The Vrill were an intelligent winged six-legged insect species. The Seventh Doctor and Elizabeth Klein encountered them on one of their colonies, just outside the Milky Way, after the death of their queen, or "Authority". The scent which the Vrill used to represent their human attackers was translated by the TARDIS' translation circuits as "the Winterlack". They killed the vast majority of the Vrill, which they refer to as xenomorphs, using weapons grade nerve gases. (AUDIO: Survival of the Fittest)
They were communal, with various members organised into different functions: female workers, male drones, neuter warriors, watchers, and healers. Just as with bees, over all of them was a fertile female queen, or "Vrill Authority". The Vrill did not have individual names, but the Doctor gave the two worker Vrill the names "Rose" and "Lilly" due to their smells and the newly hatched warrior Vrill the name "Butterfly" due to its magnificent wings.
At the centre of the colony was a hatching yard where the Authority laid thousands of eggs in a geothermal pool of hormonal fluids which cured the eggs and the resulting pupae became neuter warriors, female workers or male drones. Without fertilisation, the eggs hatched in their default warrior state. It was possible to create a new Authority by releasing certain nutrients into an egg, but only an existing Authority possessed this ability. (AUDIO: Survival of the Fittest)