Spider Silk is the Strongest Natural Fiber — It’s Even Stronger Than Steel!

Whether you are a fan of spiders or terrified of them, you can’t help but admire their ability to make webs. These fascinating and unique structures are incredible feats of engineering that combine complicated concepts of both chemistry and physics. Yet we are happy to brush them away with a swipe of our hands and view them as a nuisance in our homes. Here we will shed light on the true marvel of these creations.

Spider Web Silk: An Incredible Biomaterial

Many materials made by nature (biomaterials) far surpass anything that humans can currently make, and spider silk is a notable example. When compared weight-for-weight, spider silk has a higher tensile strength than steel yet is almost as elastic as rubber. Leading manmade fibers such as nylon are less than half as tough. What’s more, spider silk is totally biodegradable, as well as being antimicrobial and hypoallergenic.

Spider silk is made of proteins and specifically, it contains large amounts of the amino acids glycine and alanine. The exact arrangement of the amino acids is responsible for the different properties of spider silk. As long as the spider consumes protein and is healthy and uninjured, it will never run out of silk.

Spider Silk-Making Organs

Spider silk is produced in a special organ with a charming name: the ‘spinneret.’ These are usually found on the spider’s abdomen – towards the rear and on the underside. Often, the spinnerets are in a cluster but the exact number varies by species. Each spinneret has microscopic nozzle-like structures that produce one filament of silk. There may be one spigot or several allowing spiders to combine filaments to get silk with the exact properties that they need. It is a little like a cable. Some types of spigots are unique to particular species of spiders.  

When it is inside the spider, the silk is a watery fluid known as dope. It travels through progressively narrowing ducts where water is extracted. Then it passes through a mildly acidic bath and this together with the pulling stress converts the liquid protein to a solid. The silk is pulled out either by gravity or by the spider’s legs. However, the diameter of each strand is controlled by a muscular action on the spigot valve, which acts like both a pump and a brake. If the strand is drawn faster and tighter, the silk will be stronger.

Different Types of Spider Silk

Not all spider silk is the same. There are seven different types of silk glands, but not all spiders have them all. To illustrate, the different types of silk with the gland that produces it in the female orb-weaving spider are:

• Major Ampullate Silk Gland: structural and dragline silk
• Flagelliform Silk Gland: capture spiral thread
• Minor Ampullate Silk Gland: auxiliary spiral thread
• Piriform Silk Gland: attachment cement
• Cylindrical Silk Gland: tough outer egg case
• Aciniform Silk Gland: soft inner egg and wrapping
• Aggregate Silk Gland: sticky aqueous coating

The different silks have different properties to fulfill a particular purpose.

Different Uses of Spider Silk

As you can see from the above list, spiders use silk for a lot more than just building webs. Even webs consist of two types of silk — some is structural and some is for catching prey. Spider silk is also used for:

• Wrapping up and immobilizing prey
• Constructing draglines – a safety line to attach the spider to the web
• Parachuting for transportation – the silk is released into the wind and lifts the spider
• Shelter building
• Egg sac construction
• Mating – webs are used to attract a mate with pheromones and to present sperm ready for fertilization
• Breathing – the diving bell spider creates its own oxygen tank from silk and debris so that it can breathe underwater
• Specialized catching silks

Specialized Catching Silks

Snare webs contain special types of silks that are designed to trap and hold prey until the spider can get to them and kill them. These two types of silk are cribellate silk and sticky catching silk. They are both used with other types of silk that create the supporting structure on which they are laid out. Cribellate silks were used by early spiders and was the original specialized prey-catching silk. Sticky silk is a more recent evolutionary innovation.

More on Cribellate Silk

Sheet-building and net-casting spiders make their webs from cribellate silk. It is produced by many silk glands located under a flattened spinning organ called a cribellum. The cribellum consists of thousands of tiny spigots each producing a tiny fibril of silk. This means that the thread produced is actually made up of thousands of smaller fibrils. The wool-like thread is perfect for entangling the bristles and spines of insect prey. It also has electrostatic properties that cling to the insect prey’s cuticle.

More on Sticky Silk

Sticky silk is used with structural silk to build the perfect trap to secure prey. The combination of strength and elasticity ensures that fast-flying insects neither force their way through or bounce off the web. Instead, they get stuck. Added to this are the microscopic droplets of highly entangled polymers which make the web so sticky. They are both viscous and elastic and have a consistency a bit like chewing gum. They are also water-resistant. This type of silk is produced by the flagelliform gland creating the supporting line while two further aggregate glands coat it with liquid silk. Surface tension causes the liquid silk to break into the sticky droplets.

Different Types of Spider Webs

There are a large number of different spider webs that fall into seven broad categories.

• Orb webs: circular webs typically found in gardens and forests
• Funnel webs: tube webs with a wide mouth
• Tangled webs: three-dimensional criss-cross of silk that has no particular design
• Lacy webs: untidy web made from cribellate silk
• Radial webs: built around a crevice with trip wires
• Sheet webs: built low down on grassy fields
• Purse webs: look like an upright tube and are built in woodlands

Web Building Logistics

It usually takes around an hour for a spider to weave a web but this varies by species and by the type of web. If we take the classic orb web that we are all familiar with, there is usually a set construction plan. Firstly, the spider constructs a ‘Y’ shape with just a few threads. Then, a frame is constructed around it and threads are built from the frame to the Y – these are called radii or spokes. Then, the spider builds a spiral (called an auxiliary spiral) working from the center of the thread outwards. However, this is just a temporary supporting structure and is not sticky. Finally, the sticky spiral is put in place and the temporary spiral is removed.

Other webs have a zigzag structure called a stabilimentum. Most are perpendicular to the ground, but others are horizontal. Building a web on grass is more challenging because it is flexible and these webs look different. Spiders from the same species often build similar but not identical webs, and experts are not yet certain why spiders vary their techniques. A somewhat bizarre study from the 1940s exposed spiders to different types of chemicals (including LSD and motion sickness medication) and recorded the effects on web-building. Strangely, LSD made the webs more regular. A later study by NASA revealed that caffeine caused more structural disruption to web building in spiders than marijuana.

Web Longevity

Spider webs in the home are often swept away by those who live there, and outside webs can get ripped and torn by larger animals. If this does not happen, webs can last for quite some time – up to months. They have antimicrobial properties that prevent bacteria from decomposing them so they do not rot quickly. The sticky coating used to entrap prey, on the other hand, can soon be rendered ineffective by pollen and dust, so a new structure is required.

Many species of spiders make a new web every day, including the orb-weaving spider Uloborus diversus. They therefore produce nearly 200 webs over their lifetime.

The protein used to make the silk is a precious resource for the spider so it is not surprising that they do not want to waste it. Some orb weavers ingest their silk and systematically dismantle their damaged webs before building a new one. Research has shown that the spiders also use the pollen that has stuck to the web as a food source. Other spiders just abandon the web and start over.

In another approach to recycling, some spiders use the web for another purpose: they bundle it up and use it to wrap up their egg sacs.

Why Spiders Don’t Stick To Their Own Webs

It is only the catching silk that is sticky, and spiders know where it is. This means they can avoid it and not get stuck. There are areas in the webs (e.g. the hub of an orb web) that have no catching sticky silk so this is a safe place to hang out. Also, the spider makes sure that only the tips of its legs are in contact with the silk. Orb weavers clasp the silk using their middle claws, helped by bristles on their leg tips. Spiders regularly clean the sticky substance from their legs by drawing them through their jaws, which also helps, as does a secretion that lubricates their claws.

Do All Spiders Build Webs?

Of the 50,000 or so known spider species, most of them do not spin webs. They still produce silk though, and they use it for some of the other purposes we have outlined above.

Spiders who do not make webs catch their prey by sitting and waiting for unsuspecting victims to wander past. Then, they pounce. Jumping spiders actively follow their prey and grab it when the opportunity presents itself. There are also pirate spiders who pretend to be prey stuck in the webs of other spider species. When the web owner approaches hoping for a meal, the pirate spider attacks and eats them!

Do Other Animals Make Silk Webs?

In most of the insect orders, you will find creatures that can make silk. The most well-known of these is probably the domesticated silkworm (Bombyx mori). However, the list also includes ants, bees, butterflies, moths, and fly larvae. Immature mayflies, thrips, and fleas can also produce silk as can adult silverfish, ‘raspy crickets’, and male centipedes (non-insect arthropods).

Importantly, insect silk is different to spider silk in many ways. It is produced by different anatomical structures, the chemical makeup is different, and they use it for different purposes. Insects use silk to protect metamorphosing pupae (e.g. moth caterpillars), to make skin-shedding easier (e.g. butterfly caterpillars), to reproduce (e.g. silverfish), and for locomotion (e.g. black fly larvae). In common with spiders, some insects also use silk to catch prey, and caddisflies actually spin webs under water! Also in common with spiders, several insects such as the tent caterpillars and raspy crickets use silk to make a home.

There are also a few other types of species that can produce silk. The hagfish is an eel-shaped jawless fish that lives at the bottom of the ocean and produces slime/silk consisting of tens of thousands of very thin protein threads very similar to spider silk. Finally, the Mediterranean fan mussel sticks itself to rocks using a type of silk. However, no other creature does it quite as well as the spider!