Spider Silk

Properties, Uses and Production

Properties

Spider 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.

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Uses

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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.

• 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-sacs

• 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:

• 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

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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.

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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.

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