by Grant Allen
And here the wonder of their lives begins all over again. For while the gorse was old and woody, it grew like gorse, all stern and prickly. But as soon as the young seedlings start afresh in life, they seem to forget their parents: they revert once more to the old trefoil condition. All young plants and animals, at least in their embryonic stages, show this strange tendency to throw back at first to the ancestral form; and it is fortunate for us that they do so, for it often enables us to perceive underlying relationships which in the adult form escape our notice. Nobody who looked at a furze-bush in its stiff and prickly old age would ever suspect it at first sight of a cousinship with clover. Yet when we consider the trefoil leaves of the seedling, and the shape of the separate peaflowers in the adult form, we can see for ourselves that the two plants are far closer together than we might be tempted to imagine. Indeed between the little creeping yellow clovers and the aggressive furze or the tall and beautiful laburnum, we can find even now a regular series of connecting links which show clearly that all alike are slightly divergent descendants of a single common ancestor.
We may conclude, then, that gorse in every particular lays itself out in life to fight its own battle, and to meet the peculiarities of its special situation by its own exertions. Born a trefoil-bearing plant, unarmed and undefended, it produces spines instead of leaves as soon as its growth exposes it to the attacks of enemies. It defends its buds alike from the attacks of cattle and the assaults of insects; it wraps them up from the cold in efficient overcoats. It cares for its young and lays up food in its beans on their account; it scatters its seed upon unoccupied spots where they may stand the best chance of picking up a living. All these acts are analogous to those produced by intelligence in animals; and though the intelligence is here no doubt unconscious and inherited, I think we are justified in applying the same word in both cases to operations whose effects are so closely similar. Gorse, in short, may fairly be called a clever and successful plant, just as the bee may be called a clever and successful insect, because it works out its own way through life with such conspicuous wisdom.
XII. A FOREIGN INVASION OF ENGLAND
OUR worst enemies are not always the most apparent ones. It is easy enough to build forts for the protection of our towns and harbours against French or Germans, but it is very difficult to devise means of defence against such insidious foreign invaders as the influenza germ or the Colorado beetle. France lost much by the war with Germany, but she probably lost more by the silent onslaught of the tiny phylloxera, which attacked her vineyards — attacked them, literally, root and branch, and paralysed for several years one of her richest industries. Yet invasions like these, being less obvious to the eye than the landing of a boat-load of French or German marines on some bare rock in the Pacific claimed by Britain, attract far less attention than aggressions on the Niger or advances in Central Africa. The smallness of the foe makes us overlook its real strength — it has the force of numbers. We forget that while we can exterminate hostile human bands with Armstrongs and torpedo-boats, the resources of civilisation are still all but powerless against the potato blight, the vine disease, and the destroying microbe.
The enemies of our corn crops in particular are many and various. There is the wheat-beetle, for example, which ravages the wheat-fields in two ways at once, the grub devouring the growing young leaves, while the perfect winged insect eats up at leisure the grain as it ripens. There are the various cockchafers, which vie with one another in their cruel depredations on the standing corn. There are the skip-jacks and wire-worms, and other queerly named beasties, which attack the roots of the plant underground. There is the corn saw-fly, whose larva feeds on the stalk of rye and wheat, till it finally cuts off the whole haulm altogether close to the soil at the bottom. There are the midges which lay their eggs in the swelling ear, where the maggots develop and prevent the proper growth of the impregnated grain. There is the gout-fly, which causes a gouty swelling at the joints, and the corn-moth, which devours the stored wheat in the granary. There are the red-maggot, and the grain-aphis, and the thrips, and the daddy-longlegs, all of which in various ways prove themselves serious enemies of the agricultural interest. And there are dozens more known only to men of science by dry Latin names, and duly chronicled by the farmer’s friend, Miss Ormerod, in many learned and exhaustive monographs.
But as if these were not enough for our “depressed” neighbours, the agriculturists, the last ten years or so have seen England invaded by a foreign foe, either from Germany or America — a foe whose life-history has been made a special subject of study by my collaborator, Mr. Enock, and whose strange story I shall detail (largely from his materials) with no unnecessary scientific verbiage in this present chapter.
The new invader is called the Hessian fly; and he made his first appearance in Britain, or at least first attracted official entomological attention in this country, in 1886. If he was here earlier, he skulked incognito. For more than a century, however, he had already been a great scourge in America, where he first acquired the name of Hessian fly during the revolutionary war, through the popular belief that he had been imported from Europe into Pennsylvania by the Hessian troops employed as mercenaries by George III. in his fruitless struggle against the revolted colonies. The Hessians were the bêtes noires of the patriotic Americans; and the farmers, finding their crops devastated by a pest till then unknown, came at once to the conclusion that their enemy, King George, had sent the two plagues, human and entomological, over sea together. They regarded the question much in the same spirit as that of the loyal poet in the “Rejected Addresses,” when he asks about Napoleon, “Who fills the butchers’ shops with large blue flies?” The Briton set down every natural misfortune to “the Corsican ogre”; the American set down all evils that befell him to the Rhenish mercenaries.
Ever since that day, much controversy has raged in America and Germany as to the original home of the destructive creature. One school of disputants hotly maintains that the Hessian fly, which now abounds in parts of France, Austria, and Russia, is a native of the Old World, and that its first home coincided with that of our primitive cereals, Southern Europe and Western Asia. Another school, anxious to make out the enemy an American citizen, fights hard for its being an aboriginal inhabitant of the United States. Thus much, at least, is certain, that at the present day the “fly” is found in both hemispheres in too great abundance, and that in America in particular in certain disastrous years it has almost ruined the entire wheat crop. I have seen whole fields upon fields there simply pillaged by its ravages. The loss produced by this insignificant little creature, indeed, has in some seasons been measured by millions of pounds sterling.
If you go out into a barley-field in England where the Hessian fly has effected his entrance, you will probably find a large number of plants of barley, like that delineated in No. 1, with the stem bent down sharply toward the ground at the second joint. At first sight you might imagine these stalks were merely broken by the wind or fallen by their own weight; but if you examine them closely in the neighbourhood of the bend, which occurs with singular unanimity in all the affected plants at about the same point, you will find inside the sheath of the blade, where it encircles the stem, a curious little body which the farmers with rough eloquence have agreed to describe as a “flax-seed.” If you watch the development of the “flax-seed” again, you will find that it is not a seed at all, but the pupa-case (or rather the grub-shell) of a small winged insect; and it is the life-history of this insect, the Hessian fly, that I now propose to sketch for you in brief outline.
No. 2 shows the mother fly herself, very much enlarged, for in nature she is but a small black gnat, belonging to the same group as our old friend (and foe) the mosquito. You will observe that she is a fairy-like creature, for all her wickedness: she has two delicately fringed wings (with “poisers” behind them), a pair of long antennæ with beaded joints, six spindle legs, and a very full and swollen body. She needs that swollen body, for she
is a mighty egg-layer. She flies about on the stubbles in September, and lays her eggs on the self-sown barley plants and on the aftergrowth of the cut crops; as well as in spring (a second brood) on the new sprouting barley. One industrious female which Mr. Enock watched when so employed laid no less than 158 eggs on six distinct plants; while another laid eighty on a single leaf. He has noted in detail many cases in the same way, and all show an astonishingly high level of maturity. The eggs are extremely minute, and are pale orange in colour, with reddish dots. Most of them are deposited on the leaf itself, or on the sheath or tube which forms its lower portion.
And now see how clever this dainty little creature is! She lays her eggs with the head end downward; and as soon as the tiny grub hatches, which it does about the fourth day, it emerges from the shell, and walks straight down towards the stem, at the point where the protecting leaf-sheath is wrapped closely round it. The worm forces itself in between the stem and the sheath, and after walking steadily for four hours, at the end of which time it has covered a record space of nearly three inches, it arrives at the joint, where the sheath begins, and so finds its way blocked by the partition wall; it can get no further. Here then the young grub stops, as you see in No. 3, wedged tightly in between the leaf-sheath and the stem, and with its head pointing downward. Being a hungry, and therefore an industrious creature, it at once sets to work to eat the barley-plant. This it does by fixing its sucker-like mouth on the soft, sweet, and juicy portion of the stem just above the joint — that same soft, sweet, and juicy portion which children love to pull out and suck, and from which the grub, too, sucks the life-juice of the barley-plant. Naturally, however, you can’t suck a plant’s life-blood without injuring its growth; so, after a very short time, the enfeebled stem begins to bend, as you see in No. 3, a little distance above the point where the grub is devouring it. It has been undermined, and its vitality sapped, so it gives way at once near the source of the injury.
How much damage this action does to the crop you can best understand by a glance at the two next contrasted illustrations. No. 4 represents “seven well-favoured ears” of barley, unaffected by Hessian fly, and with the grains richly filled out as the farmer desires them; No. 5, on the contrary, shows you “seven lean ears,” attacked by the fly, and bent and ruined in various degrees by the indirect action of the silently gnawing larva. Look on this picture and on that, and you will then appreciate the British farmer’s horror of his insignificant opponent. You will observe, by the way, that I speak throughout of barley, not of wheat. This is because in England, where these sketches are studied, the time of wheat-sowing is such that the wheat has so far escaped the pest; the female flies are all dead before the crop is sprouted: whereas in America the “fall wheat” comes up at the exact moment when the female Hessian fly is abroad and scouring the fields in search of plants on which to lay the eggs of her future generations. In England, therefore, it is barley alone which is largely attacked; and since barley is mainly used for malting, to make beer or whisky, the teetotaler may perhaps reflect with complacency that the fly is merely playing the game of the United Kingdom Temperance Alliance. His joy, however, is fallacious, for, on the other hand, if we don’t raise enough barley at home to brew our ale, we don’t on that account refrain from malt liquors: we buy it from elsewhere; so that, in the eyes of the impartial political economist at least, the Hessian fly in Britain must be regarded as an unmitigated national misfortune.
The grub eats and eats, in his safe cradle between the sheath and the stem, till he is ready to pass into the adult condition. But he does this by various and complicated stages, all of which I do not propose to set forth in full with the tedious minuteness of a scientific treatise, lest I weary that fastidious and somewhat lazy person, the “general reader.” It must suffice here to say, in brief, that the larva is at first soft and free, but that before becoming a true pupa or chrysalis he passes through an intermediate encased or “flax-seed” stage, in which he performs some curious evolutions. The young larva when he starts in life is whitish or yellowish; in the “flax-seed” stage he becomes a rich chestnut brown, and seems externally quiescent. But the fact is, he arrives at full growth in the white form, and then leaves off feeding; his skin now hardens and darkens, and he looks from outside very much like a pupa. Indeed, his outer covering is now a sort of solid pupa-case, in shape just the same as the original grub, but more sombre in colour. No. 6 shows you the portrait of the grub in this curious intermediate condition. If you compare it with No. 3, you will see that the outer skin still preserves the original shape of the fat young larva; but that the enclosed grub himself, here shown as if the case were transparent, has shrunk away from his own old skin, just as a ripe nut shrinks away from its shell, to borrow Mr. Enock’s admirable phrase for describing the process. And this strange shrinkage is connected with a very curious fact in the eventful life-history of the Hessian fly; it tells us of a problem which the grub has to face, and for which it has devised a most unexpected solution.
You remember that the young maggot had necessarily to work its way head downward along the stalk, in order to fix itself in the only place where it can find the soft food needful for it, between the sheath and the stem, where the tissue is tenderest. But when it emerges later on in the open air as a fly, it has to walk back again to the outer world above the joint; and this it could not do if it had still to go head downward. Yet there seems no room for it to turn in. Somehow or other, in that restricted space, it must reverse its position; it must get itself head upward. How is it to do so? This difficulty early struck Mr. Enock in his examination of the creature’s life; and with characteristic patience he determined to investigate it. His researches not only answered the question itself, but also discovered a meaning and purpose in a certain organ of the adult grub, the nature of which had heretofore been a standing puzzle to that section of society which interests itself prominently in the Hessian fly question. The larva in its “flax-seed” stage develops an odd and very hard organ, known as “the anchor-process,” near the head; and this “anchor-process,” as Mr. Enock has shown, is used by the grub to turn it round completely within its hardened pupa-case. (The last phrase, I will admit, is not quite scientifically correct, but I do not wish to complicate the subject by introducing a multiplicity of technical terms unknown to my readers.) In No. 6 you can see the adult grub in the very act of thus turning round, head to tail, within his outer skin, so that he may be able to emerge as a full-grown fly, head upward. A tiger is nothing to it, though a tiger moves within his own integuments more freely than most of us. You will note that during the feeding stage the grub’s mouth and under side were pressed against the stem; when he has performed this curious somersault on his own axis, so to speak, the head is uppermost, but the mouth and under side of the body are turned outward towards the sheath, not inward towards the stem and hollow centre of the barley-plant. He wants now to bite his way out, not to suck at the stalk for its nutritive juices.
I need hardly add that it takes some watching to detect such invisible movements inside a hard dark case; and only by the closest and most unwearying attention was Mr. Enock enabled to discover the true use and meaning of the so-called “anchor-process.” It is really not an anchor, but a sort of hooked foot or lever, by whose aid the apparently dormant grub turns himself bodily over within his own hardened skin, now become too large for his shrunken body.
Discoveries like these are hard to make; yet they bring little return in money or glory. But it is only by such patient and careful investigation that a way can be discovered to get rid of pests which cost civilisation many hundreds of thousands, nay, many millions, annually.
The grub in the turning stage is thus by no means what he looks — a dormant creature; on the contrary, he is a gymnast of no small skill and activity. The muscular contortions by which he seeks to free himself of discomfort when disturbed by man show that he possesses great power of contraction, and that he can exercise a considerable force of le
verage.
After the grub has succeeded in putting itself in position for assuming the winged stage, and emerging from its home head upward, it begins next to grow into a true pupa, or chrysalis. It is in the pupa, of course, that all winged insects acquire their wings and become definitely male or female, and this stage is, therefore, one of the most important. As soon as the grub begins to reach it, he swells once more and grows quite tight inside his larval skin, which is stretched so much that it seems to be bursting. At last, as he wriggles and twists within it, the skin does burst, first over the mouth and head, and then over the central joints of the body. Again the insect twists and wriggles inside this half-broken skin, and again he pushes it backward toward his tail, till at last he has sloughed it all off entirely, and it remains a shrivelled relic — an empty case — in the spot where he has hitherto lived and breathed and had his being. He is now a true pupa, white at first, but gradually growing a delicate pink, and then rosy.
Just at first, however, the pupa looks almost as formless as the grub it replaces, revealing no limbs or distinct segments. But little by little, feet and legs and eyes and wings begin to be visible through the semi-transparent shell of the chrysalis. He is changing slowly into a winged insect, and you can watch the change through the delicate horny coverings.
Stranger still, the Hessian fly at this stage is not torpid and quiescent like most ordinary insects. The pupa, as in many of this family, is locomotive. It has legs and feet, and it can wriggle its way up, as you see in No. 7, where the lower object is the empty larval skin, now deserted by its inmate, while the upper one is the pupa, emerging from the sheath, and making its first experiences of the wide, wide world outside its native leaf-bound hollow. It is ready now to come forth from the pupa stage, and to fly forth in the open air in search of a mate with whom to carry on the serious business of replenishing the fields with new generations of similar larvæ.
