In particular, do they get a double move and THEN get their auto-attack, or would the auto-attack not happen as they were 'moving' for their entire round? Rules state they deal damage at the end of their move with no mention of what kind of move, and since they auto-attack there is no need for taking a standard action, so they'd always double move into someone's square. For my purposes, I'd think a centipede moving at 30 IS running but that's just my own observations. Lastly, a creature that can always move at maximum speed so you can't even run away, still gets auto-attack with no roll needed, and is immune to weapon damage... A single swarm would wipe out a village!
