Cybermites were insect-like robots developed and used by the Cybermen of the Cyber Legions (PROSE: A Girl Called Doubt) as advanced Cybermats. (PROSE: The Secret Lives of Monsters) They were often used in huge swarms to fully cyber-convert people and repair damaged Cybermen, (PROSE: A Girl Called Doubt) but were also capable of partial conversions in small numbers, and were highly adept at covert tasks, such as observation and intelligence gathering. (TV: Nightmare in Silver)
Resembling tiny, insect-like creatures with a pair of "antennae" and numerous legs, Cybermites were theorised by the Eleventh Doctor to be a highly evolved variant of Cybermat. (TV: Nightmare in Silver) This theory was correct, with alientologist Justin Richards pondering whether they were the ultimate form of Cybermat technology. (PROSE: The Secret Lives of Monsters) They were developed by the Cyber Legions, (PROSE: A Girl Called Doubt) who themselves also used Cybermats. (TV: Closing Time, GAME: The Eternity Clock, PROSE: The Scruffy Piper)
After conquering a planet, the Cyber Legions seeded small numbers of Cybermites into the planet's surface, and left them to naturally reproduce; unlike Cybermen and their related variants, Cybermites were created from natural minerals. The Cybermites seeded into a conquered planet would harvest such minerals to create new Cybermites, that would repeat this process over several months or even years, until a swarm of billions of Cybermites had been created. The swarm would then begin converting any remaining people on the planet into Cybermen. In addition, the Cybermites' hive served as a repair facility for damaged Cybermen; the Cybermites would completely envelop the damaged Cybermen and graft replacement parts onto them. (PROSE: A Girl Called Doubt) Interestingly, Cybermites were even capable of somewhat reanimating the empty Cyber-bodies of dead Cybermen; indeed, on Hedgewick's World of Wonders following the Cyber-Wars, a handful of Cybermites were able to puppeteer the empty body of a dead Cyberman to seize Webley.
However, as also demonstrated on Hedgewick's World of Wonders, Cybermites were capable of limited partial conversions in much smaller numbers. On these occasions, Cybermites would directly attack the victim's head to upgrade their brain. Some partial conversions of this kind involved a simple diode being grafted onto the left-hand side of the victim's head, that left the victim comatose, but able to walk, as Angie and Artie Maitland experienced. Other partial conversions involved grafting large implants of Cyber-Technology onto the left-hand side of a victim's head, as Webley experienced. The Eleventh Doctor also experienced this form of partial conversion, albeit with significantly smaller implants than Webley experienced, when a handful of Cybermites converted him into the Cyber-Planner "Mr Clever".
Due to their small size, the Cybermites were highly adept for spying on others and were thus also used by the Cybermen for reconnaissance and intelligence gathering. Each Cybermite possessed an "eye" in the form of a blue light, allowing them to scan environments and individuals, and transmit this data back to the Cybermen. The Eleventh Doctor's sonic screwdriver was capable of discovering the exact location that the Cybermites were sending data to and transmatting him to it. (TV: Nightmare in Silver)
Interestingly, a damaged Cyberman of the "Early" CyberFaction - who survived a failed Cyberman invasion of Earth in the 20th century - used Cybermites long before they would be developed by the Cyber Legions. (PROSE: The Piper)
In the 20th century, following a failed invasion of Earth by the "Early" CyberFaction, (TV: The Invasion) one Cyberman managed to cannibalise garbage it found in the London sewers to repair itself and create a considerable amount of Cybermats. The Third Doctor and Jo Grant stopped it from infecting an entire hospital with Cybermites. (PROSE: The Piper)
The Cyber Legions
The Cyber Legions developed Cybermites during the Cyber-Wars. Following the Eleventh Cyber Legion's invasion of Agrippina's capital city, Agrippina Primus, a swarm of billions of Cybermites inhabited the ruins of the city and established a hive inside the largest of Agrippina's old churches, where they repaired the highly-damaged Cybermen left behind. A human resistance force, including a rogue Cyberman nicknamed "Doubt", attempted to destroy the Cybermite hive, shortly before the Fifth Doctor's TARDIS crashed inside the church. (PROSE: A Girl Called Doubt)
A thousand years after the end of the Cyber-Wars, a small number of Cybermites monitored and surveyed the Eleventh Doctor, Clara Oswald, Angie and Artie Maitland, and Webley, in Hedgewick's World of Wonders, while reporting to an underground repair facility for an army of three million dormant Cybermen underneath the planet. The Cybermites drained Angie's phone of its power, and, later, slightly re-animated the empty body of a chess-playing Cyberman to restrain Webley and partially convert him, before being discovered by the Doctor, who used his sonic screwdriver to locate the receiver of the Cybermites' feedback signals and initiate a transmat to it. Inside the Cyberman repair facility, the converted Webley used Cybermites to convert the Doctor into the Cyber-Planner "Mr Clever".
Following the destruction of Hedgewick's World of Wonders, as well as the planet it was located on, and the three million Cybermen hidden there, a single Cybermite managed to survive the explosion, (TV: Nightmare in Silver) allowing the Cybermen to begin again elsewhere in the universe. (WC: Who Are The Cybermen?)
In an alternate timeline where the Doctor was killed on Agrippina during the Cyber-Wars, a swarm of Cybermites, numbering in the billions, attempted to convert the Doctor's TARDIS. As a result, a war broke out between the Cybermites and the TARDIS's systems, during which the TARDIS's chameleon circuit shut down and caused the ship to swell to the size of a planet. (PROSE: The Paradox Moon)
Behind the scenes
Doctor Who: Legacy