Originally, rachni are an insect-like species that threatened Citadel space two thousand years ago, which called the Rachni Wars. Most of the units are intelligent but very aggressive, which finally become a threat to the whole galaxy. The story told that rachni finally defeated by the krogan. Krogan was uplifted by salarians to confront them directly in their worlds. Krogan was chosen because of their combat prowess as well as physical resilience. However, there is some accidental discovery that shows that Citadel races curbing this race expansion in order to prevent another galactic war to be happening.
In Mass Effect 3, this race still holding a big role as it appear in one of the main story. That story is when Reaper invade, their hive is found by krogan Aralakh company in Utukku, which led by Grunt if he survive from Suicide Mission. Eventually, Reapers also found the rachni and succeed on indoctrinated of most of the units to fight for their sake.
Meanwhile, the Queen, which is the leader of all rachni found in Utukku also appear to be important in game. It is because Shepard will have a choice either to save or let the queen die during one of the mission. If Shepard chooses to save the Queen, then rachni workers will be working on the Crucible project. Having those workers definitely is a huge boost on finishing Crucible project as the workers are known for their skill in engineering, industrious nature, as well as decent teamwork. But if Shepard just let her die on Noveria, the Reapers will construct an artificial Queen in order to take control of all the army of rachni. Eventually, the Queen will betray Shepard and take all the Rachni Workers War Asset away, which decrease the numerical values of Allliance fleet War Assets.
On Mass Effect 3, there are also some sub-forms of rachni that appear, they are:
Worker; the smallest types of all that has green appearance. Their main weapon is suicide explosion that contain toxic
Soldier; although slower than workers, but it is way larger than workers. Their main weapon is the toxic spit
Brood Warrior; stronger and much larger than Soldiers, Brood Warrior has some biotic ability as well as pretty difficult to take down
Queen; the most important of all, as well as the largest and most intelligent. No wonder that Queen is the leader of all rachni races that guide as well as command all rachni species.
Although the Queen, whether the real or the artificial, will eventually betrays Shepard during one of the missions, it is recommended to spare both of them when you have a chance as it will gave a huge boost in Crucible project as well as War Assets. Also, seeing the subform of Rachni that appear in Mass Effect 3, it is recommended to be well prepare before catching combat with one of Brood Warrior as this type of Rachni is probably the toughest of all. So, what do you think about Rachni?