Spider silk is a protein fibre spun by spiders. Spiders use their silk to make webs or other structures, which function as nets to catch other animals, or as nests or cocoons for protection for their offspring. They can also suspend themselves using their silk, normally for the same reasons.
Many small spiders use silk threads for ballooning, the scientific term for the dynamic kiting spiderlings (mostly) use for dispersal. They extrude several threads into the air and let themselves become carried away with upward winds. Although most rides will end a few yards later, it seems to be a common way for spiders to invade islands. Many sailors have reported that spiders have been caught in their ship's sails, even when far from land. The extremely fine silk used by spiders for ballooning is known as gossamer.
In some cases, spiders may even use silk as a source of food.
Methods have been developed to silk a spider forcibly.
Spider silk is a remarkably strong material. Its tensile strength is comparable to that of high-grade steel (1500 MPa), and about half as strong as aramid filaments, such as Twaron or Kevlar (3000 MPa). Spider silk is about a fifth of the density of steel; a strand long enough to circle the Earth would weigh less than 500 grams (18 oz).
Spider silk is also especially ductile, able to stretch up to 1.4 times its relaxed length without breaking; this gives it a very high toughness (or work to fracture), which "equals that of commercial polyaramid (aromatic nylon) filaments, which themselves are benchmarks of modern polymer fibre technology". It can hold its strength below −40 °C.
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