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VI: THE PINE PROCESSIONARY:
THE STINGING POWER
THE Pine Processionary has three costumes: that of infancy, a scanty, ragged fleece, a mixture of black and white; that of middle age, the richest of the three, when the segments deck themselves on their dorsal surface with golden tufts and a mosaic of bare patches, scarlet in colour; and that of maturity, when the rings are cleft by slits which one by one open and close their thick lips, champing and grinding their bristling russet beards and chewing them into little pellets, which are thrown out on the creature's sides when the bottom of the pocket swells up like a tumour.
When wearing this last costume, the caterpillar is very disagreeable to handle, or even to observe at close quarters. I happened, quite unexpectedly, to learn this more thoroughly than I wished.
After unsuspectingly passing a whole
morning with my insects, stooping over them, magnifying-glass in hand, to examine the working of their slits, I found my forehead and eyelids suffering with redness for twenty-four hours and afflicted with an itching even more painful and persistent than that produced by sting of a nettle. On seeing me come to dinner in this sad plight, with my reddened and swollen and my face recognizable, the family anxiously enquired what had happened to me and were not reassured until I told them of my mishap.
I unhesitatingly attribute my painful experience to the red hairs ground to powder and collected into flakes. My breath sought them out in the open pockets and carried them my face, which was very near. The unthinking intervention of my hands, which now and again sought to ease the discomfort, merely aggravated the ill by spreading the irritating dust.
No, the search for truth on the back of the Processionary is not all sunshine. It was only after a night's rest that I found myself pretty well recovered, the incident having no other ill effects. Let us continue, however.
It is well to substitute premeditated experiments for chance facts.
The little pockets of which the dorsal slits form the entrance are encumbered, as I have said, with hairy refuse, either scattered or gathered into flakes. With the point of a paint-brush I collect, when they gape open, a little of their contents and rub it on my wrist or on the inside of my fore-arm.
I have not long to wait for the result. Soon the skin turns red and is covered with pale lenticular swellings, similar to those produced by a nettle-sting. Without being very sharp, the pain was extremely unpleasant. By the following day, itching, redness and lenticular swellings had all disappeared. This is the usual sequence of events; but let me not omit to say that the experiment does not always succeed. The efficacy of the fluffy dust appears subject to great variations.
There have been occasions when I have rubbed myself with the whole caterpillar, or with his cast skin, or with the broken hairs gathered on a paint-brush, without producing any unpleasant results. The irritant dust seems to vary in quality according to certain
circumstances which I have not been able to discover.
From my various tests it is evident that the discomfort is caused by the delicate hairs which the lips of the dorsal mouths, gaping and closing again, never cease grinding, to the detriment of their beards and moustaches. The edges of these slits, as their bristles rub off, furnish the stinging dust.
Having established this fact, let us proceed to more serious experiments. In the middle of March, when the Processionaries for the most part have migrated underground, I decide to open a few nests, as I wish to collect their last inhabitants for the purpose of my investigations. Without taking any precautions, my fingers tug at the silken dwelling, which is made of solid stuff; they tear it into shreds, search it through and through, turn it inside out and back again.
Once more and this time in a more serious fashion I am the victim of my unthinking enthusiasm. Hardly is the operation completed when the tips of my fingers begin to hurt in good earnest, especially in the more delicate part protected by the edge of the nail. The feeling is like the sharp pain of a sore
that is beginning to fester. All the rest of the day and all through the night, the pain persists, troublesome enough to rob me of my sleep. It does not quiet down until the following day, after twenty-four hours of petty torment.
How did this new misadventure befall me? I had not handled the caterpillars: indeed, there were very few of them in the nest at the time. I had come upon no shed skins, for the moults do not take place inside the silken purse. When the moment has come to doff the second costume, that of the red mosaic, the caterpillars cluster outside, on the dome of their dwelling, and there leave in a single heap their old clothes entangled with bits of silk. What is left to explain the unpleasant consequences to which the handling of the nest exposes us?
The broken red bristles are left, the fallen hairs forming a dust that is invisible without a very careful examination. For a long time the Processionaries crawl and swarm about the nest; they pass to and fro, penetrating the thickness of the wall when they go to the pastures and when they return to their dormitory. Whether motionless or
on the move, they are constantly opening and closing their apparatus of information, the dorsal mouths. At the moment of closing, the lips of these slits, rolling on each other like the cylinders of a flattening-mill, catch hold of the fluff near them, tear it out and break it into fragments which the bottom of the pocket, presently reascending, shoots outside.
Thus myriads of irritant particles are disseminated and subtly introduced into every part of the nest. The shirt of Nessus burnt the veins of whoso wore it; the silk of the Processionary, another poisoned fabric, sets on fire the fingers that handle it.
The loathsome hairs long retain their virulence. I was once sorting out some handfuls of cocoons, many of which were diseased. As the hardness of the contents was usually an indication that something was wrong, I tore open the doubtful cocoons with my fingers, in order to save the non-contaminated chrysalids. My sorting was rewarded with the same kind of pain, especially under the edges of the nails, as I had already suffered when tearing the nests.
The cause of the irritation on this occasion was sometimes the dry skin discarded by the
Processionary on becoming a chrysalis and sometimes the shrivelled caterpillar turned into a sort of chalky cylinder through the invasion of the malignant fungus. Six months later, these wretched cocoons were still capable of producing redness and irritation.
Examined under the microscope, the russet hairs, the cause of the itching, are stiff rods, very sharp at either end and armed with barbs along the upper half. Their structure has absolutely nothing in common with nettle-hairs, those tapering phials whose hard point snaps off, pouring an irritant fluid into the tiny wound.
The plant from whose Latin name, Urtica, we derive the word urtication borrowed the design of its weapon from the fangs of the venomous serpents; it obtains its effect, not by the wound, but by the poison introduced into the wound. The Processionary employs a different method. The hairs, which have naught resembling the ampullary reservoir of the nettle-hairs, must be poisoned on the surface, like the assegais of the Kafirs and Zulus.
Do they really penetrate the epidermis? Are they like the savage's javelin, which can
not be extracted once it has gone in? With their barbs, do they enter all the more deeply because of the quivering of the outraged flesh? There is no ground for believing anything of the kind. In vain do I scrutinize the injured spot through the magnifying-glass; I can see no sign of the implanted dart. Neither could Réaumur, when an encounter with the Oak Processionary set him scratching himself. He had his suspicions, but could state nothing definitely.
No; despite their sharp points and their barbs, which make them, under the microscope, such formidable spears, the Processionary's russet hairs are not darts designed to imbed themselves in the skin and to provoke irritation by pricking.
Many caterpillars, all most inoffensive, have a coat of bristles which, under the microscope, resolve themselves into barbed javelins, quite harmless in spite of their threatening aspect. Let me mention a couple of these peaceable halberdiers.
Early in spring, we see, crossing the paths, a briskly-moving caterpillar who inspires repugnance by his ferocious hairiness, which ripples like ripe corn. The ancient naturalists, with
their artless and picturesque nomenclature, called him the Hedgehog. The term is worthy of the creature, which, in the moment of danger, rolls itself up like a Hedgehog, presenting its spiny armour on all sides to the enemy. On its back is a dense mixture of black hairs and hairs of ashen-gray; while on the sides and fore-part of the body is a stiff mane of bright russet. Black, grey or russet, all this fierce-looking coat is heavily barbed.
One hesitates to touch this horror with the finger-tips. Still, encouraged by my example, seven-year-old Paul, with his tender child's skin, gathers handfuls of the repulsive insect with no more apprehension than if he were picking a bunch of violets. He fills his boxes with it; he rears it on elm-leaves and handles it daily, for he knows that from this frightful creature he will one day obtain a superb Moth (Chelonia caja, LINN.), clad in scarlet velvet, with the lower wings red and the upper white, sprinkled with brown spots.
What resulted from the child's familiarity with the shaggy creature? Not even a trace of itching on his delicate skin. I do not speak of mine, which is tanned by the years.
In the osier-beds of our local stream, the
rushing Aygues, a thorny shrub abounds which, at the advent of autumn, is covered with an infinity of very sour red berries. Its crabbed boughs, which bear but little verdure, are hidden under their clusters of vermilion balls. It is the sallow thorn or sea buckthorn (Hippophaë rhamnoides).
In April, a very hairy but rather pretty caterpillar lives at the expense of this shrub's budding leaves. He has on his back five dense tufts of hair, set side by side and arranged like the bristles of a brush, tufts deep-black in the centre and white at the edges. He waves two divergent plumes in front of him and sports a third on his crupper, like a feathery tail. These three are black hair-pencils of extreme delicacy.
His greyish Moth, flattened motionless on the bark, stretches his long fore-legs, one against the other, in front of him. You would take them, at a first glance, for antennae of exaggerated proportions. This pose of the extended limbs has won the insect the scientific label of Orgyia, arm's length; and also the vulgar and more expressive denomination of Patte étendue, or outstretched paw.
Little Paul has not failed, with my aid, to
rear the pretty bearer of the tufts and brushes. How many times, with his sensitive finger, has he not stroked the creature's furry costume? He found it softer than velvet. And yet, enlarged under the microscope, the caterpillar's hairs are horrible barbed spears, no less menacing than those of the Processionary. The resemblance goes no farther: handled without precautions, the tufted caterpillar does not provoke even a simple rash. Nothing could be more harmless than his coat.
It is evident, then, that the cause of the irritation lies elsewhere than in the barbs. If the barbed bristles were enough to poison the fingers, most hairy caterpillars would be dangerous, for nearly all have spiny bristles. We find, on the contrary, that virulence is bestowed upon a very small number, which are not distinguished from the rest by any special structure of the hair.
That the barbs have a part to play, that of fixing the irritant atom upon the epidermis, of keeping it anchored in its place, is, after all, possible; but the shooting pains cannot by any means be caused by the mere prick of so delicate a harpoon.
Much less slender, the hairs clustered
into pads on the prickly pears are ferociously barbed. Woe to the fingers that handle this kind of velvet too confidently! At the least touch they are pierced with harpoons whose extraction involves a severe tax upon our patience. Other inconvenience there is little or none, for the action of the barb is in this case purely mechanical. Supposing--a very doubtful thing--that the Processionary's hairs could penetrate our skin, they would act likewise, only with less effect, if they had merely their sharp points and their barbs. What then do they possess in addition?
They must have, not inside them, like the hairs of the nettle, but outside, on the surface, an irritant agent; they must be coated with a poisonous mixture, which makes them act by simple contact.
Let us remove this virus, by means of a solvent; and the Processionary's darts, reduced to their insignificant mechanical action, will be harmless. The solvent, on the other hand, rid of all hairs by filtration, will be charged with the irritant element, which we shall be able to test without the agency of the hairs. Isolated and concentrated, the stinging element, far from losing by this
treatment, ought to gain in virulence. So reflection tells us.
The solvents tried are confined to three: water, spirits of wine and sulphuric ether. I employ the latter by preference, although the other two, spirits of wine especially, have yielded satisfactory results. To simplify the experiment, instead of submitting to the action of the solvent the entire caterpillar, who would complicate the extract with his fats and his nutritive juices, I prefer to employ the cast skin alone.
I therefore collect, on the one hand, the heap of dry skins which the moult of the second phase has left on the dome of the silken dwelling and, on the other hand, the skins which the caterpillars have rejected in their cocoons before becoming chrysalids; and I leave the two lots to infuse, separately, in sulphuric ether for twenty-four hours. The infusion is colourless. The liquid, carefully filtered, is exposed to spontaneous evaporation; and the skins are rinsed with ether in the filter, several times over.
There are now two tests to be made: one with the skins and one with the product of maceration. The first is as conclusive as can
be. Hairy as in the normal state and perfectly dried, the skins of both lots, drained by the ether, produce not the slightest effect, although I rub myself with them, without the least caution, at the juncture of the fingers, a spot very sensitive to stinging.
The hairs are the same as before the action of the solvent: they have lost none of their barbs, of their javelin-points; and yet they are ineffectual. They produce no pain or inconvenience whatever. Deprived of their toxic smearing, these thousands of darts become so much harmless velvet. The Hedgehog Caterpillar and the Brush Caterpillar are not more inoffensive.
The second test is more positive and so conclusive in its painful effects that one hardly likes to try it a second time. When the ethereal infusion is reduced by spontaneous evaporation to a few drops, I soak in it a slip of blotting-paper folded in four, so as to form a square measuring something over an inch. Too unsuspecting of my product, I do things on a lavish scale, both as regards the superficial area of my poor epidermis and the quantity of the virus. To any one who might wish to renew the investigation I should
recommend a less generous dose. Lastly, the square of paper, that novel sort of mustard-plaster, is applied to the under surface of the fore-arm. A thin waterproof sheeting covers it, to prevent it from drying too rapidly; and a bandage holds it in place.
For the space of ten hours, I feel nothing; then I experience an increasing itch and a burning sensation acute enough to keep me awake for the greater part of the night. Next day, after twenty-four hours of contact, the poultice is removed. A red mark, slightly swollen and very clearly outlined, occupies the square which the poisoned paper covered.
The skin feels sore, as though it had been cauterized, and looks as rough as shagreen. From each of its tiny pustules trickles a drop of serous fluid, which hardens into a substance similar in colour to gum-arabic. This oozing continues for a couple of days and more. Then the inflamation abates; the pain, hitherto very trying, quiets down; the skin dries and comes off in little flakes. All is over, except the red mark, which remains for a long time, so tenacious in its effects is this extract of Processionary. Three weeks after the
experiment, the little square on the fore-arm subjected to the poison is still discoloured.
For thus branding one's self, does one at least obtain some small reward? Yes. A little truth is the balm spread upon the wound; and indeed truth is a sovereign balm. It will come presently to solace us for much greater sufferings.
For the moment, this painful experiment shows us that the irritation has not as its primary cause the hairiness of the Processionary. Here is no hair, no barb, no dart. All of that has been retained by the filter. We have nothing now but a poisonous agent extracted by the solvent, the ether. This irritant element recalls, to a certain extent, that of cantharides, which acts by simple contact. My square of poisoned blotting-paper was a sort of plaster, which, instead of raising the epidermis in great blisters, makes it bristle with tiny pustules.
The part played by the barbed hairs, those atoms which the least movement of the air disseminates in all directions, is confined to conveying to our face and hands the irritant substance in which they are impregnated. Their barbs hold them in place and thus
permit the virus to act. It is even probable that, by means of slight scratches which would otherwise pass unnoticed, they assist the action of the stinging fluid.
Shortly after handling the Processionaries, a delicate epidermis becomes tumefied, red and painful. Without being immediate, the action of the caterpillar is prompt. The extract made with ether, on the other hand, causes pain and rubefaction only after a longish interval. What does it need to produce more rapid ulceration? To all appearances, the action of the hairs.
The direct stinging caused by the caterpillar is nothing like so serious as that produced by the ethereal extract concentrated in a few drops. Never before, in my most painful misadventures, whether with the silken purses or their inhabitants, have I seen my skin covered with serous pustules and peeling off in flakes. This time it is a veritable sore, anything but pleasing to the eye.
The aggravation is easily explained. I soaked in the ether some fifty discarded skins. The few drops which remained after the evaporation and which were absorbed by the square of blotting-paper represented,
therefore, the virulence of a single insect fifty times increased. My little blistering-plaster was equivalent to the contact of fifty caterpillars at the same spot. There is no doubt that, if we left them to steep in considerable numbers, we should obtain extracts of really formidable strength. It is quite possible that medical science will one day make good use of this powerful counter-irritant, which is utterly different from cantharides.
Whether voluntary victims of our curiosity, which, while affording no other satisfaction than that of knowledge, exposes us to an intolerable itch, or sufferers through an accident, what can we do to give a little relief to the irritation caused by the Processionary? It is good to know the origin of the evil, but it would be better to apply a remedy.
One day, with both hands sore from the prolonged examination of a nest, I try without success lotions of alcohol, glycerine, oil and soapsuds. Nothing does any good. I then remember a palliative employed by Réaumur against the sting of the Oak Processionary. Without telling us how he came to know of the strange specific, the master rubbed himself with parsley and felt a good deal the better
for it. He adds that any other leaf would probably assuage the irritation in the same way.
This is a fitting occasion for reopening the subject. Here, in a corner of the garden, is parsley, green and abundant as one could wish. What other plant can we compare with it? I choose the purslain, the spontaneous guest of my vegetable-beds. Mucilaginous and fleshy as it is, it readily crushes, yielding an emollient liniment. I rub one hand with parsley and the other with purslain, pressing hard enough to reduce the leaves to a paste. The result deserves attention.
With the parsley, the burning is a little less acute, it is true, but, though relieved, it persists for a long time yet and continues troublesome. With the purslain, the petty torture ceases almost at once and so completely that I no longer notice it. My nostrum possesses incontestable virtues. I recommend it quietly, without blatant advertisement, to any one who may be persecuted by the Processionary. Foresters, in their war upon caterpillars' nests, should find great relief from it.
I have also obtained good results with the leaves of the tomato and the lettuce; and,
without pursuing this botanical survey further, I remain convinced, with Reaumur, that any tender juicy foliage would possess a certain efficacy.
As for the mode of action of this specific, I admit that I do not understand it, any more than I can perceive the mode of action of the caterpillar's virus. Molière's medical student explained the soporific properties of opium by saying:
"Quia est in eo virtus dormitava cujus est proprietas sensus assoupire."
Let us say likewise: the crushed herb calms the burning itch because it possesses a calming virtue whose property is to assuage itching.
The quip is a good deal more philosophical than it looks. What do we know of our remedies or of anything? We perceive effects, but we cannot get back to their causes.
In my village and for some distance around it, there is a popular belief that to relieve the pain of a Wasp's or Bee's sting all that we need do is to rub the part stung with three sorts of herbs. Take, they say, three kinds of herbs, the first that come to hand, make them
into a bunch and rub hard. The prescription, by all accounts, is infallible.
I thought at first that this was one of those therapeutic absurdities which have their birth in rustic imaginations. After making a trial, I admit that what sounds like a nonsensical remedy sometimes has something genuine about it. Friction with three kinds of herbs does actually deaden the sting of the Wasp or Bee.
I hasten to add that the same success is achieved with a single herb; and so the result agrees with what the parsley and purslain have taught us in respect of the irritation caused by the Processionary.
Why three herbs when one is enough? Three is the preeminently lucky number; it smacks of witchcraft, which is far from detracting from the virtues of the unguent. All rustic medicine has a touch of magic about it; and there is merit in doing things by threes.
Perhaps the specific of the three herbs may even date back to the materia medica of antiquity. Dioscorides recommends triphyllon: it is, he states, good for the bite of venomous serpents. To determine this celebrated three-leaved plant exactly would not be easy. Is it
a common clover? The psoralea, with its pitchy odour? The menyanthes, or uck-bean, that inmate of the chilly peat-bogs? The oxalis, the wood-sorrel of the country-side? We cannot tell for certain. The botany of those days was innocent of the descriptive conscientiousness of ours. The plant which acted as a poison-antidote grouped its leaves by threes. That is its essential characteristic.
Again the cabalistic number, essential to medical virtues as conceived by the first healers. The peasant, a tenacious conservative, has preserved the ancient remedy, but, by a happy inspiration, has changed the three original leaves into three different herbs; he has elaborated the triphyllon into the threefold foliage which he crushes on the Bee's sting. I seem to perceive a certain relation between these artless ways and the crushing of parsley as described by Réaumur.