Bugs are robotic probes used by the Machines to track a bluepill's location in the Matrix. They are inserted onto human flesh, where they will burrow underneath and take shelter inside the human body.
While activated they appear to be a small dark gray steel robot with a single red eye. However, once deactivated the squirming probe will appear to be back in a motionless steel probe and the red eye turns off.
- In The Matrix, after refusing to cooperate during interrogation by Agents Smith, Brown and Jones, Neo has a bug placed on his navel, which burrows into his abdomen. It is later removed by Trinity and Switch.
- Ash in the Animatrix short film "A Detective Story" has a bug planted behind his eye during what he believed was an eye examination dream which was removed by Trinity when he found her.