Originally encountered by SG-1 as technologically advanced "bugs," the term "Replicator" refers to a number of different groups. They were originally engineered in nanite form by the Ancients in the Pegasus Galaxy as a weapon against the Wraith.
Programmed to replicate and evolve, they eventually emulated their creators by forming humanoid entities -- machines made up of many trillions of microscopic nanites. The Asuran race was born.
Thousands of years later the Replicators were reborn in the Milky Way Galaxy, when an android girl named Reese (presumably herself invented by an Ancient scientist) created more simplistic, spider-like "bugs" as toys to do her bidding ("Menace"). The insect-like Replicators are made up of much larger blocks, and continued to evolve and create Replicators in other, larger insect forms ("Enemies"). Eventually they discovered the deactivated Reese, and recognizing her as their creator they remade themselves in her image. Thus the first human-form Replicators, again made up of tiny nanites, were born in the Milky Way.
The insect-form Replicators are a keron-based technology, programmed with the objective of self-replication. Anything and everything that interferes with this plan of infestation, annihilation and duplication is immediately at risk.
Reese, a corrupt and childlike android, created the first insectoid Replicators to be her "toys," but they were also capable of defending her from threats. As their creator, she also passed on to them her own design flaw. As the inhabitants of her world feared the toys, and tried to destroy them, she ordered them to "replicate at all costs." She quickly lost control, and the Replicators annihilated her civilization, and her own creator in the process. After the Replicators left, Reese went into a deep depression and deactivated herself.
Replicators are composed of hundreds of individual blocks, which are comprised of whatever substance the original Replicator has consumed. It is unclear how many blocks make up a standard Replicator, but two Replicators are required to make a third, and thus exponential growth. It takes only a few blocks to replicate new blocks, and in a few hours Replicators can breed from a handful into hundreds.
There are different kinds of Replicators, depending on what the creatures require to achieve their goals. The most common was originally the "spider" Replicator, with four legs and wing-like flippers on its dorsal side. Another Replicator is much larger and insect-like, appearing in the general shape of a giant fly. It also has four legs, but enormous "wings" allowing it to roll on its back when it lands on the floor, then righting itself. It can also leap great distances.
A third type of Replicator is an assemblage of many thousands of Replicator blocks, which comprise a single, story-tall mother Replicator, capable of siphoning, regulating and energizing an enormous amount of power. Logically, the mother Replicator may also coordinate the activities of all other Replicators.
Replicators generally operate in groups, but it is unclear how their social structure is organized, and there seems to be no apparent means by which they transmit and receive information to and from one another. Despite this absence of information, they are very effective at cooperating with each other to achieve their objectives. If need be, Replicator blocks can simply form a "wall" to block aggressors who would impede their objectives.
Goa'uld and other energy weapons are incapable of stopping Replicators, though the sheer force of such a blast does "halt" them for less than a second. Human weapons, which launch projectiles, are much more effective, causing the blocks to break apart from one another. However, this can only solve problems for a few moments; the Replicators reassemble with a vengeance and come again. Once shot a second time, the blocks do not tend to reorganize.
Generally, Replicators will not attack unless they are threatened. They can, in fact, clamor over and past an individual without a hint of malice; but once they are fired upon, a vengeful subroutine in the Replicator makeup emerges, and the creature becomes a serious threat. Their highly acidic "spray," used to help break down metal alloys to create new blocks, is deadly.
Replicators migrated to the Ida Galaxy, where they were a serious threat to the technologically advanced Asgard empire. The Asgard exhausted all unilateral options within their arsenal of ships and weapons, and have come to Earth seeking help on more than one occasion.
The Asgard, through studying the android Reese, eventually located an additional command within her: "Come forth." The Asgard used this command on every known Replicator throughout the galaxies, and once they were all gathered in one place (the Asgard home planet, Halla), the Asgard intended to activate a time dilation device and trap the Replicators in a time bubble. Unfortunately, the Replicators reversed the device before it could be activated, allotting themselves hundreds of years (in a matter of normal days, relatively speaking) to consume every technological component on the planet and conserve energy -- evolving into humanoid forms, made up of microscopic nanites rather than blocks. SG-1 infiltrated this world and successfully reversed the device with help from the Replicator Fifth, but not before billions of Replicator blocks were forged, covering the entire surface of Halla, perhaps miles beneath as well.
It took less than two years for the humanoid Replicators to escape the trap and resume their plans to destroy the Asgard empire and conquer the universe. Under the command of Fifth and his own creation, a Replicator duplicate of Samantha Carter, they sought neutronium, the raw material necessary to create more human-form Replicators. Replicator Carter soon turned on Fifth and destroyed him, taking control of the Replicator forces in the Milky Way.
The Replicators entered our galaxy and attacked the Goa'uld empire, crippling it. But SG-1 discovered an Ancient weapon on the planet Dakara, which they used to destroy all surviving Replicators throughout the Milky Way Galaxy. It was believed that the threat was gone once and for all until John Sheppard and the team from Earth's expedition to Atlantis met a massive, 10,000-year-old civilization of Replicators living in the Pegasus Galaxy. These are the original Replicators, the creation of the Ancients during their war with the Wraith.
FIRST APPEARED: Nemesis
Nemesis - SG-1 encounters the unstoppable enemy of the Asgard when Replicators bring Thor's ship to Earth in an effort to infest it.
Small Victories - A single Replicator survives the destruction of Thor's ship, and the team must destroy its new colony on board a Russian submarine. Meanwhile, Carter helps the Asgard stop an attack on one of their planets.
Enemies - Stranded in a distant galaxy, SG-1's only hope of defeating Apophis lies with the surprise arrival of the Replicators.
Menace - The team finds an advanced android alone on a desimated planet, and learn that Reese was the origin of the Replicators -- which she creates to take over the SGC and try to escape Earth.
Unnatural Selection - At the request of Thor, SG-1 takes the Prometheus into Asgard-infested territory to reverse a time dilation device so the bugs can be bottled-up indefinitely.
New Order, Part 1 - Sam and Teal'c go in search of the Asgard, only to find that the Replicators have escaped their trap.
New Order, Part 2 - Fifth captures Sam and tortures her, finally releasing her when she refuses to love him. Jack O'Neill creates a weapon capable of destroying the Replicators.
Gemini - Carter's Replicator double tricks the team into helping her overcome the anti-Replicator weapon, using it to destroy Fifth and take over the Replicator forces.
Reckoning, Part 1 - Led by Replicator Carter, the Replicators invade the Milky Way Galaxy and begin destroying the Goa'uld empire.
Reckoning, Part 2 - SG-1 discovers a weapon on Dakara that allows them to destroy all Replicators in the galaxy at once.
Progeny - John Sheppard's team meets the Asurans, an advanced race in the Pegasus Galaxy, only to discover that they are the original Replicators, created by the Ancients.
The Real World - The team learns that Elizabeth Weir has been infected with Replicator nanites.
The Return, Part 1 - The Asurans attack Atlantis and wipe out the Ancients who have taken up residence there.
The Return, Part 2 - The team takes back Atlantis from the Replicator invaders.
First Strike - Earth launches a preemptive attack on Asuras after intelligence reveals that they are building warships.
Lifeline - To save Atlantis the team visits Asuras to steal a Z.P.M., and with help from Dr. Weir, the Asurans can do nothing to stop them. The Replicators launch a war with the Wraith.
Reunion - A Replicator who has been captured by the Wraith is set free when a team from Atlantis attacks their research lab.
The Seer - Colonel Carter is shown a vision of a Replicator warship destroying Atlantis.
This Mortal Coil - A mysterious drone crashes into Atlantis, indicating that the Replicators may have found the city. But when Elizabeth Weir appears, the team realizes that the probe is just a part of a bigger mystery.
Be All My Sins Remembered - A plan to destroy the Asurans is devised and McKay creates a new human-form Replicator.
Ghost In the Machine - A group of disembodied Replicators invade Atlantis, using the technology by which they were originally created to build new, simple bodies for themselves.
Stargate: The Ark of Truth - A preserved Replicator bug escapes its containment on board the Odyssey during a mission to the Ori home galaxy, taking over the I.O.A. representative James Marrick.