Delphi collected works o.., p.1022
Delphi Collected Works of Grant Allen, page 1022
You can best observe the mosquito in action, however, by letting one settle undisturbed on the back of your hand, and waiting while she fills herself with your blood; you can easily watch her doing so with a pocket lens. Like the old lady in “Pickwick,” she is soon “swelling wisibly.” She gorges herself with blood, indeed, which she straightway digests, assimilates, and converts into the 300 eggs aforesaid. But if, while she is sucking, you gently and unobtrusively tighten the skin of your hand by clenching your fist hard, you will find that she cannot any longer withdraw her mandibles; they are caught fast in your flesh by their own harpoon-like teeth, and there she must stop accordingly till you choose to release her. If you then kill her in the usual manner, by a smart slap of the hand, you will see that she is literally full of blood, having sucked a good drop of it.
The humming sound itself by which the mosquito announces her approaching visit is produced in two distinct manners. The deeper notes which go to make up her droning song are due to the rapid vibration of the female insect’s wings as she flies; and these vibrations are found by means of a siren (an instrument which measures the frequency of the waves in notes) to amount to about 3000 in a minute. The mosquito’s wings must, therefore, move with this extraordinary rapidity, which sufficiently accounts for the difficulty we have in catching one. But the higher and shriller notes of the complex melody are due to special stridulating organs situated like little drums on the openings of the air-tubes; for the adult mosquito breathes no longer by one or two air-entrances on the tail or back, like the larva, but by a number of spiracles, as they are called, arranged in rows along the sides of the body, and communicating with the network of internal air-chambers. The curious mosquito music thus generated by the little drums serves almost beyond a doubt as a means of attracting male mosquitoes, for it is known that the long hairs on the antennæ of the males, shown in No. 9, Fig. A, vibrate sympathetically in unison with the notes of a tuning-fork, within the range of the sounds emitted by the female. In other words, hairs and drums just answer to one another. We may, therefore, reasonably conclude that the female sings in order to please and attract her wandering mate, and that the antennæ of the male are organs of hearing which catch and respond to the buzzing music she pours forth for her lover’s ears. A whole swarm of gnats can be brought down, indeed, by uttering the appropriate note of the race; you can call them somewhat as you can call male glow-worms by showing a light which they mistake for the female.
A much larger and more powerful British bloodsucker than the mosquito, again, is the gadfly or horse-fly, whose life-size portrait Mr. Enock has drawn for us in No. 10. Most people know this fearsome beast well in the fields in summer. He has a trick of settling on the back of one’s neck, and making a hole in one’s skin with his sharp mandibles; after which he quietly sucks one’s blood almost without one’s perceiving him. Horses in pastures are often terribly troubled by these persistent creatures, which make no noise, but creep silently up and settle on the most exposed parts of the legs and flanks. They are very voracious, and manage to devour an amount of blood which is truly surprising.
A little examination of the gadfly will show you, too, one important point in which it and all other true flies differ from the bees, wasps, butterflies, and the vast mass of ordinary insects. All the other races have four wings, and I showed you in the case of the wasp the beautiful mechanism of hooks and grooves by which the fore and hind wings are often locked together in one great group, so as to insure uniformity and fixity in flying. Among the true flies, however, including not only the house-fly and the meat-fly, but also the gadflies and the mosquitoes, only one pair of wings — the front pair — is ever developed. The second or hind pair is feebly represented by a couple of tiny rudimentary wings, known as poisers or balancers, which you can just make out in the sketch, like a couple of stalked knobs, in the space between the true wings and the tail or abdomen. It is pretty clear that the common ancestor of all these two-winged flies must have had four wings, like the rest of the great class to which he belonged; but he found it in some way more convenient for his purpose to get rid of one pair, and he has handed down that singular modification of structure to all his descendants. Yet whenever an organ or set of organs is suppressed in this way, it almost always happens that rudiments or relics of the suppressed part remain to the latest generations; and thus the true flies still retain, in most cases, the two tiny poisers or balancers, just to remind us of their descent from four-winged ancestors. Nature has no habit more interesting than this retention of parts long since disused or almost disused; by their aid we are able to trace the genealogy of plants and animals.
In No. 11 we have a dissected view of the mouth-organs and blood-sucking apparatus of the gadfly, immensely enlarged, so as to show in detail the minute structure. In life, all these separate parts are combined together into a compound sucker (commonly called the proboscis), which forms practically a single tube or sheath; they are dissected out here for facility of comprehension. The longest part, marked LA in the sketch, is the labium or lower lip, which makes up the mass of the tube; it ends in two soft finger-like pads, which are fleshy in texture, and which enable it to fix itself firmly (like a camel’s foot) on the skin of the victim. The grooved and dagger-shaped organ, marked LBR, is the labrum, or upper lip; and the tube or sheath formed by the shutting together of these two parts encloses all the other organs. Combined, they form a trunk or proboscis, not unlike that of the elephant. But the elephant is not a bloodsucker; his trunk encircles no dangerous cutting weapon. It is otherwise with the gadfly, which has a pair of sharp knives within, for lancing the thick skin of its unhappy victims. These knives are known as mandibles, and are marked MD in the sketch, one on either side of the labrum. They first pierce the skin; the maxillæ, marked MX, of which there are also a pair, then lap up the blood from the internal tissues. Finally, there is the true tongue or lingua, marked L, which is the organ for tasting it. As to the maxillary palps, marked MP, they do not form part of the tube at all, but stand outside it, and assist like hands in the work of manipulation.
This is how the mouth looks when fully opened out for microscopic examination. But as the fly uses it, it forms a closed tube, of which the labium and the labrum are the two walls, enfolding the lances or mandibles, and the lickers or maxillæ, as well as the tongue. Pack them all away mentally, from MX to MX, within the two covers, and you will then understand the nature of the mechanism. Look back at Fig. B in No. 9, and you will there observe that all the parts in the mosquito answer to those in the gadfly. The long upper sheath is the upper lip: then come the lancets, the lappers, and the tongue, and last of all, the lower lip.
In No. 12, which is still more highly magnified, we have the essential parts of the blood-sucking apparatus made quite clear for us. Here LBR is the tip of the labrum, or upper lip, forming the front of the groove or sheath in which the lances work. Its end is blunt, so as to enable it to be pressed close against the minute hole formed by the lances. MD is the sharp tip of one of the two lances, with its serrated or saw-like cutting-edge; this is the organ that does the serious work of imperceptibly piercing the skin and the tissues beneath it. MX is the tip of one of the maxillæ, or blood lappers, which suck or lap up the blood from the wound after the lances have opened it. I need hardly call your attention to the extraordinary delicacy and minuteness of these hard, sharp weapons, strong enough to pierce the tough hide of a horse, yet so small that if represented on the same scale as the insect itself, you would fail to perceive them.
Is it not marvellous, too, that the same set of organs about the mouth, which we saw employed by the wasp for cutting paper from wood, and by the ant for the varied functions of civilised ant-life, should be capable of modification in the butterfly into a sucker for honey, and in the gadfly into a cunning mechanism for piercing thick hides and feeding on the life-blood of superior animals. Nature, it seems, is sparing of ground-plan, but strangely lavish of minor modifications. She will take a single set of organs, inherited from some early common ancestor, and keep them true in the main through infinite varieties; but as habits alter in one species or another, she will adapt one of these sets to one piece of work and another to a second wholly unlike it. While she preserves throughout the similarity due to a common origin, she will vary infinitely the details and the minor structures so as to make them apply to the most diverse functions. Nothing shows this truth more beautifully, and more variously, than the mouths of insects; and though the names by which we call the different parts are, I will admit, somewhat harsh and technical, I feel sure that anybody who once masters their meaning cannot fail to be delighted by the endless modifications by which a few small instruments are made to fit an ever-increasing and infinite diversity of circumstances.
XI. A VERY INTELLIGENT PLANT
PEOPLE who have never had occasion to observe plants closely often fall into the error of regarding them as practically dead — dead, that is to say, in the sense of never doing or contriving anything active. They know, of course, that herbs and trees grow and increase; that they flower and fruit; that they put forth green leaves in spring and lose them again in autumn. But they picture all this as taking place without the knowledge or co-operation of the plant itself — they think of it as done for the tree or shrub rather than by it. Those, however, who have kept a close watch upon living green things in their native condition have generally learned by slow degrees to take quite a different view of plant morals and plant economy. They begin to find out in the course of their observations that the life of a herb is pretty much as the life of an animal in almost everything save one small particular. The plant, as a rule, is rooted to a single spot; the animal, as a rule, is free and locomotive.
Yet even this difference itself is not quite absolute: for there are on the one hand locomotive plants, such as that quaint microscopic vegetable tumbler, the floating green volvox, which whirls about quickly through the water like a living wheel, by means of its rapid vibratory hairs; and there are, on the other hand, fixed animals, such as the oyster and the sea-anemone, which are far more rigidly attached to one spot for life than, say, the common field-orchid or the yellow crocus. For field-orchids and crocuses do travel very slightly from place to place each season, by putting out fresh bulbs or tubers at the sides of the old ones, and springing up next year in a spot a few inches away from their last year’s foothold; whereas the oyster and the sea-anemone settle down early in life on a particular rock, and never stir one step from it during their whole existence. Thus the distinction which seems to most people most fundamental as marking off plants from animals — the distinction of movement — turns out on examination to be purely fallacious. There are sedentary animals and moving plants; there are herbs that catch and eat insects, and there are insects that live a life more uneventful and more stagnant than that of any herb in a summer meadow.
Again, everybody who has studied plants in a broad spirit is well aware that each act of the plant’s is just as truly purposive, as full of practical import, as any act of an animal’s. If a child sees a cat lying in wait at a mouse’s hole, it asks you why she does so; it is told, in reply, and truly told, “Because she wants to catch her prey for dinner.” But even imaginative children seldom or never ask of a rose or a narcissus, “Why does it produce this notch on its petals? Why does it make this curious crown inside the cup of its flower?” Those things are thought of as purely ornamental; as parts of the plant, not as organs made by it. Yet the rose and the narcissus have just as much a reason of their own for everything they do and everything they make as the cat or the bird; they are just as much governed by ancestral wisdom, though the wisdom may in one case be conscious, in the other hereditary.
The rose, for example, produces prickles for its own defence, and scented blossoms to attract the fertilising insects for its own propagation. It does everything in life for some good and sufficient reason of its own, and takes as little heed of other people’s convenience as the tiger or the snake does. “Each species for itself,” is the rule of nature; no species ever undertakes anything for the sake of any other, except in the expectation of a corresponding advantage. If the wild thyme lays by in its throat abundant honey for the bumble-bee, that is because it counts upon the bumble-bee to carry its pollen from blossom to blossom; if the holly puts forth bright red berries for the robin to eat, that is not because it cares for the robin’s distress, but because it looks upon the bird as a paid disperser of its stony seeds, and gives him in return a pittance of pulp for his pains, as stingy payment for the service rendered. The holly and the thyme are confirmed sweaters. Indeed, you will find that no plant ever wastes one drop more of nectar on its flowers, or one atom more of sweet pulp on its fruit, than is absolutely necessary to secure its own purely selfish object. It offers the bird or the insect the minimum wage for which bird or insect will consent to do the work it contracts for; and it never wastes one farthing’s worth of useful material on tips or generosities. The rose, for all that poets have said of it, is strictly utilitarian. “You help me and I will help you,” it says to the butterfly; and it keeps the sternest possible debtor-and-creditor account with all its benefactors.
As a familiar example of this purposive character in all plant life, I am going, in the present chapter, to take one of the most utilitarian shrubs — the common gorse — and try to show you why it behaves as it does in the conduct of its affairs; who help it in life and who hinder it, what friends it strives to buy or conciliate, what enemies it repels by what violent acts of armed hostility.
Everybody knows gorse; and everybody also knows that it is almost never out of flower. This last peculiarity, however, is due to a cause that not everybody has noticed. We have two distinct kinds of gorse at least — the larger and the smaller. It is the larger sort that one observes most when it is not in blossom, though it is the smaller kind whose golden bloom contrasts so beautifully in autumn with the rich purple of the upland heather. Now, the larger gorse begins to flower in October or November; it goes on opening its buds spasmodically in every fine spell throughout the winter, reaching its fullest glory of blossom in April and May; while the smaller kind begins to flower in summer, as soon as its larger cousin has fixed its attention on setting seed; and it goes on yellowing our heaths with its wealth of gold till October or November, when the bigger sort once more replaces it and takes up the running. In this way there is no bright day throughout the year — that is to say, no day fit for insects to gather honey — on which one kind of gorse or the other does not seek to cater for the friendly allies which help it to set its precious seeds, as we shall see in the sequel. It is the larger and better-known gorse with which I shall deal chiefly here, though I may occasionally refer by way of illustration or contrast to its smaller neighbour.
If we begin at the beginning in the life-history of the gorse, it may surprise you to find that each plant sets out on its way through life, not as a prickly gorse plant, but as a sort of quiet and unarmed little flat trefoil. No. 1 shows you the young furze-bush in its earliest infantile stage, when it is still essentially a two-leaved seedling. This seedling grows from a small bean scattered by the parent plant in a very curious way, which I will explain later. Thousands of the beans lie on the ground in every common, and only a few germinate, under favourable circumstances, into two-leaved seedlings, like those represented in these illustrations. The leaves of the first pair spread out flat on the surface of the unoccupied soil and drink in the sunlight. They also drink in, what is equally important to them, the carbonic acid of the air, and manufacture from it the living material of fresh leaves by the aid of the sunlight. For the first few days of its life, the young gorse plant lives mainly on the food laid up for it in the bean by the parent bush; but as soon as this is exhausted, and it has accumulated a little stock of its own by its private exertions, it begins to manufacture new leaves and branches that it may rise above the tangled mass of competitors by which its birthplace is surrounded.
No. 2 shows us this second stage in the young shrub’s development. At first sight you would hardly suppose it was a gorse at all; you might take it for the young of some such allied species as a broom or a genista. You will observe that at this point in its history the young gorse has trefoil leaves, not very unlike those of some kinds of clover. Why is this? Well, we have many good reasons for supposing that the ancestors of gorse were originally soft-leaved and unarmed shrubs, like the ornamental genistas which we grow in pots for drawing-room decoration; but as they were much exposed on open moors and commons, where they were liable to be grazed down and browsed upon by rabbits, sheep, and other herbivorous animals, the tenderer and more luscious among them stood little chance of surviving. Indeed, so hard is it for plants to grow in such situations, that one not uncommonly finds tiny trees of Scotch fir, close-cropped to the ground, yet with many years’ growth exhibited by the annual rings of wood in their underground root-stock. These poor persistent little trees have been nibbled down, year after year, as soon as they appeared, by rabbits or donkeys; yet year after year they have gone on sprouting afresh, as well as they could, and laying by an annual ring of woody tissue in buried root-stock.
To some such attacks the ancestral gorses must always have been exposed on the open moors and hillsides of primitive Europe, at first, no doubt, from deer and wild oxen and beavers, but later on from the sheep and cows and goats and donkeys which followed in the wake of aggressive civilisation. Under these circumstances, most of the soft-leaved and unprotected plants got eaten down and killed off; but any shrub which showed a nascent tendency to develop stout spines or prickles on their branches must have been favoured by nature in the struggle for existence. The consequence was that in the end our upland slopes and open spaces all over Western Europe came to be occupied by nothing but strongly armed plants — brambles, thistles, blackthorns, may-bushes, nettles, butcher’s-broom, and the various kinds of furze, all of which can hold their own with ease against the attacks of quadrupeds. Indeed, there is one not uncommon English herb, the little purple-flowered rest-harrow, which very well illustrates this curious connection between the production of thorns and the habit of growing in much-browsed-over spots; for when it settles in enclosed and protected fields it produces smooth and unarmed creeping branches, but when it happens to find its lot cast in places where donkeys and rabbits abound, it defends itself against the dreaded enemy by covering its shoots with stout woody prickles.
