Fly

Flies were non-sentient insects native to Earth (TV: "Dangerous Journey") and Gallifrey. (TV: Heaven Sent) 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")

The Twelfth Doctor recognised 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)