Webs are the source of silk, and appear in caverns. The web itself looks like a light-grey gem, and is most often labeled as being from cave spiders, giant cave spiders, or (in evil biomes) phantom spiders. If an embark region is populated by web-making creatures, a number of its webs will be present immediately after embark, though further webs will only appear if the creature itself shows up and leaves them.
Webs will be collected automatically by dwarves with the weaving labor enabled, but not until the fortress has a loom. A collect webs task will be triggered at the loom as long as webs are present within the loom's burrow (even if they are not actually accessible Bug:3525). This can be comfortably deactivated with - - , an important option since webs often occur in dangerous places you might want to secure first. The weaver will drop the collected thread in the loom and, depending on the loom setting and availability of a stockpile, either leave it there, immediately weave it to cloth, or put it on a stockpile. Other ways to prevent collecting are preventing any dwarf from using the loom ( - - requires manager), deactivating the weaving labor on all dwarves, or removing all looms (lack of stockpiles alone will not prevent this). An easier alternative is just to suspend the collect webs task in each loom ( - ). It may be a good idea to only allow non-skilled weavers to collect webs and only skilled ones to actually weave cloth. Set high minimal skill on the actual weaving loom ( - ) and just as high maximal skill on the collecting loom. Suspend weaving on collecting loom and collecting on weaving loom. This allows higher efficiency, since collecting is not in any way affected by weaving skill (it happens instantly), but trains it and the skill affects quality of woven cloth.
Webs become silk thread the moment they are picked up. (They are, however, shown in the -stocks menu under -> thread.) Cave spider and phantom spider silk thread are worth 6☼; giant cave spider silk webs and thread are worth 24☼.
Giant cave spiders shoot long strings of dangerous entangling webs at targets prior to attacking - this behaviour can be used to create giant cave spider silk farms. It is not advised to allow weavers to randomly gather webs on maps where giant cave spiders might be lurking nearby. Certain types of titans, forgotten beasts, and demons also shoot webs, and just as other spider webs are found naturally in their environments, HFS may also contain demon webs.
Webs are delicate, and can be destroyed by contact with any creature (except a dwarf that is gathering that web), or by any other rough treatment such as irrigating, etc. It also appears that if open to the outside, rain will destroy the webs remarkably quickly.
Vermin that come into contact with a web will immediately turn into remains, and large creatures caught in thick webs (as left by giant cave spiders, titans, forgotten beasts, etc.) will be immobilized for a short period of time, forcibly activating any traps they are standing on (even if they are immune to traps). Creatures that spin webs, however, are generally also immune to them - this means that cave spiders and phantom spiders are immune to webs' vermin-killing effect, and that web-spinning forgotten beasts cannot be captured by webbed cage traps.