by Grant Allen
Similar results were obtained with wasps. At 6 A.M. on September 13, 1875, Sir John observes, “I put a wasp to some honey on green paper and about a foot off I put some more honey on orange paper. The wasp kept returning to the honey at the usual intervals. At 8.30 I transposed the papers; but the wasp followed the colour. At 9 o’clock I transposed the papers again, but not the honey; she returned again to the green, from which it would appear that she was following the colour, not the honey. At 10.20 I again transposed them, with the same result.”
It should be mentioned, however, that later experiments led Sir John Lubbock to the conclusion that bees do not so easily discriminate between blue and green as between other colours — a very natural fact, considering how slight is the objective difference between these shades. It would also appear that though wasps can distinguish colours, they are less guided by them than is the case with bees. This we might have expected a priori from the diversity of habits of the two insects, and we shall see hereafter that the difference has important bearings on the question of relative tastes.
With respect to ants, Sir John Lubbock’s experiments, though read before the Linnean Society, have not at the date of writing yet been published in its Transactions. It must suffice, therefore, without anticipating the author’s statement, to notice here, that though these insects are very defective in the sense of vision, depending much more largely upon touch and smell, they are evidently affected in a distinctive manner by the red and violet ends of the spectrum. Of course the habits of ants would not lead us to credit them with the necessary circumstances for giving rise to a developed colour-sense; nor are their reactions on external nature of the same startling character as those which we observe in the case of the fully-winged insects, like bees and butterflies. Nevertheless, it is interesting to observe that even here a groundwork of faint discrimination evidently exists, which might possibly have been developed into a perfect sense had the circumstances of ant-life so determined its evolution.
Observation in many ways confirms the results thus obtained by experiment. Let us look first at the evidence on this head which can be derived from the visits of bees and butterflies to flowers.
We have seen already that one main difference between anemophilous and entomophilous flowers consists in the fact that while the former are usually small, greenish, and inconspicuous, the latter are usually large, brilliant, and deeply coloured with white, red, blue, or yellow. We have the high authority of Mr. Darwin for the statement, as an invariable rule, “That when a flower is fertilised by the wind it never has a gaily-coloured corolla.” And though the converse proposition is not strictly true in every case, yet a large proportion of those blossoms which depend for fertilisation upon insects are noticeable for their bright and flaunting hues.
Now, the structures in which the brilliant pigments reside have absolutely no function except that of attracting the insect agents for fertilisation. But these structures, as we have already seen, are produced by the plant at an enormous physiological cost, often so as to engage by far the greater portion of all its energies. Unless we allow, then, that roses, tulips, lilies, and rhododendrons have developed their large and showy corollas for the sake of alluring their insect allies, we shall be reduced to believe that they have produced these expensive and useless adjuncts for no other purpose than that of wasting their substance in riotous living. Such a supposition involves a simple physical absurdity. If any plant could, by any accidental combination of circumstances, once acquire so bad a habit, it must necessarily stand at a disadvantage in comparison with all other plants, and so be quickly extinguished in the ceaseless struggle and competition of terrestrial life. We cannot for a moment believe that any structure could exist at all, far less spread itself through all the most successful species of a dominant class, unless it subserved some function of great utility to the organism in which it is found.
Nor are we left here entirely to such a priori reasoning. It happens that a few aberrant plants possess two forms of flowers — the one entomophilous, and often appearing in the spring; the other self-fertilised, and often appearing in the autumn. Of this phenomenon the common violet offers a well-known example. The entomophilous blossoms, which seem to be frequently sterile, and to answer the purpose of such occasional cross-fertilisation as may keep up the vigour of the stock, are distinguished by the usual coloured corolla, as well as by a sweet and attractive perfume in certain species. But the self-fertilised or cleistogamous blossoms, which in many ways recall the spore-cases of cryptogams, are quite green and inconspicuous, so much so, that no ordinary observer would call them flowers at all, but would set them down as fruits or seed-vessels. These latter blossoms do most of the effective reproductive work; but as they do not aim at cross-fertilisation, and so do not require the aid of insects, they have entirely lost all semblance of coloured floral whorls, and consist merely of hidden fructifying elements. Thus nature herself, as it were, makes an experiment for us which clearly demonstrates, by the Method of Difference, the real function of the pigmented corolla.
Like results are shown in the case of many plants belonging to mainly entomophilous families. True anemophilous flowers, whose ancestors have been anemophilous for all past generations, and have never learnt to depend upon insects at all, possess only the two effective sexual whorls, with at most some appendage having more or less the nature of a calyx. But certain plants, which apparently belong by descent to mainly entomophilous tribes, seem to have dropped the habit of insect-fertilisation, and to have reverted once more to wind-fertilisation. In all (or almost all) these cases, where the corolla still exists at all in a rudimentary form, bearing witness to their ancestral habit, the petals have grown quite small and dwarfed (because their original function is gone), and have once more resumed the green appearance of ordinary leaves. In short, we see that where flowers require the aid of insects they almost always bid for it by assuming bright hues; but that those flowers, anemophilous or cleistogamous, which do not need their aid, are normally destitute of such hues, while those plants which once required their aid, but have ceased to do so, rapidly lose their coloured adjuncts, in accordance with that Law of Parsimony whereby all structures whose functions are no longer necessary become finally obsolescent. How these facts can be accounted for if we suppose insects to be destitute of a colour-sense it would baffle the cleverest theorist to say.
In some few instances we even possess actual proof that insects are attracted by the bright hues of petals. Thus Mr. Anderson noticed that when the corolla of certain blossoms, so constructed as to favour or almost ensure cross-fertilisation, had been cut away, the insects never discovered or visited the flowers. “I proved the importance of the gaily coloured corolla,” says Mr. Darwin, “by cutting off the large lower petals of several flowers of Lobelia erinus, and these flowers were neglected by the hive-bees, which were incessantly visiting the other flowers.”
Descending to particular instances, we find that while most bright-coloured blossoms offer the visiting insect some real advantage in the shape of honey, other unprincipled plants, trading upon the general faith in a connection between colour and food, delude insects into visiting them by their hue alone. Again, certain insects, as Müller has observed, visit certain flowers only; and in other cases, a particular insect, during a single day, confines himself, for some reason of his own, to some one chosen species. Numerous naturalists have put on record the preferences which individual insects have shown on special occasions for one kind of blossom alone. A single case must suffice for all. That careful observer, Mr. H. O. Forbes, saw “by the roadside, near Kew Bridge Station, several species of Hymenoptera, of the genus Bombus principally; one visited thirty flowers of Lamium purpureum in succession, passing over without notice all the other plants on the same bank — species of Convolvulus, Rubus, Solanum. Two other species of Bombus and a Pieris rapæ also patronised the Lamium, seeking it out deep in the thicket, thrusting their probosces even into withered cups, although the
Rubus flowers were far more accessible, and seemed much more attractive, being fresh and well-expanded.” The pages of scientific journals during the last few years have positively teemed with similar instances from all parts of the world.
Furthermore, the varying colours of flowers seem adapted, as we saw in the last chapter, to attract particular insects at particular periods of inflorescence. I have already mentioned the case of a Lantana described by F. Müller as altering in hue at different times during its maturation, being yellow on the first, orange on the second, and purple on the third day. “This plant,” says Müller, “is visited by various butterflies. As far as I have seen, the purple flowers are never touched. Some species inserted their proboscis both into yellow and into orange flowers (Danais erippus, Pieris aripa); others, as far as I have hitherto observed, exclusively into the yellow flowers of the first day (Heliconius apseudes, Colænis julia, Eurema leuce). . . . If the flowers fell off at the end of the first day, the inflorescence would be much less conspicuous: if they did not change their colour, much time would be lost by the butterflies inserting their proboscis in already fertilised flowers.” In another species of the same genus the flowers are lilac, but the entrance of the tube is marked with yellow, surrounded by a white circle. These yellow and white markings, which probably serve as guides for the insect allies, disappear entirely on the second day.
And now, from the evidence supplied by flowers, we may pass on to the evidence supplied by the colours of insects themselves. I do not here propose to enter upon the consideration of those hues which depend for their origin upon sexual selection; that part of our cumulative argument must be delayed to a later chapter. But we may fittingly consider at the present point the proofs of a colour-sense in insects afforded by the curious phenomenon of mimicry, so fully illustrated by Mr. Wallace and Mr. Bates.
It is now an established fact that certain animals have survived in the struggle for existence by means of some special resemblance to other species or to objects in the environment which gives them a special chance of deceiving prey, escaping the notice of enemies, or adopting some similar protective device. Colour enters largely into the special adaptations thus produced, and many insects have been largely modified in their colouration by the action of such mimetic selection. But, as a rule, the particular hues or lines have reference less to the eyes of insects themselves than to the eyes of the reptiles, birds, or mammals which prey upon them; and these cases will therefore be more fully considered when we come to treat of the colour-sense in vertebrates. In a few instances, however, protective imitation seems to have been produced in certain insects with reference to the eyes of other species in their own class; and these latter cases may properly be treated under the present heading.
First of all, we may take the instance of those flies which live in a sort of social parasitism among the hives or nests of bees. These flies have acquired belts of colour and other imitative appendages closely resembling those of the host upon whose stores they commit their depredations, while their larvæ actually live by devouring the larvæ of the bees themselves. Obviously, any fly which entered a beehive could only escape detection and extermination at the hands of its inhabitants, provided it so far resembled them as to be mistaken at a first glance by the community for one of their fellows. Thus any fly which showed the slightest superficial resemblance to a bee might at first be enabled to rob their storehouses with impunity, while such flies would escape continued detection from generation to generation just in proportion as they more and more closely approximated to the appearance of their unwilling hosts. For, as Mr. Belt has well pointed out, while the mimicking species would become naturally more numerous from age to age, the senses of the mimicked species would become naturally sharpened by the habit of detecting flimsy pretences; and so at last very close resemblance might be expected to arise. In the particular instance now under notice, I learn from Mr. Lowne, who has carefully measured the curvature of the facets in the compound eyes of insects (upon which, of course, depends the minimal size of apprehensible objects), that the mimicry has proceeded just so far as the structure of the bee’s eye would lead him to expect, and no further. In other words, Mr. Lowne is inclined to suppose, if measurements of angular distance subtended can guide him, that such a fly is indistinguishable by a bee from one of his own species within the limits of ordinary vision.
Mr. Bates mentions a still more interesting case of some showy coloured Brazilian spiders (which, of course, are not themselves insects in the scientific sense), “who double themselves up at the base of leaf-stalks, so as to resemble flower-buds, and thus deceive the insects upon which they prey.” Sir Joseph Hooker believes that an Indian Mantis, or praying-insect, similarly deludes the little creatures which form its food by its extraordinary likeness to a leaf. Another rapacious example of the same genus exactly mimics the white ants, whom it devours quietly and unsuspected. But all these curious facts are thrown into the shade by a third Mantis, for which Sir Charles Dilke stands as voucher — a bloodthirsty wretch, whose head and fangs have been moulded into the image of an orchid, with a deceptive blossom which closes upon the insect who seeks for food in its treacherous arms.
Sometimes even higher animals seem to have acquired similar disguises in order to deceive the insects for which they lie in wait. Thus Mr. Belt notices a green Nicaraguan lizard, looking like the herbage by which it is surrounded, and decked with leaf-like expansions, which serve to conceal its predacious nature from passing beetles or flies. How far the greenness of lizards and forestine birds in general, or the sandy hue of those which frequent deserts, may serve them as aids in escaping the notice of prey upon which they creep, is too uncertain a question to be urged in evidence as to the colour-sense of insects, and yet too interesting to be passed by without at least an allusion.
In all these instances, various predatory species have acquired mimetic resemblances for the sake of deceiving their quarry; but in other cases the defenceless booty turns the tables upon the tyrants, and is accordingly enabled to elude their hungry quest. Thus Mr. Wallace tells us of a cricket which exactly reproduces the features of its foe the sand wasp; while Mr. Belt saw a green leaf-like locust, overrun by foraging ants, yet remaining as motionless as the leaves, whose colour and texture it so faithfully mimicked. This latter creature appeared to have some dim instinctive knowledge of the fact that its safety depended upon its absolute immobility, for even when lifted by the hand from the ground, it continued strenuously to preserve its rigid attitude.
But indeed, to any mind unbiassed by preconceptions derived from another sphere of thought, the grand evidence in favour of an insect colour-sense is to be found, not in isolated instances, but in the broad expanse of meadow and hill-side around us. The million hues of spring or summer flowers have no meaning and no explanation on any other hypothesis. The colour-sense of bees and butterflies has metamorphosed the world; and we must seek for its indications on every plain and mountain of every country in the earth.
A human analogy will make clear the magnitude of the changes in the face of nature due to the reactive effects of the insect colour-sense. Man has wrought many alterations in the aspect of the fauna and flora of all countries which he has brought under his sway. He has cut down the forests, cleared the jungle, irrigated the desert, reclaimed the stony waste. Whatever plants or animals served him for food, clothing, or other useful purposes, he has selected, propagated, and carried with him in his wanderings from shore to shore. Whatever others proved useless or positively noxious, he has extirpated from their native haunts. His fields glow with golden corn or dark-green maize; with millet, barley, oats, or rice; with vines, hops, or sugar-cane; with yams, potatoes, plantain, or bananas; with flax, hemp, cotton, oil-seeds, or fibres. His orchards are laden with apples, plums, peaches, pears, oranges, olives, mangoes, and papaws. His meadows are stocked with the clover, lucerne, vetches, and grass which feed his flocks and herds. The woodland itself is spared only to supply him with timber f
or firewood or building. In like manner, the wild beasts of the forest have given place to his cattle, his horses, and his sheep. Camels, elephants, and llamas bear his burdens on their patient backs. Alpacas and merinos, the goats of Kashmir and the yaks of Tibet, supply his woven fabrics. Even insects are not exempt; silkworms fatten on his mulberry trees, and nopaleries, specially planted for that purpose, afford sustenance to his cochineal flies. The wolves, bears, foxes, tigers, and venomous snakes retire before his face, and he wages a perpetual warfare with the intrusive weeds which seek to find a root-hold among his growing crops. Such are the changes which man has impressed upon the lands where he has made his settled home.
But all these alterations are mere surface scratches compared with the immense revolution wrought in the features of nature by the unobtrusive insect. Half the flora of the earth has taken the imprint of his likes and his necessities. While man has only tilled a few level plains, a few great river valleys, a few peninsular mountain slopes, leaving the vast mass of earth untouched by his hand, the insect has spread himself over every land in a thousand shapes, and has made the whole flowering creation subservient to his daily wants. His buttercup, his dandelion, and his meadow-sweet grow thick in every English field. His mint clothes the hill-side; his heather purples the bleak grey moorland. High up among the Alpine heights his gentian spreads its lakes of blue; amid the snows of the Himalayas his rhododendrons gleam with crimson light. Even the wayside pond yields him the crowfoot and the arrowhead, while the broad expanses of Brazilian streams are beautified by his gorgeous water-lilies. The insect has thus turned the whole surface of the earth into a boundless flower-garden, which supplies him from year to year with pollen or honey, and itself in turn gains perpetuation by the baits that it offers for his allurement.