Well, at the moment the spider isn't doing anything. I have a tunnel with a cage trap and a tethered horse in it, but nothing seems to be showing interest. I hope it hasn't fallen into the magma or anything. I think I'll send someone to look for it when I get the first migrant wave.
I'll know if it is trappable when it either gets trapped or eats the horse. They don't destroy buildings, right? The only thing keeping my dwarves from the magma pool are flimsy wooden doors.
***UPDATE***
Okay, looks like a fire man triggered the trap, and is now hopelessly stuck in a wooden cage. If only it had something to burn the cage away, it could just walk free. Anyway, I can't choose whether this is a good thing or a bad thing. On one hand, I caught a fire man, which is nice. On the other hand, I'll have to send someone to the invisible monster room to retrieve the cage. Oh, well.
***UPDATE***
The fire man did break out somehow. The cage is gone, presumably burnt away. I guess I shouldn't be all that surprised, but I was really under the impression that things in cages were completely isolated from the outside world and unable to do anything to free themselves. What happened here? Could a fire snake or something have burned the cage away, freeing the fire man to murder the horse? It is a mystery.
***UPDATE***
Woe! Calamity! There is in fact no Giant Cave Spider present!
I sent one of the new immigrants to scout the perimeter, and nowhere, in the pool room itself or the tunnel system I dug before I found it did anything jump out of the shadows to try and eat him. It appears that I will have to make do with ogres and magma dwellers for my intruder devouring solutions.
How did this happen? The room clearly had two units of giant cave spider silk in it. I collected one of them, and some magma creature tore down the other. Could the Spawn of Ungoliant have fallen into the magma? And more importantly, could it come back if I abandon and reclaim?