The Queen Bat is a chiropteranoid creature native to the cave systems of Androzani Minor. Their life cycle spans three years and includes a stage spent concealed within a chrysalis. This chrysalis contains large quantities of brute spectrox; once described as the most valuable substance in the galaxy, which in its refined state has the properties to slow the ageing process to the point of doubling the length of one's life.
Unfortunately, exposure to its unrefined state can lead to the development of spectrox toxaemia; a lethal ailment to which humans and Time Lords are equally susceptible. The milk of the Queen Bat provides the only known cure for the disease, explaining how the bats themselves are unaffected by it.