Quark

Quarks were the robot slaves of the Dominators' galactic empire.

An offshoot of rebel Quarks would go on to conquer worlds on their own. These Quarks were dangerous enough that the Doctor was forewarned so he could place John and Gillian in a safer location. (TVC: Invasion of the Quarks)

Appearance
Quarks were cuboid in shape with a sphere on top. Two 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. (DW: 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 flashes when that Quark was destroyed. Dominator Quarks generally communicated with bleeps, shrill squeaks, and squawks, although they were capable of high-pitched speech. (DW: The Dominators)

On several occasions, the independent Quarks used Giant Wasps to cover for their limited agility. (TVC: The Killer Wasps)

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. (BFA: Flip-Flop)

History
Quarks were originally the robot servants of the Dominators. The Dominators Rago and Toba took a party of Quarks to the planet Dulkis. There, they had the Quarks 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, Jamie McCrimmon and Zoe Heriot were on Dulkis when this party arrived. Along with 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. (DW: The Dominators)

Other accounts show the Quarks working alone, without reference to the Dominators. They were famed for their brutality and conquests. (TVC: Invasion of the Quarks) Jack Harkness referred to "rebel Quarks" in a report on the Vespiform, implying these were Quarks who rebelled against and broke away from their Dominator masters. At some point in their history, these Quarks waged war with the Vespiforms. (WEB: Monster Files: Vespiform)

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. (TVC: Invasion of the Quarks) After this encounter, the Quarks were all ordered to search for and kill the Doctor.

At some point, they attacked the planet Gano. When the Doctor and Jamie arrived there, seemingly in 1968 as well, the Quarks unleashed their Giant Wasps. (TVC: 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. (TVC: Jungle of Doom)

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. (TVC: Martha the Mechanical Housemaid)

In 1984, slave Quarks were part of a Dominator secret invasion of Earth. They came up against UNIT forces under Iceland. UNIT's Major Whitaker ensured their destruction. (DWM: Fires Down Below)

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

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

In 3090, the 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. (BFA: Flip-Flop)

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. (DW: The War Games)
 * The Sixth Doctor and Melanie Bush fought the Quarks and their Giant Wasps again. (MA: Millennial Rites)
 * Iris Wildthyme was once trapped in the Death Zone and encountered Quarks. (EDA: The Scarlet Empress)
 * The Master once attached Graak with a Quark, which was inside one of the Celestial Toymaker's Toys. It could be stopped by a Water Pistol that squted some sort of Blue liquid. (VG: Destiny of the Doctors)

Behind the scenes

 * The Quarks were originally created by Mervyn Haisman and Henry Lincoln. They left the project as the 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 cut from six to five parts. Haisman and Lincoln later felt cheated out of royalties from merchandise featuring them.
 * A Quark, with a Dalek, a Yeti, an Ice Warrior and a Cyberman appeared briefly in studio (rather than in previously seen footage) in the last episode of The War Games. One also appeared in publicity photos for that story.
 * Despite the Dominators being the titular enemy and the leaders, the Quarks were seen as the more memorable. They appeared many times in TV Comic as a solo villain, without the Dominators.