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THE GARDEN SPIDERS: THE LIME-SNARE




The spiral network of the Epeirae possesses contrivances of fearsome
cunning. Let us give our attention by preference to that of the Banded
Epeira or that of the Silky Epeira, both of which can be observed at
early morning in all their freshness.

The thread that forms them is seen with the naked eye to differ from that
of the framework and the spokes. It glitters in the sun, looks as though
it were knotted and gives the impression of a chaplet of atoms. To
examine it through the lens on the web itself is scarcely feasible,
because of the shaking of the fabric, which trembles at the least breath.
By passing a sheet of glass under the web and lifting it, I take away a
few pieces of thread to study, pieces that remain fixed to the glass in
parallel lines. Lens and microscope can now play their part.

The sight is perfectly astounding. Those threads, on the borderland
between the visible and the invisible, are very closely twisted twine,
similar to the gold cord of our officers' sword-knots. Moreover, they
are hollow. The infinitely slender is a tube, a channel full of a
viscous moisture resembling a strong solution of gum arabic. I can see a
diaphanous trail of this moisture trickling through the broken ends.
Under the pressure of the thin glass slide that covers them on the stage
of the microscope, the twists lengthen out, become crinkled ribbons,
traversed from end to end, through the middle, by a dark streak, which is
the empty container.

The fluid contents must ooze slowly through the side of those tubular
threads, rolled into twisted strings, and thus render the network sticky.
It is sticky, in fact, and in such a way as to provoke surprise. I bring
a fine straw flat down upon three or four rungs of a sector. However
gentle the contact, adhesion is at once established. When I lift the
straw, the threads come with it and stretch to twice or three times their
length, like a thread of India-rubber. At last, when over-taut, they
loosen without breaking and resume their original form. They lengthen by
unrolling their twist, they shorten by rolling it again; lastly, they
become adhesive by taking the glaze of the gummy moisture wherewith they
are filled.

In short, the spiral thread is a capillary tube finer than any that our
physics will ever know. It is rolled into a twist so as to possess an
elasticity that allows it, without breaking, to yield to the tugs of the
captured prey; it holds a supply of sticky matter in reserve in its tube,
so as to renew the adhesive properties of the surface by incessant
exudation, as they become impaired by exposure to the air. It is simply
marvellous.

The Epeira hunts not with springs, but with lime-snares. And such lime-
snares! Everything is caught in them, down to the dandelion-plume that
barely brushes against them. Nevertheless, the Epeira, who is in
constant touch with her web, is not caught in them. Why?

Let us first of all remember that the Spider has contrived for herself,
in the middle of her trap, a floor in whose construction the sticky
spiral thread plays no part. We saw how this thread stops suddenly at
some distance from the centre. There is here, covering a space which, in
the larger webs, is about equal to the palm of one's hand, a fabric
formed of spokes and of the commencement of the auxiliary spiral, a
neutral fabric in which the exploring straw finds no adhesiveness
anywhere.

Here, on this central resting-floor, and here only, the Epeira takes her
stand, waiting whole days for the arrival of the game. However close,
however prolonged her contact with this portion of the web, she runs no
risk of sticking to it, because the gummy coating is lacking, as is the
twisted and tubular structure, throughout the length of the spokes and
throughout the extent of the auxiliary spiral. These pieces, together
with the rest of the framework, are made of plain, straight, solid
thread.

But, when a victim is caught, sometimes right at the edge of the web, the
Spider has to rush up quickly, to bind it and overcome its attempts to
free itself. She is walking then upon her network; and I do not find
that she suffers the least inconvenience. The lime-threads are not even
lifted by the movements of her legs.

In my boyhood, when a troop of us would go, on Thursdays, {31} to try and
catch a Goldfinch in the hemp-fields, we used, before covering the twigs
with glue, to grease our fingers with a few drops of oil, lest we should
get them caught in the sticky matter. Does the Epeira know the secret of
fatty substances? Let us try.

I rub my exploring straw with slightly oiled paper. When applied to the
spiral thread of the web, it now no longer sticks to it. The principle
is discovered. I pull out the leg of a live Epeira. Brought just as it
is into contact with the lime-threads, it does not stick to them any more
than to the neutral cords, whether spokes or parts of the framework. We
were entitled to expect this, judging by the Spider's general immunity.

But here is something that wholly alters the result. I put the leg to
soak for a quarter of an hour in disulphide of carbon, the best solvent
of fatty matters. I wash it carefully with a brush dipped in the same
fluid. When this washing is finished, the leg sticks to the
snaring-thread quite easily and adheres to it just as well as anything
else would, the unoiled straw, for instance.

Did I guess aright when I judged that it was a fatty substance that
preserved the Epeira from the snares of her sticky Catherine-wheel? The
action of the carbon disulphide seems to say yes. Besides, there is no
reason why a substance of this kind, which plays so frequent a part in
animal economy, should not coat the Spider very slightly by the mere act
of perspiration. We used to rub our fingers with a little oil before
handling the twigs in which the Goldfinch was to be caught; even so the
Epeira varnishes herself with a special sweat, to operate on any part of
her web without fear of the lime-threads.

However, an unduly protracted stay on the sticky threads would have its
drawbacks. In the long run, continual contact with those threads might
produce a certain adhesion and inconvenience the Spider, who must
preserve all her agility in order to rush upon the prey before it can
release itself. For this reason, gummy threads are never used in
building the post of interminable waiting.

It is only on her resting-floor that the Epeira sits, motionless and with
her eight legs outspread, ready to mark the least quiver in the net. It
is here, again, that she takes her meals, often long-drawn-out, when the
joint is a substantial one; it is hither that, after trussing and
nibbling it, she drags her prey at the end of a thread, to consume it at
her ease on a non-viscous mat. As a hunting-post and refectory, the
Epeira has contrived a central space, free from glue.

As for the glue itself, it is hardly possible to study its chemical
properties, because the quantity is so slight. The microscope shows it
trickling from the broken threads in the form of a transparent and more
or less granular streak. The following experiment will tell us more
about it.

With a sheet of glass passed across the web, I gather a series of lime-
threads which remain fixed in parallel lines. I cover this sheet with a
bell-jar standing in a depth of water. Soon, in this atmosphere
saturated with humidity, the threads become enveloped in a watery sheath,
which gradually increases and begins to flow. The twisted shape has by
this time disappeared; and the channel of the thread reveals a chaplet of
translucent orbs, that is to say, a series of extremely fine drops.

In twenty-four hours, the threads have lost their contents and are
reduced to almost invisible streaks. If I then lay a drop of water on
the glass, I get a sticky solution, similar to that which a particle of
gum arabic might yield. The conclusion is evident: the Epeira's glue is
a substance that absorbs moisture freely. In an atmosphere with a high
degree of humidity, it becomes saturated and percolates by sweating
through the side of the tubular threads.

These data explain certain facts relating to the work of the net. The
full-grown Banded and Silky Epeirae weave at very early hours, long
before dawn. Should the air turn misty, they sometimes leave that part
of the task unfinished: they build the general framework, they lay the
spokes, they even draw the auxiliary spiral, for all these parts are
unaffected by excess of moisture; but they are very careful not to work
at the lime-threads, which, if soaked by the fog, would dissolve into
sticky shreds and lose their efficacy by being wetted. The net that was
started will be finished to-morrow, if the atmosphere be favourable.

While the highly-absorbent character of the snaring-thread has its
drawbacks, it also has compensating advantages. Both Epeirae, when
hunting by day, affect those hot places, exposed to the fierce rays of
the sun, wherein the Crickets delight. In the torrid heats of the dog-
days, therefore, the lime-threads, but for special provisions, would be
liable to dry up, to shrivel into stiff and lifeless filaments. But the
very opposite happens. At the most scorching times of the day, they
continue supple, elastic and more and more adhesive.

How is this brought about? By their very powers of absorption. The
moisture of which the air is never deprived penetrates them slowly; it
dilutes the thick contents of their tubes to the requisite degree and
causes it to ooze through, as and when the earlier stickiness decreases.
What bird-catcher could vie with the Garden Spider in the art of laying
lime-snares? And all this industry and cunning for the capture of a
Moth!

Then, too, what a passion for production! Knowing the diameter of the
orb and the number of coils, we can easily calculate the total length of
the sticky spiral. We find that, in one sitting, each time that she
remakes her web, the Angular Epeira produces some twenty yards of gummy
thread. The more skilful Silky Epeira produces thirty. Well, during two
months, the Angular Epeira, my neighbour, renewed her snare nearly every
evening. During that period, she manufactured something like
three-quarters of a mile of this tubular thread, rolled into a tight
twist and bulging with glue.

I should like an anatomist endowed with better implements than mine and
with less tired eyesight to explain to us the work of the marvellous rope-
yard. How is the silky matter moulded into a capillary tube? How is
this tube filled with glue and tightly twisted? And how does this same
wire-mill also turn out plain threads, wrought first into a framework and
then into muslin and satin; next, a russet foam, such as fills the wallet
of the Banded Epeira; next, the black stripes stretched in meridian
curves on that same wallet? What a number of products to come from that
curious factory, a Spider's belly! I behold the results, but fail to
understand the working of the machine. I leave the problem to the
masters of the microtome and the scalpel.





Next: THE GARDEN SPIDERS: THE TELEGRAPH-WIRE

Previous: THE GARDEN SPIDERS: MY NEIGHBOUR



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