The Wirrn were roughly Human-sized, though with an anatomy more like an Earth-based insect. Their head was made of two large golden eyes, long, floppy antennae and a three small mandibles. Underneath the head were larger mandibles which they use to talk. They communicate by squeaking, though Noah was able to speak Human languages. The head is attached to a long thorax which has six long arms attached to it. Though thin, these arms are as strong as a human's and can be used for similar tasks. Underneath the thorax was a long abdomen.
Wirrn lived in space for most of their life and thus they could reuse their waste products with special enzymes, allowing them to survive without fresh air for years. They would only occasionally land on planets to gather air or food or to breed. The Wirrn were a very tough species, being able to survive in a wide variety of environment, energy blasts and apparently diamond-tipped blades. The Wirrn's most important weakness was their vulnerability to electricity.
Wirrn had an unusual life-cycle which was shown on the Ark. The Wirrn start breeding with a queen, who searches for a suitable planet. The Wirrn used cattle-like creatures in the Andromeda Galaxy, but they can also substitute Humans. When the queen finds a suitable host, it lays an egg in the host. The larva gains all the racial memories of the Wirrn, as well as the host's knowledge. Eventually, the larva eats the host and escapes.
This larva is a large, green, slug-like creature with several small limbs. These larva, while not as tough as an adult, are still resistant to a large amount of damage. If damaged, they could simply shed their skin, revealing an undamaged skin underneath. The larva produce a green slime, which can infect other creatures, turning them into larvae as well. These infected individuals gain all the knowledge of the Wirrn and the previous host.
Eventually, the larva starts to metamorphasize, turning into large pupa. After a few hours, full grown Wirrn come out of the pupae and set out to do the bidding of the swarm. One of the these adult Wirrn, generally the first one infected, will become a Swarm Leader and will control the actions of the other larvae which were infested at the same time. Some will also be turned into Secondary Queens, allowing them to lay eggs in more hosts.
The Wirrn have had a presence in the universe for a long time. A dead Wirrn was found in a prison ship that launched in 2000 BCE.
The Wirrn left the Andromeda Galaxy at some point before the end of the 40th century, looking for new sources of hosts. They eventually made their way to the Mutter's Spiral, where they occasionally infested the odd planet. As they gained new hosts, they were able to learn about the political structure, allowing them to plan out an invasion. Eventually, they reached Earth and were able to infest Miles Mason. Using his connections and scientific skills, they were able to produce drugs to be given to the athletes of the Olympics which would infect them. Those who were not infected immediately would then return to their home planets and infect others.
The Swarm Queen then moved to Micawber's World to better organise the swarm. She converted the people who came to investigate the tunnels of Micawber's World, adding to the swarm. As well, Reverend Lukas of the Church of the Way Forward picked up her telepathic communications and believed that she was his Goddess.
Using the murder of the Duchess of Auckland as a distraction, the drug was the activated. Unfortunately for them, SSS was prepared and ready to drive back the larvae. The swarm was eventually stopped when the Doctor confronted the queen, exposing the underwiring of Micawber's World and electrocuting her. Despite this, some of the larvae escaped and stole a ship, instinctive escaping back to Andromeda Galaxy. As well, the body of the queen was taken by Reverend Lukas, who wanted to spread his Goddess' message throughout the galaxy.
Thousands of years later, the Humans first reached Andromeda Galaxy and started to expand there, destroying the Wirrn Breeding Colonies along the way. After thousand years of fighting between the two species, the Wirrn were driven out of their own galaxy and forced to drift into the universe.
One queen ended up in Mutter's Spiral and discovered the frozen survivors of an irradiated Earth in the Ark. Though the queen was killed by a security device, it was able to implant an egg in one of the Humans (Dune).
Eventually, the larva escaped and was able to infect Noah. The Wirrn intended to use the sleeping humans as a food source and to gain their advanced knowledge to help take over the "empty" Earth. However, Noah's human side reasserted itself and led the Wirrn into the Ark's transport ship, even though he knew it was rigged to explode.
Human colonists in Wirrn space began sacrificing one of their number to have Wirrn queens gestated inside them; due to the influence of human minds on the queens, peace was achieved between the two species. Following the expansion of GalSec and their takeover of these existing colonies, however, the sacrifices were ended. As a result, a huge and vicious war erupted between human and the Wirrn, lasting generations.