The Stone Spider’s special rule mentions successful Attacks or Feints with the spider’s fang weapon. The Kill and Drive-Off weapons entry lists only Feint and Maneuver, however. Is there a conflict weapon missing or mislabeled, or am I missing something?
You choose a weapon that you use for the entire round. If a stone spider chooses to use its Fangs weapon for the round and takes away stones with an Attack or Feint, its special power goes off.
It’s just that the Fangs weapon is only listed for Feint and Maneuver, not for Attacks; hence my question.
But you can still Attack when you have that weapon selected. It doesn’t give you a bonus, but it does apply the special effect.
It’s like the kobolds’ bombs – I see. Thanks! That had me confused.
Very importantly, using the weapon for actions that it doesn’t give an explicit bonus to also (in a way) prevents you from being unarmed. 
Well, in the Stone Spider’s case, the Fang weapon (where listed) gives a penalty of -1D, so Attack without a bonus is still a better deal than that …
Also, for a creature to suffer the -1D for being unarmed you have to actually disarm it, I think. Even if a monster has no weapons listed for a particular type of conflict, it doesn’t count as unarmed (p. 150).