THE TEREBINTH-LOUSE: THE MIGRATION

By the end of September the horn-shaped gall is full, almost as full as a keg of anchovies. There would not be room for them all were the Lice to form only one layer, side by side, with their suckers implanted. They lie in strata according to the length of their probe: uppermost are the big Lice, in the second layer the medium-sized and between their legs the small ones, all of them motionless, with their trunks at work. Above those engaged in drinking is the shifting horde, seeking a place at the refreshment bar. Eddies occur in the crowd: those at the top dive down, those underneath return to the surface; and this continual ebb and flow gives each one time for a little tippling.

In this rough and tumble the white waxen finery turns to flour, which fills up the interstices and makes of the whole a swarming conglomerate in which the metamorphosis is [272]effected. Here, without a moment’s quiet, the moult takes place and not a leg is out of joint: here, when there is no free space, wide wings are unfurled and not a wing is torn. To achieve transfiguration without a hitch in such a tumult the insect must be peculiarly favoured by fortune.

The pot-bellied orange Lice are now handsome, black, slender midges, provided with four wings. Their secluded life is over; the time has come for soaring in the open air. But how will they get out? The internees are quite incapable of making a breach in the ramparts: they have no tools. Well, what the prisoners cannot accomplish the fortress itself will do. When the population is ripe the gall is ripe too, so closely does the calendar of the bush synchronize with that of the insect.

The hems raise their upper folds a little; the spindles open like so many purses, each lined with pink satin; the auricles part their thick gnarled lips. The doors open of themselves for the impatient inmates, by the mere action of the sap. In the other galls, the globular and horn-shaped ones, the mechanism does not work so easily; the unclosing is [273]a violent affair. More and more distended day by day, the globes burst their sides in star-shaped rents, while the horns split open at the top.

The exodus is worth close observation. I choose a few of the horn-shaped galls whose cracked tips announces the coming rupture. I expose them to the sun, in my study, facing a window, at a distance of a few paces from the closed casements. In the intervening space I set up a thick branch of leafy terebinth. I reckon upon this bait to attract the flying Lice, at least as a resting-spot. Next morning one of the horns opens, and by midday, in radiant sunlight, in calm, hot weather, the winged Lice are emerging.

They come forth in small companies, without hurrying. It is a quiet, gently-flowing stream. They are dusted over with a waxy flour, all that remains of the sometime powder-puffs. When barely on the threshold of the cranny, they spread their wings and are off, shedding a faint trail of dust from their shoulders, shaken by the vibrations of their wings. With an undulating flight they all make straight for the window, where the light is brighter than elsewhere. [274]They dash against the panes and slip down upon the cross-bars. There, bathed in the sunlight, without attempting to go further afield, they remain, collecting in a drift.

Although the rest of the room is thoroughly well lit in all directions, the flight of the departing Lice is always directed towards the window facing the sun. There are thousands upon thousands of them; and not one takes another path, veering ever so little to the right or left. You feel a certain surprise at the invariable route pursued by these atoms which, when released, in a space well lit on every side, all, from the first to the last, rush towards the delights of a ray of sunshine. A handful of shot dropped from a height does not return to earth with greater certainty. The leaden pellets are attracted by gravity, to which all dead matter is subject, while the specks of living matter obey the light.

My window-panes check them. In the absence of this obstacle, where would they go? Certainly not to the terebinth-trees near by. I have definite proof of this here, before my eyes. As a resting-place I have [275]set up a bough of the cherished bush. None of the newly emerged insects takes notice of it; none of them pauses there. If on the way to the window one of them collides with the green thicket and falls upon a leaf, it quickly picks itself up again and makes off in a hurry to join the others in the sunlit window. Freed henceforth from the demands of the stomach, they are no longer interested in the terebinth; they all avoid it.

The exodus lasts a couple of days. When the last loiterers have gone, let us open the gall entirely. The population has been rigorously sorted. At first it was a mixture of wingless red and winged black Lice. The latter have all left their dwelling; the others are still there. Those faithful to their home are small as before, squat, wrinkled and vermilion. Some of them bear the dorsal wallet, the maternal pouch. In them I recognize the legion of the mothers, now left alone in the house. For some time yet they linger on languidly, the gall being open to wind and weather; those less exhausted continue to produce offspring; mere abortions without a future; the time is too short and [276]the house is falling into decay. At length they perish, with their belated young. The gall is a deserted ruin.

Let us return to the emigrants, checked in their flight by the window-panes. In shape, colour and size they are all alike; the swarm is a monotonous repetition of the same individual; there is not one detail, however minute, to denote any difference. Yet we should expect to find males and females here. The Plant-louse, until this moment in the humble larval stage, has just acquired the attributes of the perfect insect. The heavy, pot-bellied Louse has become a slender midge, glorified by four iridescent wings. In any other insect this would be an infallible token of the nuptial frolics.

Well, in the children of the galls, these wings, these adornments of maturity, belie their promises. There is no wedding and there can be none. Not a Louse in all the swarm is endowed with sex, and yet each has her brood, which she brings into the world by direct reproduction as her predecessors did.

With a slip of straw moistened with saliva I pick up a winged Louse at random. I [277]press its abdomen with a pin. My brutal obstetrics produces an immediate effect: the insect’s outraged flanks eject a string of five or six fœtuses; and the process is repeated without variation no matter what specimen we deliver.

Let us, for that matter, consult the natural procedure. A couple of hours elapse and my prisoners behind the window are in the throes of childbirth on the glass of the panes, the plaster of the embrasure, the wood of the cross-bars. Matters become so urgent that any place suits them.

The Louse in the act of parturition raises her two large wings, the upper pair, and gently moves the two small ones, the lower pair. The tip of the abdomen bends downwards, touches the supporting surface and the thing is done: a fœtus is implanted perpendicularly to the support, with its head uppermost. A little farther away, a second is deposited as promptly, followed by another and yet others. In one brief sitting the distribution is over. The average number of the litter is six.

The infant, we were saying, is fixed in an upright position, at right angles to the supporting [278]surface. This nicely-balanced attitude is necessary. The new-born Louse is, in fact, wrapped in a thin tunic of which it must first of all divest itself. In a minute or two this swaddling band splits and is thrust backwards. The legs release themselves, kicking freely in all directions, which they could not do were the tiny creature lying on the ground. By this means joints that are working for the first time gain strength and suppleness. After a few moments of these gymnastic exercises, the tiny insect drops on its feet and wanders forth into the wide world.

While it is struggling in an upright position, passers-by sometimes knock it over, without consideration for its tender age. Then the danger is great. Thrown from its sticky pedestal, the little insect often perishes, incapable of casting off its slough. There are a few threads of cobweb in the corner of the window. Some winged Lice have been caught in them. The garlands of hanging Lice give birth to their offspring all the same, but the young ones, falling on the sill of the embrasure, cannot manage to strip, because they are not in a standing position.[279]

Soon the cross-bars of the window are peopled with vermin, jogging along with great activity, promiscuously with the winged Lice. What a to-do on the borderland of the invisible! What are they seeking, these busy atoms? What do they want? My ignorance will be their undoing. In two or three days the winged Lice die. Their part is played. That of the children is beginning. For some time yet the latter wander about, but at last nothing stirs at the window; the legion of Lice is dead. Before sweeping them away with a camel’s-hair brush, let us give a brief description of them. The new-born insects are pale green and slender in shape. Their length is not far short of a millimetre.1 Nimble and standing fairly high on their legs, they trot about busily.

The globular galls burst and the hems, auricles and spindles begin to gape a little earlier than the horn-shaped galls, about the middle of September. The five gall-makers of the terebinth all have the same customs. After emerging from their open dwellings, all the adults, or winged black Lice, give [280]birth, within twenty-four hours, to a small number of young, some five or six, as do those of the horn-shaped galls.

The auricles yield a dumpy Louse, wider behind than before and of a dark olive colour. Her most remarkable feature is her sucker, which, folded underneath the insect, sticks out behind, recalling after a fashion a Grasshopper’s oviscapt. What can the puny creatures want with this mechanism? It is a sword, a sabre. Held erect, the implement would prevent any attempt at walking. To drive it into the food-plant, the insect apparently hoists itself on its legs, which correspond in length with the enormous probe. I should like to see this inordinate beak at work. My captives refuse what I give them: leaves and fresh galls. They lie huddled on the plug of cotton-wool which closes the tube. They have business to attend to. They want to get away; but to what?

Likewise squat of build, packed, not without a certain prettiness, into the shape of miniature Toads, the Lice from the globular galls are a pale yellowish brown, while those of the folded leaves are greenish black. [281]Neither the first nor the second have beaks of exaggerated length. That extraordinary rostrum, which sticks out behind, and, when at rest, resembles a caudal appendage, recurs in the young Lice from the spindle-shaped galls; but this time the little creature is oblong and its colour is pale green.

Let us cut short these dry details. It is enough if we recognize that these five fellow-guests of the terebinth are not of one race following different trades, but separate species. If the earlier generations, which all resemble one another, seemed to bear witness to a specific unity, the family of the winged Lice testifies to the contrary. These thickset insects and these slender ones; these bearers of the rostrum, sometimes of normal length and sometimes fantastically prolonged into the semblance of a caudal beak; these pale-green, olive-green, light-yellow insects are obviously independent forms.

A meticulous examination might find here preeminently all the characteristic features of the five categories; but the reader, repelled by prose descriptions, would soon turn the page. Let us pass on. Let us leave the insect laboratory, with its jars and test-tubes; [282]let us go out of doors to see how matters come to pass under natural conditions on the terebinth in the grounds.

The galls, frequently inspected during the hottest hours of the day, open before my eyes; the horns are splitting at the top, the globes are opening their sides, the others are parting their lips. The moment the fissure is wide enough the black emigrants appear, without haste, one by one, in absolute composure, despite the fierceness of the sun. The exodus was not accomplished with greater sobriety in the comparative darkness of my study. For a few seconds they linger in the breach; then, shedding a dusty trail from their floury backs, they spread their wings and are off. Their flight, favoured by the least breath of air, promptly carries them to a distance at which I soon lose sight of them.

As a rule the exodus is partial, being distributed over several days. When the whole swarm has disappeared there are still the wingless red Lice, the hump-backed pigmies, the progenitors of the big migrants. Some of them come to enjoy a little sunlight on the brink of the aperture. They soon go in again. Others follow them; perhaps they [283]too are attracted by the brilliant sunshine. Then we see none at all. The festival of the light is not for them. For a week or two longer they lead a hand-to-mouth existence in the ruined gall, but their end is not far off. The withered gall starves them and old age kills them where they stand.

So far there is nothing new: my laboratory experiments have already shown me what the terebinth in the garden tells me. The window-panes and test-tubes have even taught me more than the tree: they have enabled me to realize the part played by the winged Lice. In the liberty of the open air one fundamental detail of their story escapes me, for parturition takes place at a distance, I do not know where. The new-born Lice must be scattered everywhere, often at a considerable distance, as the emigrant’s flight informs me. Shall I then not find on the tree itself the little Lice with which my indoor observations have made me familiar? Yes: and in circumstances which are worth recording.

Let me recapitulate: to escape from their galls, strongly-built dungeons without any outlet, the Terebinth-Lice have no means of [284]breaking through. Though very clever at tickling vegetable tissues and making them swell into excrescences, they can do nothing with the walls of their prison. When it is time to go, however impatient they may be to get out, they must wait until the gall opens of itself, until the horn, in particular, splits into jagged segments at the top and the globe bursts open at the side. Until the fort is thus spontaneously dismantled, there is no possibility of escape.

Now it may happen that the winged population is ripe and ready to increase and multiply before there is a breach in the wall, either because the gall is not yet sufficiently distended, or because it has dried up before its time and is henceforth unable to open.

What do the captives do in the event of such a disaster? Precisely what they would do in the open air. Their business cannot be postponed. When the imperious hour has struck they bring forth their young, one on top of another, in such a crush that it is hardly possible to move. For good, or ill, the great task is accomplished.

In this tangle of wings a-flutter in the midst of a waxy powder, this skirmish of [285]legs seeking equilibrium on an ever-shifting support, many young Lice are trampled underfoot and injured, many are unable to strip and shrivel into grains of dust. The majority, none the less, so tenacious of life are they, contrive to escape in the swarming confusion.

Let us, in October, open a globular or horn-shaped gall which has dried up without bursting. We shall find it crammed with black Lice, all winged and all dead; a mass of procreators who have died after parturition. Beneath the heap of corpses, more especially against the walls of the dwelling, the lens, in amazement, discovers thousands of young ones. This is a new people: it is the future struggling amidst the cadaveric relics of the past; it is the progeny of the winged Lice, the family born in prison. Here and there, in the midst of this bustling youth, are vermilion-coloured specks, more awkward in their gait but as lively as the rest. These are the grandmothers of the colony, still doing fairly well and capable, I should say, of surviving the winter.

I have some hope of keeping them alive, they look so healthy. Perhaps their part is [286]not yet fully played. I set them aside, together with their galls, opened with a penknife. If left to the inclemencies of the weather in their ruined cells, they would die when the cold sets in; but may they not hold out if sheltered under glass? I almost think they will.

And indeed at the outset things do not go so badly. My little red insects continue to look in the best of health. Then, at the first frosts, they become motionless, though still fresh in appearance as though they meant to return to life in the spring. Appearances deceive; the motionless Lice never move again. Long before April the whole herd is dead. My care has slightly delayed the dissolution, without preventing the inevitable end. None the less I marvel at the tenacious vitality of the little red grandmothers. They live half the year, their daughters but a few days.

Released henceforward from the necessity of feeding themselves, the black emigrants, the winged Lice, leave their terebinth and need not search for another, as is proved by my bough, which, placed in the path of the emerging insects, does not even serve them [287]as a temporary resting-place. They seem equally heedless in selecting a spot for the establishment of their family. Before my window the young Lice are dropped at random, at any point to which the hazards of flight have led: on the window-panes, the plaster of the embrasure, the wood of the cross-bars or the threads of cobweb indifferently. There is nothing to show that the unfamiliar spot is regarded as inopportune. There is no sign of uneasiness, no attempt to fly off elsewhither, to a more propitious place. Soberly and serenely, the winged legion brings forth its young and goes its way.

In the open country things must happen no otherwise. The moment they are free, the emigrants shake off their waxen dust and flit away in this direction or in that, according to the prevailing breeze. A flying-machine has sprouted from their shoulders, a remarkable contrast to the clumsy paunch of their early days. Quick, for the sunlight, for flight, for the joys of the ballet in mid-air! Off they go, hovering as long as their feeble wings allow; then, wearied of merry-making in the sun, they alight on the first object that [288]offers, without henceforth renewing their flight as do my prisoners behind the closed window. Here, no matter what the nature of the site, parturition takes place. There is nothing left for them but to die.

With these urgent methods, disdainful of deliberate selection, the wastage among the emigrants’ tiny offspring must be great. On the bare soil, on stones, on dry bark, the little Lice undoubtedly perish. They need food quickly; and they are scarcely capable of wandering in quest of it themselves. Their sucker, sometimes of inordinate length, projecting beyond the tip of the abdomen like a caudal rapier, demands that the wearer shall erect it, shall drive it into some yielding source of sap. The insect must drink or die. In the test-tubes wherein I collect the young Lice born before my eyes, my captives die in less than a fortnight from want of food.

I try various kinds of green stuff. I have no success with any of them. But here, if direct observation fails me, logic comes to my assistance. There is no doubt that the tiny Lice, at the present moment the sole representatives of their race, must live [289]through the winter and serve as the origin of the population which will occupy the terebinth in the spring. These puny creatures cannot remain exposed to the severities of the winter. A shelter is indispensable, a shelter that will afford them both food and lodging. Where will they find it? Only one shelter is possible: it must be underground, beneath some sort of grass that will retain a little green in winter.

It is, in fact, to be presumed that the thick tufts of certain grasses will afford them shelter. This abiding-place, where the sucker will sink into the sweet root-fibres, and where the drip of rain or snow does not easily find access, is beloved by several Plant-lice. Those of the terebinth also may very well take up their winter-quarters there. As for what happens in these subterranean lairs, we are reduced to more or less probable conjectures.