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Spider Silk

This page summarizes the known facts about the silk spun by spiders.

Spider's silk has been of great interest to biotechnologists in recent years because of its remarkable strength and durability. The following paragraphs outline some important properties of spider silk and how spiders make and use it.

What is spider silk?
Spider silk is primarily composed of a class of proteins collectively known as fibroin. This type of protein has much in common with fibrous proteins found in the human body, including the collagen of tendons and skin and the elastin of artery walls. Fibroin is a moderately large protein with a molecular weight of 200,000 - 300,000 Daltons. Glycine constitutes almost half of its amino acid content and another 25 percent is alanine. The rest of the molecule is almost entirely composed of seven other amino acids, including proline and tyrosine. It is said that changes in the amounts of these last two amino acids in particular samples of silk lead to differences in strength and other properties but different kinds of silk also vary in the other amino acids they contain and in the presence of carbohydrate components (glycoproteins) and other substances. The presence of these other materials is the reason why only some silk glands secrete sticky silk and why certain forms of silk are used only for special functions such as sperm transfer by males or egg sac construction by females.

It is now clear that the amino acid sequence in the fibroin molecule is far from random but is different not only from species to species but also from silk gland to silk gland within an individual spider. At least for the strong 'dragline' silk that spiders use when forced to escape by jumping from a high surface, there are multiple repeats of sequences of several amino acids plus long polymeric strings of alanine or glycine. Fibroin also contains amorphous areas which are rich in glycine. The amino acid sequences in a strand of silk are typically arranged in a spiral configuration and this makes the silk highly elastic. In addition, alanine polymers in the form of highly organized crystalline sheets cross-link the individual protein strands and thereby give the silk its high tensile strength and very low water solubility.

How is this silk made by a spider?
Individual strands of silk are extruded from minute spigots on the 4-6 small appendages found underneath the rear end of a spider's abdomen. These are the spinnerets which differ in shape and function from species to species. Several types of glands supply them so the properties of the silk extruded can be varied according to the particular use to which the silk is to be put. Many spiders also add materials to the forming silk to enhance its properties. For example, the golden orb-weaver, Nephila edulis, adds a yellow pigment to its silk. Many other species secrete an adhesive substance that makes at least some strands in a web so sticky that insects are held until the spider can fully immobilize them. Bolas spiders like Ordgarius magnificus even add pheromone-like attractants that lure certain insect species to their deaths.

Silk is extruded as a viscous solution that quickly solidifies as its proteins cross-link. It is claimed that the more rapid the extrusion the stronger the threads formed, and this makes sense because we know that many spiders extrude silk to slow down a fall from a high surface. The silk is not actually forced out of each spigot but instead is drawn out, sometimes by waving the spinnerets over surfaces or by employing special structures such as the tarsal combs of the redback spider, Latrodectus hasseltii, or the calamistrum of cribellate spiders. Depending on the arrangement of silk spigots on the spinnerets and on the chemistry of the extruded silk the fibres of the web may be straight or have a lacy or woolly appearance, the latter being typical of cribellate webs.

What variations in spinneret arrangements can be found among spiders?
For some of their anatomical structures arachnids have sets of eight. Thus, most spiders have eight legs and eight eyes. They should also have eight spinnerets but in fact at least one pair is missing on almost all successful present-day species. In the case of cribellate genera such as Badumna the 'missing' pair is actually not missing at all but is present as a cribellum. For those genera that lack a cribellum there is normally a very small tapering process called a colulus located just in front of the spinnerets. This is a vestige of the missing pair of spinnerets.

Today's spiders also have fewer segments (often only two segments) on each spinneret than primitive ones had. However, the length and position of the spinnerets are extremely variable among the various species, this presumably being at least partly because of the different habitat and behavioural patterns each species has chosen to adopt. Such differences are of considerable value for the recognition of the family an individual specimen belongs to. For most species the set of spinnerets is at or near the rear end of the spider's abdomen but enlargement of the abdomen sometimes causes it to appear to have moved forward along the underside of the abdomen. Indeed, in the unusual spider family known as the Prodidomidae one pair of very long spinnerets is attached about half way along the abdomen, the others being in their normal location.

What do spiders use their silk for?
At least five important uses deserve mention plus several others that are characteristics of only a few species:

Provision of a dragline support. Spiders extrude from their anterior lateral spinnerets a single or multiple strand of silk when they want to start building a suspended web and when they 'jump' or are accidentally dislodged from a high surface. Being skilled acrobats, they can easily climb back up such threads if motivated to do this. Even in a finished web, some of the strands of silk are used for resting or walking on the web.

Spiderlings also use dragline silk as a means of migrating relatively large distances in a short period of time. The tiny spiders secrete an unattached strand of silk then allow the wind to make them airborne with the strand of silk trailing behind them and acting like a kind of hang glider. This process is called ballooning and sometimes allows spiders to travel several kilometres in less than an hour. Evidence of ballooning is often seen early in the morning as dew-laden threads lying across lawns or stretched across open surfaces and between the branches of shrubs.

Formation of an insect-trapping net. Most araneids and theridiids as well as many other kinds of spiders build some form of web as a means of catching insect prey. The web may simply be strung across an insect's flight path, the spider waiting patiently in the middle of the web or somewhere nearby. In this situation the spider is very aware of all vibrations transmitted through the web and quickly investigates all stimuli that seem likely to be a trapped insect. However, some spiders take a more active approach in their insect-trapping. The net-casting spider Deinopis subrufa stretches a small rectangular web on the ends of two pairs of its legs and then flings this over any insect that comes close enough.

Many studies of the sequence of steps used by those spiders that make a structured web such as a wheel, cone or uniform sheet have been published. The first step probably will be to extrude some sticky silk that will float on air currents until it contacts and sticks to a nearby structure that can serve as one of the major anchoring points of the web. From this initial suspension thread the spider can then drop down and attach to lower objects until a basic 'loom' for web-spinning has been established. The further refinements to this web will then vary according to what kind of spider is involved in the web-building. One of the most intriguing aspects of the building of large suspended webs by spiders is how they know where they are both on the web and in space. There is good evidence that many web-building spiders are well equipped with orientation and position-in-space sensory receptors and therefore instinctively know how to construct a web that is typical for their species.

Temporary immobilization of insects until the spider can inject its venom. Spiders have potent nerve poisons in their venoms but none of these work instantly so it is necessary to restrain a captured insect until it is fully immobilized. Some spiders have strong legs and chelicerae that are adequate for this purpose but many others catch insects that are much larger and stronger than they are. To avoid losing the insect and to minimize damage to the web by the struggling insect, the majority of spiders possess the skill to quickly tie up their prey with silk that is more than strong enough to keep them secure despite their attempts to escape. This same process also facilitates the injection of venom and digestive secretions.

Construction of retreats and lining of burrows. Many spider species prefer to have a 'safe' place to hide in when the open spaces around them are perceived as unsafe. A burrow in the ground or a crevice in a wall or tree trunk is a very popular option. However, this may still need some form of lining, silk being the usual material employed for this purpose. Many of the primitive mygalomorph spiders line their burrows with silk to keep water out and to minimize the risk of cave-ins. Some spiders, including several lycosid species, even add a door for additional protection. Modern spiders of many different families also build retreats that serve as hiding places to surprise insects and to avoid predators. An additional benefit of such retreats is that they are useful places in which to deposit egg sacs and to keep the newly hatched spiderlings safe until they are mature enough to survive on their own.

Formation of egg sacs. Among the many spider families a variety of ways of protecting newly laid eggs are used. Egg sacs are often round or pillow-shaped containers fashioned out of white silk. If they are fragile they are normally constructed under bark or in a retreat where they have some protection from predators and mechanical hazards. Some spiders make egg sacs that are much more exposed and are left undefended. Additional protection may be provided by enclosing the egg sacs in a mass of fluffy silk or building them of very tough silk that is resistant to desiccation and attack by predators. Quite often these exposed egg sacs have interesting shapes, most of which serve to camouflage them so they will be overlooked or misinterpreted by other predatory animals.

How durable is spider silk?
We know that in relative terms spider silk is stronger than mild steel yet remarkably elastic. It does not dry out and become brittle and neither is it prone to rapid microbial degradation by either bacteria or fungi. Although silk does not dissolve when it gets wet it is very quickly digested by enzymes in the spider's digestive secretions and those spiders that build or rebuild their webs every evening usually digest old silk in order to reuse its protein components. This recycling of a valuable resource allows a spider to spin a remarkable amount of webbing in a short space of time and with little need for a meal before or during the process. Such efficiency is vital for survival since most spiders frequently have a long wait between meals.

Some related sources of information
The pages on spider movements and growth and reproduction contain some information that is related to what is covered in the above paragraphs. In addition, the following articles are worth reading:

The article by the Smithsonian National Museum of Natural History

Dr Ricki Lewis' article entitled "Unravelling the Weave of Spider Silk"

The Spider Spinning System page of the Clemson University website

Email Ron Atkinson for more information. Last updated 30 December 2009.