Should spiderines be allowed in 'A Dig Too Far?'
From the rules (p.7)


  Troop Redeployment:
  
  Once a player's conquests for the turn have ended, he may
  freely redeploy the Race tokens he has on the board, moving
  them from one Region to any other Region occupied by his race
  (not necessarily just an adjacent or contiguous Region), provided
  that at least one Race token remains in each Region under his
  control. 


At the end of the Spiderine player's turn there should be at least one token per region, which means that


He cannot "abandon the regions and let the monster races flood out"
Other players should be able to attack that player, giving him losses and thus forcing him to go into decline eventually.