The rachi are a nearly extinct race in Mass Effect.
Rachni are race of sapient, insect-like creatures with a eusocial organization. Rachni hives have a population of workers, warriors, and drones (also called "brood warriors") that follow the direction of a queen. They vary widely in size, with workers smaller than a human and queens as much as 17 meters in length. Rachni are strong, intelligent, and dangerous adversaries. In particular, they are capable of surviving in many environments that are toxic or otherwise intolerable to humanoids.
Rachni queens can transfer memories to their progeny, allowing them to maintain a sophisticated culture and technology base without a written language. Rachni can communicate telepathically, a form of contact that they call "singing". This telepathy works with some other species, like asari, that the rachni describe as "receptive".
The rachni posed a threat to the galaxy roughly 2000 years ago, when the only members of the Citadel Council were the asari and salarians. Territorial and aggressive, the rachni began expanding when the mass relay in their system was accidently activated by the salarians, initiating the Rachni Wars. There are some indications that older races had been selectively breeding them for such aggression.
The salarians and asari did not have the numbers or durability to battle the rachni in their toxic homeworlds, so the salarians "uplifted" the krogan race to fight the war for them, giving them space flight technology so they could carry the fighting to the rachni homeworlds. The krogan were immensely successful, killing the rachni queens in their nests and bringing the race to the brink of extinction.
Rachni technology is not as advanced as that of most other species in Mass Effect, but they are physically strong and durable, and their queens are able to reproduce quickly enough to give them an advantage in a war of attrition. Direct transfer of knowledge from the queens to their descendants allows young rachni to be combat-effective without extensive training.