The Mantis were a humanoid-sized insectoid species similar in appearance to Earth mantises, but with bill-like mouths and fangs instead of mandibles. They were skilled at inventing, building and calculating, but lacked the faculty of purpose. They were the servants of the Tyrryxians until one mutation was born among them with the power of free will. The Mantis leader took control of the others and had them build a spacecraft for the Tyrryxians, then stole it.
The Tyrryxians sent their battle fleet in pursuit of the Mantis, but one of the ship's devices could shrink or enlarge the ship or any object aboard it. They used it to shrink themselves to particulate size to avoid detection, but while they were still in giant mode, the Doctor's TARDIS materialised on board. The Mantis leader discovered the TARDIS and experimented on it to find out what it was, subjecting it to acid and x-rays. Learning that it was a dimensionally transcendental craft, he forced the Third Doctor out and demanded that he reveal his scientific secrets. The Doctor escaped, made contact with miniaturised Tyrryxians, and they worked together to get the leader on board the TARDIS and then re-enlarge it, destroying the stolen spacecraft. The leader was miniaturised and imprisoned. (COMIC: Size Control)