The Zarbi appeared in the 1965 First Doctor story The Web Planet written by Bill Strutton, and are an (ant-like) insectoid species, with some characteristics associated with beetles, from the planet Vortis, which were controlled by the power of the Animus. They are roughly eight feet long, and the Menoptra claim, perhaps a little callously, that they are "little more than cattle".
They possess little intelligence but were not at all aggressive until the Animus arrived. They were enslaved to the alien consciousness and considered the butterfly-like Menoptra (with which they once lived peacefully) their mortal enemies. Only they could control the woodlouse-like venom grubs (also known as larvae guns).
They returned to their normal ways after the Animus was defeated by the First Doctor, Ian Chesterton, Barbara Wright and Vicki. It is presumed that the various species on Vortis are now living peacefully together.