Spider silk is a protein fibre spun by spiders. Spiders use their silk to make webs or other structures, which function as sticky nets to catch other animals, or as nests or cocoons to protect their offspring, or to wrap up prey. They can also use their silk to suspend themselves, to float through the air, or to glide away from predators. Most spiders vary the thickness and stickiness of their silk for different uses.
A single spider can produce up to seven different types of silk for different uses. Consisting of mainly protein, silks are about a sixth of the density of steel (1.3 g/cm3). As a result, a strand long enough to circle the Earth would weigh less than 500 grams (18 oz). Making the silk acidic (pH 4) is a protection from fungi and bacteria that would otherwise digest the protein.
The spider silk shown here results from combination of four fibres. This hand-painted picture was produced using a scanning electron microscope, by Micronaut (Martin Oeggerli).