Rachni

The rachni are an extinct insect-like species that threatened to defeat and overthrow the Citadel Council roughly two thousand years ago. They are the only sapient arthropods known to exist in the galaxy (the keepers not being known to be sapient).

Intelligent but highly aggressive, the spacefaring rachni were driven to expand and defend their territory. They were eventually defeated and completely eradicated by the krogan, who had been groomed by the salarians to effectively do the Council's dirty work during the Rachni Wars. The accidental discovery of the rachni led to the Citadel races to curb their rapid expansion, in fear of being plunged into another galactic war.

History
Though now extinct, the rachni once threatened every species in Citadel space. Around 1 CE, explorers foolishly opened a mass relay to a previously-unknown system and encountered something never seen before or since: a species of space-faring insects guided by a hive-mind intelligence.

The rachni were not peaceful, and the galaxy was plunged into a series of conflicts known as the Rachni Wars. Attempts to negotiate were futile, as it was impossible to make contact with the hive queens that guided the race from beneath the surface of their toxic homeworld.

The emergence of the krogan in 80 CE turned the war against the rachni. Able to survive the harshest environments, the krogan were able to strike at the queens in their lairs and reclaim conquered Council worlds. But when krogan fleets pressed them back to their homeworld, the rachni refused to surrender. The krogan response was swift and brutal; the rachni were declared extinct in 300 CE.

The rachni home cluster is unknown, but Hawking Eta is known as formerly rachni-controlled. The relay that led to rachni space might have been the relay from Horse Head Nebula to Hawking Eta; alternatively, the relay the explorers opened may have been one of the dozens not depicted on the Galaxy Map.

Rachni Workers
Rachni Workers are tiny green rachni (similar to aphids) which usually act as a first wave for Rachni Soldiers. Acting in swarms, they rush up to the squad and explode in suicide attacks, causing heavy toxic damage which ignores shields. One shot or a tech proximity mine will cause them to explode at a distance. Given their name, they likely fill the niche occupied by drones in other hive species, tending to the needs of the queen.

Rachni Soldiers
Rachni Soldiers are slower, but much larger creatures than Rachni Workers, with thin tentacles ending in little pods. These tentacles are used to impale enemies - the rachni can use weapons but prefer to use these tentacles to impale their victims. They can also spit acid, causing heavy toxic damage. Rachni soldiers are cunning and like to ambush their enemies. They are most at home in vents and tunnels.

Rachni Brood Warriors
Rachni Brood Warriors are rachni who are much larger and stronger than normal Rachni Soldiers. They are the 'elder males' of the hive that usually mate with the queen. It is said that they only attack when the hive is particularly strained for warriors. The Brood Warrior has abilities comparable to a standard Rachni Soldier, although its attacks inflict more damage. It is also unique among rachni in that it possesses some biotic ability; it will occasionally place a squad member in Stasis and appears to use a form of Barrier.

Rachni Queen
The largest and most intelligent of the species.

Behaviour and Culture
The rachni are territorial, determined to remain isolated from the rest of the galaxy. They normally inhabit extremely hazardous worlds, able to survive environments that kill most sentient species. Should their territory be invaded on purpose or even by accident, they respond with brutal force.

However, many make the mistake of underestimating the rachni as mindless animals. In fact the rachni are an extremely intelligent sentient species that achieved space flight and a form of cryogenic suspension, developed weapons and carved out a huge swathe of galactic territory. Far from treating soldiers as disposable resources of the hive, rachni are carefully nurtured as part of the group.

The queens are the leaders of the rachni hive mind. Their ways of perception, thinking and communications seems to be somewhat different from that of other races, as shown by the strange metaphors used when attempting to communicate, for example, "songs the color of oily shadow". The speech patterns used by them in such occasions are also unusual; "When we speak, one moves all". They refer to their manner of communication as "singing", which soothes and nurtures their young, and this appears to colour their entire psychology - for example, the rachni speak of death as "the great silence", and refer to their homeworld as the "singing planet". The rachni ability to genetically share knowledge across generations makes queens a vast repository of information.

Tactics
Rachni workers and soldiers use bio-attacks, which appear to be acid-based: workers rush up to the squad and explode, while soldiers spit acid from a distance. These attacks ignore shields and Barrier, and directly damage health. The best tactic with workers is to take them out from a distance - a single shot will make them explode - and Throw them back if they get too close. Soldiers are tough and deadly but vulnerable to biotics. Lift will prevent them using their acid attack and allow them to be hit with Warp, Neural Shock or a hail of gunfire.

Plot
The rachni aren't as extinct as the galaxy had been led to believe. At the remote Peak 15 facility on Noveria, Commander Shepard encounters rachni workers and soldiers that have escaped from the labs. The company renting Peak 15, Binary Helix, had found a rachni ship adrift in space carrying eggs in cryogenic suspension. One of the eggs was recovered and brought to Noveria for study. Binary Helix planned to clone the rachni inside and create an army, but when the egg was hatched the researchers realized this was no ordinary egg, but the egg of a rachni queen. Matriarch Benezia used the genetic memory of the queen to find the location of the Mu Relay on the orders of Saren Arterius.

After she had reached maturity, this resurrected queen began laying eggs, but the rachni were removed from her presence by Binary Helix once hatched, in an attempt to control them. As the queen later explains to Shepard, the rachni elders communicate by 'singing' and this comforts their children. Without the queen's 'song' to soothe them, the rachni soldiers and workers have been driven insane from fear and attack anything on sight. They are beyond help, so the queen asks Shepard to euthanise them.

The queen then asks what Shepard intends to do with her: either use the acid tanks in the lab to kill her, or let her go. She says she knows nothing about what happened in the war: "we were only an egg, hearing mother cry in our dreams". The queen promises that if she is released, she will not attack other races again but find somewhere to live in peace, and teach her children about Shepard's forgiveness. Liara favors releasing the rachni, claiming the krogan went too far. However, both Wrex and Ashley are angry at the prospect of releasing the queen, if they are present alongside a more compassionate squadmate - Wrex isn't impressed by "bugs writing songs about you" and Ashley wants to protect her sisters from horrors like the rachni, not release more of them - but the final decision is up to Shepard.

Cerberus have rachni imprisoned on Binthu as part of their 'super soldier' research. More rachni are encountered on Nepmos, at Listening Posts Alpha and Theta. They have infested various sites across Nepmos and have taken over a nearby mine. After clearing them out, Shepard discovers they were specimens shipped from Noveria by Cerberus, and tracks them back to a depot in the Gorgon system. The depot is overrun with rachni and there are no survivors. After killing the rachni, Shepard finds the logs of a scientist named Flores who was involved in the project. She angrily claims they treated the rachni like animals when they should have treated them like POWs and leaves a recommendation that Cerberus pursue other projects; the rachni are too smart.

Speculation
The Rachni Queen on Noveria mentions 'a tone from space' that forced the rachni to "resonate with its own sour yellow note". This hints that there may have been more to the Rachni Wars than the Council was aware of. This is confirmed in Mass Effect 2, depending on whether you released the Queen, by a message given to Shepherd by an asari who is in service to the newly freed Queen. The asari tells Shepherd that the Queen believes he is fighting the enemy who "soured" the rachni. Shepherd jumps to the conclusion that the message is referring to the Reapers, either through agents, such as the Collectors, or directly. This makes it reasonable to assume that the rachni were indoctrinated into servitude in a manner similiar to the Collectors.

Additionally, a news report can be heard on the Citadel, Illium, and Omega that reveals several mysterious ships have been sighted that match rachni designs, and the Citadel has pledged to investigate these sightings. This appears to indicate that the rachni have progressed extensively in the two years since Shepard freed the Queen.

Trivia

 * The rachni resemble several insectoid alien races in various science fiction series, but most closely resemble the Arachnids from Robert A. Heinlein's Starship Troopers. The name 'rachni' even forms part of the name 'Arachnids', the scientist Flores is also a reference to Private Isabel "Dizzy" Flores. The story of nearly complete extinction of a telepathic, sentient insectoid race and then a single Queen egg surviving to resurrect the race may be inspired by the Formics in Orson Scott Card's Ender's Game and subsequent sequel novels.
 * The rachni can be heard 'singing' on Altahe and near the abandoned mining camp on Nepmos, but this 'song' has also been reported on worlds like Luna, where it should be impossible for any rachni to be present.