THE PINE PROCESSIONARY: THE MOTH
When March comes, the caterpillars reared in domesticity never cease
processioning. Many leave the greenhouse, which remains open; they go
in search of a suitable spot for the approaching metamorphosis. This is
the final exodus, the definite abandonment of the nest and the
pine-tree. The pilgrims are much faded, whitish, with a few russet
hairs on their backs.
On the 20th of March I spend a whole morning watching the evolutions of
a file some three yards in length, containing about a hundred
emigrants. The procession toils grimly along, undulating over the dusty
ground, where it leaves a furrow. Then it breaks into a small number of
groups, which crowd together and remain quiescent save for sudden
oscillations of the hind-quarters. After a halt of varying duration,
these groups resume their march, henceforward forming independent
processions.
They take no settled direction. This one goes forward, that one goes
back; one turns to the left and another to the right. There is no rule
about their marching, no positive goal. One procession, after
describing a loop, retraces its steps. Yet there is a general tendency
towards that wall of the greenhouse which faces the south and reflects
the sun’s rays with added fervour. The sole guide, it would seem, is
the amount of sun which a place obtains; the directions whence the
greatest heat comes are preferred.
After a couple of hours of marching and countermarching, the
fragmentary processions, comprising each a score of caterpillars, reach
the foot of the wall. Here the soil is powdery, very dry, easy to
burrow in, although made somewhat firmer by tufts of grass. The
caterpillar at the head of the row explores with his mandibles, digs a
little, investigates the nature of the ground. The others, trusting
their leader, follow him with docility, making no attempts of their
own. Whatever the foremost decides will be adopted by all. Here, in the
choice of a matter so important as the spot whereat the transformation
shall take place, there is no individual initiative. There is only one
will, the leader’s. There is only one head, so to speak; the procession
may be compared with the chain of segments of an enormous worm.
Finally some spot is recognized as propitious. The leading caterpillar
halts, pushes with his head, digs with his mandibles. The others, still
in a continuous line, arrive one by one and likewise come to a halt.
Then the file breaks up into a swarming heap, in which each of the
caterpillars resumes his liberty. All their backs are joggling
pell-mell; all their heads are plunged into the dust; all their feet
are raking, all their mandibles excavating the soil. The worm has
chopped itself into a gang of independent workers.
An excavation is formed in which, little by little, the caterpillars
bury themselves. For some time to come, the undermined soil cracks and
rises and covers itself with little mole-hills; then all is still. The
caterpillars have descended to a depth of three inches. This is as far
as the roughness of the soil permits them to go. In looser soil, the
excavation would attain a much greater depth. The greenhouse shelf,
supplied with fine sand, has provided me with cocoons placed at a depth
of from eight to twelve inches. I would not assert that the interment
might not be made still lower down. For the most part, the burial is
effected in common, by more or less numerous clusters and at depths
which vary greatly, according to the nature of the soil.
A fortnight later, let us dig at the point where the descent
underground was made. Here we shall find the cocoons assembled in
bunches, cocoons of sorry appearance, soiled as they are with earthy
particles held by silken threads. When stripped of their rough
exterior, they are not without a certain elegance. They are narrow
ellipsoids, pointed at both ends, measuring twenty-five millimetres in
length and nine millimetres [21] in thickness. The silk of which they
are composed is very fine and of a dull white. The fragility of the
walls is remarkable when we have seen the enormous quantity of silk
expended on the construction of the nest.
A prodigious spinner where his winter habitation is concerned, the
caterpillar finds his glands exhausted and is reduced to the strictly
necessary amount when the time comes for making the cocoon. Too poor in
silk, he strengthens his flimsy cell with a facing of earth. With him
it is not the industry of the Bembex [22], who inserts grains of sand
in her silky web and makes a solid casket of the whole; it is a summary
sort of art, devoid of delicacy, which just casually sticks together
the surrounding earthy refuse.
Moreover, if circumstances demand it, the Pine Caterpillar can do
without earth. In the very midst of the nest I have sometimes—very
rarely, it is true—discovered cocoons which were perfectly clean. Not a
scrap of alien matter defiled their fine white silk. I have obtained
similar specimens by placing caterpillars under a bell-glass in a pan
provided only with a few pine-twigs. Better still: an entire
procession, a good-sized one too, gathered at the opportune moment and
enclosed in a large box containing no sand nor any material whatever,
spun its cocoons with no other support than the bare walls. These
exceptions, provoked by circumstances in which the caterpillar is not
free to act according to his wont, does not in any way invalidate the
rule. To prepare for the transformation, the Processionary buries
himself, to the depth of nine inches and more, if the soil permit.
Here a curious problem forces itself upon the observer’s mind. How does
the Moth contrive to ascend from the catacombs into which the
caterpillar has descended? Not in the finery of her perfect state—the
big wings with their delicate scales, the sweeping antenna-plumes—dare
she brave the asperities of the soil, or she would issue thence all
tattered, rumpled and unrecognizable. And this is not the case: far
from it. Moreover, what means can she employ, she so feeble, to break
the crust of earth into which the original dust will have turned after
the slightest of showers?
The Moth appears at the end of July or in August. The burial took place
in March. Rain must have fallen during this lapse of time, rain which
beats down the soil, cements it and leaves it to harden once
evaporation has set in. Never could a Moth, unless attired and equipped
with tools for the purpose, break her way through such an obstacle. She
would perforce require a boring-tool and a costume of extreme
simplicity. Guided by these considerations, I institute a few
experiments which will give me the key to the riddle.
In April I make a copious collection of cocoons. Of these I place ten
or twelve at the bottom of test-tubes of different diameters and, last
of all, I fill the apparatus with sandy soil, sifted and very slightly
moistened. The contents are pressed down, but in moderation, for fear
of injuring the cocoons below. When the month of August comes, the
column of earth, damp at the outset, has set so firmly, thanks to
evaporation, that, when I reverse the test-tube, nothing trickles out.
On the other hand, some cocoons have been kept naked under a metallic
cover. These will teach me what the buried cocoons would not be able to
show. They furnish me, in fact, with records of the greatest interest.
On issuing from the cocoon, the Pine Bombyx has her finery bundled up
and presents the appearance of a cylinder with rounded ends. The wings,
the principal obstacle to underground labour, are pressed against the
breast like narrow scarves; the antennæ, another serious embarrassment,
have not yet unfolded their plumes and are turned back along the Moth’s
sides. The hair, which later forms a dense fleece, is laid flat,
pointing backwards. The legs alone are free, fairly active and endowed
with a certain vigour. Thanks to this arrangement, which does away with
all awkward projections, the ascent through the soil is made possible.
True, every Moth, at the moment of quitting her shell, is this sort of
swathed mummy; but the Pine Bombyx has in addition an exceptional
aptitude rendered necessary by the fact that she hatches under the
ground. While the others, once out of the cocoon, hasten to spread
their wings and are powerless to defer their development, she, by
virtue of an indispensable privilege, remains in her compact and
wrapped-up condition as long as circumstances demand it. Under my
bell-glasses I see some who, though born upon the surface, for
twenty-four hours drag themselves over the sand or cling to the
pine-branches, before untying their sashes and unfurling them as wings.
This delay is evidently essential. To ascend from beneath the earth and
reach the open air, the Moth has to bore a long tunnel, which requires
time. She will take good care not to spread her finery before emerging,
for it would hamper her and would itself be rumpled and badly creased.
Therefore the cylindrical mummy persists until the deliverance is
effected; and, if liberty happen to be acquired before the appointed
moment, the final evolution does not take place until after a lapse of
time in conformity with usage.
We are acquainted with the equipment for emergence, the tight-fitting
jerkin indispensable in a narrow gallery. Now, where is the
boring-tool? The legs, though free, would here be insufficient: they
would scrape the earth laterally, enlarging the diameter of the shaft,
but could not prolong the exit vertically, above the insect’s head.
This tool must be in front.
Pass the tip of your finger over the Moth’s head. You will feel a few
very rough wrinkles. The magnifying-glass shows us more. We find,
between the eyes and higher up, four or five transversal scales, so set
as to overlap one another; they are hard and black and are trimmed
crescent-wise at the ends. The longest and strongest is the uppermost,
which is in the middle of the forehead. There you have the centre-bit
of your boring-tool.
To make our tunnels in granitic rocks we tip our drills with diamond
points. For a similar task the Bombyx, a living drill, wears implanted
on her forehead a row of crescents, hard and durable as steel, a
regular twist-bit. Without suspecting its use, Réaumur was perfectly
aware of this marvellous implement, which he called scaly stairs:
“What does it profit this Moth,” he asks, “that she should thus have
the front of her head formed like scaly stairs? That is just what I do
not know.”
My test-tubes, learned master, will tell us. By good fortune, of the
numerous Moths ascending from the bottom of my apparatus through a
column of sand solidified by the evaporation of the original moisture,
some are making their way upwards against the side of the tube,
enabling me to follow their manœuvres. I see them raising their
cylindrical bodies, butting with their heads, jerking now in one
direction, now in another. The nature of their task is obvious. The
centre-bits, with an alternating movement, are boring into the
agglutinated sand. The powdery wreckage trickles down from overhead and
is at once thrust backward by the legs. A little space forms at the top
of the vault; and the Moth moves so much nearer to the surface. By the
following day, the whole column, ten inches in height, will be
perforated with a straight, perpendicular shaft.
Shall we now form an idea of the total work performed? Let us turn the
test-tube upside down. The contents, as I have said, will not fall out,
for they have set into a block; but from the tunnels bored by the Moth
trickles all the sand crumbled by the crescents of the drill. The
result is a cylindrical gallery, of the width of a lead-pencil, very
cleanly cut and reaching to the bottom of the solid mass.
Are you satisfied, my master? Do you now perceive the great utility of
the scaly stairs? Would you not say that we have here a magnificent
example of an instrument superlatively fitted for a definite task? I
share this opinion, for I think, with you, that a sovereign Reason has
in all things coordinated the means and the end.
But let me tell you: we are called old-fashioned, you and I; with our
conception of a world ruled by an Intelligence, we are quite out of the
swim. Order, balance, harmony: that is all silly nonsense. The universe
is a fortuitous arrangement in the chaos of the possible. What is white
might as easily be black, what is round might be angular, what is
regular might be shapeless and harmony might just as well be discord.
Chance has decided all things.
Yes, we are a pair of prejudiced old fogeys when we linger with a
certain fondness over the marvels of perfection. Who troubles about
these futilities nowadays? So-called serious science, the science which
spells honour, profit and renown, consists in slicing your animal with
very costly instruments into tiny circular sections. My housekeeper
does as much with a bunch of carrots, with no higher pretension than to
concoct a modest dish, which is not an invariable success. In the
problem of life are we more successful when we have split a fibre into
four and cut a cell into shavings? It hardly seems so. The riddle is as
dark as ever. Ah, how much better is your method, my dear master; above
all, how much loftier your philosophy, how much more wholesome and
invigorating!
Here at last is the Moth at the surface. With the deliberate slowness
demanded by so delicate an operation, she spreads her bunched wings,
extends her antennæ and puffs out her fleece. Her costume is a modest
one: upper wings grey, striped with a few crinkly brown streaks;
under-wings white; thorax covered with thick grey fur; abdomen clad in
bright-russet velvet. The last segment has a pale-gold sheen. At first
sight it appears bare. It is not, however; but, in place of hairs like
those of the other segments, it has, on its dorsal surface, scales so
well assembled and so close together that the whole seems to form a
continuous block, like a nugget.
Let us touch this trinket with the point of a needle. However gently we
rub, a multitude of scales come off and flutter at the least breath,
shining like mica spangles. Their concave form, their shape, an
elongated oval, their colouring, white in the lower half but reddish
gold in the upper, give them, if we allow for the difference in size, a
certain resemblance to the scales surrounding the heads of some of the
centaury tribe. Such is the golden fleece of which the mother will
despoil herself in order to cover the cylinder of her eggs. The nugget
of her hind-quarters, exfoliated spangle by spangle, will form a roof
for the germs arranged like the grain in a corn-cob.
I was anxious to watch the actual placing of these pretty tiles, which
are fixed at the pale end with a speck of cement, leaving the coloured
end free. Circumstances did not favour me. Inactive all day, motionless
on some needle of the lower branches, the Moth, whose life is very
short, moves only in the darkness of the night. Both her mating and
egg-laying are nocturnal. On the morrow, all is finished: the Bombyx
has lived. Under these conditions, it was impossible, by the doubtful
beams of a lantern, to follow satisfactorily the labour of the mother
on the pine-trees in the garden.
I was no more fortunate with the captives in my bell-glasses. A few did
lay their eggs, but always at a very advanced hour of the night, an
hour which found my vigilance at fault. The light of a candle and eyes
heavy with sleep were of little avail when it came to analysing the
subtle operations of the mother as she puts her scales in place. We
will say nothing of the little that was imperfectly seen.
Let us close with a few words of sylvicultural practice. The Pine
Processionary is a voracious caterpillar who, while respecting the
terminal bud, protected by its scales and its resinous varnish,
completely denudes the bough and imperils the tree by leaving it bald.
The green pine-needles, that mane in which the vegetable vigour of the
tree resides, are shorn to the roots. How are we to remedy this?
When consulted on the subject, the forest-ranger of my parish told me
that the custom is to go from tree to tree with pruning-shears fitted
on a long pole and to cut down the nests, afterwards burning them. The
method is a troublesome one, for the silken purses are often at
considerable heights. Moreover, it is not without danger. Attacked by
the hairy dust, the destroyers soon experience intolerable discomfort,
a torture of irritation which makes them refuse to continue the work.
To my thinking it would be better to operate before the appearance of
the nests.
The Pine Bombyx is a very bad flyer. Incapable of soaring, almost like
the Silk-moth, she flutters about and blunders to earth again; and her
best efforts barely succeed in bringing her to the lower branches,
which almost drag along the ground. Here are deposited the cylinders of
eggs, at a height of six feet at most. It is the young caterpillars
who, from one provisional encampment to another, gradually ascend,
attaining, stage by stage, the summits upon which they weave their
final dwellings. Once we grasp this peculiarity, the rest is plain
sailing.
In August we inspect the lower foliage of the tree: an easy
examination, for it is carried on no higher than our heads. Towards the
far end of the twigs it is easy to espy the Bombyx’ eggs, packed into
cylinders that resemble scaly catkins. Their size and their whitish
colour make them show up amid the sombre green. Gathered with the
double pine-needle that bears them, these cylinders are crushed under
foot, a summary fashion of stamping out an evil before it spreads.
This I have done in the case of the few pine-trees in my enclosure. And
the same might be done in the wider forest expanses and more especially
in parks and gardens, where symmetrical foliation is one of the great
beauties of the tree. I will add that it is wise to prune every bough
that droops to earth and to keep the foot of the conifer bare to a
height of six feet or so. In the absence of these lower stairs, the
only ones that the Bombyx with her clumsy flight can reach, she will
not be able to populate the tree.