Roots: Starship Troopers (ignorant human galactic colonists launch retaliatory attack on an intelligent and vicious insect species), Aliens. Aladdin ("your wish is my command")
Fluffs: Admiral Farroll says "It's dissecting our orbit" (shouldn't that be bisecting?)
Goofs: Should Lucie be able to hear the Doctor at all in space? Not usually, no.
Double Entendres: "Hold on to me tightly, Miss Miller. Brace yourself!"
"Just get your big gun out and look macho"
Continuity: The Doctor describes this point in time as one of the bloodiest periods in Earth's history (although he admits that he hasn't exactly seen it first-hand before) GalSec colonists.
There are sixty-eight ships in the GalSec fleet, their most recent addition being the Repulse. Outsiders in the GalSec community are 'Indigs', a pejorative term for the so-called indigenous settlers and precursors of the GalSec colonists. The Indigs of Carista VII maintained peaceful relations with the local Wirrn through a sacrifice at the beginning of every harvest season. Initially cattle were sacrificed, until it was discovered that the race memory of a human from the community was more effective, promoting greater instinctive understanding between the two species and ensuring their mutual survival.
Wirrn are "all over" GalSec territory, leading to a war of attrition between them and the colonists. Wirrns inherit the race memories of their swarm leaders. Their queens are larger Wirrn individuals and (it is implied) are the only individuals capable of laying eggs. Queens gestate at a quick rate, absorbing the host consciousness as they do so. There doesn't appear to be an instinctive enmity or rivalry between queens of the Wirrn, and it may be inferred that either populations are so widespread that territorial battles do not occur as in natural social insects (ants, bees, wasps), or that old queens are not threatened by younger queens and therefore defer to them when their 'replacement' emerges. It is not explained whether the queen which attacked the Admiral is the same individual who later appears on the planet (it would appear not) - such understandings between established and potential queens may be indicative of the species' shared consciousness.
Lucie knows her way around a space suit [having worn arguably the equivalent of one in Phobos]
Links: Lucie says she used to be "great friends" with a centipede (Sisters of the Flame/The Vengeance of Morbius.) This is the second Doctor Who audio by Nicholas Briggs to mention ichneumen wasps (the first being The Mutant Phase), and the first in which the Doctor pronounces it with a German accent...
Location: Carista VII, in Galsec Territory "long after" the evacuation of Earth.
The Bottom Line: "Survival is a messy business"
Nick Briggs gives the Wirrn a Dalek War/Sword of Orion treatment with his usual space opera furniture. The resolution is well-telegraphed for the listener familiar with the Wirrn, and fittingly for a story about survival of the fittest supporting characters are dispatched once their narrative function come to an end. Very nice sound design, an epic scale, but apart from (we presume) some scene-setting conversation between Solway and Lucie, there's nothing essential here.