Rachni are a race a strange insect-like creatures originating from the planet Suen. They are a hive mind species controlled by Rachni Queens, who live underground in the Rachni Hives/Cities and command their underlings to do their bidding. Rachni are divided into ranks in the Hives:
Worker: Much smaller than other Rachni, workers are little green thingys that scurry around doing domestic work for other Rachni, like cleaning or cooking. They have no form of natural defence, however they can explode at will so their main tactic is suicide bombing.
Soldier: Large, red creatures and the bulk of the Rachni military. They have 2-4 large tentacle-like weapons attached to their backs and usually spit poisonous acids at their foes.
Brood Warrior: The alpha males of a Hive, they are larger than a soldier and are much more intelligent. They are capable of communicating with other races, albiet limitedly. They have much stronger bodies than soldiers and their acid is much more corrosive. They almost always have some sort of Biotic ability and are very powerful combatants.
Brood Mother: Females responsible for raising newborn Rachni and ensuring that they are brought up to fulfill their role in the Hive. They are large but physically weak, and therefore almost never leave their Hives.
Queen: Leaders of the Hives. They command the actions of their Hive from deep underground. They can reproduce assexually but the new-born will always be a Worker or Soldier. A fertilised egg will result in a Brood Warrior. They cannot communicate with other species directly, but can take control of willing vessels in order to speak. They are massive creatures, and although not suited for combat, they are well capable of defending themselves.
Rachni like music. When new young are born, the Brood Mothers 'sing' to them to ensure they are brought up peacefully and can fulfill their role. When a young Rachni is brought up in 'silence', they are driven insane and tend to attempt mad killing sprees. In 3 PLF, the Reapers 'soured the song' of the Rachni and so drove all of them insane at once. The resulting chaos from this insanity led to the Rachni Wars.