THE CICADA: LEAVING THE BURROW
To come back to the Cicada after Réaumur [9] has told the insect’s
story would be waste of time, save that the disciple enjoys an
advantage unknown to the master. The great naturalist received the
materials for his work from my part of the world; his subjects came by
barge after being carefully preserved in spirits. I, on the other hand,
live in the Cicada’s company. When July comes, he takes possession of
the enclosure right up to the threshold of the house. The hermitage is
our joint property. I remain master indoors; but out of doors he is the
sovereign lord and an extremely noisy and abusive one. Our near
neighbourhood and constant association have enabled me to enter into
certain details of which Réaumur could not dream.
The first Cicadæ appear at the time of the summer solstice. Along the
much-trodden paths baked by the sun and hardened by the frequent
passage of feet there open, level with the ground, round orifices about
the size of a man’s thumb. These are the exit-holes of the
Cicada-larvæ, who come up from the depths to undergo their
transformation on the surface. They are more or less everywhere, except
in soil turned over by the plough. Their usual position is in the
driest spots, those most exposed to the sun, especially by the side of
the roads. Equipped with powerful tools to pass, if necessary, through
sandstone and dried clay, the larva, on leaving the earth, has a fancy
for the hardest places.
One of the garden-paths, converted into a little inferno by the glare
from a wall facing south, abounds in such exit-holes. I proceed, in the
last days of June, to examine these recently abandoned pits. The soil
is so hard that I have to take my pickaxe to tackle it.
The orifices are round and nearly an inch in diameter. There is
absolutely no rubbish around them, no mound of earth thrown up outside.
This is invariably the case: the Cicada’s hole is never surmounted with
a mole-hill, as are the burrows of the Geotrupes, [10] or Dorbeetles,
those other sturdy excavators. The manner of working accounts for this
difference. The Dung-beetle progresses from the outside inwards; he
commences his digging at the mouth of the well, which allows him to
ascend and heap up on the surface the material which he has extracted.
The larva of the Cicada, on the other hand, goes from the inside
outwards; the last thing that it does is to open the exit-door, which,
remaining closed until the very end of the work, cannot be used for
getting rid of the rubbish. The former goes in and makes a mound on the
threshold of the home; the latter comes out and cannot heap up anything
on a threshold that does not yet exist.
The Cicada’s tunnel runs to a depth of between fifteen and sixteen
inches. It is cylindrical, winds slightly, according to the exigencies
of the soil, and is always nearly perpendicular, for it is shorter to
go that way. The passage is quite open throughout its length. It is
useless to search for the rubbish which this excavation ought, one
would think, to produce; we see none anywhere. The tunnel ends in a
blind alley, in a rather wider chamber, with level walls and not the
least vestige of communication with any gallery prolonging the well.
Reckoned by its length and its diameter, the excavation represents a
volume of about twelve cubic inches. What has become of the earth
removed? Sunk in very dry and very loose soil, the well and the chamber
at the bottom ought to have crumbly walls, which would easily fall in,
if nothing else had taken place but the work of boring. My surprise was
great to find, on the contrary, coated surfaces, washed with a paste of
clayey earth. They are not by a long way what one could call smooth,
but at any rate their irregularities are covered with a layer of
plaster; and their slippery materials, soaked with some agglutinant,
are kept in position.
The larva can move about and climb nearly up to the surface and down
again to its refuge at the bottom without producing, with its clawed
legs, landslips which would block the tube, making ascent difficult and
retreat impossible. The miner shores up his galleries with pit-props
and cross-beams; the builder of underground railways strengthens his
tunnels with a casing of brickwork; the Cicada’s larva, which is quite
as clever an engineer, cements its shaft so as to keep it open however
long it may have to serve.
If I surprise the creature at the moment when it emerges from the soil
to make for a neighbouring branch and there undergo its transformation,
I see it at once beat a prudent retreat and, without the slightest
difficulty, run down again to the bottom of its gallery, proving that,
even when the dwelling is on the point of being abandoned for good, it
does not become blocked with earth.
The ascending-shaft is not a piece of work improvised in a hurry, in
the insect’s impatience to reach the sunlight; it is a regular
manor-house, an abode in which the grub is meant to make a long stay.
So the plastered walls tell us. Any such precaution would be
superfluous in the case of a mere exit abandoned as soon as bored.
There is not a doubt but that we have here a sort of meteorological
station in which observations are taken of the weather outside.
Underground, fifteen inches down, or more, the larva ripe for its
emergence is hardly able to judge whether the climatic conditions be
favourable. Its subterranean weather is too gradual in its changes to
be able to supply it with the precise indications necessary for the
most important action of its life, its escape into the sunlight for the
metamorphosis.
Patiently, for weeks, perhaps for months, it digs, clears and
strengthens a perpendicular chimney, leaving at the surface, to keep it
sequestered from the world without, a layer as thick as one’s finger.
At the bottom it makes itself a recess more carefully built than the
remainder. This is its refuge, its waiting-room, where it rests if its
reconnoitring lead it to defer its emigration. At the least suspicion
of fine weather, it scrambles up, tests the exterior through the thin
layer of earth forming a lid and enquires into the temperature and the
degree of humidity of the air.
If things do not bode well, if a heavy shower threaten or a blustering
storm—events of supreme importance when the delicate Cicada throws off
her skin—the prudent insect slips back to the bottom of the tube and
goes on waiting. If, on the other hand, the atmospheric conditions be
favourable, then the ceiling is smashed with a few strokes of the claws
and the larva emerges from the well.
Everything seems to confirm that the Cicada’s gallery is a
waiting-room, a meteorological station where the larva stays for a long
time, now hoisting itself near the surface to discover the state of the
weather, now retreating to the depths for better shelter. This explains
the convenience of a resting-place at the base and the need for a
strong cement on walls which, without it, would certainly give way
under continual comings and goings.
What is not so easily explained is the complete disappearance of the
rubbish corresponding with the space excavated. What has become of the
twelve cubic inches of earth yielded by an average well? There is
nothing outside to represent them, nor anything inside either. And then
how, in a soil dry as cinders, is the plaster obtained with which the
walls are glazed?
Larvæ that gnaw into wood, such as those of the Capricorn and the
Buprestes, [11] for instance, ought to be able to answer the first
question. They make their way inside a tree-trunk, boring galleries by
eating the materials of the road which they open. Detached in tiny
fragments by the mandibles, these materials are digested. They pass
through the pioneer’s body from end to end, yielding up their meagre
nutritive elements on the way, and accumulate behind, completely
blocking the road which the grub will never take again. The work of
excessive division and subdivision, done either by the mandibles or the
stomach, causes the digested materials to take up less room than the
untouched wood; and the result is a space in front of the gallery, a
chamber in which the grub works, a chamber which is greatly restricted
in length, giving the prisoner just enough room to move about.
Can it not be in a similar fashion that the Cicada-grub bores its
tunnel? Certainly the waste material flung up as it digs its way does
not pass through its body; even if the soil were of the softest and
most yielding character, earth plays no part whatever in the larva’s
food. But, after all, cannot the materials removed be simply shot back
as the work proceeds? The Cicada remains four years in the ground. This
long life is not, of course, spent at the bottom of the well which we
have described: this is just a place where the larva prepares for its
emergence. It comes from elsewhere, doubtless from some distance. It is
a vagabond, going from one root to another and driving its sucker into
each. When it moves, either to escape from the upper layers, which are
too cold in winter, or to settle down at a better drinking-bar, it
clears a road by flinging behind it the materials broken up by its
pickaxes. This is undoubtedly the method.
As with the larvæ of the Capricorn and the Buprestes, the traveller
needs around him only the small amount of free room which his movements
require. Damp, soft, easily compressed earth is to this larva what the
digested pap is to the others. Such earth is heaped up without
difficulty; it condenses and leaves a vacant space.
The difficulty is one of a different kind with the exit-well bored in a
very dry soil, which offers a marked resistance to compression so long
as it retains its aridity. That the larva, when beginning to dig its
passage, flung back part of the excavated materials into an earlier
gallery which has now disappeared is fairly probable, though there is
nothing in the condition of things to tell us so; but, if we consider
the capacity of the well and the extreme difficulty of finding room for
so great a volume of rubbish, our doubts return and we say to
ourselves:
“This rubbish demanded a large empty space, which itself was obtained
by shifting other refuse no less difficult to house. The room required
presupposes the existence of another space into which the earth
extracted was shot.”
And so we find ourselves in a vicious circle, for the mere subsidence
of materials flung behind would not be enough to explain so great a
void. The Cicada must have a special method of disposing of the
superfluous earth. Let us try and surprise his secret.
Examine a larva at the moment when it emerges from the ground. It is
nearly always more or less soiled with mud, sometimes wet, sometimes
dry. The digging-implements, the fore-feet, have the points of their
pickaxes stuck in a globule of slime; its other legs are cased in mud;
its back is spotted with clay. We are reminded of a scavenger who has
been stirring up sewage. These stains are the more striking inasmuch as
the creature comes out of exceedingly dry ground. We expected to see it
covered with dust and we find it covered with mud.
One more step in this direction and the problem of the well is solved.
I exhume a larva which happens to be working at its exit-gallery. Very
occasionally, I get a piece of luck like this, in the course of my
digging; it would be useless for me to try for it, as there is nothing
outside to guide my search. My welcome prize is just beginning its
excavations. An inch of tunnel, free from any rubbish, and the
waiting-room at the bottom represent all the work for the moment. In
what condition is the worker? We shall see.
The grub is much paler in colour than those which I catch as they
emerge. Its big eyes in particular are whitish, cloudy, squinting and
apparently of little use for seeing. What good is sight underground?
The eyes of the larvæ issuing from the earth are, on the contrary,
black and shining and indicate ability to see. When it makes its
appearance in the sunshine, the future Cicada has to seek, occasionally
at some distance from the exit-hole, the hanging branch on which the
metamorphosis will be performed; and here sight will manifestly be
useful. This maturity of vision attained during the preparation for the
release is enough to show us that the larva, far from hastily
improvising its ascending-shaft, works at it for a long time.
Moreover, the pale and blind larva is bulkier than it is in the state
of maturity. It is swollen with liquid and looks dropsical. If you take
it in your fingers, a limpid humour oozes from the hinder part and
moistens the whole body. Is this fluid, expelled from the intestines, a
urinary product? Is it just the residue of a stomach fed solely on sap?
I will not decide the question and will content myself with calling it
urine, merely for convenience.
Well, this fountain of urine is the key to the mystery. The larva, as
it goes on and digs, sprinkles the dusty materials and makes them into
paste, which is forthwith applied to the walls by abdominal pressure.
The original dryness is succeeded by plasticity. The mud obtained
penetrates the interstices of a rough soil; the more liquid part of it
trickles in front; the remainder is compressed and packed and occupies
the empty spaces in between. Thus is an unblocked tunnel obtained,
without any refuse, because the dust and rubbish are used on the spot
in the form of a mortar which is more compact and more homogeneous than
the soil traversed.
The larva therefore works in the midst of clayey mire; and this is the
cause of the stains that astonish us so much when we see it issuing
from excessively dry soil. The perfect insect, though relieved
henceforth from all mining labour, does not utterly abandon the use of
its bladder; a few drains of urine are preserved as a weapon of
defence. When too closely observed, it discharges a spray at the
intruder and quickly flies away. In either form, the Cicada, his dry
constitution notwithstanding, proves himself a skilled irrigator.
Dropsical though it be, the larva cannot carry sufficient liquid to
moisten and turn into compressible mud the long column of earth which
has to be tunnelled. The reservoir becomes exhausted and the supply has
to be renewed. How is this done and when? I think I see.
The few wells which I have laid bare throughout their length, with the
painstaking care which this sort of digging demands, show me at the
bottom, encrusted in the wall of the terminal chamber, a live root,
sometimes as big as a lead-pencil, sometimes no thicker than a straw.
The visible part of this root is quite small, barely a fraction of an
inch. The rest is contained in the surrounding earth. Is the discovery
of this sort of sap fortuitous? Or is it the result of a special search
on the larva’s part? The presence of a rootlet is so frequent, at least
when my digging is skilfully conducted, that I rather favour the latter
alternative.
Yes, the Cicada-grub, when hollowing out its cell, the starting-point
of the future chimney, seeks the immediate neighbourhood of a small
live root; it lays bare a certain portion, which continues the side
wall without projecting. This live spot in the wall is, I think, the
fount from which the contents of the urinary bladder are renewed as the
need arises. When its reserves are exhausted by the conversion of dry
dust into mud, the miner goes down to his chamber, drives in his sucker
and takes a deep draught from the cask built into the wall. With his
jug well filled, he goes up again. He resumes his work, wetting the
hard earth the better to flatten it with his claws and reducing the
dusty rubbish to mud which can be heaped up around him and leave a
clear thoroughfare. That is how things must happen. So logic and the
circumstances of the case tell us, in the absence of direct
observation, which is not feasible here.
If this root-cask fail, if moreover the reservoir of the intestine be
exhausted, what will happen then? We shall learn from the following
experiment. I catch a grub as it is leaving the ground. I put it at the
bottom of a test-tube and cover it with a column of dry earth, not too
closely packed. The column is nearly six inches high. The larva has
just quitted an excavation thrice as deep, in soil of the same nature,
but offering a much greater resistance. Now that it is buried under my
short, sandy column, will it be capable of climbing to the surface? If
it were a mere matter of strength, the issue would be certain. What can
an obstacle without cohesion be to one that has just bored a hole
through the hard ground?
And yet I am assailed by doubts. To break down the screen that still
separated it from the outer air, the larva has expended its last
reserves of fluid. The flask is dry; and there is no way of
replenishing it in the absence of a live root. My suspicion of failure
is well-founded. For three days I see the entombed one wasting itself
in efforts without succeeding in rising an inch higher. The materials
removed refuse to stay in position for lack of anything to bind them;
they are no sooner pushed aside than they slip down again under the
insect’s legs. The labour has no perceptible result and has always to
be done all over again. On the fourth day, the creature dies.
With the water-can full, the result is quite different. I subject to
the same experiment an insect whose work of self-deliverance is just
beginning. It is all swollen with urinary humours which ooze out and
moisten its whole body. This one’s task is easy. The materials offer
hardly any resistance. A little moisture, supplied by the miner’s
flask, converts them into mud, sticks them together and keeps them out
of the way. The passage is opened, very irregular in shape, it is true,
and almost filled up at the back as the ascent proceeds. It is as
though the larva, recognizing the impossibility of renewing its store
of fluid, were saving up the little which it possesses and spending no
more than is strictly necessary to enable it to escape as quickly as
possible from its unfamiliar surroundings. This economy is so well
arranged that the insect reaches the surface at the end of ten days.