The Caretakers, as they were named, were an insectoid species native to a sandy world with two suns in another reality. They were roughly humanoid, with two arms and two legs, and young Caretakers could almost seem human except for having exaggerated ears.
They bled through a portal from their world into the council estate where Roger lived, infesting the hallway and creating nests. The Second Doctor and Zoe Heriot encountered them, using Roger's help to recreate Zoe using memory meat, and defeated them. (AUDIO: A Stain of Red in the Sand)