Why and how do spiders spin threads?

From fingerlike organs called spinnerets, all spiders produce what is called spider silk. Scientists estimate that for a given diameter, their tensile strength can be greater than that of steel. Some use this silk for constructing symmetrical fly-trapping webs, but almost all use it for mobility – even sailing, or ‘’ballooning’’ on winds.

Airborne spiders have been known to land on ships sailing 300 kilometers off the land and have been found on altitudes of 3,000 meters. To help them navigate, spiders spin draglines, at least two or sometimes four or more silken threads meshed closely together. On vertical surfaces they use these threads as a mountain climber uses a rope. The silk is anchored to a surface at intervals by specially fabricated attachment disks, giving the climber both guidance and a safety rope, should the surface ahead prove impassable. Encountering danger, the spider maneuvers in midair, assured that the dragline will bring it back against the surface. Wherever spiders go, they leave a silken trail behind. The strands we encounter floating between objects are relics of their daredevil flights – gossamer draglines discarded once they have served their purpose and the spider has moved on. In the open air these are soon brushed or blown away. But in seldom-frequented places, the threads accumulate in the familiar sheets and nonsymmetrical masses of silk we call cobwebs.

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