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The Wonders of Spider Silk


 

  The Nature of Silk     Spiders Using Silk  

Introduction

Spider silk is one of the 7 great wonders of the animal kingdom. That a small animal, often less than a millimetre across can make a substance that we humans with all our technology are unable to reproduce, a substance that is tough, stronger and more flexible than anything else we can make is surely a humble reminder of the fact that nature created us and not the other way around.

One of the most astounding thing about spiders is their ability to make silk, their webs amaze and fascinate us, but they are also used by us in some cultures. All spiders possess spinning glands and make silk, they use it for a safety dragline, to fly, for making a highly durable cocoon for their eggs, to line their homes, to trap their prey and to immobilise their victims. In some species males use silk to ritually immobilise the females before mating, and all male spiders make a special sperm-web or sperm-line to allow them to transfer the sperm from their genitals to their copulatory organs. Not all species use silk for all these reasons, but all species make egg cocoons.

 

Amongst the most amazing and commonly admired webs are the various orb-webs. The largest orb-webs in the world are spun spiders in the genus Nephila which may be 2 metres (6 ft) in diametre and are capable of catching small birds and bats. However the largest webs of all are built by communal spiders. Ixeuticus socialis in Australia builds webs that may be 1.2 metres wide and 3.7 metres long (12 ft by 4 ft) and is claimed by some authorities to build the largest nest. However G. F. Masterman, once the British Ambassador to Paraguay, described social spiders there building webs 9 metres (30 ft) long and 2.4 metres (8 ft) wide in his book Seven Eventful Years in Paraguay (1888). Whatever the truth here, it is a fact that there are billions of kilometres of spider silk spread across the globe and it is all amazing.

The Nature of Silk

Spider silk is both light and strong. Hillyard says "A typical strand of garden spider silk has a diametre of about 0.003 mm (0.00012 in) in diametre, compare this with silkworm silk which is 0.03 mm in diametre" or ten times as thick.

Spiders can produce different kinds of silks, at least 7 or 8 different kinds. However generally individual spider species use 5 to 6 types or less, in other words not all spiders produce all the possible types of silk. When assessing the strengths of silk scientists usually refer to dragline silk. It is also generally accepted that spiders in the genus Nephila produce the strongest draglines, however this may reflect their large size. Knowing this we should accept that any measurements we come across on the strength of spider silk will not apply to all the silks produced by one spider, and definitely not to the silk of all spiders. The data we do have applies to the toughest spider silk we know of, just how much variation there is between different silks and species I have not been able to find out.

Never-the -less the strength and resilience of spider silk has amazed people for thousands of years, reading around, on and off the web, we get a lot of facts about the strength of spider silk that sometimes seem contradictory, although they all agree that spider silk is way stronger than any other silk or natural material. Here are some quotes.

Paul Hillyard says in The Book of the Spider 1994. "For an equal diametre, spider silk is stronger than steel and about as strong as nylon. It is, however much more resilient and can stretch several times before breaking - it is twice as elastic as nylon and more difficult to break than rubber. The energy required to break spider silk (its 'toughness') is about ten times that of other natural materials such as cellulose, collagen and chitin. Dragline silk (about .00032 inch (.008 mm) in Nephila) is especially strong - approximately twice that of silk from silkworms."
Mark Carwardine says in The Guiness Book of Animal Records 1995. "Spider silk is the strongest of all natural and man-made fibres.... It is even stronger than steel: the dragline of a European garden spider (Araneus diadematus), for example, can support a weight of 0.5 g (0.002 oz) without snapping, whereas a steel strand of similar thickness will snap under the strain of just 0.25 g (0.01 oz)."
Rainer Foelix says in Biology of Spiders 2nd Ed 1996. "The tenacity of spider silk is slightly less than that of Nylon (Lucas 1964), yet its elasticity is twice as high (31% vs 16%). .... In terms of tensile strength it is clearly superior to bone, tendon or cellulose, and it is half as strong as the best steel. .....a dragline would have to be 80km long before it would break under its own weight."
At "How Stuff Works" we are told. "Spider silk is extremely strong -- it is about five times stronger than steel and twice as strong as Kevlar of the same weight. Spider silk also has the ability to stretch about 30-percent longer than its original length without breaking, which makes it very resilient."
At Access Excellence we are told. "It has been estimated by scientists to be at least five times as strong as steel, twice as elastic as nylon, waterproof and stretchable."
 
"Dragline spider silk is actually stronger than Kevlar synthetic fibre and Kevlar is several times stronger than steel," says polymer scientist David Tirrell who wrote a review for the journal Science."

If all that is a little confusing, is it as strong as steel, stronger than steel or less strong, is it stronger than Kevlar or not, perhaps we can find some facts somewhere. The truth is it depends on how you measure it and what condition the silk is in. Silk absorbs moisture (a property given it by the large amount of the amino acid Alanine it contains). The more water silk contains the less brittle and the more elastic it is. The range is from about 30% to 300% elasticity depending on the amount of water it contains. Thus we can see that changing the humidity of the environment the silk is spun in, and the length of time it exists in that environment before it is tested will effect the results of the tests.

With all these things considered we learn that the tensile strength of spider silk under normal laboratory conditions is slightly less than steel if you compare it in terms of the threads diametre, but it is far greater than that of steel if you compare it in terms of the weight of the thread. Obviously spider silk is lighter per unit area than steel. When we make the comparison with Kevlar we find that it is three times harder to break than spider silk, however spider silk is five times as elastic.

OK enough about the strength of spider silk, where does it come from and what is it. It is an unusual substance in that it is quite acidic and is not attacked by bacteria or fungi, which is why cobwebs remain around for so long. Silk is created as a proteinaceous liquid within special silk glands within the spider's abdomen. The proteins have a molecular weight of 30,000 daltons (the units that the weights of atoms and molecules are measured in). However when the silk is released from the spiders spinnerets it instantly solidifies and changes into a much larger molecule with a weight of between 200,000 and 300,000 daltons. Quite how this happens is not yet understood.

The proteins that make up dragline silk are secreted in long, sac-like glands, from which they are funnelled as a watery solution into a long, looping duct. This duct leads to a small spigot or nozzle on the end of a spinneret. There are tens or even hundreds of these spigots, and therefore of their accompanying glands, for each spinneret, and most spiders have 6 (3 pairs) of spinnerets. Examining the complex structure of silk we see that its proteins come in two different forms called α-chains and β-pleated sheets. The β-pleated sheets are about 10 nm by 15 nm and are interspersed in a matrix of the α-chains.

As I said above, spiders possess different glands that produce different silks, scientists distinguish the following glands and silks. Not all spiders have all these glands, in fact the last two are only found in orb-web weavers.

Table of Spider Silk Glands
Gland Name Spinnerets Used Type of Silk
Piriform Anterior Disk Attachment
Ampullate Anterior/Median Dragline and Web Frame
Aciniform Median/Posterior Wrapping silk, Sperm-web, Egg-cocoon (outer wall)
Tubuliform Median/Posterior Egg-cocoon
Aggregate Posterior Spiral parts of Sticky-web
Flagelliform Posterior Axial thread of Sticky-web

As if this wasn't enough some spiders have an additional silk spinning organ called a cribellum. The cribellum is situated in front of the three pairs of spinnerets and consists of between one and four plates covered in spigots. The spider Stegodyphus pacificus has 40,000 spigots on its cribellum, but other species have much fewer. Spiders that possess a cribellum are called 'Cribellate', as compared with the remaining spiders which are sometimes referred to as 'Ecribellate'. The silk is combed out of the spigots with a set of special hairs located on the 4th metatarsi called a 'Calamistrum'. The silk produced by the cribellum is especially fine and is used to help entangle prey. Spiders can control the thickness of a strand of silk using muscles and valves in the necks of the spigots, larger spiders for instance need thicker draglines. Most strands of spider web are only a few micrometres in diametre, for instance the dragline of an adult orb-web has a diametre of about 2µm. In comparison with this hackle band silk is normally only 0.01µm to 0.02µm in diametre.

How spiders Use Silk

So now that we know something of what silk really is, lets have a look at how spiders actually use it. Naturally enough different spiders use silk in different ways. However there are three nearly universal usages. The first is the dragline, all spiders, as far as I know, use a dragline at sometime in their life. In small species it is also safety line that will catch them if they jump into space in order to avoid a predator. Many spiders do not use silk to make a catching web, but do use it to make themselves a nice cosy cocoon or cell to live in when they aren't out hunting. Such a neatly made hideaway helps them avoid predators and offers some protection from both excessive moisture and lack of moisture. The other two uses are gender specific, males spin a sperm web before mating to allow them to charge their pedipalps (read more about this in Reproduction) and females spin a web cocoon around their eggs, often it is composed of more than one kind of silk, you can read more about this in the page on reproduction as well.

 

An unusual use of silk is flying, when small spiderlings stand head down and abdomen up to release a strand of silk which is caught by the breeze and lifted up, as the silk strand lengthens it eventually creates enough lift to raise the spiderling and it turns and grabs the thread and away it goes. Flying like this is a risky business as there is no way the spider can control where it ends, many must die as a result of landing in inappropriate places such as rivers and seas. However for those that land somewhere more accessible a new life is offered far away from all those brothers and sisters.

Spiders can travel huge distances like this and their ability to live without food and water for long periods of time undoubtedly help. In May 1984 the first scientist to visit the island of Krakatoa, which had exploded violently 9 months earlier exterminating all life on the island, reported that the only, and there fore the first, animal he found on the planet was a spider. Krakatoa is 37 km (25 miles) from the nearest other land. However spiders fly far further than this, scientific research has recorded spiders over 4.500 metres (14,000 ft) above sea level and also in the middle of the ocean 1,500 km (1,000 miles) from the nearest land.

In late summer over grasslands in temperate climates you can experience huge numbers of these aeronautic spiders, the silk they deposit across the landscape as they descend towards evenings as the air cools and descends can also become very noticeable if you are out and about. This is commonly called gossamer. In the past huge falls of gossamer were well known, but while they can be experienced anywhere in the world on warm days they appear to be less intense these days. In France gossamer is known as fils de la Vierge and in Germany as Marienfaden (both meaning 'Our Lady's threads'). Its presence is recorded in ancient literature, Pliny the Roman Historian and Encyclopaedist wrote what is now the earliest known reference to gossamer :-
 
              "in the year that L. Paulus and C. Marcellus were consuls it rained wool",
 
a few hundred years later Chaucer wrote:-
 
                           Sore wondren some on cause of thonder,
                           On ebb and floud, on gossomer, and mist.

 
proving that even then people were amazd by the phenomenom of gossamer.

The general types of catching webs and their evolution are discusses in another page as this one is getting long enough, see The Evolution of Webs but here I will mention a few uses of webs that are not mentioned there.

The Water Spider, Argyroneta aquatica, lives in temperate Europe and is the only truly aquatic spider. It lives under the water, but it still breathes air, and it has a wonderful way of getting enough. Using aquatic vegetation as an anchor it builds a special dome shaped web under the water. Then it brings bubbles of air (trapped in the hairs of its body) down from the surface and slowly fills the dome, then it lives inside the dome breathing air normally.

The next two sets of spiders are highly unusual and I am unsure which I think is the most amazing, however both of them are fascinating in the unusual methods they have developed to use their wondrous silk.

Bolas spiders (Mastophora sp.) are a small group of spiders in the family Araneidae, a family that otherwise contains many orb-web weavers. They hunt only at night and do so by dangling a thread of silk with a sticky blob on the end and swinging this to entrap moths that come too near. Thus they have been named after the weapon used by South American cowboys or gauchos. It may seem improbable that this would catch them many moths, but the spiders have another trick that makes it work. They secrete a pheromone that is the same as that emitted by female 'Armyworm' moths (Spodoptera). This attracts the male moths which are then easy for the spider to catch as they are going to keep flying around until entangled in the bolas. Taking this practice one step further the spiders in the genus Caelenia in Australia do not even bother with a bolas. Instead they simply hang from a thread (presumably emitting a pheromone) legs and jaws at the ready, and the moths just fly straight to them. Catching dinner couldn't be easier.

The second of my fabulously inventive spiders is Dinopis guatemalensis. These spiders, commonly called Ogre-eyed spiders because they have huge eyes which are extremely light sensitive. They hunt by night. They hunt by building a small platform of strands they can hang from, and then a they make a small net. They hold the net open and hang quietly waiting for a passing insect. If an insect passes on the ground they will see it and throw the net over it, tugging it a few times to make sure the prey is well entangled, then it is bitten, wrapped a bit and eaten. If however a fly comes past the spider can detect the vibrations caused by its flight and will through its web net upwards to catch it, if successfully entangled it too gets a bite and a wrap before being invited to diner.

 

 

 

 

Well I hope that has helped you to appreciate that spiders are truly amazing animals. Don't forget there is a lot more info available on spiders at the Earthlife Web, see the menu below.

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Introduction Spider Anatomy Bibliography and Reviews
Feeding Ecology Reproductive Ecology Spiders and Man
Ecological Considerations Tarantula Myths Spiders and Evolution
Caring for your Tarantula A House-spider Safari Fear of Spiders

 

 

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