Insect

Insects were a class of animal. Insects or analogues to them, some sentient, appeared on Earth and many other planets. They were preyed upon by birds. (COMIC: Dr Who in "Lilliput")

On Earth
On Earth, the population of insects was many times greater than humanity. It was mentioned that had Miracle Day, which caused death in humans to cease to exist, affected insects, they would have overrun the Earth in 48 hours. (TV: The New World)

Insecticide was used to kill insects. (TV: Planet of Giants)

Elsewhere
The Wirrn were sapient insects native to the Andromeda Galaxy. (TV: The Ark in Space)

On one planet, a crashed spacecraft spilled fuel which caused nearby insects and other life forms to mutate to giant sizes. These mutants plagued the native Meerags until the Fourth Doctor arrived and helped destroy them with fire. (COMIC: The Mutants)

The First Doctor did not like insects, and was thus frightened of the Menoptera when he first encountered them. (PROSE: The Lost Ones)

Other references
The First Doctor, Ian Chesterton and Barbara Wright found themselves mistaken for insects as they were menaced by a bird after being shrunk. (COMIC: Dr Who in "Lilliput")