The Zarbi are a species of human-sized insectoids resembling bipedal black ants native to the planet Vortis in the Isop Galaxy. Despite their hive society, they are acknowledged (at least by the Menoptera) as non-sapient creatures devoid of intelligence and individuality, and communicate by rubbing their legs producing loud sounds like a grasshopper. They also share their communities with creatures known as Larvae Guns, which might represent their larval stages or a different morphological caste, or even an unrelated but symbiotic species which simply shares homes with them. After the fungoid Animus came to Vortis she took control of the minds of the Zarbi and the Larvae Guns and used them to drive the intelligent Menoptera off the planet. When the Animus was destroyed with the help of the Doctor, the Zarbi slaves were freed and lived alongside the Menoptera in peace again.
- It is curious that the Doctor seems to connect the rarefied atmosphere of planet Vortis with the evolution of gigantic insectoids. In the real world, insects are known to have a lungless respiratory system which is only effective when oxygen is highly abundant compared to the animal's body size, hence the existence of giant insects in the Carboniferous period of Earth's past, when the concentration of atmospheric oxygen is known to have been much higher. Today, the very reason insects are unable to grow to larger sizes on land is that the Earth atmosphere is too rarefied for their metabolism.