Quark

Quarks were robots that served in the Dominators' multi-galactic empire. An offshoot of rebel Quarks went on to conquer worlds on their own. These Quarks were dangerous enough that the Doctor was forewarned so he could place John and Gillian Who in a safer location. (COMIC: Invasion of the Quarks)

Appearance
Quarks were cuboid in shape with a sphere on top. Four spikes came from opposite sides of this sphere, with one on top. They had rectangular compartments on their chests into which their cuboid arms folded. Their arms were equipped with weaponry and manipulating appendages. The spore spikes were red along with the rim attaching them all. The rest of the sphere was black and the body was grey. The Quarks' feet were large pieces of metal in the shape of a block; poles attached these to the body. (TV: The Dominators)

Abilities
Quarks were able to use their guns to kill directly, stun, electrocute, communicate and to power up objects. They could collect data from a survey just by turning 360 degrees. They emitted warning signals when tripped or covered.

The Dominators had symbols representing each Quark on a wall, the symbol for the corresponding Quark flashing when that Quark was destroyed. Dominator Quarks generally communicated with bleeps, shrill squeaks, and squawks, although they were capable of high-pitched speech. (TV: The Dominators)

Quarks operating independently made use of Quark craft and ray guns separate from their in-built weaponry. (COMIC: Death Race) On several occasions, the independent Quarks used Giant Wasps to cover for their limited agility (COMIC: The Killer Wasps) and as their servants. (PROSE: The Quantum Archangel)

The Seventh Doctor once described a specific weakness the Quarks had. According to him, they were effectively "allergic" to leptonite crystals. If they came within half a mile of leptonite, they would explode. (AUDIO: Flip-Flop)

History
According to the Fourth Doctor, the Quarks inhabited a completely different era and region of the universe to the Krotons. Thus, to witness a conflict between the two races would not "make sense". (COMIC: The Lost Dimension)

Quarks were originally the robot servants of the Dominators. The Dominators Rago and Toba took a party of Quarks to the planet Dulkis and had them drill a shaft, through which a device could be dropped that would cause a planet-wide volcanic eruption and make the planet an ideal source of power for the Dominators.

The Second Doctor and his companions Jamie McCrimmon and Zoe Heriot were on Dulkis when this party arrived. Along with the Dulcian Cully, Jamie destroyed a number of Quarks through various methods. He destroyed one by shooting it with a gun, distracted one while Cully smashed it with a large boulder, and used explosives to destroy a third. The Doctor managed to catch the Dominators' bomb as it fell down the mineshaft and, as the Quarks and Dominators prepared to leave, placed it aboard their craft. Once the ship was in flight, the bomb detonated, destroying the remaining Quarks and their masters. (TV: The Dominators)

In 1943, Eileen le Croisette discovered a steelwork's manager, Huxtable, was creating Quarks to use in his own personal war. (PROSE: Home Fires Burn)

The Dominators took the Quarks to Earth in the 1950s to try and enslave the human race. They used the Quarks to stun anyone who discovered them earlier than intended. The Quarks and the Dominators were defeated by the Tenth Doctor. (COMIC: Quiet on the Set)

Other accounts show the Quarks working alone, without reference to the Dominators. They were famed for their brutality and conquests. (COMIC: Invasion of the Quarks) Jack Harkness referred to "rebel Quarks" in a report on the Vespiform, revealing rumours of the Quark-Vespiform War. (WC: )

The rebel Quarks had multiple battles with the Second Doctor, especially in their attempts to invade Earth. Their first attempt was in 1968, where a Quark spearhead destroyed a radar tracking station in Scotland and made a landing at an old castle. The Doctor seized their ship and used it to take out the invasion fleet. (COMIC: Invasion of the Quarks) After this encounter, the Quarks were all ordered to search for and kill the Doctor.

At some point, the rebel Quarks attacked the planet Gano. When the Doctor and Jamie arrived there, seemingly in 1968 as well, the Quarks unleashed their Giant Wasps. (COMIC: The Killer Wasps) At another time, apparently on contemporary Earth again, the Quarks ambushed the Doctor and buried the TARDIS under rubble in an attempt to prevent him escaping. (COMIC: Jungle of Doom)

The Dominators used a squad of slave Quarks to guard their nuclear power station when they infiltrated 1969 Britain. These Quarks were modified to open up into ten-feet tall killing machines. Colonel Lethbridge-Stewart led a small commando raid on the site, finding the Quarks could shrug off some bullets but were vulnerable to grenade launchers and Mills bombs. The surviving Quarks were taken into the custody of the Vault. (PROSE: Mutually Assured Domination)

In 1971, when the Doctor invented the Martha series of houseworker robots and sold them to an American company, the Quarks waited until they were widespread across the country and then turned them against humanity. This was intended as a prelude to their invasion, but they were routed when the Doctor reprogrammed the Marthas to engage the Quark armies. (COMIC: Martha the Mechanical Housemaid)

At some point, the Brigadier and UNIT reprogrammed Quarks to fight Rill defence drones. One of those Quarks eventually wound up at an arcade in Porthcawl. (PROSE: The Arcade of Doom)

In 1984, slave Quarks were part of a failed Dominator secret invasion of Earth. Under Iceland, Major Whitaker's UNIT forces destroyed them. (COMIC: The Fires Down Below)

In 2018, Lucy Wilson accidentally activated the Porthcawl Quark. It blew up most of the arcade before shutting down after recognising her as a Lethbridge-Stewart.(PROSE: The Arcade of Doom) This Quark ended up at a skip. (PROSE: Beyond the Sunset) Later, at an eSports expose, Lucy Wilson was exposed to the memories of a Quark that had rampaged for the Dominators across the galaxy. The Quark's remains tried to use her to escape. (PROSE: The Expo of Terror) That incident reactivated the Porthcawl Quark, and Lucy and Hobo recovered it only to learn the Quark's mind had been upgraded: it now had freedom from its programming, enough to feel grief it was the last of its kind on Earth, to desire death, and to wonder what lay 'beyond'. Lucy Wilson sat with it until it died. (PROSE: Beyond the Sunset)

In 2044, a Quark squad arrived on the planet Hekton to kill the Second Doctor, but were defeated by the human colonists. (COMIC: The Duellists)

In 2053, a Quark assassin pursued the Doctor when he participated in an automobile race on Earth. (COMIC: Death Race)

In 3090, the Seventh Doctor and Mel arrived on the planet Puxatornee looking for leptonite crystals with which to defeat the Quarks in an ongoing battle they were having on the space yacht Pinto. It was unknown whether this battle also took place in 3090. (AUDIO: Flip-Flop)

A film adaptation of H.G. Wells' The Time Machine, which according to the Sixth Doctor "hadn't even been bad enough to be good", featured Quarks and the Giant Wasps. (PROSE: Synthespians™)

Rassilon Era
As part of a failed Dominator invasion of Gallifrey, Quarks "flooded" the Capitol. They were fought by Leela and K9 Mark I, who destroyed 39 Quarks between them. Though the Fifth Doctor would claim that there was never "any serious risk of us being conquered", one of the Quarks was fitted with a bomb which caused severe damage to the Capitol, necessitating the construction of a replacement building. (AUDIO: Time in Office)

Unknown period
The Eighth Doctor and Guy shared an adventure involving the Quarks. Like Edgar Allan Poe, Charlemagne and the Boston Strangler, the Quarks remarked it was odd that the Doctor was travelling with a gorilla in a cravat and a waistcoat. (AUDIO: The Mummy Speaks!)

Alternate universes
In an alternate universe, the Quarks were the dominant power of their reality, theorised by the Fourth Doctor to be the result of a hypothetical absence of the Daleks. A fleet passed into a gate through the Void to N-Space and engaged in battle with forces of the Kroton Imperium who had arrived the same way. Discovering the Doctor's TARDIS, they engaged a transmat opposing that of the Quarks to bring it aboard, with the Fourth Doctor and Romana II taking the chance and boarding one ship, which they found inhabited by advanced Krotons, to stop the tug of war. A boarding party of Quarks was transmated to the ship to intercept but was quickly destroyed by the Krotons. Soon after, both Quark and Kroton fleets were attacked by forces of the Ogron Confederation of Planets who, coming from yet another universe, sought to cease hostilities between what they saw as "lesser-evolved species" via the use of deadly force. Ultimately, all three sides retreated to out of fear when the Doctor and Romana revealed the existence of the Daleks and their own people, the Time Lords. (COMIC: The Lost Dimension)

Other information
During his trial, the Second Doctor recalled the Quarks as one of the adversaries he had broken the Time Lord code of non-interference to fight. (TV: The War Games)

The Sixth Doctor encountered the Quarks with Mathew Sharpe, (AUDIO: The Lure of the Nomad) and later fought them and their Giant Wasps with Melanie Bush. (PROSE: Millennial Rites)

Iris Wildthyme was once trapped in the Death Zone and encountered Quarks. (PROSE: The Scarlet Empress)

once attacked the Graak with Quarks, which was inside one of the Celestial Toymaker's toys. They could be stopped by a water pistol that squirted some sort of blue liquid. (GAME: Destiny of the Doctors)

Panda once met someone dressed in a Quark outfit. (AUDIO: Whatever Happened to Iris Wildthyme?)

In the video game Happy Deathday, played by Izzy Sinclair on the Time-Space Visualiser, a Quark was among a host of "every single enemy" that the Doctor had ever defeated, who were assembled by the Beige Guardian and pitted against the Doctor's first eight incarnations. (COMIC: Happy Deathday)

Writing a new afterword for The Secret Lives of Monsters, a book about aliens by Justin Richards, the Twelfth Doctor noted that the Quarks were one of the monsters who had not been mentioned in the text. The Doctor went onto write that, given the missing information, the book was only "a good start" to the story of threats. (PROSE: The Secret Lives of Monsters)

Behind the scenes
The Quarks were originally created by Mervyn Haisman and Henry Lincoln for The Dominators. They left the project as they felt it was no longer faithful to their original draft. A major factor in this was the design of the Quarks and the fact that The Dominators had been shortened from six to five parts. As a result, the story aired with the writers' names taken off and a pseudonym substituted.

Haisman and Lincoln later felt cheated out of royalties from merchandise featuring the Quarks. Owing to their small size, children inhabited the costumes. Despite the Dominators being the titular enemy and their leaders, the Quarks appeared in TV Comic in an uncharacteristic manner, as solo villains, without the Dominators.

A Quark, along with a Dalek, a Yeti, an Ice Warrior and a Cyberman, appeared briefly in studio (versus previously seen footage from past stories) in the last episode of The War Games  and in publicity photos taken during the shoot during which the monsters posed with the Doctor, played by Patrick Troughton.

The Monster Vault hypothesizes that the Quarks are not truly robots, using it as an explanation as why the Dominators needed to use slaves if they could seeminly make robot minions. Instead, the book notes the Quarks' love of destruction, child-like voices, and small sizes as evidence for a much darker origin.

The trailer Legend on P.R.O.B.E. Case Files - Volume 2 claimed Alistair Gordon Lethbridge-Stewart had defended the Earth from Quarks and Dominators.