This is the first fort I've had that had cave spiders in it. I'm a bit curious about the mechanics of vermin, now.
My fort has a couple of neat things on the map, which turned out to be really close together, and right where I was building my fort. The bottomless pit with the lava glob was obvious from the start, and I built next to it. The underground pool I discovered while my miners were digging out a housing block... which made it a really frustrating discovery, but interesting as well. I walled it up so the monsters couldn't get out, but a cave spider had already escaped and I started finding occasional spider webs in my fort.
There's a lot of spider webs around the pool, and the monsters don't seem to be interested in leaving the water so they don't break them.
My first question about these things is, do they work as real spider webs? As in, do vermin get caught in the webs and killed by the spiders? I've noticed miasma clouds appearing in regions of the bottomless pit where I can only see spider webs, and a number appear in a certain corridor in my fort. The corridor is just four blocks away from the underground pool.
So my other question is, are vermin calculated in larger areas than block-by-block? Along with the cave spiders and webs that occasionally appear just in that small area of corridor, I've seen a few olms appearing. It would make sense if the region around the water was spawning olms where it could, even though the pool is completely walled in.
What in the heck ARE olms anyway?