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 the sting of a nettle. On seeing me
come down to dinner in this sad plight, with my eyes reddened and
swollen and my face unrecognizable, 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 to 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 further 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 cannot 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 inflammation 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 sovran 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 Réaumur, 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 τρίφυλλον: 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 τρίφυλλον 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.