THE SPINNING APPARATUS, AND THE FEET
Seeing that the possession of spinnerets is a characteristic of all
spiders, and that a great deal of the interest attaching to their
life-history arises from their spinning operations, any account of
the group, however brief, would be incomplete without some attempt to
describe these remarkable organs.
Among the spiders to which the attention of the reader has been
directed, some have been highly accomplished spinners, constructing
complicated snares, retreats and egg-cocoons, while in the case of
others the spinning work is very meagre and employed chiefly for the
protection of the eggs. As might be expected, the organs attain a very
much higher development in some spiders than in others, and the most
complex of all are those of the Epeiridae, the constructors of the
circular snare.
Now in the first place it is rather striking that the spiders with the
most conspicuous spinnerets are by no means the most able spinners. The
“bird-eating” spiders are a case in point, for they spin very little,
yet two of their spinnerets are much more obvious than anything Epeira
has to show, for they protrude behind the body and strike the eye at
the first glance. Indeed excessive length has nothing to do with
complexity but is found wherever a wide sweep is necessary in laying
down the threads—as we saw in the case of Agelena, when constructing
its sheet-web.
Roughly speaking, the spinnerets are very mobile finger-like
projections, generally situated under the hind end of the abdomen and,
bearing more or less numerous tubes from which the silken threads
proceed. The usual number of spinnerets is six, but there is a pretty
wide range, one group of spiders having only two, while a few possess
eight.
The spinnerets, then, are only the bearers of the actual tubes which
emit the silk. The distribution of the tubes themselves is different
in the different kinds of spiders, but it is usually possible to
distinguish two kinds. There are generally present a large number of
very fine cylindrical tubes or “spools” and a few conical tubes of much
larger base, which are called spigots. Each of these orifices, whether
on spool or spigot, is connected by a fine tube with a separate silk
gland, or organ for manufacturing silk, situated within the spider’s
abdomen. Epeira has about 600 of such glands, each with its own
terminal spool or spigot, and the large number of these tubes has given
rise to a misconception that is very widely spread—namely that the
spider’s line, fine as it is, is “woven” of hundreds of threads of very
much finer silk. This is not so, as we shall presently see.
Though Epeira has some 600 silk-glands, it has only five different
_kinds_ of gland, manufacturing silk of different properties. No other
family of spiders has so many, though two other kinds of gland have
been found in less elaborate spinners. Within the spider the silk is
fluid but it solidifies on meeting the air, each thread hardening as it
emerges though still continuous with the fluid contents of the gland,
so that the drawing out of a silken thread is just like the operation
so familiar with the glue-pot, or with spun glass, except that the
hardening is not due to cooling but to exposure to the air. This
general description will, it is hoped, make an account of the organs in
Epeira more comprehensible.
The spinnerets of Epeira are so small and inconspicuous that their
disposition is not very easy to make out. When not in use they form
a tiny cone under the tip of the abdomen, and only four are visible,
their free ends being so brought together as entirely to conceal a
small central pair. There are really, then, three pairs of spinnerets
which we may call at once the anterior, median and posterior pairs,
though when at rest only the anteriors and posteriors can be seen.
If the spider is observed with a pocket-lens as it crawls about in a
glass tube it will be noticed that the spinnerets are capable of great
mobility. Their ends can be separated or brought together, or they may
be made to rub against each other or against the sides of the tube.
The anteriors and posteriors, moreover, are two-jointed though the
medians consist only of a single joint.
So much can be seen without any great magnification, but the microscope
will be necessary if a complete understanding of their mechanism is to
be arrived at. What it reveals will now be briefly described, and will,
it is hoped, be made tolerably clear by the accompanying figures which
are simplified by the omission of a large number of bristles which tend
to hide the essential structure, and by a great reduction in the number
of “spools,” though the spigots are all indicated.
The anterior spinneret (that nearest the head end of the animal) is a
sort of cone, divided into a large basal joint and a small terminal
joint. The latter bears on its inner side a single spigot (fig. 12 _a_)
and is crowned with a battery of spools, about a hundred in number.
The median spinneret has three spigots, two at the tip and one on the
inner side (fig. 12 _b_), and about a hundred spools, mostly on its
inner surface.
The posterior spinneret is divided very obliquely into two joints, so
that the terminal joint extends much lower down on the inner than on
the outer side. It has five spigots in groups of three and two, and
again there are about a hundred spools.
Now the point that I wish to make clear is that there is no
interweaving of the output of these various spools and spigots. At the
moment of emission the threads are adhesive, and can be made to stick
to the glass or to one another, but they are not in any sense either
fused or interwoven. For ordinary operations the brunt of the work is
borne by the spigots marked _a_ in the figure, sometimes reinforced
by silk from the spigots on the median spinnerets marked _b_, the
functions of all the other spools and spigots being special and
occasional. For instance, when Epeira is laying down a foundation line,
this is what happens. The spider sits down, so to speak, on a twig,
separating its spinnerets and rubbing them on the surface. As it raises
its abdomen a multitude of little threads are seen merging into what
appears to be a single line.
[Illustration: Fig. 12. View, from the inner side, of one of each of
the three spinnerets of _Epeira_. _A_, anterior; _B_, median; _C_,
posterior spinneret.]
In reality the line is double, emerging from the spigots (_a_) on
the anterior spinnerets, and it can easily be separated into two—and
two only—any where along its length. The multitudinous spools have
emitted short lengths of silk to anchor the foundation line at its
commencement, but they are then closed and have no share in the
ever-lengthening line as the spider lets itself drop or crawls away
to attach it to a new spot. One of their uses, then, is to anchor
the main lines from the spigots to external objects, but they have
another function not less important. Everybody has seen a garden-spider
trussing up a captured fly. It is held in the jaws and front legs and
slowly revolved while with its hind legs the spider draws out bands of
silk from the spinnerets and swathes it like a mummy. No silken rope,
this, of fused or interwoven threads, but a broad band, every strand
of which is separate and distinct and proceeds from a different spool.
Two or three hundred fine threads wound simultaneously round the insect
form a much more effectual winding sheet than would a single cord
composed of them all.
So far we have accounted for the spools, and for one pair of
spigots—those on the anterior spinnerets. The lower spigot (_b_) on
the middle spinneret often assists in laying down a foundation line
when extra strength is required. In that case the line is fourfold, and
can easily be split into four along its whole length, the threads from
the middle spinnerets being rather finer than those from the anterior,
but composed of the same kind of silk.
There remain seven pairs of spigots whose function has still to be
explained, two on the middle and five on the posterior spinnerets. The
three which are clustered together on the posterior spinneret do not
form _silk_ at all, that is, the material they emit does not harden on
exposure to the air but remains fluid and adhesive. When the spider is
spinning the “viscid spiral” of its web it is from these spigots that
the sticky matter oozes, enveloping the true silken lines and presently
resolving itself into little globules in the manner already described.
The remaining spigots—two on the middle and two on the posterior
spinnerets are employed only in spinning the egg-cocoon, and the silk
they produce is unlike that used in making the snare, being much
stronger and less elastic, and—in the case of the garden-spider—of
a yellow colour. In the occasional attempts which have been made to
substitute spiders for silkworms as commercial silk producers, it is
only this cocoon silk that has given any considerable results, the
produce of the other glands being far too frail for profitable use.
Such attempts, however, have always failed, principally for a reason
quite unconnected with the particular nature of the silk, namely,
the difficulty of keeping the spiders in captivity. It is a simple
matter to supply dozens of silkworms in the same box with mulberry
leaves, but spiders require separate compartments or they will fight
and devour each other, and the provision of suitable food for them is
such a troublesome matter that it has proved quite impracticable on a
commercial scale.
We have incidentally seen that there are quite a number of different
operations in which the spinning apparatus takes part. There is the
line which most spiders lay down as they wander, and which secures
them from the danger of a fall if they lose their footing; there is
the snare for catching prey, the nest or retreat, and the egg-cocoon,
and in addition, silk from the spinnerets may be used to enwrap and
paralyse captured insects, or to assist the young spider to migrate.
Since the Epeiridae perform all these operations, and are, moreover,
the most finished of snare-makers, it does not surprise us to find in
them the highest development of the silk glands and the most complete
battery of spools and spigots on the spinnerets. Many spiders, as
we know, make no snare at all, and in the case of some, very little
spinning is attempted beyond the manufacture of a rather rudimentary
covering for the eggs. Naturally a less complex spinning apparatus is
required, and we accordingly find that jumping spiders, for instance,
have only about fifty silk-glands comprising three different kinds of
gland, while the glands found in such of the large Aviculariidae as
have been examined have been all alike.
There is in some spiders a spinning organ, not to be found in
Epeira, which deserves a passing notice. It does not take the place
of spinnerets, of which the usual three pairs are present, but it
is situated in front of them, and only occurs in the female of the
species. Its peculiarity is that the silk does not emerge from
projecting spools; but through fine holes in a sieve-like plate, called
a _cribellum_, which is flush with the surface of the abdomen. It has
no mobility, therefore, and the threads from it have to be combed out
and distributed by the spider’s hind leg. For the better accomplishment
of this purpose there is a special comb of stiff hairs or bristles,
called a _calamistrum_, on each of the fourth pair of legs.
The web of these spiders is not unlike that of Agelena, but of a rather
finer texture, and it can be seen, on magnification, to consist of
an irregular ground-work over which have been spread wavy bands of
excessively fine silk, combed out from the orifices of the cribellum
glands. Some of these cribellate-spiders, of the genus _Amaurobius_,
are not uncommon in our cellars and out-houses; their bodies are of
stouter build and their legs much shorter than those of the common
house-spider.
We have no space for anything approaching a full description of the
anatomy of spiders, but there is one other point of structure of which
the reader has been promised some account. Attention was directed to
the fact that while some spiders are helpless on smooth perpendicular
surfaces unless they have lines to cling to, others can run with ease
upon the walls or even the ceiling, of a room.
[Illustration: Fig. 13. Foot of Jumping Spider (on left), foot of
Garden Spider (on right).]
The last joint or _tarsus_ of the spider’s leg is very different in
the two cases. It always terminates in claws—either two or three—so
that any species can make some show of climbing where the surface is
rough and there is anything to cling to, but to obtain a hold on a
polished surface it needs a special contrivance. This takes the form
of a pad of curiously modified hairs, called a _scopula_. The hairs
are club-shaped, narrow at their stalk and swelling towards the tip,
and their clinging power seems to be due to a viscid secretion. The
foot of any jumping spider will show this structure well. Epeira has no
scopula, and its climbing is always laborious unless it has a thread
to cling to, but it is supreme as a rope walker, treading daintily on
the most delicate threads, mounting a line “hand over hand” with great
agility, and manipulating the silk in its various spinning operations
with unerring skill and facility.