Silk factories - the spider's silk glands
The silk glands can occupy a large amount of abdominal space, especially in web-building species. Ten different types of silk glands and their spigots are known in spiders. Up to eight silk glands may be present in a single species, each producing silk with different properties and uses. For example: attachment disc silk; strong dragline (safety line) and web frame silk; the orb web spiral line; glue-like sticky catching silk; swathing silk; tangling cribellate catching silk; protective egg sac silk.
Although silk is produced as a liquid within the silk glands, it usually emerges from the spigots (as the spider moves away from an attachment point or pulls the silk out with its leg claws and bristles) as solid silk fibres. Different parts of the silk gland secrete different types of proteins (spidroins) into the gland cavity. These form an inner core and then outer sheathing layers. The very viscous silk flows as a liquid crystal fluid through a long, progressively narrowing duct whose lining cells extract water from the protein. It is then subjected to a mild acidic bath and increased pulling stress which help convert the liquid protein into a solid fibre. The final section of the duct provides a thin, fatty coating to the silk line. The large drag-line gland duct has a valve at this point, just before entering the spigot .This valve probably provides both a means of braking when the spider drops on its dragline, and a pump to move silk forward into the spigot duct after a silk line has broken. The only silk that remains in a liquid state after leaving the spigot is that produced by the aggregate glands - the sticky catching silk of the orb web weavers and their relatives.
