Biosynthesis
The production of silks, including spider silk, differs in an important respect from the production of most other fibrous biological materials: rather than being continuously grown as keratin in hair, cellulose in the cell walls of plants, or even the fibers formed from the compacted faecal matter of beetles, it is "spun" on demand from liquid silk precursor sometimes referred to as unspun silk dope, out of specialised glands.
The spinning process occurs when a fiber is pulled away from the body of a spider, be that by the spider’s legs, by the spider's falling and using its own weight, or by any other method including being pulled by humans. The name "spinning" is misleading as no rotation of any component occurs, but the name comes from when it was thought that spiders produced their thread in a similar manner to the spinning wheels of old. In fact the process is a pulltrusion—similar to extrusion, with the subtlety that the force is induced by pulling at the finished fiber rather than being squeezed out of a reservoir of some kind.
The unspun silk dope is pulled through silk glands, of which there may be both numerous duplicates and also different types on any one spider species.
Read more about this topic: Spider Silk