Silk: the spider's success story

Spiders use silk for many purposes - to protect their young, catch food, make homes and move around. They are the only animals which use silk in almost every part of their daily lives.

Evolution of spinnerets

The original spiders, represented today by the primitive, segmented, mesothelid spiders, had eight pairs of silk spinning organs or spinnerets placed under the middle of the abdomen (Mesothelae). In mygalomorph and araneomorph spiders the spinnerets have moved to the end of the abdomen (Opisthothelae). In these spiders the anterior median spinnerets (AMS) are either no longer present or remain in some araneomorph spiders as a modified spinning structure called the cribellum. Mygalomorph spiders have four or six spinnerets, compared to six in most araneomorphs.

Structure of spinnerets

The paired spinnerets have one to three segments. They are highly manoeuvrable for silk spinning and may be quite short or relatively long (as long as the abdomen in some mygalomorph spiders). The end segment of each spinneret has many spigots - hollow, hair-like silk outlets connected to particular paired silk glands within the abdomen. Each gland opens on the spinnerets either via one or two spigots (ampullate glands), several spigots (cylindrical glands), or many spigots (pyriform and aciniform glands). In addition, the cylindrical and aciniform glands open onto two spinnerets.

Other silk-spinning organs

As well as the spinnerets, many male spiders have an area of hair-like spigots near the gonopore (epiandrous spigots) that produce silk for the spider's sperm web.

The silk glands can occupy a large amount of abdominal space, especially in web-building species. Ten different types of silk glands and their spigots are known in spiders. Up to eight silk glands may be present in a single species, each producing silk with different properties and uses, including:

• attachment disc silk
• strong dragline (safety line) and web frame silk
• the orb web spiral line
• glue-like sticky catching silk
• swathing silk
• tangling cribellate catching silk
• protective egg sac silk

How silk is produced

Although silk is produced as a liquid within the silk glands, it usually emerges from the spigots (as the spider moves away from an attachment point or pulls the silk out with its leg claws and bristles) as solid silk fibres. Different parts of the silk gland secrete different types of proteins (spidroins) into the gland cavity. These form an inner core and then outer sheathing layers. The very viscous silk flows as a liquid crystal fluid through a long, progressively narrowing duct whose lining cells extract water from the protein. It is then subjected to a mild acidic bath and increased pulling stress which help convert the liquid protein into a solid fibre. The final section of the duct provides a thin, fatty coating to the silk line. The large drag-line gland duct has a valve at this point, just before entering the spigot .This valve probably provides both a means of braking when the spider drops on its dragline, and a pump to move silk forward into the spigot duct after a silk line has broken. The only silk that remains in a liquid state after leaving the spigot is that produced by the aggregate glands - the sticky catching silk of the orb web weavers and their relatives.

Silk structure

Typically, a spider's silk line is only about 0.001 mm - 0.004 mm thick. It is made up of different spidroin proteins whose structures provide silk with unique properties. Silk fibres get their stretchiness from the disordered, loose, coil-like protein chains of glycine peptides (amino acids) that stretch when pulled, giving silk its elasticity; and it gets stiffness and strength from highly ordered, 'brick-like' protein crystals of alanine peptides that are spread throughout the silk line. The structural properties of different silks vary with the composition and arrangement of these proteins.

The orb web

Despite its light and delicate appearance spider silk has amazing structural properties. These properties vary considerably between silk types and usage.

The functional use of silk is well illustrated by the orb web. Catching a fast flying insect in an orb web can be likened to snaring a jumbo-jet in a scaled-up web with silk lines only a few millimetres thick.

How is this done?

The typical orb web consists of outer frame lines to which radial (spoke-like) lines are attached, providing support for the characteristic spiral sticky line that occupies most of the web's surface. Dragline silk from the major ampullate silk glands (which also provide the spider's safety line) is used to make the frame and radial lines of the web. Its tensile strength is greater than that of mild steel and almost all artificial fibres (kevlar is the exception). By contrast, the spiral line silk (from the flagelliform glands) is weaker but very much more stretchy (elastic) than the framework silk. The use of these two very different silks gives the orb web the required strength and stretchiness to cope with the impact of fast flying insects and the struggles of captured prey. The absorption of an insect's impact energy by the stretching silk means that the insect neither breaks through nor simply bounces off the web surface - such webs would not make good trampolines! And finally, of course, the droplets of sticky silk (from the aggregate silk glands) on the spiral line stick the prey onto the web.

The orb web framelines have another property - when moistened in humid air they contract, a characteristic that may help keep the web under tension and in shape after deformation by wind, rain or prey. The relative stiffness of the frame and radial lines also makes then excellent transmitters for signalling the twitches of struggling prey back to the spider at the web's centre.

Making a dragline

Different silks are often used sequentially or together. If you watch a huntsman spider moving across a window you may see it apply its front pair of spinnerets (anterior lateral spinnerets, ALS) to the glass, leaving behind a zig-zag patch of silk. This is called the attachment disc and it is made up of numerous, very short threads of silk from the piriform gland spigots. The spider's 'safety line' or dragline (a strong silk from the large major ampullate gland spigots adjacent to the piriform spigots) fuses with the attachment disc silk, giving the drag-line a secure anchor point as the spider walks or drops away, letting out the dragline (under muscular valve control in the silk duct) as it goes.

Silk is often drawn out of the spigots simply by movement of the spinnerets in relation to a fixed attachment point - as in the case of the dragline. Other examples can be seen during prey wrapping and egg sac construction. Silk is pulled from the aciniform spigots, often in broad swathes, as the spider's legs rotate the prey 'package' or egg sac. The terminal leg claws and bristles also are used to pull out silk and manipulate silk lines during these and many other web building (e.g., combing out cribellate silk) and prey catching activities (e.g., combing out cribellate silk and throwing out sticky swathing silk).

Why don't spiders stick to their webs?

Spiders keep their bodies clear of web surfaces, especially those made with catching silk - they also have areas in their webs, like the hub of an orb web or the retreat funnel of a window spider's web, which lack catching silk. When moving about in the web, the spider has only a tiny area of its body in contact with the silk lines - the tips of its legs. For example, orb weavers clasp a silk line using only the middle claw and the adjacent toothed bristles on the leg tips. This small contact area, aided by regular cleaning of the leg tips (watch a spider drawing its legs through its jaws) and the probable secretory lubrication of the claws, combine to ensure that spiders don't stick to their webs.

The specialised catching silks

Cribellate and sticky catching silks evolved to increase the prey holding efficiency of snare webs. They represent totally different solutions to the problem of web-based prey capture. Both are spun in association with supporting lines to maintain their structural integrity in the web.

Cribellate silk

Sheet web building spiders, like the common black house or window spiders, make their webs with this sort of catching silk (sheet or shawl webs), as do the net casting spiders and their relatives. Cribellate silk is produced from many tiny, silk glands placed beneath a specialised, flattened spinning organ called the cribellum. The cribellum is placed in front of the spinnerets and is derived from spinnerets (the anterior median spinnerets) present in ancestral araneomorphs. Its surface is covered by hundreds or thousands of tiny, elongate spigots, each producing a single fibril of cribellate silk about 0.00001 mm thick. All of these spigots act together to produce a single cribellate thread made up of thousands of the silk fibrils. They are supported on thicker lines produced by spigots on the posterior and median spinnerets. A web made with a meshwork of these composite 'wool-like' threads is particularly effective at tangling the bristles, spines and claws of insect prey. The fine fibrils of cribellate silk also appear to have some type of 'dry adhesive' properties (possibly electrostatic in nature) and will even cling to smooth beetle cuticle.

Cribellate spiders all possess a row of toothed bristles (the calamistrum) on the metatarsal segment of the last legThese bristles are used to simultaneously comb out the mass of cribellate fibrils and their supporting silk lines from the cribellum and spinnerets.

This remarkable innovation allowed spiders to produce the first specialised prey catching silk. All araneomorph spiders were once cribellate, and a lot still are, but the cribellum has been lost in many descendent lineages. These include the hunting spider groups (wolf, huntsman and jumping spiders, etc.) and most of the orb weavers and their relatives, the latter having evolved a quite different sort of catching silk.

Sticky silk

A more recent evolutionary innovation was the development of a glue-like silk for use in prey capture - that is, a silk that remained liquid rather than being produced as a fibre. This type of catching silk evolved in the ancestors of the highly successful orb web weaving and comb-footed spider families and their relatives. As with cribellate silk, the sticky liquid catching silk had to be carried on fibrous silk support lines - for example, the spiral line of the orb web or the vertical lines of a redback spider's web.

Sticky silk and its supporting line are produced simultaneously from a 'triad' of spigots on each of the posterior lateral spinnerets (PLS) - a central spigot provides the supporting line from the flagelliform silk gland, while two spigots on either side coat the line with liquid silk from the two aggregate silk glands. The two coated lines from each PLS coalesce a little way from the spinnerets, forming a doubled sticky line. Surface tension effects subsequently cause the sticky silk coating to break up into droplets. At the centre of each droplet is a core of very sticky glycoprotein material. In orb weavers the sticky silk is also hygroscopic (absorbs moisture from the air) and this 'wetting' of the spiral line is probably a significant factor in increasing its ability to stretch.