40d:Web

Webs are the source of silk, and appear randomly in fortress areas that are near chasm tiles, including the end-point of underground rivers. The web itself looks like a light-grey gem, and is most often labeled as being from cave spiders, but the presence of the actual vermin is not necessary for the web to be formed on a given spot (though cave spiders will appear in the general biome where the webs are found). Similar webs can be found in a biome shared by giant cave spiders - an actual GCS will be present if these are found.

Webs will be collected automatically by dwarves with the weaving labor enabled, but not until the fortress has both a loom and a stockpile that accepts silk thread. To prevent this, either all weaving labor must be un-designated, all looms must be removed, or all silk stockpiles removed or sealed from a path. (If the webs are outside, or it requires an above-ground journey to path to them, changing orders to "Dwarves stay indoors" will also prevent this.)

Webs become silk thread the moment they are picked up. Cave spider silk webs and thread are worth 12☼; giant cave spider silk webs and thread are worth 140☼.

Giant cave spiders leave similar, individual webs behind, but also 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.

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 flooding, etc.