Spider Silk

Properties,
Uses and Production

Properties

Nephilia clavipesSpider
silk is incredibly tough and is stronger by weight than steel.
Quantitatively, spider silk is five times stronger than steel of the same
diameter. It has been suggested that a Boeing 747 could be stopped in
flight by a single pencil-width strand and spider silk is almost as strong as
Kevlar, the toughest man-made polymer. It is finer than the human hair
(most threads are a few microns in diameter) and is able to keep its strength
below -40°C. The toughest silk is the dragline silk from the Golden
Orb-Weaving spider (Nephilia clavipes), so-called
because it uses silk of a golden hue to make orb webs.

Spider
silk is also very elastic and capture silk (sticky silk for catching prey)
remains unbroken after being stretched 2-4 times its original length.
Spider silk is tougher, more elastic and more waterproof than silkworm silk so
it could have a much wider range of applications. It is simple to see why
spider silk is of such interest to materials chemists since new
ultra-strong fibres based on the silk could be developed.

Garden spider with preyUses

Spiders
use silk for a variety of functions:

  • Swathing
    silk
    for the wrapping and immobilisation of prey.

  • Webs
    for catching prey using sticky silk – it is elastic to prevent the prey from
    rebounding off the web.Orb web

  • Draglines
    which are used to connect the spider to the web, as safety lines in case a
    spider should fall and as the non-sticky spokes of the web. Dragline
    silk is the strongest kind of silk because it must support the weight of the
    spider.

  • Parachuting
    or ballooning
    which is used to aid the dispersal of young and to find
    new areas as a food source. Silk is released and is caught by the wind
    to lift the spider up into the air – flying spiders!

  • Shelters
    such as burrows or
    nests

  • Egg-sacsWolf spider and eggs

  • Mating:
    male spiders weave sperm webs on which they deposit sperm and subsequently
    transfer it to their front palps, ready for placing on a females genital
    organs. Some species make a web and coat it with sex pheromones to
    attract a mate.

Production

There
are seven types of silk produced by seven silk glands. A single spider
does not possess all seven glands but has at least three if it is male
(dragline, attachment and swathing silk) or four if it is female. The
additional one is for egg sac silk. The seven types of gland are:

Spigots

  • Achniform
    gland
    : swathing silk.

  • Cylindriform
    gland
    : egg sac silk.

  • Ampullate
    glands (major and minor)
    : non-sticky dragline silk. Silk from the
    minor ampullate gland is only half as strong as that from the major gland.

  • Pyriform
    gland
    : attaching threads – attachment discs are made which anchor a
    thread to a surface or another thread.

  • Flagelliform
    gland
    : core fibres of sticky silk.

  • Aggregate
    gland
    : outer part of sticky silk – droplets of an adhesive substance are
    deposited along the threads.

Courtesy
of Tina Carvlho, Microangela

The
glands are located on the lower side of the abdomen (see diagram below) and
contain a watery fluid known as ‘dope’. This fluid passes through to the
spinneret via a multitude of microscopic tubes where water recovery and
solidification begins. Fluid from different glands can lead to the same
spinneret so silk with specific properties required for a particular function
can be produced. There are usually three pairs of spinnerets but this can
vary between 1 and 4 pairs depending on the species. The substance exits
through the spiggots which are mobile, finger-like protrusions and the resulting
silk emerges as a solid. There are many spigots so many fibres are bound
together like a cable. The diameter of a single fibre is controlled by the
muscular action of a valve. The faster and tighter the strand is drawn,
the stronger the silk.

Spider's anatomy

Picture from reference 11

Some
interesting web facts:

  1. Not
    all spiders weave webs.

  2. Spiders
    do not stick to their own web because only the central spiral part of the
    web is sticky, not the spokes. The spider knows where to tread!

  3. Webs
    lose their stickiness after about a day due to factors such as dust
    accumulation and exposure to air. In order to save energy the spider
    eats its own web before making a new one so the protein used for the silk
    threads is recycled.

Source

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