From : Andromeda Galaxy
"We shall absorb the human's..........the Earth is OURS!" Swarm Leader - Ark In Space
The Wirrn Swarm were an intelligent insect race who came from the Andromeda galaxy and used other organisms as as food and host hatcheries for their babies. They could recycle their bodily wastes and thus could sustain themselves for years living in the vacuum of space. Occaisonally, they would land on worlds to scavenge for food or breed.
They begin life as a huge green maggot, having been implanted into a hosts body which upon hatching becomes a food source. The maggot would then pupate before the final tall insect. The maggot could also infect a live organism by touch causing the unfortunate victim to metamorphise themselves into a maggot.
The adult Wirrn were about 7 foot tall with a Wasp-ish face and long attenae's their body was wormlike with a claw on the end of their tail and they had long spider like legs. They would automatically inherit the memories and conciousness of their race and their former host, which meant they learned quickly and were dangerous opponents.
Their maggot stage was susceptible to laser fire but their adult stage had no known weakness, except a strong dislike of concentrated electricity.
The Fourth Doctor happened across them in the Ark In Space - 1975. The Wirrn had infested humanities last refuge of humanity; the Ark - a huge floating space station where the humans were in hypersleep. Earth had been bombarded by solar radiation and was uninhabitable so the human race went into hibernation, till such time as the radiation subsided.
A Wirrn queen managed to infiltrate the Ark but was mortally wounded by the automatic sentry. With its last vestige of strength it impregnated a comatosed human in hypersleep before dying in a broom cupboard. This unfortunate individual was an engineer aboard the Ark and this gave the Wirrn detailed knowledge of the spacestation and its workings.
A recently hatched maggot also touched the leader of the Ark - Noah, (who was woken from hypersleep by the Doctor) leaving a green slimy redisue on his hand. Over time, the residue spread up his arm until his body morphed into the maggot stage of the Wirrn.
The Doctor used electricity to electrify the spacestation and drove the rest of the swarm into space when Noah, in his new Wirrn adult phase was pesuaded by the doctor to board the Ark's lifeboat shuttle.
Noah/Wirrn's last act was to blow the shuttle up, his human side finally overcoming his Wirrn survival instinct long enough to save the remaining sleeping humans.
The Wirrn Swarm's still scavenge the far reachs of space but have not made a followup appearance in Doctor Who, to date.