About the size of a pinhead, a bug has a microphone and a radio transmitter.  Wrist units can be set to detect bugs’ signals.  A bug’s range is short (about one kilometre) and the tiny battery doesn’t last long (less than an Earth day).

On Agla 5, Tony planted a bug on Bo-Gwarric without her knowing.  He hid nearby, and used his wrist unit to listen to what she said, and what people said to her.  See Report 11.