How Do Spiders Make Silk? Spinnerets and Seven Silk Types

From liquid to thread: how silk is spun

Inside each silk gland, the spider stores silk proteins called spidroins as a thick, concentrated liquid, sometimes described as spinning dope. In this state the long protein molecules float freely and are not yet a fiber.

When silk is needed, the liquid flows down a long, narrowing S-shaped duct towards a spigot. Along the way several changes happen at once. Water is drawn out, the chemical balance of salts shifts, the liquid becomes more acidic, and the narrowing duct stretches the flow. Together these changes make the protein chains align side by side and bond into tiny, very orderly crystals held in a more flexible matrix.

The final, crucial step is pulling. A spider does not squirt silk out like toothpaste. The thread is drawn out by the spider’s legs, by its body weight as it drops, or by the wind. The pulling force finishes aligning the molecules, which is why silk becomes solid at room temperature, in air and without any heating. Spinning is faster and gentler than any industrial process for making synthetic fibers.

What are spinnerets?

Spinnerets are small, movable, finger-like organs at the tip of the abdomen. Most spiders have three pairs, though some have one, two or four pairs. Each spinneret is covered with dozens to hundreds of spigots, microscopic nozzles each connected to a single silk gland.

Because the spinnerets can move independently, a spider can combine threads from several glands, lay them side by side, glue them at a point, or spread them into a broad band for wrapping prey. Hunting spiders and web builders both use them constantly, which is why spinnerets are visible as little bumps under the rear of the abdomen.

The seven types of spider silk

An orb weaver such as the European garden spider can have up to seven kinds of silk glands, and each makes a silk with a different job. Not every spider has all seven, and some other spiders make extra types, such as the woolly cribellate silk of certain web builders.

The dragline is the spider’s lifeline. A spider trails it almost everywhere it goes and can drop and hang on it, then climb back up. The capture spiral is the opposite: thin, very elastic and covered in sticky glue so that insects hitting the web are held rather than bouncing off. How a spider combines these silks into a web is explained in how do spiders spin webs.

How strong is spider silk?

The strongest dragline silks have a breaking strength in the same range as high-grade steel. Because silk is about six times less dense than steel, it is much stronger than steel weight for weight. Silk is also highly elastic, so it can absorb a lot of energy before it breaks. That toughness, more than raw strength, is what lets an orb web stop a fast-flying insect.

Capture spiral silk can stretch to roughly two or three times its original length before breaking. Dragline silk stretches less but is stiffer and stronger. Silk properties vary with species, temperature, humidity and the speed at which the spider pulls the thread.

The toughest silk known comes from Darwin’s bark spider (Caerostris darwini) of Madagascar, whose webs can span rivers. Its dragline silk is roughly ten times tougher than Kevlar, the material used in protective vests.

Despite all this, a single thread is very fine, usually only a few thousandths of a millimeter thick, much thinner than a human hair.

What do spiders use silk for?

All spiders make silk, even those that never build a web. Silk is used for:

• Webs to catch prey, from orb webs and sheet webs to tangled cobwebs.
• Safety lines, so a spider that falls or jumps is caught on its dragline.
• Egg sacs that protect eggs from drying out, predators and parasites.
• Wrapping prey, so a struggling insect can be held safely before it is bitten.
• Shelters and burrow linings, used by many hunting spiders.
• Ballooning, in which young spiders release threads that catch the breeze and carry them long distances.
• Sperm webs, small sheets males use when charging their pedipalps before mating.

Web builders also keep the web non-sticky where it matters, which is part of the answer to why spiders don’t stick to their own webs.

Can people make spider silk?

People have long been fascinated by spider silk, but farming spiders is impractical because most are solitary and many will eat each other. A famous exception is a golden cape made from the silk of more than a million golden orb-weaving spiders collected in Madagascar, displayed at the Victoria and Albert Museum in London in 2012.

Today scientists insert spider silk genes into bacteria, yeast and other organisms to produce silk proteins, then try to spin them into fibers. The results are promising for medical sutures, textiles and lightweight materials, but artificial fibers still rarely match the performance of silk spun by a living spider. The spider’s secret lies not only in the proteins but in the way it spins them, something that is explained further in spider vs insect, which compares spider silk with the silk made by insects.