Rachni

The rachni are a presumably extinct race of intelligent insectoids that threatened to defeat and overthrow the Citadel races roughly two thousand years before the Mass Effect's timeline.

History
They were first encountered by accident due to the activation of a long-range mass relay with a counterpart in what was, at that time, an unknown location. The extremely xenophobic rachni(see below), angered by the encroachment of what it believed were inferior species upon its living space, counter-attacked, determined to destroy the perceived invaders. Negotiation was impossible with the rachni queens, who could not be contacted in their underground lairs on the toxic rachni worlds. Eventually the krogan, discovered and elevated to civilization by the salarians due to their fighting prowess, hardiness, and high birth rate, were able to push back and eventually exterminate the rachni. With the conclusion of the Rachni Wars, the Citadel Council declared the rachni to be extinct. By the time of Mass Effect, a rachni egg was found by a science team on a derelict ship and brought to the Peak 15 research labs on Noveria, where lax research regulations permitted a significant and dangerous amount of research and experimentation; the rachni were effectively brought back from extinction for the purpose of serving as weapons of war sold to the highest bidder. The rachni queen discovered by the player on Noveria explains that the rachni's tendency for extreme belligerence was not intrinsic to their nature and that, were it permitted to live and escape from the facility, the queen would raise a peaceful new breed of rachni with respect for life and other life-forms.

If the rachni queen survives, it contacts Shepard in Mass Effect 2, saying that it managed to escape from Noveria and reestablish the rachni race. It also believes that the previous rachni were manipulated by the Reapers, and promises to help him fight them. Meanwhile, there are increased sightings of Rachni ships, and the Council is taking these reports very seriously.

Physiology
Different types of rachni have different physiology. They are all, however, a carbon-based hive-mind species of insectile creatures, though they have certain crustacean and reptilian traits.

Workers are small, brightly coloured beetle-shaped creatures, capable of moving at very high speeds. They explode when they come into contact with hostile life forms, and are thought to develop from the spiked 'Mermaid's Purse' type objects the player meets in most Rachni levels. The explosion creates a toxic gunge, presumeably fatal to the unprotected human, which is expelled forcefully enough to have several metres' range. It seems to have less effect on other Rachni. The conversion to gunge seems near-total. They can also build extensive tunnel systems very quickly.

Warriors are far larger, taller than a man. They are somewhat like lobsters, with legs coming down from the hindmost section and their back curving forwards over that. They have several long appendages tipped with spikes, easily capable of punching through a man (though this should require more musculature) and presumably strong armour. They also spit toxic fluid, seemingly the same as that from exploding workers. They often come from large mounds above the nest, smashing through its roof to attack foes.

The Brood Warriors are large warriors, with greater endurance. They are higher up the caste system, and are said to only come forwards when the hive is on its last legs.

The Queen is the commander of a Rachni Hive, far larger and differently shaped. They seem to be about 4m long, and are more intelligent than their compatriots, capable of language and problem-solving. They also can learn the gestures and speech of other species. It is said that there were many Queens during the rachni wars, and that they sheltered deep within the planets. They probably worked together to manage their rachni. They also seem to be able to pass on memories genetically or epigenetically, such as the location of the Mu Relay.

Communication
The Rachni communicate through gestures, noises and some kind of transmission- perhaps pheromones, bioelectricity, or biotics. Even when feral, they can distinguish their own species. The queen was capable of controlling the dying body of an asari commando, whether this could occur with a healthy being is unknown.

The Rachni also implanted a message in an asari's mind, who the player meets on Illium. This was probably biotically done.

Senses
Rachni have eyes, antennae and shriek when damaged, implying touch. It is not known whether they atatcked the Geth on Noveria at all. If not, they probably cannot detect inorganic beings, or otherwise do not consider them worth paying attention too.