THE PINE PROCESSIONARY: METEOROLOGY
In January a second moult occurs, leaving the caterpillar less fair to
the eye, while at the same time endowing him with some very peculiar
organs. When the moment has come to shed their skins, the
Processionaries cluster higgledy-piggledy on the dome of the nest and
there, if the weather be mild, remain motionless day and night. It
would seem as though the fact of their contact, of their mutual
discomfort, while thus heaped together, furnishes a resistance, a
fulcrum, which favours the process of excoriation.
After this second moult, the hairs on the middle of the back are of a
dull reddish colour, which is made paler still by the interposition of
numerous long white hairs. But this faded costume is accompanied by the
singular organs which attracted the attention of Réaumur, who was
greatly perplexed as to their function. In the place originally
occupied by the scarlet mosaic, eight segments of the caterpillar are
now cleft by a broad transversal gash, a sort of thick-lipped mouth,
which opens and gapes wide at the caterpillar’s will, or closes without
leaving a visible trace.
From each of these expanding mouths rises a tumour with a fine,
colourless skin, as though the creature were exposing its tender inside
and inflating it, for the appearance is almost that which would be
presented by the viscera protruding through skin incised by the
scalpel. Two large dark-brown dots occupy the front face of the
protuberance. At the back are two short, flat tufts of russet bristles,
which in the sunlight shine with a rich brilliancy. All around is a
radiating border of long white hairs, spread almost flat.
This protuberance is extremely sensitive. At the slightest irritation
it goes in again and disappears under the dark integument. In its place
opens an oval crater, a sort of huge stoma, which swiftly brings its
lips together, closes and entirely disappears. The long white hairs
that form a moustache and imperial around this mouth follow the
movements of the contracting lips. After first radiating from a centre
and lying flat, these hairs rise like levelled wheat which the wind has
caught from beneath and meet to form a transversal crest, perpendicular
to the creature’s back.
This hairy erection produces a sudden modification in the caterpillar’s
aspect. The red shiny bristles have disappeared, buried under the dark
skin; the white hairs, now standing on end, form a hirsute mane; an
ashy tinge has crept into the general colour of the costume.
When calm is restored, as soon happens, the slits open and yawn afresh;
the sensitive protuberances emerge, quick to disappear once more should
any cause for alarm occur. These alternate expansions and contractions
are rapidly repeated. I provoke them at will in various ways. A slight
puff of tobacco-smoke immediately causes the stomata to yawn and the
protuberances to emerge. One would think that the insect was putting
itself on its guard and displaying some special apparatus of
information. Before long the protuberances go in again. A second puff
of smoke brings them out once more. But, if the smoke is too abundant,
too acrid, the caterpillar wriggles and writhes without opening his
apparatus.
Or else I touch one or other of these uncovered protuberances, very
delicately, with a bit of straw. The pimple affected immediately
contracts, draws into itself, like the horns of the Snail, and is
replaced by a gaping mouth, which in its turn closes. Usually, but not
always, the segment excited by the contact of my straw is imitated by
the others, both front and back, which close their apparatus one by
one.
When undisturbed and in repose, the caterpillar generally has his
dorsal slits expanded; in moving, he sometimes opens and sometimes
closes them. In either case expansion and contraction are frequently
repeated. Constantly coming together and retreating under the skin, the
lips of the mouth-like opening therefore end by losing their brittle
moustaches of russet hairs, which break off. In this way a sort of dust
collects at the bottom of the crater, a dust formed of broken hairs,
which, thanks to their barbs, soon collect into little tufts. When the
slit expands rather suddenly, the central projection shoots out on the
insect’s sides its load of hairy remnants, which the least breath blows
into a cloud of golden atoms highly disagreeable to the observer. I
shall have something to say presently of the itch to which he is at
such times exposed.
Are these peculiar stomata designed merely to collect the adjoining
bristles and to grind them to powder? Are these fine-skinned papillæ,
which inflate and ascend from the depths of their hiding-place,
intended to get rid of the accumulation of broken hairs? Or is it the
sole function of this peculiar apparatus to prepare, at the expense of
the caterpillar’s fleece, an irritant dust which shall act as a means
of defence? Nothing tells us so.
Certainly the caterpillar is not armed against the enquirer who from
time to time takes it into his head to come and examine him through a
magnifying-glass. It is even very doubtful whether he troubles at all
about those passionate caterpillar-lovers, Calosoma sycophanta [18]
among insects and the Cuckoo among birds. Those who consume such fare
have a stomach expressly fashioned for the purpose, a stomach that
laughs at blistering hairs and possibly finds an appetizing stimulant
in their sting. No, I do not see the motives that prompted the
Processionary to cleave his back with so many slits, if he merely
strips himself of his hair to throw an irritating dust in our eyes.
There must certainly be something else in question.
Réaumur mentions these openings, of which he made a brief study. He
calls them stigmata and is inclined to take them for exceptional
breathing-holes. That they are not, O my master; no insect contrives
air-holes on its back! Moreover, the magnifying-glass reveals no
channel of communication with the interior. Respiration plays no part
here; the solution of the enigma must lie elsewhere.
The protuberances that rise from those expanded cavities are formed of
a soft, pale, hairless membrane, which gives the impression of a
visceral hernia, as though the caterpillar were wounded and exposing
its delicate entrails to the air. The sensitiveness just here is great.
The lightest touch with the point of a hair-pencil causes the immediate
indrawing of the protuberances and the closing of the containing lips.
The touch of a solid object even is not essential. I pick up a tiny
drop of water on the point of a pin and, without shaking it off,
present this drop to the sensitive projection. At the moment when
contact occurs the apparatus contracts and closes up. The recoil of the
Snail’s horns, withdrawing the visual and olfactory organs into their
sheaths, is no prompter.
Everything seems to prove that these optional tumours, appearing and
disappearing at the caterpillar’s will, are instruments of sensorial
perception. The caterpillar exposes them to obtain information; he
shelters them under his skin to preserve their delicate functions. Now
what is it that they perceive? This is a difficult question, in which
the habits of the Processionary alone can afford us a little guidance.
During the whole winter, the Pine Caterpillars are active only at
night. In the daytime, when the weather is fine, they readily repair to
the dome of the nest and there remain motionless, gathered into heaps.
It is the hour of the open-air siesta, under the pale December and
January sun. As yet none leaves the home. It is quite late in the
evening, towards nine o’clock, when they set out, marching in an
irregular procession, to browse on the leaves of the branches hard by.
Their grazing is a protracted affair. The flock returns late, some time
after midnight, when the temperature falls too low.
Secondly, it is in the heart of winter, during the roughest months,
that the Processionary displays his full activity. Indefatigably at
this time of year he spins, adding each night a new web to his silken
tent; at this time, whenever the weather permits, he ventures abroad on
the neighbouring boughs to feed, to grow and to renew his skein of
silk.
By a very remarkable exception, the harsh season marked by inactivity
and lethargic repose in other insects is for him the season of bustle
and labour, on condition, of course, that the inclemencies of the
weather do not exceed certain limits. If the north wind blow too
violently, so that it is like to sweep the flock away; if the cold be
too piercing, so that there is a risk of freezing to death; if it snow,
or rain, or if the mist thicken into an icy drizzle, the caterpillars
prudently stay at home, sheltering under their weatherproof tent.
It would be convenient to some extent to foresee these inclemencies.
The caterpillar dreads them. A drop of rain sets him in a flutter; a
snowflake exasperates him. To start for the grazing-grounds at dark of
night, in uncertain weather, would be dangerous, for the procession
goes some distance and travels slowly. The flock would fare ill before
regaining shelter did any sudden atmospheric trouble supervene, an
event of some frequency in the bad season of the year. So that he may
be informed in this particular during his nocturnal winter rambles, can
the Pine Caterpillar be endowed with some sort of meteorological
aptitudes? Let us describe how the suspicion occurred to me.
Divulged I know not how, my rearing of caterpillars under glass
acquired a certain renown. It was talked about in the village. The
forest-ranger, a sworn enemy to destructive insects, wanted to see the
grazing of the famous caterpillars, of whom he had retained a too
poignant memory ever since the day when he gathered and destroyed their
nests in a pine-wood under his charge. It was arranged that he should
call the same evening.
He arrives at the appointed hour, accompanied by a friend. For a moment
we sit and chat in front of the fire; then, when the clock strikes
nine, the lantern is lit and we all three enter the greenhouse. The
visitors are eager for the spectacle of which they have heard such
wonderful things, while I am certain of satisfying their curiosity.
But, but ... what is this? Not a caterpillar on the nests, not one on
the fresh ration of branches! Last night and on the previous nights
they came out in countless numbers; to-night not one reveals himself.
Can it be that they are merely late in going to dinner? Can their
habitual punctuality be at fault because appetite has not yet arrived?
We must be patient.... Ten o’clock. Nothing. Eleven. Still nothing.
Midnight was at hand when we abandoned our watch, convinced that it
would be vain to prolong the sitting. You can imagine what an abject
fool I looked at having thus to send my guests away.
Next day I thought that I dimly perceived the explanation of this
disappointment. It rained in the night and again in the morning. Snow,
not the earliest of the year, but so far the most abundant, whitened
the brow of the Ventoux. [19] Had the caterpillars, more sensitive than
any of us to atmospheric changes, refused to venture forth because they
anticipated what was about to happen? Had they foreseen the rain and
the snow, which nothing seemed to announce, at all events to us? After
all, why not? Let us continue to observe them and we shall see whether
the coincidence is fortuitous or not.
On this memorable day, therefore, the 13th of December, 1895, I
institute the caterpillars’ meteorological observatory. I have at my
disposal absolutely none of the apparatus dear to science, not even a
modest thermometer, for my unlucky star continues in the ascendant,
proving as unkind to-day as when I learnt chemistry with pipe-bowls for
crucibles and bottles that once contained sweets for retorts. I confine
myself to visiting nightly the Processionaries in the greenhouse and
those in the garden. It is a hard task, especially as I have to go to
the far end of the enclosure, often in weather when one would not turn
a Dog out of doors. I set down the acts of the caterpillars, whether
they come out or stay at home; I note the state of the sky during the
day and at the moment of my evening examination.
To this list I add the meteorological chart of Europe which the Temps
publishes daily. If I want more precise data, I request the Normal
School at Avignon to send me, on occasions of violent disturbances, the
barometrical records of its observatory. These are the only documents
at my disposal.
Before we come to the results obtained, let me once more repeat that my
caterpillars’ meteorological institute has two stations: one in the
greenhouse and one in the open air, on the pines in the enclosure. The
first, protected against the wind and rain, is that which I prefer: it
provides more regular and more continuous information. In fact, the
open-air caterpillars often enough refuse to come out, even though the
general conditions be favourable. It is enough to keep them at home if
there be too strong a wind shaking the boughs, or even a little
moisture dripping on the web of the nests. Saved from these two perils,
the greenhouse caterpillars have only to consider atmospheric incidents
of a higher order. The small variations escape them; the great alone
make an impression on them: a most useful point for the observer and
going a long way towards solving the problem for him. The colonies
under glass, therefore, provide most of the material for my notes; the
colonies in the open air add their testimony, which is not always quite
clear.
Now what did they tell me, those greenhouse caterpillars who, on the
13th of December, refused to show themselves to my guest, the
forest-ranger? The rain that was to fall that night could hardly have
alarmed them: they were so well sheltered. The snow about to whiten
Mont Ventoux was nothing to them: it was so far away. Moreover, it was
neither snowing yet nor raining. Some extraordinary atmospheric event,
profound and of vast extent, must have been occurring. The charts in
the Temps and the bulletin of the Normal School told me as much.
A cyclonic disturbance, coming from the British Isles, was passing over
our district; an atmospheric depression the like of which the season
had not as yet known, had spread in our direction, reaching us on the
13th and persisting, in a more or less accentuated form, until the
22nd. At Avignon the barometer suddenly fell half an inch, to 29.1 in.,
on the 13th and lower still, to 29 in., on the 19th.
During this period of ten days, the garden caterpillars made no sortie
on the pine-trees. True, the weather was changeable. There were a few
showers of fine rain and some violent gusts of the mistral; but more
frequently there were days and nights when the sky was superb and the
temperature moderate. The prudent anchorites would not allow themselves
to be caught. The low pressure persisted, menacing them; and so they
stopped at home.
In the greenhouse things happen rather differently. Sorties take place,
but the staying-in days are still more numerous. It looks as though the
caterpillars, alarmed at first by the unexpected things happening
overhead, had reassured themselves and resumed work, feeling nothing,
in their shelter, of what they would have suffered out of doors—rain,
snow and furious mistral blasts—and had then suspended their work again
when the threats of bad weather increased.
There is, indeed, a fairly accurate agreement between the oscillations
of the barometer and the decisions of the herd. When the column of
mercury rises a little, they come out; when it falls they remain at
home. Thus on the 19th, the night of the lowest pressure, 29 in., not a
caterpillar ventures outside.
As the wind and rain can have no effect on my colonies under glass, one
is led to suppose that atmospheric pressure, with its physiological
results, so difficult to define, is here the principal factor. As for
the temperature, within moderate limits there is no need to discuss it.
The Processionaries have a robust constitution, as behoves spinners who
work in the open air in midwinter. However piercing the cold, so long
as it does not freeze, when the hour comes for working or feeding they
spin on the surface of the nest or browse on the neighbouring branches.
Another example. According to the meteorological chart in the Temps, a
depression whose centre is near the Iles Sanguinaires, at the entrance
of the Gulf of Ajaccio, reaches my neighbourhood, with a minimum of
29.2 in., on the 9th of January. A tempestuous wind gets up. For the
first time this year there is a respectable frost. The ice on the large
pond in the garden is two or three inches thick. This wild weather
lasts for five days. Of course, the garden caterpillars do not sally
forth on the pine-trees while these are battered by such a gale.
The remarkable part of the business is that the greenhouse caterpillars
do not venture out of their nests either. And yet for them there are no
boughs dangerously shaken, no cold piercing beyond endurance, for it is
not freezing under the glass. What keeps them in can be only the
passage of that wave of depression. On the 15th the storm ceases; and
the barometer remains between 29.6 and 30 in. for the rest of the month
and a good part of February. During this long period there are
magnificent sorties every evening, especially in the greenhouse.
On the 23rd and 24th of February, suddenly the Processionaries stop at
home again, for no apparent reason. Of the six nests under cover, only
two have a few rare caterpillars out on the pine-branches, while
previously, in the case of all six, I used every night to see the
leaves bending under the weight of an innumerable multitude. Warned by
this forecast, I enter in my notes:
“Some deep depression is about to reach us.”
And I have guessed right. Two days later, sure enough, the
meteorological record of the Temps gives me the following information:
a minimum of 29.2 in., coming from the Bay of Biscay on the 22nd,
reaches Algeria on the 23rd and spreads over the Provence coast on the
24th. There is a heavy snowfall at Marseilles on the 25th.
“The ships,” I read in my paper, “present a curious spectacle, with
their yards and rigging white. That is how the people of
Marseilles, little used to such sights, picture Spitzbergen and the
North Pole.”
Here certainly is the gale which my caterpillars foresaw when they
refused to go out last night and the night before; here is the centre
of disturbance which revealed itself at Sérignan by a violent and icy
north wind on the 25th and the following days. Again I perceive that
the greenhouse caterpillars are alarmed only at the approach of the
wave of atmospheric disturbance. Once the first uneasiness caused by
the depression had abated, they came out again, on the 25th and the
following days, in the midst of the gale, as though nothing
extraordinary were happening.
From the sum of my observations it appears that the Pine Processionary
is eminently sensitive to atmospheric vicissitudes, an excellent
quality, having regard to his way of life in the sharp winter nights.
He foresees the storm which would imperil his excursions.
His capacity for scenting bad weather very soon won the confidence of
the household. When we had to go into Orange to renew our provisions,
it became the rule to consult him the night before; and, according to
his verdict, we went or stayed at home. His oracle never deceived us.
In the same way, simple folk that we were, we used in the old days to
interrogate the Dor-beetle, [20] another doughty nocturnal worker. But,
a little demoralized by imprisonment in a cage and apparently devoid of
any special sensitive apparatus, performing his evolutions, moreover,
in the mild autumn evenings, the celebrated Dung-beetle could never
rival the Pine Caterpillar, who is active during the roughest season of
the year and endowed, as everything would seem to affirm, with organs
quick to perceive the great atmospheric fluctuations.
Rural lore abounds in meteorological forecasts derived from animals.
The Cat, sitting in front of the fire and washing behind her ears with
a saliva-smeared paw, foretells another cold snap; the Cock, crowing at
unusual hours, announces the return of fine weather; the Guinea-fowl,
with her screeching, as of a scythe on the grindstone, points to rain;
the Hen, standing on one leg, her plumage ruffled, her head sunk on her
neck, feels a hard frost coming; the pretty green Tree-frog inflates
his throat like a bladder at the approach of a storm and, according to
the Provençal peasant, says:
“Ploùra, ploùra; it will rain, it will rain!”
This rustic meteorology, the heritage of the centuries, does not show
up so badly beside our scientific meteorology.
Are not we ourselves living barometers? Every veteran complains of his
glorious scars when the weather is about to break. One man, though
unwounded, suffers from insomnia or from bad dreams; another, though a
brain-worker, cannot drag an idea out of his impotent head. Each of us,
in his own way, is tried by the passage of those huge funnels which
form in the atmosphere and hatch the storm.
Could the insect, with its exceptionally delicate organization, escape
this kind of impression? It is unbelievable. The insect, more than any
other creature, should be an animated meteorological instrument, as
truthful in its forecasts, if we knew how to read them, as the lifeless
instruments of our observatories, with their mercury and their catgut.
All, in different degrees, possess a general impressionability
analogous to our own and exercised without the aid of specific organs.
Some, better-gifted because of their mode of life, might well be
furnished with special meteorological apparatus.
The Pine Processionary seems to belong to this number. In his second
costume, when the segments bear on their dorsal faces an elegant red
mosaic, he differs apparently from other caterpillars only by a more
delicate general impressionability, unless this mosaic be endowed with
aptitudes unknown elsewhere. If the nocturnal spinner is still none too
generously equipped, it must be remembered that the season which he
passes in this condition is nearly always clement. The really
formidable nights hardly set in before January. But then, as a
safeguard in his peregrinations, the Pine Processionary cleaves his
back with a series of mouths which yawn open to sample the air from
time to time and to give a warning of the sudden storm.
Until further evidence is forthcoming, therefore, the dorsal slits are,
to my mind, meteorological instruments, barometers influenced by the
main fluctuations of the atmosphere. To go beyond suspicions, though
these are well based, is for me impossible. I lack the equipment
necessary to delve more deeply into the subject. But I have given a
hint. It is for those who are better favoured in the matter of
resources to find the final solution of this interesting problem.