I'm playing the Legendary Lands mod, the only relevant change of which is the common inclusion of "large cave spiders" in chasm areas. These basically behave just like giant cave spiders, but are a bit less dangerous. For example, I watched six of them (out of the eight discovered thus far) swarm a bison for quite some time before finally managing to kill it. It was raining at the time, so the webs they threw continually got destroyed.
Later one of my stray animals stumbled into the webs left by these spiders. I know it was them, because no spider was revealed along with the webs (i.e. the count of known spiders remained at 8), whereas it's been my experience in the past that revealing a stealthed web also reveals the spider with it. These webs were outside and should have been destroyed in the rain earlier. Unfortunately, it wasn't raining when they were discovered, or else I suspect I would have seen them vanish as soon as they aappeared.
For your amusement: