In a system of underground caverns in England, a species of insect mutated to gigantic size due to exposure to radioactive minerals. They were approximately ten feet long and resembled locusts with tentacles surrounding their mandibles, but they also seemed to possess some ant-like qualities. They may have possessed some form of intelligence.

When UNIT was called in to investigate some mysterious earth tremors in the area, the Third Doctor and Liz Shaw joined them in exploring the caves. A sentry and Liz separately were attacked by an insect, both being badly frightened by the ordeal. Knowing the threat the insects posed to the world's agriculture, the Doctor advised the Brigadier to bring in cyanide gas. While searching for the insects, the Doctor was captured and taken to their main warren, where he was almost made into a food source for their young. Overwhelmed by the horror of his situation, the Doctor used his laser pistol to shoot his way out and escaped, grateful that UNIT soldiers would soon destroy the creatures. (PROSE: Caverns of Horror)