Instinctive Behaviour

If all models in a Tyranid brood begin their movement phase more than 12" away from a Synapse Creature, and that unit is not falling back or already in combat, it will revert to Instinctive Behaviour. Choose each brood in this situation in turn, and apply the following rules:

• If you want to move that brood that turn for any reason, it must take a Leadership test at the start of its Movement phase. If this is failed, the brood will fall back as if it had failed a Morale test. If it is passed, the brood may act as normal.
• Alternatively, the brood may Lurk. this means that it will remain stationary that turn but may fire its weapons as normal. Lurking units that are not Monstrous Creatures add +1 to any cover save they may benefit from. Lurking Tyranids may not claim objectives or hold table quarters. If fearless Tyranid is outside Synapse range and wishing to move, fails its Leadership test, it must Lurk instead of falling back. 
• Tyranids always fall back towards the nearest Synapse Creature if possible - if there are no Synapse Creatures on the board they will fall back towards the nearest Tyranid table edge.