The Glarn were, according to Romana I, a race of "intelligent molluscs". They were skilled tacticians. It was their practice to provoke a planet's species to destroy themselves, after which the Glarn would take over the planet for themselves.
When the Glarn wanted to take over Earth, they tested their program on the employees of Morgan Heggarty by sending them offensive emails and telephone calls. This plan provoked the humans to aggression, but the Fourth Doctor and Romana helped the employees defeat the Glarn. (PROSE: The Glarn Strategy)