MANUFACTURE OF SILK BY SPIDERS.
Our history of the silk manufacture among insects would be incomplete
without a notice of the labours of spiders. Not only do these insects
produce filmy webs to entrap their prey, but they also spin, for the
protection of their eggs, a bag not much unlike the cocoon of the
silkworm. At the beginning of the last century a method was discovered
of procuring silk from these spiders’ bags, and of making it into
several useful articles. The experiments took place in France, and it
was there discovered that two species of spider in particular produced
strong and beautiful silk, capable of being usefully employed. The
structure of these insects was closely examined by the celebrated
naturalist Réaumur, and he found that the silk is spun from five
papillæ, or small nipples, placed in the hinder part of the body; these
serve the purpose of so many wire-drawing irons, to mould a gummy
liquor, which dries as it is drawn out and exposed to the air.
[Illustration:
SPINNING APPARATUS OF THE SPIDER.
(_Greatly magnified._)
]
On pressing the body of a spider, the liquor flows into these nipples,
by applying the finger against which, distinct threads may be drawn out
through the numerous openings; and, what is very astonishing, every
separate thread is made up of innumerable smaller threads, so that
Réaumur thought himself far within the limits of the truth when he
stated that each of the five nipples supplied one thousand separate
fibres, in which case the slender filament of the spider’s nest must be
made up of five thousand fibres. By applying the whole, or a part, of
this apparatus to her work, the spider can make the thread stout or fine
at pleasure: thus the webs for entrapping flies are very slight and
fragile; but the nest for securing the eggs is much stronger, to afford
them shelter from the cold. The threads are wound loosely round the eggs
in a shape similar to that of the silkworm’s cocoon. The colour of the
silk is generally grey, becoming blackish on exposure to the air:
sometimes it is pale yellow, and also of very fine quality; but this is
the production of comparatively rare species, which could not be
depended on for the purposes of manufacture. A spider’s nest preserved
by the writer during the last winter was of a beautiful yellow, almost
approaching that of the cocoon of the silkworm. As spring approached it
increased in bulk and became rather paler, until at last a dark
appearance in the centre betokened the bursting of the eggs. At the
present time (April 10th) ninety-six small yellow-bodied spiders have
come forth, and are actively engaged in weaving their delicate webs
across the glass which contains them. A muslin cover admits air to the
interior, and these minute insects appear perfectly healthy although
deprived of their natural food. Some sugar was placed in the glass, but
they do not appear to have consumed any of it, although some of them
have been hatched for more than a fortnight. From the appearance of the
nest, more of these spiders yet remain to be hatched.
In the French experiments, spiders’ nests in large quantities were
collected from the trunks of trees, corners of windows and vaults, and
eaves of houses at the time above mentioned, and from these a new kind
of silk was obtained by M. Bon, who declared it to be in no respect
inferior to that of the silkworm. It was afterwards proved that he was
greatly mistaken in this respect; yet the spiders’ silk readily took all
kinds of dyes, and was actually wrought into stockings and gloves,
specimens of which were presented by M. Bon to the Royal Academy of
Paris, and also to the Royal Society of London. His method of preparing
the silk was as follows:—
Twelve or thirteen ounces of the bags were beaten with the hand, or by a
stick, until they were entirely free from dust. They were then washed in
warm water, which was frequently changed, until it was no longer
discoloured by the bags. They were next steeped in a large quantity of
water, wherein soap, saltpetre, and gum-arabic had been dissolved. The
whole was then set to boil over a gentle fire for three hours. Lastly,
the bags were rinsed in clear warm water, and set out to dry. They were
then fit for the operation of carding, which was performed with very
fine cards, and thus silk of a peculiar ash colour was obtained, which
was spun without difficulty.
[Illustration:
SPIDER’S NEST ATTACHED TO A FLAT SURFACE.
]
[Illustration:
SPIDER’S NEST LAID OPEN.
]
The great obstacle which prevented the establishment of any considerable
manufacture from these spiders’ bags, was the difficulty of obtaining
them in sufficient abundance; but M. Bon, who was enthusiastic
respecting the value of his discovery, fancied that he could easily
overcome this obstacle, and at first his efforts appeared remarkably
successful. He formed a large spider establishment, which, for a time,
was very prosperous. Having ordered all the short-legged spiders (which
are the most industrious spinners) to be collected for him by persons
employed for the purpose, he enclosed them in paper boxes, with
pin-holes pricked in them to admit the air to the prisoners. The insects
were regularly fed with flies, and prospered well on their diet. In due
time most of them laid their eggs, and spun their silken bags. M. Bon
affirmed that each female produced from six to seven hundred eggs,
whereas the silkworm moth lays only about one hundred. He also stated,
that out of seven hundred or eight hundred young spiders which he kept,
scarcely one died in a year; while of one hundred silkworms, not forty
lived to form their cocoons.
These favourable statements led the Royal Academy of Paris to take the
subject into consideration, and Réaumur was appointed to inquire into
the merits of the new scheme. This careful inquirer found many serious
obstacles in the way of such establishments. The fierceness of spiders,
and their propensity to destroy each other, were noticed as unfitting
them to be bred and reared together. On distributing about five thousand
spiders in cells, in companies of about fifty or a hundred, it was found
that the larger spiders quickly killed and ate the smaller, until there
were only one or two occupiers of each cell. The silk of the spider was
also found inferior in lustre and strength to that of the silkworm, and
had the disadvantage of being incapable of winding off the ball, but
must necessarily be carded.
[Illustration:
GARDEN SPIDER—(_Natural Size._)
]
Indeed, it could require no very great consideration to decide, that
spiders’ silk, when compared with that of the silkworm, was vastly
inferior for manufacturing purposes, though employed in many useful and
highly ingenious ways by the insect itself. A few of these we must not
omit to notice. Every one must have seen the common garden spider
(_Epeira diadema_) suspended by its silken rope, or forming its
beautiful web; but every one is not aware that that silken rope is made
up of a multiplicity of threads, and that when the spider attaches the
rope to any object by pressing her spinneret against it, she spreads out
these threads over an area of some diameter, thus securing a much
greater degree of strength than could be gained by merely fixing her
thread to one point. This contrivance may be seen best when the threads
are attached to a black object. Under the microscope they appear thus:—
[Illustration:
SPIDER’S METHOD OF ATTACHING HER THREAD.
]
The uses of silk in the webs and nests of common spiders are too obvious
to be dwelt on, but there is a most clever and surprising adaptation of
the same material by several foreign species of spider which must be
briefly stated. In the Ionian islands, and also in the West Indies,
there are found certain spiders, commonly known as trap-door spiders,
which make a cylindrical nest in the earth, and cover the entrance with
a door of their own construction, framed of alternate layers of silk and
earth, and fastened to the opening by a hinge of stout silk. These
spiders also line their nests throughout with numerous layers of silken
web to the thickness of stout cartridge paper, and finish it with the
greatest care. This beautiful lining is yet further strengthened in
particular parts, where the nest is likely to be exposed to danger. But
the greatest amount of skill and care is bestowed upon the trap-door and
its silken hinge. This door is about the eighth of an inch thick, rough
on the outside, not much unlike an oyster shell, which it also resembles
in being thick and strong near the hinge, but thinner towards the
circumference. The breadth of this hinge is various, but sometimes it is
very considerable, as shown in the accompanying figure. It also
possesses great elastic force, so that on being opened, it closes again
of itself. This is principally accomplished by a fold or doubling of the
web, at each end of the hinge, which permits the door to be opened
nearly to a right angle with the aperture, but no farther, unless
violence be used. The under side of the door is perfectly smooth and
firm, being shaped so as to fit accurately, and yet to offer no
resistance when pushed open by the insect.
[Illustration:
TRAP-DOOR SPIDER.
]
[Illustration:
NEST OF TRAP-DOOR SPIDER.
]
[Illustration:
TRAP-DOOR OPENING BY A LEVER.
]
As might be expected, there are varieties in the shape and size of these
nests. Some specimens found in the island of Zante had the silken layers
of the lid extended into a sort of handle or lever just above the hinge,
on pressing which in ever so slight a degree the trap-door opened. From
this it would appear that the entrance to such a nest could be effected
as easily by the enemies of the spider as by the spider itself: this,
however, is not the case; for repeated observation has shown that the
spider keeps guard at the entrance, and actually holds the door with her
fore feet and palpi, while the hinder feet are extended down the side of
the nest, and the mandibles are thrust into the opposite side near the
door. By this means the insect gets such power as to resist with
considerable force the opening of the door. If it be asked how this is
known, we are able to refer to the experiments of careful observers, who
extracted a number of nests from the ground, and opening them at the
lower end, looked up, and saw the spider so occupied. A sectional view
of the nest will show that the curved form of the cover, and the shape
of the side walls, must favour this method of keeping the door shut. In
some cases, small hollows were formed round the interior edge of the
lid, into which the spider thrust its feet when keeping guard. It is a
curious fact, that when several of these spiders enclosed in their nests
were kept as a matter of curiosity in a box of earth, and the doors
frequently opened to examine their proceedings, one or two of them, as
if wearied at these repeated interruptions, effectually closed their
doors by weaving a piece of silken tapestry, which was spread over the
interior of the opening, and rounded like the inside of a thimble. This
was so strongly attached to the door and to the side walls, that no
opening could be made without destroying the nest.
[Illustration:
SECTION OF NEST.
]
It was long a matter of surprise to the observer to find, that in the
case of some nests, and not of others, there was a trap-door at the
bottom as well as at the top of the nest: this was at last explained by
the following fact. A spider’s nest, which was accidentally broken off
in being extracted from the ground, was purposely restored to the earth
in a reversed position, with the trap-door downwards, and the broken and
exposed part level with the surface. The spider immediately set to work
to make a new door over the broken part, and finished it with as much
completeness as the other. Doubtless, then, those nests which were
provided with two doors were such as had been upset and broken. This is
the more likely because in Zante, where such nests were found, the earth
is annually dug up round the roots of the olive tree, a favourite
nesting place of these trap-door spiders.
[Illustration:
NEST WITH TWO OPENINGS.
]
We must not omit to mention, that in some parts of South America,
especially in Paraguay, there is a spider which forms a spherical cocoon
for its eggs, an inch in diameter, of a yellow silk, which the
inhabitants spin on account of the permanency of the colour. It must
also be observed that the silk of spiders is useful to the astronomer,
who employs the strongest thread (the one, namely, which supports the
web) for the divisions of the micrometer. By its ductility, this thread
acquires about a fifth of its ordinary length.
[Illustration:
WHITE WAX INSECT OF CHINA.
]