Fireworm

Fireworms could be inserted into one's head to enable them to eavesdrop. They could literally cause one's ears to burn when their name was said near them, something that could be exploited to knock one unconscious.

Yvonne Hartman used a fireworm to help in taking over Torchwood Three. She inserted another into Xander Vaughn's head which Orr found, allowing them to play back Xander's conversation with Jack Harkness. (AUDIO: Poker Face)