Tyranids are really just bioengineered animals, and being so, they need instructions to survive out of the reach of their synsapse creature counterparts. If a Tyranid unit,at the beginning of their movement phase, is not in range of a synapse creature, such as a Tyranid Warrior, it must take a morale check. If it fails that morale check, than it converts back to its natural instincts for this turn. You must roll a D6 and get one of the following results:
1-2:The creature runs toward the nearest synapse creature on the board. If there are not any, than the creature runs toward the nearest Tyranid player's table edge.
3-4: The creature retreats toward the nearest bit of land that provides a cover save. If there are none in range, the creature retreats using the normal rules.
5-6: This is the good one. The creature lunges toward the nearest enemy unit! Roll 2D6" and if the creature makes contact with the model, it counts as a sweeping advance. If the creature is already in assault, than it counts as a charge in the next assault phase.
Tyranids also have a special rule called "Fast and Deadly". Pretty neat sounding rule, ain't it? Anyway, this rule gives Tyranids the ability to move an additional D6" through difficult terrain, such as jungles. The downside to this is that is a brood has more than eight units, and they roll more than 2 6's, one model in the brood suffers a wound from the feverish trample of its fellow Tyranids. Saving throws apply as normal. Synapse creatures get the rule but can suffer no wounds because of their control over the other Tyranids around it.