Know Your Cybermen - Cybermats

Before we move on to the 21st Century Cybermen, let's pause to take a look at the evolution of the Cybermats.
The origins of these creatures is unclear. In their first story - Tomb of the Cybermen - their true nature is never discussed. Are they purely machine, or are they cybernetically enhanced animals? The ones which attack the Doctor's party are all of the same size, but earlier Victoria finds a much smaller one - which attacks Kaftan. Is it based on a smaller animal, or do Cybermats grow?

We are not even sure what specific threat the Cybermats pose to the humans. They home in on brain waves, but we don't see them hurt or kill anyone.
These Cybermats are destroyed when subjected to a powerful electric field. Once the Cybermen have been returned to hibernation, there is at least one active Cybermat still on the surface of Telos.

In their second outing - in The Wheel In Space - they have a specific purpose. They are sent ahead to break into the space station to sabotage the Bernalium supplies. They kill a crewman who stumbles upon them - appearing to paralyse his limbs first.
There is a slight design change - they no longer have antennae, and the eyes are not patterned. The Telos Cybermats' eyes could flex. The ones on the Wheel have eyes that light up. The Wheel ones also have sharp spine ridges.

When next seen - in Revenge of the Cybermen - the Cybermats have a specific purpose once more. This time it is to spread a deadly virus among the crew of Nerva Beacon. They are still metal with segmented bodies, but these ones are longer and more serpentine. They can leap up off the ground and physically bite their victims. They are being operated by remote control by the Cybermen's human agent. The Doctor destroys one with gold dust. Rather strangely, he later fills it with gold dust and uses it to attack a Cyberman. How could it have survived this, if gold is inimical to it?

If the Cybermats seen in Revenge appeared to be purely mechanical, the ones introduced in Closing Time are the most clearly part-organic. They have mouths filled with sharp fangs, and can also generate an electrical charge. Smaller than the standard ones seen before, they have eyes based on the design of their Cyberman masters - with the teardrop oil ducts - but retain the segmented bodies.

The Doctor is able to disable a Cybermat using his sonic screwdriver. He reprogrammes it to drain the Cybership's power supply, but it is destroyed by a Cyberman before it can achieve this.

Not Cybermats as such - the Doctor states so - but the Cybermites seen in Nightmare In Silver do appear to be a further evolution of the Cybermats. They have similar segmented bodies. However, these appear to be designed for conversion - of people and technology - rather than for use as weapons.
The Master seems to have developed the Cybermites further, reducing them to microscopic size. Presumably it is nano-mites that inhabit the water which turns the Earth's dead into Cybermen in Death In Heaven.