to attract them, and which seem to possess the highest æsthetic sensibility. It is hardly necessary to say that butterflies are also the most beautiful of all insects; and are, moreover, noticeable for the most highly developed ornamental adjuncts. Those butterflies make the best matches in their world of fashion which have the brightest crimson on their wings or the most exquisite gloss in their changeful golden scales. With us, an eligible young man is too often a young man with a handsome estate in the country, and with no other attractions mental or physical. Among insects, which have no estates, an eligible young butterfly is one with a peculiarly deep and rich orange band upon the tip of his wings. Thus the cumulative proof of the æsthetic superiority of butterflies seems well-nigh complete.
If we examine the lepidoptera or butterfly order in detail, we shall find some striking conclusions of the same sort forced upon us. The lepidoptera are divided into two great groups, the moths and the butterflies. Now, the moths fly about in the dusk or late at night; the flowers which attract them are pale, lacking in brilliancy, and, above all, destitute of honey-guides in the shape of lines or spots; and the insects themselves are generally dark and dingy in coloration. Whenever they possess any beauty of color, it takes the form of silvery scales which reflect what little light there may be in the gray gloaming. The butterflies, on the other hand, fly by day, and display, as we know, the most beautiful colors of all insects. Here we must once more recall that difference between the structure of the eye in nocturnal and diurnal species which Mr. Lowne has pointed out. Nor is this all. While most moths are night-fliers, there are a few tropical genera which have taken to the same open daylight existence as the butterflies. In these cases, the moths, unlike their nocturnal congeners, are clad in the most gorgeous possible mixtures of brilliant metallic colors.
Other instances of like kind occur in other orders. Thus, among the beetles, there is one family, the rose-chafers, which has been specialized for flower-haunting; and these are conspicuous for the beauty of their coloring, including a vast number of the most brilliant exotic species. Their allies, the common cock-chafers, however, which are not specialized in the same manner, are mere black and inconspicuous insects. So among the flies: most of the omnivorous families are dull and ugly; but several of the flower-haunting tribes are adorned with brilliant colors, and live upon honey. In fact, an immense majority of the brightest insects are honey-suckers, and seem to have derived their taste for beautiful hues from the nature of the objects among which they seek their food.
There is one striking and obvious exception, however, which has doubtless already suggested itself to the minds of readers. I mean the bees. These are the most flower-loving of all insects, and yet they are comparatively plain in their coloration. We must remember, however, that the peculiar nature of the commonwealth among the social bees prevents the free action of the selective preference by which we account for the brilliancy of all other flower-haunting species. The queen or mother bee is a prisoner for life; her Majesty's domestic arrangements are all made for her by the state; she does not herself seek honey among flowers, and those bees which do so have no power of transmitting their tastes to descendants, as they live and die mere household drudges. On the other hand, the solitary bees are in many cases exquisitely colored, as we might expect from their power of free choice; and one flower-haunting family of the same order, the Chrysidæ, are aptly compared to the humming-birds in the richness of their coloring.
One more peculiarity of great interest must also be noted. It appears that many insects have two sets of colors, seemingly for different purposes; the one set protective from the attacks of enemies, the other set attractive to their own mates. Thus several butterflies have the lower side of their wings colored like the leaves or bark on which they rest, while the upper sides are rich with crimson, orange, and gold, which gleam in the bright sunlight as they flit about among their fellows. Butterflies, of course, fold their wings with the under side outward. On the other hand, moths, which fold their wings in the opposite manner, often have their upper surfaces imitative or protective, while the lower sides are bright and beautiful. One Malayan butterfly, the Kallima paralecta, has wings of purple and orange above, but it exactly mimics dead foliage when its vans are folded; and, as it always rests among dry leaves, it can hardly be distinguished from them, as it is even apparently spotted with small fungi. In these and many other cases one can not help believing that, while imitative coloring has been acquired for protective purposes, the bright hues of the concealed portion must be similarly useful to the insect as a personal decoration.
It would seem, then, that we owe half the loveliest objects in our modern world to the insect color-sense. It is the bee and the butterfly which have given us the gorgeous orchids and massive creepers of the tropics, the gentians and rhododendrons of the Alps, the camellias and heathers of our conservatories, the may and primroses of our English meadows. To the same primitive taste, exerted in a slightly different direction, are due the gilded wings of Brazilian moths, and the exquisite tints of our own ruby or sapphire colored summer insects. The beauty and the glory of the world are not for the eyes of man alone; they appeal equally to the bee and the butterfly, to the bird and the child. To some people it strangely seems a nobler belief that one animal only out of all the earth enjoys and appreciates this perpetual pageant of natural loveliness; to me it appears, on the contrary, a prettier and more modest creed, as well as a truer one, that in those higher and purer delights we are but participants with the vast mass of our humbler dumb fellow-creatures.—Gentleman's Magazine.