@Tyranids, also known as The Great Devourer are an extragalactic alien race, whose sole purpose is the consumption of all forms of genetic and biological material in order to evolve and reproduce. Tyranid technology is based entirely on biological engineering. Every function is carried out by living, engineered creatures, each of which collectively forms the Hive Fleet, directed by a single Hive Mind.
The Tyranids are seen as one of the gravest threats to the entire Galaxy. They seek only to consume all organic life and cannot be reasoned with or deterred in this quest. Worse still for the Galaxy, thus far the Tyranid Hive Fleets that have been encountered are merely the furthest stretched tendril of the main invasion fleet that is still traveling in the void of space.
The Tyranids do not communicate with the other races of the Galaxy. The Tyranids cannot be reasoned with, appeased or surrendered to. There can be no hope of mercy from such a foe. To face the Tyranids is simply a matter of survival: kill or be consumed.[14b]
The Tyranid forces are constantly changing and evolving at an unnatural speed. It is unlikely that all types of Tyranids have been seen by the Imperium, and less likely still that they will ever all be seen. However a few key traits are identical to nearly all Tyranids, such as tough chitinous armour, a hexapodal anatomy, and multiple redundant organs which make them incredibly difficult to kill.
Within the Tyranid swarm there is no idea of individualism as each unit is linked to the next in a form of swarm consciousness. There is only one reason for the creation of any of the forces involved in Tyranid armies: to implement the will of the swarm, be it a Hive Tyrant or Ripper Swarm. This gestalt consciousness is commonly referred to as the Hive Mind.
Certain Tyranid units are considered Synapse Creatures, whose job it is to control the lesser species within the swarm and exert the will of the Hive Mind. They are capable of receiving and sending orders via the Hive Mind and this connection is vital to the effective running of the Tyranid race for, without it, the swarm would collapse into disarray. Usually Synapse Creatures are fielded in relatively large numbers to allow a rapid advance of the lesser creatures to remain in range of a synapse creature.