Wirrn

Wirrns are a sapient insectoid species native to the Andromeda Galaxy.

Biology and life cycle
A remarkable species in many ways, Wirrns are able to survive and travel in the vacuum of space for several years in their adult stage. Their lungs possess a complex enzymatic apparatus which allows them to fully recycle wastes, much like a plant uses carbon dioxide to repose the oxygen it consumes and vice-versa. Thermodynamically, such a system cannot go on forever, and accordingly, every few years or decades the Wirrn swarms must land on a planet to renew their food and oxygen. They also need to be on a planet for the early stages of their life cycle.

They are a hive species, the Wirrn queen having to infect a host with an endo-parasitoidic larva, much like some species of Earth wasps. Normally, the chosen host will be a non-sapient herbivore from their homeworld. The Wirrn larva consume the host from inside out, absorbing the host's tissues until transforming it into a large green maggot-like lifeform; which them undergoes a brief pupal stage to emerge as an adult Wirrn. The maggot-like larvae also appear to be able to infect new hosts directly.

Another remarkable feature is their ability to inherit racial memories both from their Wirrn ancestors and from their larval hosts. Thus, if the chosen host is of a sapient species, the Wirrns formed will inherit all of its knowledge; although apparently only a few of its emotional memories.

Adult Wirrns are highly resistant to most kinds of weapons, but can be easily killed by a strong electric discharge.

Like the Alpha Centaurans, the Wirrns have six limbs protruding from the thorax but usually do not use any of them for locomotion; instead rising on their strong abdomen. This is not the case in space, however, showing that their spidery limbs would probably be useless for supporting their weight under Earth-like gravity.

History
At some point in their history, Human settlers arrived on the Andromeda Galaxy and for some reason became involved in a thousand year-long war against the Wirrns, resulting in the destruction of the Wirrn breeding colonies and ultimately their withdrawal from their home galaxy. After that, they traveled to the Milky Way / Mutter's Spiral to get revenge on the Humans.

At that time, the entire population of Earth was being kept dormant in an artificial satellite construction known as the Ark in order to survive a 5000 years period of intense solar flares which would leave the Earth temporarily uninhabitable. The vengeful Wirrns invaded the Ark and infected some of its inhabitants with their larvae, thus hoping to destroy the Human race and at the same time inherit their more advanced scientific knowledge.

As is so often the case, their plan was foiled with the arrival of the Doctor and his companions, although also with a little help from the newborn Wirrn commander, who remembered enough from the life of his Human host to be convinced to sabotage the Wirrns' plan and save humanity.

Appearances

 * Doctor Who season 12 (fourth Doctor), "The Ark in Space"