Flies were known to consume waste matter. As such, the Tenth Doctor deemed it "perfect natural" that the Tritovores, a race of fly-like humanoids, fed on what others "[left] behind from their behind. (TV: Planet of the Dead)

In an alternate 1804, the true Marquis de Sade once witnessed a bluebottle fly being eaten by a spider. (PROSE: The Man in the Velvet Mask)

Barbara Wright and Ian Chesterton encountered a fly when they were miniaturised in 1960s England. Although flies were ordinarily of insignificant mass to humans, this particular fly was a genuine threat to Barbara and Ian. Fortunately for them, it lit upon something coated with the insecticide DN6 and died. (TV: "Dangerous Journey")

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)

The Twelfth Doctor recognised the the Veil, who was swarmed by flies, as having been stolen from his first incarnation's boyhood nightmares on Gallifrey. The Veil was based on an old lady who died and was covered in veils, but because it was a hot, sunny day, the flies came. This gave the young Doctor nightmares for years. (TV: Heaven Sent) Following the Last Great Time War, the flies were all that was confirmed to remain of Gallifrey's fauna. (PROSE: A Brief History of Time Lords)

In the 82nd century, Frobisher took on the form of a fly to search bars for the Sixth Doctor. He was spotted by a waiter, who swatted him with a rolled-up newspaper. (COMIC: The Shape Shifter)