The Zarbi resembled giant ants, though with four small arms and two powerful legs. They communicated by screeching and rubbing their legs together. They were naturally simple-minded creatures, no smarter than cattle.
Socially, they were even more similar to the ants of Earth. They lived in massive nests, with millions of members. There was also a similiar caste system, with powerful warrior Zarbi, common workers and giant, bloated queens. They were born as larva, which were little more than grey blobs.
The Zarbi were used by the Menoptera as cattle until the Animus arrived and took control of them, forcing them to fight their former masters. They reverted to their original nature after the Doctor, Ian Chesterton, Barbara Wright and Vicki defeated the Animus.
Many years later, the relationship between the Menoptera and the Zarbi had patched up, with both forces helping the other. Unfortunately, a strange mutation led to the creation of the Zarbi Supremo, a twenty-foot tall Zarbi who took control of the other Zarbi. It was able to devise means to move Vortis, turning it into a rogue planet.
Over several generations, Vortis left the Isop Galaxy and found itself near Jupiter. The Zarbi Supremo planned to take over Earth for its resources. The Menoptera had tried to deal with this, building up weaponry and infiltrating the hives, but the agents were captured or killed. At the same time, the Earth ship Solar Queen came to investigate the rogue planet, but when the scientists went to investigate they were also captured. They were able to send a message out, which attracted the Doctor. With the help of Gordon Hamilton, he was able to free the scientists, who killed the Zarbi Supremo and released the Zarbi from their subjugation. With that, the scientists were returned to their ship and the Menoptera left the solar system for their own galaxy.
The Menoptera and the Zarbi worked peacefully together for many years, until the Zarbi apparently developed flight and deadly stingers, which they used to attack Menoptera. The Doctor, John and Gillian arrived and, investigating, discovered that the flying Zarbi were actually Zarbi-shaped vehicles piloted by Skirkons, who had come to Vortis for its supplies of Galvinium X, and had enslaved the real Zarbi to oversee their Menopteran slaves. The Doctor and his grandchildren freed the Menoptera and defeated the Skirkons, who died when their base was destroyed.