This page summarizes the known facts about the silk spun by spiders.
Spider 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.
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
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.
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.
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.
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.
Why do spiders' webs vary so much in appearance from species to species?
There are a number of reasons for this. Some species build partly sticky, vertical or near-vertical sheet webs that they hope insect prey will unintentionally fly into. A recent spider web study came up with the interesting observation that when orb weaver webs are positioned so that strong winds flow past them they gradually acquire a static electricity charge and this draws in nearby flying insects and causes them to make contact with the web even though they could see it and were intending to avoid it. Other spiders, notably the Stiphidiidae, spin horizontal sheets that the spider hides under, presumably intending to be less visible to any prey that ventures onto the web. And then there are the species such as Cyrtophora hirta that build cone- or dome-shaped webs which an insect may fly into but then is less able to exit. Theridiosoma makes a vertically oriented circular web then draws the centre of it sideways to form a cone. When an insect flies into the cone the spider then releases the web which springs forward to engulf the insect. The net-casting spider Deinopis uses a somewhat similar trick except that it stretches its web using some of its legs then literally flings the web over any prey that comes close enough to be caught in this way.
Of course, many flying insects have very good vision and can readily see a sheet web spread across their flight path before they have become trapped in it. What can spiders do to overcome this problem? Well, orb-weavers like Eriophora transmarina build their webs in the early evening which allows them to hide both themselves and their web in near darkness. This is also a good idea in that many insects find it otherwise safer to fly at night and in addition the web is less likely to be damaged by small birds that mainly fly during the daylight hours. But there are many web-building spiders that successfully trap flying insects during the daylight hours so how do these species make their webs less obvious to the prey they are trying to catch? It might seem that a good way is to use only very thin silk and to have relatively large gaps between individual strands. This probably is helpful but may make the web less robust and even very thin strands of silk become easy to see if coated with dust or fine droplets of dew or mist.
In fact, many web-building spiders construct daylight webs that are dense enough to be seen easily by flying insects yet still manage to make good catches if there are plenty of insects around. How can this be? There are several ways a spider can 'hide' a web that fling insects should have no difficulty seeing. One thing they can do is to make the web strands vibrate slightly (or maybe the prevailing winds will do this for them) so the individual strands become effectively blurred. But this is a rather basic and only moderately successful way to use web vibrations for catching insects. A far more sophisticated technique is used by the salticid Portia fimbriata. This spider has excellent vision but it also is clever enough to vibrate the web of some orb weavers which it intends to feed on, its pattern of vibrations serving to deceive the orb weaver into believing that either its web has caught an insect or its matching male has come visiting. It is also said that at least in the case of some Argiope species Portia can generate web vibrations that actually have a calming effect so that when Portia approaches it Argiope makes no move to escape or to defend itself or even to attack Portia as potential prey.
Some spiders that build robust and very visible webs have come up with other ways to persuade insect prey that it is safe to approach the web. In the case of the
bolas spider Ordgarius magnificus just a single strand of sticky silk is enough to allow it to catch the moth species it mostly feeds on because this silk
has the odour of the pheromone that is used by the moth's mate to bring the two sexes together for mating. It is likely that many other spiders also have
learned to make their webs attractive to flying insects by giving the silk an odour that is either a sex pheromone or food that the insects can be expected to
be interested in. Nephila edulis is an example of a web builder that decorates its web with detritus (debris from insects it has previously caught and
eaten). This may give off odours that some flying insects find attractive and it is said that the yellow colour of Nephila silk actually is attractive to bees.
But for many smaller orb weavers the detritus masses and the spider's body (with legs retracted) are so similar in appearance that flying insects are
unaware there is a spider in the web until it is too late.
There are quite a few orb weavers that decorate their webs with dense patterns of silk. These have been termed stabilimenta becaused it was initially
assumed they stabilized the web. It now is clear the main purpose of such decorations are to make the web less obviously an insect-trapping net. A good
example of this is the spiral stabilimentum built by some Cyclosa species. An alternative use of a stabilimentum is to provide a dense structure on which
the spider can hide or at least have its outline less distinctive to potential prey.
Some related sources of information
Email Ron Atkinson for more information. Last updated 21 April 2015.