Cybermat

Cybermats were small cyborgs used as advance guards and plague carriers by the Cybermen.

Variants
From early in the Cyberman's history before they froze themselves on Telos, some variants had crystalline eyes and antennae through which they receive commands. These Cybermats tended to resemble oversized metallic silverfish and have segmented bodies with cilia-like tactile sensor probes along the base of their heads. Some, though not all, small Cybermats of this kind could easily fit in the palm of a humanoid hand. (DW: The Tomb of the Cybermen)

In the 21st century when the Cybermen attempted to infiltrate Space Station W3 they had photoreceptors for eyes rather than crystalline based eyes. Otherwise they looked similar. (DW: The Wheel in Space)

When the Cybermen infiltrated Nerva Beacon in a plot to destroy Voga the Cybermats had segmented, eyeless, flat worm-like bodies without cilia. (DW: Revenge of the Cybermen)

Cybermen from the 30th century who traveled back to 1940 also employed Cybermats and harvested Humans to make more of them. (PDA: Illegal Alien)

The Cybermat's also appeared in Blood of the Cybermen. Heaps of them were in the game. In Blood of the Cybermen, we find out the Cybermat's weakness.

Weaknesses
Cybermats shared the Cyberman's weakness for gold. (DW: Revenge of the Cybermen) They could also be disabled with electric charges. (DW: The Tomb of the Cybermen)

History
The Cybermats originated on Mondas during the earliest phases of Cyberman history, when most of the planet had not yet converted. (BFA: Spare Parts)

The Cyber-tombs on Telos had tunnels for the Cybermats to move from the tombs into the main control rooms. (DW: The Tomb of the Cybermen)

Some accounts show that the Cybermats were created by Cybermen using bodies (or parts of bodies) of humanoids not suitable for Cyber-conversion: pets, very young children or babies. (BFA: Small Parts, PDA: Illegal Alien, BFBS: The Crystal of Cantus)

Behind the scenes

 * Wood lice inspired the creation and look of the Cybermats. In their first appearances in The Tomb of the Cybermen and The Wheel in Space, they were realized in the studio by a number of techniques, including clockwork and radio-controlled models. In some shots they were pulled by wires or simply pushed into shot. Shots of the Cybermats leaping onto their victims were achieved by pulling the Cybermat away via wires and then playing the film backwards.
 * Cybermat effects in Revenge of the Cybermen used a combination of strings, puppetry and CSO, which did not look very convincing. The Cybermats (or rather the single Cybermat which appeared) went through a radical re-design and resembled a cybernetic silverfish. To date, the Cybermats have not appeared in any other television stories.