How Do Spiders Make Webs?

Silk is produced as a liquid protein solution inside specialised glands. As it is drawn out through the spinnerets, the pulling force and changes in acidity cause the protein molecules to align and lock together into a solid fibre.

The change happens during extrusion rather than by drying, which is why a spider can produce thread on demand underwater or in humid air. Most orb weavers have several gland types producing different silks for different jobs.

The construction sequence is consistent enough to describe step by step:

• •Release a fine thread into the breeze until it catches on something
• •Reinforce that first bridge line and build a frame around it
• •Lay radial spokes from the centre outwards, like wheel spokes
• •Spin a temporary non sticky spiral from the middle outwards as scaffolding
• •Replace it with the sticky capture spiral, working inwards and eating the temporary one

Only the capture spiral is coated in glue droplets. The radial spokes and frame are dry, and a spider moving around the web keeps to those lines.

There is more to it. Spider legs have a coating that reduces adhesion, fine hairs that minimise contact area, and careful movement that lets them peel off rather than pull straight away from a sticky thread. Spiders can and do get stuck occasionally, but the combination makes it rare.

Dragline silk has a tensile strength comparable to high grade steel by weight, and it far outperforms steel in toughness, which measures how much energy it absorbs before breaking.

That toughness is the point. A web has to stop a flying insect without snapping or bouncing it back out, so the silk stretches, absorbs the impact and dissipates the energy. Materials scientists have spent decades trying to reproduce these properties artificially with only partial success.

The classic wheel shaped orb is only one design among many. Sheet webs form horizontal platforms with tangled threads above that knock insects down onto them. Funnel webs have a retreat at the back where the spider waits.

Many spiders build no web at all. Jumping spiders stalk and pounce, using silk only as a safety line. Trapdoor spiders line a burrow and ambush from it. Even web builders use silk for egg sacs, wrapping prey and dispersing as spiderlings on strands carried by air currents.

In the 1940s and again in the 1990s, researchers gave spiders various substances and photographed the resulting webs. The results were used to argue that web geometry could serve as a sensitive measure of a compound's effect on the nervous system.

Spiders have also been flown to orbit several times. Early experiments produced messy webs, but later ones showed spiders adapting within days and building near normal webs, apparently using light as an orientation cue in the absence of gravity. Both lines of research use the web as a readable record of an animal's internal state, which few behaviours allow.

Many orb-weaving spiders have poor eyesight, so the web works less like a net they watch and more like an extension of their senses. Sitting at the hub, or in a retreat with one foot on a signal thread, the spider feels vibrations travelling along the radial lines. Sensitive organs in its legs pick up tiny strains in the exoskeleton. Differences in the pattern and strength of vibrations help the spider tell a struggling insect from a falling leaf or wind, and judge roughly where on the web the disturbance is.

This sensitivity creates problems as well as opportunities. A male approaching a female's web risks being treated as prey, so males of many species pluck or drum on the threads in distinctive patterns that signal who they are. The web is also costly to make, because silk is protein. Many orb weavers take down and eat their old capture spiral before building a new one, recycling much of the protein. Some species rebuild daily, which is why a web you see in the morning may be freshly made.

People have long put spider silk to work. Because a single strand is extremely fine yet strong and uniform, it was used for the crosshairs in telescopes, surveying instruments and gun sights, and it remained in use in some optical instruments well into the twentieth century. Silk was also used in folk medicine to cover small wounds. More recently, two textile makers in Madagascar collected silk from more than a million golden orb weaver spiders, gently drawing it out by hand and then releasing them, to weave a shimmering golden cape that was displayed in museums in the 2010s.

That cape took years to make, which shows why there are no spider silk farms. Unlike silkworms, most spiders are territorial and will eat one another when kept close together, and each produces only a small amount of silk. Scientists have instead tried inserting spider silk genes into other organisms, including bacteria, yeast and silkworms, so that they produce silk proteins which can be collected and spun. One project even bred goats that secreted a silk protein in their milk. Making artificial fibres match natural dragline silk has proved the hardest part.

The capture spiral is made from a different silk from the frame. Its core thread, called flagelliform silk, can stretch to two or three times its resting length, so it gives way when an insect hits instead of snapping. As the spider lays it down, it coats the thread with a watery glue. Just as rainwater on a washing line gathers into droplets, that coating breaks up by itself into a neat row of tiny beads, an effect of surface tension. Chemicals in the beads draw moisture from the air, which keeps the glue soft and tacky.

Inside many of those droplets is a neat trick. When a stretched capture thread slackens, the spare length is reeled up and bundled inside the droplets instead of sagging, rather like a fishing reel taking in line. So the spiral stays taut whether a gust stretches the web or lets it relax, and it springs back after a struggling insect pulls on it. The glue also changes its behaviour with speed. Researchers have found that it grips harder when pulled quickly, so the frantic jerks of a trapped insect tend to hold it more firmly rather than freeing it.

Many spiders, especially young ones, travel by air. A spider climbs to a high point, raises its abdomen and lets out several fine strands of silk until they lift it away, a behaviour called ballooning. Charles Darwin noticed tiny spiders landing in large numbers on the rigging of HMS Beagle when the ship was far off the coast of South America, and ballooning spiders have since been caught by aircraft flying high in the atmosphere. Ballooning helps explain why spiders are often among the first animals to reach new volcanic islands and other freshly formed land.

How the strands produce enough lift long puzzled scientists, because spiders sometimes take off on days with very little wind. In 2018 researchers at the University of Bristol reported that spiders respond to the weak electric field that is always present in the atmosphere. In a box with no moving air, spiders began their take-off behaviour when an electric field of natural strength was switched on, and some rose into the air; when it was switched off, they drifted down. Fine hairs on their legs appear to sense the field, and electrically charged silk may provide part of the lift alongside air currents.

Spiders make webs by extruding liquid protein that solidifies as it is drawn out, following an instinctive construction sequence, and they avoid sticking by walking only on the dry radial threads with legs adapted to release easily.