A time fly was a large fly, covered in hair with wings the size of tennis rackets, (PROSE: Slow Decay) while some were completely hairless, had green eyes and wings as well as pincers. (COMIC: A Stitch in Time)
When the Twelfth Doctor accidentally crashed on top of Ethel's sister, Ethel began summoning time flies to attack both the Doctor and Clara Oswald. They would open up portals to all the Doctor's previous incarnations in an attempt to destroy him. However Gretel, Ethel's other sister, helped the Doctor by getting rid of these flies.
Behind the scenes
Their name is pun on the common phrase ‘time flies’.