Spider

Spiders were a type of arachnid commonly found on Earth. They were noted for using spinnerets for spinning webs which they used to catch prey, commonly flies. In late 2018, Jade McIntyre believed that there were a total of 21 quadrilion spiders on Earth. (TV: Arachnids in the UK)

On Earth
Earth spiders had no eyelids. (PROSE: The Stone House)

Flies were preyed upon by spiders. The Fifth Doctor quipped that the Racnoss Consort must have eaten a lot of flies to reach his size. (AUDIO: Empire of the Racnoss)

Originally, spiders were small, inoffensive creatures, none growing much larger than a rat, though some species had a venomous bite that could be deadly to humans. By the 26th century, though, radiation had caused mutations, allowing them to reach much larger sizes. (PROSE: Ship of Fools) An example of this were the Eight Legs, spiders with great mental powers, who evolved on Metebelis III from spiders that survived the crash landing of a human ship. (TV: Planet of the Spiders)

A money spider grown to enormous proportions by a Zygma beam was discovered by the Fourth Doctor in the basement of the Palace Theatre. (TV: The Talons of Weng-Chiang)

Due to their connection with the Eight Legs, (PROSE: The Ancestor Cell, Interference - Book Two) the Enemy were often associated with spider imagery. (PROSE: The Map and the Spiders, Subjective Interlock, et al.) A T. memeticus took the form of a regular-sized spider while on the Greenworld. (PROSE: T. memeticus: A Morphology)

The First Doctor, Ian and Barbara encountered a giant spider when the TARDIS landed in its web. (COMIC: Dr. Who in the Spider's Web)

Gwen Cooper once told Jack Harkness that she "never even kills spiders in the bath" when he wanted to teach her how to use a gun. (TV: Ghost Machine)

The spiders in Sheffield were behaving erratically in the year 2018 due to the presence of many large, mutated spiders spreading pheromones throughout the area. (TV: Arachnids in the UK)

On Earth, toy versions of spiders were used as Halloween decorations. The night after one Halloween, an alien possessed hundreds of toy spiders left behind in a warehouse to attack and frighten the Eleventh Doctor and Louie Rollins. (PROSE: The Night After Hallowe'en)

According to the Thirteenth Doctor, spiders disliked the smell of garlic and acetic acid. A spider repellent could also be concocted from peppermint and tea tree oil diluted with water. (TV: Arachnids in the UK)

Elsewhere
K9 once encountered spiders on a planet in another dimension, partially inhabited by ex-Tellac miners. They lived alongside other Earth animals from widely varying periods of evolutionary history. Unlike most species of spider on 21st century Earth, however, these were as big as gorillas, and their webs spanned between trees. (PROSE: K9 and the Missing Planet)

Another planet was home to giant spiders which were controlled by the Master of Spiders. When he used them to attack the Second Doctor, John and Gillian, the Doctor used his ray gun to kill one spider (while shouting "Die, hideous creature... die!") and used his recorder to keep other spiders from attacking them. When the Master was defeated, the Doctor sealed the surviving spiders back in their cave. (COMIC: Master of Spiders)

Another planet was home to large spider-like creatures. The sons of Grekk, who also lived there, viewed them and all the other intelligent life forms on the planet as vermin, and had vowed to exterminate them all. When the First Doctor was imprisoned, he and the other prisoners used the webbing produced by the spiders to set a trap for the sons. (PROSE: The Sons of Grekk)

Another planet was home to a species of flying spiders. Professor Gnat discovered one in his cell when he was imprisoned by Cybermen, and was still studying it when the Second Doctor, John and Gillian came to rescue him. (COMIC: Flower Power)

The Eleventh Doctor saw a spider on a web upon arriving at the Silurian Ark. He noted that he didn't usually see "spiders in space". (TV: Dinosaurs on a Spaceship)

Bone spiders had the ability to materialise dreams and spin spiderweb rendering a person invisible and inaudible. (PROSE: The Stone House)

Metal spiders were robots used by Cassandra O'Brien.Δ17. (TV: The End of the World, New Earth)

In non-valid sources
A spider was once present in Festive Road, with a boy greeting it by saying "Morning!". (TV: Future Generations)