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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 inorning, 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 beeswax, 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 æsthetic 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 percep
tions 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 horsechestnut. 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 waste
ful. 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, rosebeetles, 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. 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 cuter 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 adaption 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 windfertilized 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 chlorophyll, 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 corallas 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 colorsense; 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 their 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 brighthued 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 connect
ed 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 nightflying 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. ored corolla. No wind-fertilized plant