THE POPLAR-WEEVIL
Generally speaking, the mother Weevil’s attainments are limited to
slipping her eggs into places where the grubs will find food to suit
them and occasionally, with wonderfully assured botanical tact, to
varying the diet. She does little or no industrial work. The niceties
of the baby-linen or the feeding-bottle do not concern her. To this
uncouth maternity I know but one exception, appertaining to certain
Weevils who, in order to endow their young with preserved foodstuffs,
have the knack of rolling a leaf, which serves as board and lodging in
one.
Among these manufacturers of vegetable sausages the most skilful is the
Poplar-weevil (Rhynchites populi, Lin.), who is of modest size but
splendidly attired. Her back glitters with gold and copper, her abdomen
with indigo-blue. Would you see her at work, you need but inspect the
lower twigs of the common black poplar, at the edge of the meadows,
about the end of May.
While, overhead, spring’s caressing breezes stir the majestic green
distaff and set the leaves quivering on their flat stalks, down below,
in a layer of calmer air, this year’s tender shoots remain quiescent.
Here above all, far from the wind-tossed heights uncongenial to the
industrious, the Rhynchites labours. And, as the workshop is just at a
man’s height, nothing is more easy than to watch the roller’s actions.
Easy, yes, but distressing, under a blazing sun, if you wish to follow
the insect in every detail of its method and the progress of its work.
Moreover, this involves long journeys, which take up time; and again it
is none too favourable to precise observations, which demand indefinite
leisure and assiduous inspections at all hours of the day. It is
greatly preferable to pursue our studies in the comfort of our own
home; but it is above all things necessary that the insect should lend
itself to our plan.
The Rhynchites fulfils this condition excellently well. She is a
peaceable enthusiast who works on my table with the same zest as on her
poplar-tree. A few young shoots, planted in fresh sand, under a
wire-gauze cover, and renewed as and when they fade, take the place of
the tree in my work-room. The Weevil, not in the least intimidated,
devotes herself to her industry even under my magnifying-glass and
supplies me with as many cylinders as I could wish for.
Let us watch her at work. From this year’s growth, sprouting in sheaves
at the base of the trunk, she chooses the leaf to be rolled; but she
picks it not among the lower leaves, which are already of the usual
green and of a firm texture, nor yet among the end leaves, which are
still growing. Above, they are too young, not large enough; below, they
are too old, too tough, too difficult to manage.
The leaf selected belongs to the intermediate rows. Though still of a
doubtful green, in which yellow predominates, soft and shiny with
varnish, it has very nearly attained the final dimensions. Its
denticulations swell into delicate glandular pads, whence oozes a
little of the viscous matter that smears the buds at the moment when
their scales separate.
Now a word on the tools. The legs are provided with two claws shaped
like the hook of a steel-yard. The lower side of the tarsi carries a
thick brush of white bristles. Thus shod, the insect very nimbly climbs
the most slippery perpendicular walls; it can stand and run like a Fly,
back downwards, on the ceiling of a glass bell. This characteristic
alone is enough to suggest the delicate balance which its work will
demand.
The beak, the curved and powerful rostrum, without being exaggerated in
size, like those of the Balanini, expands at the tip into a spatula
ending in a pair of fine shears. It makes an excellent stylet, which
plays the first part of all.
The leaf, as a matter of fact, cannot be rolled in its actual
condition. It is a living sheet which, owing to the rush of the sap and
the resilience of the tissues, would recover its flatness while the
insect was endeavouring to bend it. The dwarf has not the strength to
master an object of this size, to roll it up so long as it retains the
elasticity of life. This is obvious to our eyes; it is obvious likewise
to the Weevil’s.
How is she to obtain the degree of lifeless flexibility required in the
circumstances? We might say:
‘The leaf must be plucked, allowed to fall to the earth and manipulated
on the ground when sufficiently faded.’
The Weevil knows more than we do about these things and does not share
our opinion. What she says to herself is:
‘On the ground, amid the intricate obstructions of the grass, my task
would be impracticable. I want elbow-room; I want the thing to hang in
the air, free from any obstacle. And there is a more important
condition: my larva would refuse a rank, withered sausage; it insists
on food that retains a certain freshness. The cylinder which I intend
for its consumption must be not a dead leaf but an enfeebled leaf, not
entirely deprived of the juices with which the tree supplies it. I must
wean my leaf and not kill it outright, so that, when dead, it will
remain in its place during the few days of the grub’s extreme youth.’
The mother therefore, having made her selection, takes up her stand on
the stalk of the leaf and there patiently inserts her rostrum, turning
it with a persistency that denotes the great importance of this
stiletto-thrust. A little wound opens, a fairly deep wound, which soon
becomes a speck of decay.
It is done: the conduits are cut and allow only a small quantity of sap
to ooze into the edge. At the injured point the leaf yields under its
own weight; it droops perpendicularly, becomes slightly withered and
soon acquires the requisite flexibility. The moment has come for
operating on it.
That stiletto-thrust represents, though much less scientifically, the
prick of the Hunting Wasp’s sting. [42] The latter wants for her
offspring a prey now dead, now paralysed: she knows, with the
thoroughness of a consummate anatomist, at what points it behoves her
to insert her lancet to procure either sudden death or merely a
suppression of movement. The Rhynchites requires for hers a leaf
rendered flexible, half-alive, in a sense paralysed, which can be
easily fashioned into a cylinder; she is perfectly familiar with the
little leaf-stalk, the petiole, in which the vessels that disperse the
energy of the foliage are gathered in a tiny bundle; and she inserts
her drill here, here only and never elsewhere. Thus at one blow,
without much trouble, she effects the ruin of the aqueduct. Where can
the long-nosed insect have learnt her clever trick of draining springs?
The leaf of the poplar is an irregular rhombus, a spear-head whose
sides are expanded into pointed wings. The manufacture of the cylinder
begins with one of these two lateral corners, the right or the left
indifferently.
Despite the hanging posture of the leaf, which makes the upper or lower
surface equally easy of access, the insect never fails to take up its
position on the upper side. It has its reasons, dictated by the laws of
mechanics. The upper surface, which is smooth and more flexible, has to
form the inside of the cylinder; the under surface, which has greater
elasticity because of its powerful veins, has to occupy the outside.
The statics of the small-brained Weevils agrees with that of the
scientists.
Watch her at work. She is standing on the line along which the leaf is
rolled, with three legs on the part already rolled and the three
opposite legs on the part still free. Firmly fixed on both with her
claws and tufts, she obtains a purchase with the legs on one side while
straining with the legs on the other side. The two halves of the
machine alternate as motive powers, so that at one moment the shaped
cylinder encroaches on the free leaf and at another the free leaf moves
and is applied to the cylinder already formed.
There is nothing regular, however, about these alternations, which
depend upon circumstances known to the insect alone. Perhaps they
merely enable the insect to take a brief rest without suspending a task
which does not allow of interruptions. In the same way our two hands
mutually relieve each other by taking it in turns to carry a burden.
It is impossible to form an exact image of the difficulties overcome
without watching, for hours on end, the obstinate straining of the
legs, which tremble with exhaustion and threaten to jeopardize
everything should one of them let go at the wrong moment, or without
seeing how prudently the leaf-roller refrains from releasing one claw
until the five others are firmly anchored. On the one hand are three
points of support, on the other three points of traction; and the six
points are shifted, one by one, little by little, without for an
instant allowing their mechanical system to become relaxed. A single
moment of forgetfulness or weariness would cause the refractory leaf to
unroll its cylinder and escape from the manipulator’s grasp.
The work is performed, moreover, in an uncomfortable position. The leaf
hangs, almost or even quite vertically. Its surface is varnished and as
smooth as glass. But the worker is shod accordingly. With her tufted
soles, she scales polished and perpendicular surfaces; with her twelve
meat-hooks, she grapples the slippery floor.
Yet this fine equipment does not rid the operation of all its
difficulties. I find it no easy matter to follow the progress through
the magnifying-glass. The hands of a watch do not move more slowly. For
a long while the insect stands still, at the same point, with its claws
firmly fixed: it is waiting for the leaf to take the curve and cease to
react. Here, of course, there is no glue to set hard and hold the fresh
surfaces stuck together. The stability depends purely on the flexion
acquired. And so it is not unusual for the elasticity of the leaf to
overcome the worker’s efforts and partly to unroll the more or less
complete work. Stubbornly, with the same impassive slowness, the insect
begins all over again, putting the unsubjected piece back into its
place. No, the Weevil is not one to allow herself to be upset by
failure: she knows too well what patience and time can do.
As a rule, the Rhynchites works backwards. When her line is finished,
she is careful not to abandon the fold which she has just made in order
to return to the starting-point and begin another. The part last folded
is not yet sufficiently subdued; if left to itself too soon, it might
prove rebellious and flatten out again. The insect therefore continues
at this extreme point, which is more exposed than the rest, and then,
without letting go, makes her way backwards to the other end, always
with patient deliberation. In this manner, an added firmness is
imparted to the new fold; and the next fold is prepared. At the end of
the line, there is a fresh prolonged halt and a fresh move backwards.
Even so does the husbandman plough the furrows in alternate directions.
Less frequently, no doubt when the leaf is found to be so limp as to
entail no risk, the insect abandons the fold which it has just made,
without going over it again conversely, and quickly scrambles back to
the starting-point to make another.
Here we are at last. Coming and going from top to bottom and from
bottom to top, the insect, by dint of stubborn dexterity, has rolled
its leaf. It is now at the extreme edge of the leaf, at the lateral
corner opposite to that whereat the work began. This is the keystone on
which the stability of the rest depends. The Rhynchites redoubles her
efforts and her patience. With the tip of her rostrum, expanded
spatula-wise, she presses, point by point, the edge to be fixed, even
as the tailor presses the rebellious edges of a seam with his iron. For
a long, a very long time, without moving, she pushes and pushes,
awaiting a proper degree of adhesion. Point by point, the whole welt of
the corner is minutely and carefully made fast.
How is adhesion obtained? If only some sort of thread were employed,
one might very well regard the rostrum as a sewing-machine, inserting
its needle at right angles into the stuff. But the comparison is not
permissible: there is no filament employed in the work. The explanation
of the adhesion lies elsewhere.
The leaf is young, we said; the fine pads of its denticulations are
glands emitting traces of liquid glue. These drops of sticky matter are
the gum, the sealing-wax. By the pressure of its beak, the insect makes
it flow more abundantly from the glands. It then has only to hold the
signet in position and wait for the viscous seal to set. Taken all
round, this is our own method of sealing a letter. If it holds ever so
lightly, the leaf, losing its resilience as it gradually withers, will
soon cease to react and will of itself retain the cylindrical shape
imposed upon it.
The work is finished. It is a cigar of the diameter of a thick straw
and about an inch long. It hangs perpendicularly from the end of the
stalk bruised and bent at a sharp angle. It has taken the whole day to
manufacture. After a short spell of rest, the mother tackles a second
leaf and, working by night, obtains another cylinder. Two in
twenty-four hours is as much as the most diligent can achieve.
Now what is the roller’s object? Can she be preparing preserves for her
own use? Obviously not: no insect, where itself alone is concerned,
devotes such care and patience to the preparation of food. It is only
with a view to the family that it hoards so industriously. The
Rhynchites’ cigar forms a dowry for the future.
Let us unroll it. Here, between the layers of the cylinder, is the egg;
often there are two, three or even four. They are oval, pale-yellow,
like fine drops of amber. Their adhesion to the leaf is very slight;
the least jerk loosens them. They are distributed without order, tucked
away more or less deeply in the thickness of the cigar and always
isolated, one at a time. We find them in the centre of the scroll,
almost at the corner where the rolling begins; we come upon them
between the different layers and even near the edge sealed in glue with
the signet of the rostrum.
Without interrupting her work on the cylinder, without relaxing the
tension of her claws, the mother laid them between the edges of the
fold which she was forming, as she felt them coming, duly matured, at
the end of her oviduct. She produces life in the very midst of her
labours, amid the wheels of the machine which would be thrown out of
gear if she snatched a moment’s rest. Industry and procreation go
hand-in-hand. Short-lived, with but two or three weeks before her and
an expensive family to establish, the mother Rhynchites would not dare
to waste time in being churched.
This is not all: on the same leaf, not far from the cylinder that is
being laboriously rolled, we almost always find the male. What is he
doing there, the lazybones? Is he watching the work as a mere onlooker,
who happened to be passing and stopped to see the wheels go round? Is
he interested in the business? Does he ever feel inclined to lend a
helping hand in case of need?
One would say so. From time to time I see him take his stand behind his
industrious mate, in the furrow of the fold, hang on to the cylinder
and join in the work for a little. But it is done listlessly and
awkwardly. A bare half-turn of the wheel; and that’s enough for him.
After all, it is not his affair. He moves away, to the other end of the
leaf; he waits, he looks on.
We will give him credit for this attempt, since paternal assistance in
settling the family is rare among insects; we will congratulate him on
the help which he gives, but not to excess: his assistance is
interested. It is a means of declaring his flame and urging his merits.
And in fact, after several refusals, notwithstanding the advances made
during a brief collaboration on the cylinder, the impatient suitor is
accepted. Everything takes place on the site of the female’s labours.
For ten minutes or so the rolling is suspended, but the worker’s legs,
violently contracted, are very careful not to let go: were their effort
to cease, the cylinder might at once come unrolled. There must be no
interruption of work for this brief diversion, the insect’s only
enjoyment.
The stoppage of the machine, which remains tense in order to keep the
recalcitrant roll in subjection, is soon over. The male retires to a
little distance, without quitting the leaf, and the task is resumed.
Sooner or later, before the seals are set upon the work, a fresh visit
is paid by the dawdler, who, under pretence of assisting, comes running
up, sticks his claws for a moment into the partly-rolled piece, plucks
up courage and renews his exploits with as much liveliness as though
nothing had yet occurred. And this is repeated three or four times
during the making of a single cigar, so much so that we begin to wonder
whether the laying of each egg may not require the direct co-operation
of the insatiable swain.
It is true that numerous couples are formed in the sunlight, on the
leaves not yet punctured. Here the nuptial gambols are really a frolic
unaffected by the stern demands of labour. The insects revel
unreservedly, hustling their rivals off the field and browsing on half
the thickness of a leaf, which becomes furrowed with bare streaks
resembling a freakish handwriting. The fatigues of the workshop are
preceded by merry-making in gay company.
According to the rules of entomology, once these rejoicings are over,
all should be quiet again and each mother should get to work on her
cigars without further disturbance. In this case the general law
relents. I have never seen a cylinder formed without a male lurking in
the neighbourhood; and if I had the patience to wait, I should not fail
to witness repeated pairings. These weddings renewed for each egg
puzzle me. Where, on the faith of the text-books, I looked for a single
mating, I find an indefinite number.
This is not an isolated instance. I will mention a second, which is
even more striking. It is supplied by the Capricorn (Cerambyx heros). I
rear a few couples in captivity, with sliced pears for food and with
oak billets wherein to lay the eggs. The pairing is continued during
almost the whole of July. For four weeks the long-horned Beetle does
nothing but mount his mate, who, gripped by her rider, wanders at will
and, with the point of her oviscapt, selects the fissures in the bark
best-suited to receive the eggs.
At long intervals, the Cerambyx steps off and goes to refresh himself
on a piece of pear. Then suddenly he stamps his feet as though he had
gone mad; he returns with a frantic rush, clambers into the saddle and
resumes his seat, of which he makes free use at all hours of the night
and day. At the moment when the egg is being placed in position, he
keeps quiet; with his hairy tongue he polishes the egg-layer’s back,
which is a Capricorn’s way of caressing; but the next instant he renews
his attempts, which are usually followed by success. There is no end to
it!
The pairing continues in this manner for a month; it does not cease
until the ovaries are exhausted. Then, mutually worn out, having no
further business on the trunk of the oak, husband and wife separate,
languish for a few days, and die.
What conclusion are we to draw from this extraordinary persistency in
the Cerambyx, the Rhynchites and many others? Simply this: our truths
are but provisional; assailed by the truths of to-morrow, they become
entangled with so many contradictory facts that the last word of
knowledge is doubt.