THE CIRCULAR SNARE
Select the most perfect circular snare at hand, and examine it
attentively. In the autumn, when the large garden-spider, _Epeira
diademata_ (fig. 2 _A_), is mature, it will probably be easy to find
such a snare a foot or more in diameter. It is stretched within an
irregular frame of _foundation lines_ of extra thickness and strength,
and consists of a large number of radii or spokes connected by what
appear to be a series of concentric circles, in reality a continuous
spiral, like the hair-spring of a watch. The central portion is
different from the rest of the wheel. Probably in the very centre
there is a vacant space and round this a hub, consisting of a spiral
line different in appearance from that of the main spiral. It does
not leave a radius exactly at the point where it strikes it, and the
rather zig-zag effect has caused it to be known as the “notched zone.”
Touch the web and it adheres to the finger, but all its lines are
not adhesive. Test this with some fine-pointed implement, and the
foundation lines, the radii and the notched zone will give negative
results; the spiral line alone is viscid, and its viscidity is due
to the presence of thousands of little beads of gummy matter strung
on a thin elastic thread. The vast number and uniformity of these
beads—estimated at 120,000 on a large web—excited the wonder and
admiration of naturalists until it was proved that they were not
deposited by the spider as beads at all, but as a uniform coating of
viscid matter which subsequently arranged itself into equidistant
globules easily explicable by the physicist. Indeed precisely the same
phenomenon is seen on a dew-laden web, where similar but very much
larger beads of water decorate all the lines.
From the hub of the wheel we shall very likely notice a rather stout
cable diverging from the plane of the snare and leading to a nest of
leaves spun together. Here the spider is to be found when not on duty
in the centre of the wheel, and here it constructs its egg-cocoons.
This, then, is the complete circular snare, but we shall understand it
much better if we watch the spider at work in its construction.
[Illustration: Fig. 2. _A_, the Garden Spider. _B_, diagram of a sector
of the snare. _f_, foundation line; _r_, radius; _v.s_, viscid spiral;
_n.z_, notched zone; _h_, hub.]
The first business of the spider is to lay down the foundation lines.
Any sort of trapezium—or even a triangle if large enough—in a more
or less vertical plane will suffice, and under some circumstances
the operation is simple enough. The spider attaches a line at the
point of departure and crawls along, spinning as it goes and holding
up the newly-spun thread by the claws of one of its hind feet, till
it reaches a suitable spot for its farthest limit. It then hauls in
the slack and makes it fast. It will probably return along the line
thus laid down—still spinning—to the starting point, thus doubling
the strength of the cable, and indeed a large spider will often repeat
this operation several times. Now the upper boundary of the future web
is secured. It is next necessary to find points of attachment for the
lower boundary, and the spider either drops or climbs down—always
carrying a line—from one of the ends of the upper line till it reaches
a spot suitable for its purpose, and the previous performance is
repeated. If there is any difficulty about a fourth attachment it is
always open to the spider to climb back along the two lines already
laid down, and by carrying a loose line with it, to secure at all
events a triangular frame-work. This frame-work, whether trapezoid
or triangular, will be reinforced several times and made thoroughly
trustworthy before the work of making the actual snare is proceeded
with.
Now the foregoing operation is obviously perfectly simple in certain
cases, as, for instance when a spider has chosen lattice work, or the
mouth of an empty barrel as its “pitch,” but snares may easily be
found in situations where such a mode of procedure seems impossible.
In a pine forest, for example, one may see huge webs stretched at a
great height from the ground between boles ten feet apart; or one may
find such a snare spread across a stream at a spot where the trees on
either side do not intermingle their boughs. How in such cases does the
spider accomplish its purpose?
There is little doubt that, wherever practicable, the spider walks
round, sometimes crawling quite an astounding distance, but that it can
at need, resort to another method, is easily proved by a very simple
experiment in the house. Fill any vessel—a basin or a bath—with water
and arrange an upright post in the middle, placing a spider upon it.
If the air in the room is absolutely still the captive is powerless to
escape, but if draughts are present it will sooner or later disappear;
and it accomplishes this feat by emitting a thread which, caught by the
air-current, is drawn out from its spinnerets till it by and by becomes
entangled in the surrounding furniture. This power of emitting silk to
some little distance and allowing the wind to draw it out is, as we
shall see, frequently exercised in the early life of many spiders.
The foundation lines which may thus have given the spider great trouble
to secure, are of extreme importance to it, and may serve for several
snares in succession. There is little hesitation or delay about the
subsequent operations. The spokes of the wheel are readily formed by
carrying lines across to opposite points of the frame-work and uniting
them where they intersect. They are laid down in no special order, but
more or less alternation is generally noticeable—apparently for the
purpose of keeping the tension equally balanced—and the spider will
occasionally desist in order to go and brace up the frame-work with
additional stays, which generally have the effect of converting it to a
polygon.
Before long the requisite number of fairly equidistant “spokes” or
radii are visible, and then the spider, starting from the centre,
rapidly spins a spiral thread consisting of a few coils only, to the
circumference, stepping from spoke to spoke. This is only a temporary
scaffolding and will not be suffered to remain in the completed snare.
If the structure is touched at this stage of the operations it does not
adhere to the finger; the viscid spiral remains to be laid down. Though
it does not hesitate for a moment, the spider now works with a peculiar
deliberation, but the operation will be much better understood by
actual observation than by any amount of description, and we shall only
recommend the reader to note that the new spiral is exceedingly elastic
and that at the moment of its attachment to a spoke it is stretched
and let go like the string of a bow. The spider seems carefully to
avoid treading on it as it proceeds, utilising the non-viscid spiral
scaffolding already described.
[Illustration: Fig. 3. Stretching the viscid spiral.]
A little attention to the centre of the wheel, and the snare is
complete. Some species of _Epeira_ entirely remove the centre, leaving
a circular empty space, while others fill it with an irregular network
of threads.
How does the garden spider avoid getting caught in its own web? We have
shown that there are many lines which are not viscid, and no doubt
these are utilised as far as possible, but it can hardly happen that
the spider never touches adhesive portions of the web with legs or body.
Possibly some explanation is furnished by an ingenious experiment
which Fabre performed. He found that a glass rod, lightly smeared with
oil, did not adhere to the viscid spiral; neither did a leg freshly
taken from a garden-spider unless allowed to remain in contact for
a considerable time. When, however, this leg had been washed with
bisulphide of carbon, which dissolves any kind of oily substance, it
adhered at once. It would seem likely, therefore, that the legs and
body of the spider itself are protected by some oily exudation from any
danger of adherence to its own lines.