Isopterus was a planet inhabited by a humanoid species. It was named after the Latin term for termite. At some point in its history, a "genetic explosion" caused the termites to grow to giant size and they began overrunning the planet, first consuming all plant life and then starting on the humanoid population.
The Fifth Doctor, Adric, Nyssa and Tegan visited Isopterus, where they were captured by termites and taken to their nest. After learning their history from one of the natives, the Doctor used his dream machine to send the termites on a search for food on the other side of the planet, allowing him and all the remaining survivors to escape. He led them to the TARDIS, where he promised to take them to a safe planet, with the hope that they would someday return to Isopterus once the termites died out from lack of food. (COMIC: On the Planet Isopterus)