LEAF-ROLLERS
The attainments of the Curculionid mother are, generally speaking,
limited to inserting her eggs at places where the grubs will find
suitable nourishment and occasionally varying the diet with a botanical
judgment of marvellous certainty. She displays little or no industry.
The niceties of the feeding-bottle or the baby-linen do not concern
her. To this rough conception of the duties of maternity, I know but
one exception, the attribute of certain Weevils, who, in order to endow
their young with an alimentary preserve, possess the art 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 modest in
proportions, but resplendent in attire. Her back is clad in gleaming
gold and copper; her back is indigo blue. Would you see her at work,
you need but visit the lower twigs of the common black poplar, at the
edge of the meadows, about the end of May.
Whereas, up at the top, the fond spring breezes shake the majestic
green distaff and set the leaves quivering on their flattened stalks,
down below, in a zone of calmer air, the tender shoots of the year
remain quiescent. Here, especially, far from the wind-tossed heights
opposed to labour, the Rhynchites works. And, as the workshop is just
at a man’s height, nothing is more easy than to observe the roller’s
actions.
Easy, yes, but distressing, under a blazing sun, if one would follow
the insect in all the detail of its methods and the progress of its
work. Moreover, this involves a great deal of walking, which takes up
time; and, again, it is not favourable to precise observations, which
require an indefinite amount of leisure and assiduous visits at all
hours of the day. It would, therefore, be greatly preferable to study
the animal comfortably at home; but it is above all things necessary
that she should lend herself to this plan.
The Rhynchites fulfils the condition excellently well. She is a
peaceable enthusiast and works on my table with the same zest as in her
poplar-tree. A few tender shoots, planted in fresh sand, under a
woven-wire cover, and renewed as soon as they begin to fade, take the
place of the tree in my study. The Weevil, not in the least
intimidated, devotes herself to her industry even under the lens of my
magnifying-glass and supplies me with as many scrolls as I could wish
for.
Let us watch her at work. She picks the leaf which she proposes to roll
from the young shoots sprouting in sheaves at the base of the trunk,
but picks it not among the lower leaves, which are already the correct
green and of a firm texture, nor yet among the terminal leaves, which
are in a fair way of growing. Above, they are too young, not wide
enough; below, they are too old, too tough, too hard to manage.
The leaf selected belongs to the intermediate rows. As yet of a
doubtful green, in which yellow predominates, soft and glossy with
varnish, it has, or has very nearly, the final dimensions. Its
denticulations swell into delicate glandular pads, whence oozes a
little of the viscous matter that tars the buds at the moment when
their bracts become disjoined.
A word now on the equipment in respect of tools. The legs are supplied
with double claws shaped like the meat-hooks of a steel-yard. The lower
side of the tarsi carries a thick tuft of white bristles. Thus shod,
the insect clambers very nimbly up the most slippery vertical walls; it
can stand and run like a fly, with its back downwards, on the ceiling
of a glass bell. This characteristic alone is enough to suggest the
subtle sense of equilibrium which the Weevil’s work will demand.
The curved and powerful beak or rostrum, without being exaggerated in
size, spreads at the tip into a spatula ending in a pair of fine,
shear-like mandibles. It makes an excellent bodkin, which plays the
first or leading part in the whole work. The leaf, in fact, cannot be
rolled in its actual condition. It is a live blade which, owing to the
afflux of the sap and the tonicity of the tissues, would resume its
flat formation in proportion as the insect endeavoured to curve it. The
dwarf insect has not the strength to master a piece of these
dimensions, to roll it up so long as it retains the elasticity of life.
This is evident to our eyes; it is evident also to the eyes of the
Weevil.
How is she to obtain the degree of inert suppleness required in the
circumstances? We ourselves would say:
“We must pluck the leaf, let it fall to the earth, and manipulate it on
the ground when it is rightly withered.”
The Curculionid is cleverer than we at this sort of business and does
not share our opinion. What she says to herself is:
“On the ground, amid the obstruction of the grass, my labours would be
impracticable. I want elbow-room; I want the thing to hang in the air,
where there are no obstacles of any kind. And there is a more serious
consideration: my grub would refuse a rank, dried-up sausage; it
insists on food that retains a certain freshness. The scroll which I
intend for its consumption must be not a dead leaf, but an impaired
leaf, not altogether deprived of the juices with which the tree
supplies it. I must wean my joint, but not kill it outright, so that
the dying leaf may remain in its place for the few days during which
the extreme youth of the worm lasts.”
The mother, therefore, having made her selection, takes up her stand on
the stalk of the leaf and there patiently drives in her rostrum,
turning it with a persistency that denotes the great importance of this
thrust of the bodkin. A little wound opens, a fairly deep wound, which
soon becomes a point of mortification.
It is done: the sap-conduits are cut and allow only a scanty proportion
to ooze through to the edge. At the injured point, the leaf gives way
under the weight; it bends vertically, withers a little and soon
acquires the requisite flexibility. The moment has come to work it.
That bodkin-thrust represents, although much less scientifically, the
prick of the hunting Hymenopteron’s sting. The latter wants for her
offspring a prey now dead, now paralyzed; she knows, with the
thoroughness of a consummate anatomist, at what points it behoves her
to insert the sting to obtain either sudden death or merely a cessation
of movement. The Rhynchites requires for her young a leaf rendered
flexible ad hoc, half-alive, paralyzed in a fashion, a leaf that can
easily be shaped into a scroll; she is wonderfully familiar with the
little leaf-stalk, the petiole, in which the vessels that dispense the
foliaceous energy are collected in a tiny bundle; and she inserts her
drill there, there only and never any elsewhere. Thus, at one blow,
without much trouble, she effects the ruin of the aqueduct. Where can
the beaked insect have learnt her astute trade as a drier-up of wells?
The leaf of the poplar is an irregular rhombus, a spear-head whose
sides widen into pointed pinions. The manufacture of the scroll begins
with one of those two lateral corners, the right or left indifferently.
Notwithstanding the hanging posture of the leaf, which makes the upper
or lower surface equally easy of access, the insect never fails to take
its position on the upper side. It has its reasons, dictated by the
laws of mechanics. The upper surface of the piece, which is smoother
and more flexible, has to form the inside of the scroll; the lower
surface, which has greater elasticity because of its powerful veins,
must occupy the outside. The statics of the small-brained Weevil agree
with those of the scientists.
See her at work. She stands on the rolling-line, with three of her legs
on the part already rolled and the three opposite on the part still
free. Solidly fixed, on both sides, with her claws and tufts, she
obtains a purchase with the legs on the one side, while making her
effort with the legs on the other. The two halves of the machine
alternate like motors, so that, at one time, the formed cylinder rolls
over the free blade and, at another, the free blade moves and is laid
upon the scroll already made.
There is nothing regular, however, about these alternations, which
depend upon circumstances known to the animal alone. Perhaps they
merely afford a means of resting for a little while without stopping a
work that does not allow of interruption. In the same way, our two
hands mutually relieve each other by taking it in turns to carry the
burden.
It is impossible to form an exact image of the difficulty overcome,
without watching, for hours on end, the obstinate straining of the
legs, which tremble with exhaustion and threaten to spoil everything if
one of them let go at the wrong moment, or without seeing with what
prudence the roller never releases one claw until the five others are
firmly fixed. On the one side are three points of support, on the other
three points of traction; and the six are shifted, one by one, little
by little, without for a moment allowing their connected mechanical
system to flag. A single instant of forgetfulness or weariness would
cause the rebellious piece to unroll its scroll and escape from the
manipulator’s grasp.
The work is accomplished, moreover, in an uncomfortable position. The
leaf hangs very much on the slant or even vertically. Its surface is
varnished, is smooth as glass. But the worker is shod accordingly. With
her tufted soles, she scales the polished perpendicular; with her
twelve meat-hooks, she tackles the slippery floor. Yet this fine set of
tools does not rid the operation of all its difficulties. I find it no
easy matter to follow the progress of the rolling with the
magnifying-glass. The hands of a watch do not move more slowly. The
insect stands for a long time, at the same point, with its claws firmly
fixed; it is waiting for the leaf to be mastered and to cease
resistance. Here, of course, there is no gumming-process to catch hold
and keep the fresh surfaces glued together. The stability depends
purely upon the flexion acquired. And so it is not unusual for the
elasticity of the piece to overcome the efforts of the worker and
partly to unroll the more or less forward work. Stubbornly, with the
same impassive slowness, the insect begins all over again, replaces the
insubordinate piece. No, the Weevil is not one to allow herself to be
upset by failure: she knows too well what patience and time will do.
The Rhynchites usually works backwards. When her line is finished, she
is careful not to abandon the fold which she has just made and return
to the starting-point to begin another. The part last folded is not yet
fastened sufficiently; if left to itself too soon, it might easily
rebel and flatten out again. The insect, therefore, persists at this
extreme point, which is more exposed than the rest; and then, without
letting go, makes her way backwards to the other end, still with
patient slowness. In this way, an added firmness is imparted to the
fold; and the next fold is prepared. At the end of the line there is a
fresh prolonged halt, followed by a fresh backward motion. Even so does
the husbandman’s plough-share alternate its work on the furrows.
Less frequently, when, no doubt, 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 in the opposite direction, and quickly
scrambles to the starting-point to contrive a new one.
There 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 on the extreme edge of the border, at the lateral
corner opposite to that whereat the work commenced. This is the
keystone upon which the stability of the rest depends. The Rhynchites
redoubles her cares and patience. With the end of her rostrum, expanded
spatulawise, she presses, at one point after the other, the edge to be
fixed, even as the tailor presses the recalcitrant edges of a seam with
his iron. For a long, a very long time, without moving, she pushes and
pushes, awaiting the proper adhesion. Point by point, the whole of the
corner welt is fastidiously sealed.
How is adhesion obtained? If only some thread or other were brought
into play, one would readily look upon the rostrum as a sewing-machine
planting its needle perpendicularly into the stuff. But the comparison
is not allowable: there is no filament employed in the work. The
explanation of the adherence lies elsewhere.
The leaf is young, we said; the fine pads of its denticulations are
glands whence ooze liquid beads of glue. These drops of viscous matter
are the gum, the sealing-wax. With the pressure of its beak, the insect
makes it gush more plentifully from the glands. It then has only to
hold the signet in position and wait for the impress to acquire
consistency. Taken all round, it is our method of sealing a letter. If
it hold ever so little, the leaf, losing its elasticity gradually as it
withers, will soon cease to fly back and will of itself retain the
scroll-form imposed upon it.
The work is done. It is a cigar of the diameter of a thick straw and
about an inch long. It hangs perpendicularly from the end of the
bruised and bent stalk. It has taken the whole of a day to make. After
a short spell of rest, the mother tackles a second leaf and, working by
night, obtains another scroll. Two in twenty-four hours are as much as
the most diligent can achieve.
Now what is the roller’s object? Would she go to the length of
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 in view of the family that it hoards so
industriously. The Rhynchites’ cigar forms the future dowry.
Let us unroll it. Here, between the layers of the scroll, is an egg;
often there are two, three, or even four. They are oval, pale-yellow
and like fine drops of amber. Their adhesion to the leaf is very
slight; the least jerk loosens them. They are distributed without
order, pushed more or less deeply in the thickness of the cigar and
always isolated, singly. 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 which is sealed in
glue with the signet of the rostrum.
Without interrupting her work on the scroll, without relaxing the
tension of her claws, the mother has laid them between the lips of the
fold in formation, as she felt them coming, one by one, duly matured,
at the end of her oviduct. She procreates in the midst of her toil in
the factory, between the wheels of the machine that would be thrown out
of gear if she snatched a moment’s rest. Manufacture and laying go
hand-in-hand. Short-lived, with but two or three weeks before her and
an expensive family to settle, the Rhynchites would fear to waste her
time in churching.
This is not all: on the same leaf, not far from the scroll that is
being laboriously rolled, we almost always find the male. What is he
doing there, the idler? Is he watching the work as a mere inquisitive
onlooker, who happened to be passing and stopped to see the machinery
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
the manufacturer, in the groove of the fold, hang on to the cylinder
and join for a little in the work. But this is done without zeal and
awkwardly. Half a turn of the wheel, or hardly; and that’s enough for
him. After all, it is not his business. He moves away, to the other end
of the leaf; he waits, he looks on.
Let us give him credit for this attempt: paternal assistance in the
settling of the family is very rare among insects; let us congratulate
him on the help he gives, but not beyond measure: his was an interested
aid. It is to him a means of declaring his flame and urging his merits.
And, in fact, after several refusals, notwithstanding the advances made
by a brief collaboration at the scroll, the impatient one is accepted.
Things happen in the work-yard. For ten minutes, the rolling is
suspended; but the workwoman’s legs, stubbornly contracted, are careful
not to let go: were their effort to cease, the scroll would unroll at
once. There must be no interruption of work for this brief diversion,
the animal’s only pleasure.
The stopping of the machine, which is always held tight so as to keep
the recalcitrant roll in subjection, does not last long. The male
withdraws to a slight distance, without quitting the leaf, and the task
is resumed. Sooner or later, before the seals are put upon the work, a
new visit is paid by the dawdler, who, under pretence of assisting,
plants his claws for a moment into the rolling piece, plucks up courage
and renews his exploits with the same vigour as though nothing had yet
happened. And this is repeated three or four times during the making of
a single cigar, so much so that one asks one’s self whether the
depositing of each germ does not demand the direct cooperation of the
insatiable suitor.
According to entomological rules, once the fun is over, everything
should relapse into calmness and each mother should to work at those
cigars without further disturbance. In this case, the general law
relents. I have never seen a scroll shaped without a male lurking in
the neighbourhood; and, when I have had the patience to wait, I have
always witnessed manifold pairings. These weddings repeated for each
germ puzzle me. Where, relying on the books, I expected uniformity, I
find uncertainty.
This is not an isolated case. I will mention a second and one that is
even more striking. It is supplied by the Capricorn (Cerambyx Heros). I
bring up a few couples in the volery, with sliced pears for food and
oak billets wherein to lay the eggs. Pairing-time lasts during nearly
the whole of July. For four weeks, the great horned one does nothing
but mount his companion, who, gripped by her rider, wanders at will
and, with the tip of her oviduct, selects the fissures in the bark
best-suited to receive the eggs.
At long intervals, the Cerambyx alights and goes to refresh himself
with 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 position, of which he makes free use at all hours of
the night and day.
At the moment when an egg is being deposited, he keeps quiet: with his
hairy tongue, he polishes the back of the egg-layer, which is a
Capricorn’s way of caressing; but, the instant after, he renews his
attempts, which are usually crowned with 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
nothing more to do 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.
In the spring, while the leaves of the poplar are being worked into
scrolls, another Rhynchites, she also gorgeously attired, makes cigars
of the leaves of the vine. She is a little stouter, of a metallic
gold-green turning to blue. Were she but larger, the splendid Vine
Weevil would occupy a very respectable place among the gems of
entomology.
To attract our eyes, she has something better than the brilliancy of
her appearance: she has her industry, which makes her hated by the
vine-grower, so jealous of his property. The peasant knows her; he even
speaks of her by a special name, an honour rarely bestowed in the world
of the smaller animals.
The rural vocabulary is rich where plants, but very poor where insects
are concerned. A couple of dozen words, inextricably confused owing to
their general character, represent the whole of entomological
nomenclature in the Provençal idiom, which becomes so expressive and so
fertile the moment it has to do with any sort of vegetation, sometimes
even with a poor blade of grass which one would believe known to the
botanist alone.
The man of the soil is interested first and foremost in the plant, the
great foster-mother; the rest leaves him indifferent. Magnificent
adornment, curious habits, marvels of instinct: all these say nothing
to him. But to touch his vine, to eat grass that doesn’t belong to one:
what a heinous crime! Quick, give the malefactor a nickname, to serve
as a penal collar!
This time, the Provençal peasant has gone out of his way to invent a
special word: he calls the cigar-roller the Bécaru. Here the scientific
expression and the rural expression agree fully. Rhynchites and Bécaru
are exact equivalents: each refers to the insect’s long beak.
The Vine Weevil adopts the same method in her work as her cousin of the
poplar. The leaf is first pricked with the rostrum at a spot in the
stalk, which provokes a stoppage of the sap and flexibility in the
withered blade. The rolling begins at the corner of one of the lower
lobes, with the smooth, green upper surface within and the cottony
strongly-veined lower surface without.
But the great width of the leaf and its deep indentations hardly ever
allow of regular work from one end to the other. Abrupt folds occur
instead and repeatedly alter the direction of the rolling, leaving now
the green and now the cottony surface on the outside, without any
appreciable order or arrangement, as though by chance. The poplar-leaf,
with its simple form and its moderate size, gives a neat scroll; the
vine-leaf, with its cumbersome girth and its complicated outline,
produces a shapeless cigar, an untidy parcel.
THE HALICTI