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 the main reasons why spiders 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.

It is now clear that the amino acid sequence in the fibroin molecule is far from random. On the contrary, there are multiple repeats of sequences of several amino acids plus long polymeric strings of alanine or glycine. Fibroin contains amorphous areas which are rich in glycine. These are arranged in a spiral configuration which 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 furcatus 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.

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

Provision of a 'dragline' support. Spiders extrude 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 silken strands as a means of migrating relatively large distances in a short period of time. The tiny spiders allow the wind to make them airborne then trail a strand of silk behind them to act 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 many other kinds of spiders use webbing 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 the latter 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. 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. Similarly, bolas spiders fish for insects by twirling a line with a pheromone-laden drop of sticky fluid on the end.

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 makes easier 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, including several lycosid species, even add a front 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 sacs of eggs 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. Sometimes they have much more interesting shapes, perhaps to disguise what they really are. The green leaf-like egg sac of the St. Andrew's Cross spider Argiope keyserlingi and the spiky round sacs of some Celaenia species are good example of this.

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 subject to 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 saliva. 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.

Email Ron Atkinson for more information. Last updated 5 July 2006.