Xenomorphs from the Alien franchise have staked their claim as some of the most terrifying extraterrestrials of all time. We fear them for their merciless hunting of people, their super acidic blood, and their (probably) awful tiny-second-mouth breath. But it's the xenomorph's reproductive cycle, which includes elements analogous to real-life parasitoidism, cancer, and good ol' fashion spermatophores, that really make them the last kind of creature you'd want to be stuck with aboard an M-Class star freighter.
The cycle endures as one of the most fascinating fictional methods of producing offspring ever, so here's a brief breakdown of the steps 'morphs go through to transform from egg into fully formed nightmare.
Ovomorph (the Egg)
Just like the original 1979 Alien film, the horror of the xenomorph's reproductive cycle all starts with eggs, a.k.a. ovomorphs. Ovomorphs are laid by a queen, and they are far more than meets the eye. In fact, they have a symbiotic relationship with their in vivo facehuggers, and are able to "'sense' or otherwise detect when a potential host creature approaches." This way they can open their tops, flower-like...
And eject a Facehugger onto an unsuspecting host's face.
The facehugger is the second stage of the xenomorph's life cycle, and is, at its core, a delivery system for inserting the xenomorph's DNA into a host. The facehugger isn't so much about delivering sperm, like real spermatophores are, but is instead used to deliver a knot of cells (basically an embryo) through the mouth of the host. It executes this goal by clinging to the host's face and tightening its tail around the throat to minimize oxygen moving to the brain, keeping the host alive but unconscious. (It also apparently delivers "a cyanose-based paralytic chemical" to help sedate the host.) Once the 'Morph DNA has been delivered via a proboscis down the throat, the facehugger detaches and dies.
Adults (Warriors, Drones, Runners, and Praetorians)
They also apparently have superhuman strength, quickness, stealth, and general combat abilities. Plus they have those inner mouths, which are fictional correlates of the pharyngeal jaws of a moray eel, but about one billion times as freaky.
Xenomorphs that eventually become queens are destined to be the most massive and powerful adults. Queens are almost three times the height of the other strange forms, standing roughly 20 feet tall, and spanning over 50 feet in length. The queens are also faster and stronger than any other xenomorph class, as well as significantly more intelligent.
Xenomorph queens also lay new ovomorphs, which thusly continues the circle of life... as well as the most horrifying method of procreating ever thought up by humanity or mother nature.
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Images: 20th Century Fox
Just how acidic is the Xenomorph's blood?