I wanted to post an example that's working in v0.47.05, at least for me. I found conflicting and sometimes outdated information about how to do it, and I had shelved the project and thought it might never work. I had tried everything I could think of using captured goblins and pets as bait, either on chains or roaming free, including a few zany pressure plate designs inspired by comments that a path from GCS to bait was necessary, which I was never able to get working. The GCS would spew webs, but never through my fortifications, only after closing in on the bait and then subsequently reaching and killing it. But using a captured mega- and semi-mega-beast I finally have success!
There's a cyclops up top and a hydra below, with the GCS in the middle. The various levers here just manage the critters, opening up the cages and controlling the cinnabar raising bridges. If I were building this from scratch, I probably would have just held the bait using one raising bridge each, but this is my 111th design as I mentioned.
I've got the microcline retracting bridge next to a raising bridge (in white), since it seems like raising bridges are more violent than retracting ones, and I didn't want to hurt the silk. Maybe a raising bridge would have worked by itself? The reason for the raising bridge is to block the view from the collection area below so that jobs don't get cancelled.
That, boys and girls, is how silk is made.
Down below I've got my collection areas that the bridges drop the webs into, near the looms and dyers. I've had it running for quite a while and I did see one cancellation message caused by "webbed", maybe because the bridge retracted above the dorfs head while he was collecting, and a web fell on him. But it didn't seem to hurt him.
My trigger mechanism for the collection bridges is dead simple. Next to my meeting hall, I've got a two-tile long minecart track with pressure plates on both tiles. I have a hauling route set up to push the cart after 1 day always, so the bridges alternate but not unnecessarily quickly. I had it set to immediately at first but that seemed like overkill.
Speaking of overkill, I'm not sure yet how much silk I really need, and I'll probably have to halt production at some point. I'm fairly sure if I just disable the hauling job, the webs will not proliferate in the upper chamber since it seems like the spider itself cannot deposit multiple webs onto a single tile. I have multiple webs stacked up in the collection area, but the webs are only ever one layer thick on the GCS floor.
Anyway, I guess that's about it. Whee!