You've lived in the 41st millennium long enough to fear the Tyranid hive fleets: that seemingly endless stream of bio-ships, soldier species and bioweapons, all psychically linked to a single Hive Mind and destined to consume all the biomass in our galaxy.
Long before a hive fleet arrives at your planet, however, it sends forth its genestealers to infiltrate the population. I've already discussed gene theft in the natural world before, so I'd like to focus here on the genestealer's bizarre life cycle and breeding strategy.
Life Cycle of the Brood
In order to weaken a world for proper invasion and assimilation, the Tyranid hive mind first deploys its genestealer species to a chosen planet. While these cunning six-limbed organisms are incredibly deadly in a stand-up fight, their ultimate aim is to infect members of a planet's intelligent population with their own genetic material.
They use their ovipositor -like tongue to inject a tiny embryonic mass into the host organism. This "seed" is largely inert, but carries out three primary functions:
1) It psychically enslaves the host's mind to a localized version of the Tyranid hive mind.
2) It alters the host's DNA, causing it to pass on hybrid genestealer genetics to its offspring, while also infecting its mate.
3) It alters the host's behavior, forcing them to care for their monstrous hybrid offspring and carry out the wishes of the Hive Mind
And so the hyrbids grow to infect new hosts. Each generation expresses less-monstrous features, till a fourth-generation hybrid is largely indistinguishable from the rest of the population. Finally, fifth-generation hybrids revert to the pure genestealer form and the cycle begins again.
As the Genestealer population grows, purestrains and hybrids alike work in secretive cults to undermine society and planetary defenses. When the hive fleet finally arrives, it deploys its forces on a largely destabilized world.
While there's nothing exactly like this in the natural world of terrestrial biology, we do see shadows of the genestealer. Nature evolves often-devious survival tactics -- tactics the Great Devourer has likely encountered and assimilated on previous worlds.
At heart, the genestealer employs an extraterrestrial (and, indeed, extragalactic) variant of kleptoparasitism and brood parasitism.
In an example of brood parasitism, the common cuckoo lays its eggs in the nests of other bird species, retaliating violently if the host birds reject its young. For an example of kleptoparasitism, consider cuckoo wasps (family Chrysididae) that lay eggs in the nests of other wasp species. The cuckoo wasp egg hatches first and the ravenous larva consumes its surrounding host eggs and larvae. Simply put, their reproduction strategy is to steal the reproductive resources of another entity.
Will of the Hive Mind
But of course genestealers depend on far more than cuckoo bird threats to insure infected humanoids raise their hybrid young. They reprogram their hosts via the influence of the local hive mind. While such psychic powers are beyond the anatomy of the natural world, we do see plenty of parasitic mind control here on Earth.
For instance, the wasp Dinocampus coccinellae not only lays its egg inside the belly of a ladybug, but also reprograms the eviscerated host to stand guard as it transitions from larva to full-grown adult. In a small way, like the genestealer, it hijacks both body and culture.
Meanwhile, the hive fleets leave only dead worlds in their wake, the sweeping tentacles of an extragalactic super-organism intent on total cosmic consumption of life. It plays the long game of conquest, and we see this reflected in the genestealer life cycle.
Monster of the Week is a - you guessed it - regular look at the denizens is of our monster-haunted world. Sometimes we'll focus on the cultural aspects, but mostly we'll look at the possible science behind a creature of myth, movie or legend. Be sure to explore the Monster Gallery as well as the Monster Science video series.