You wake in your stasis pod aboard the USCSS Nostromo following the accident on your away mission. A little foggy after cryo-sleep, you follow the rest of the crew to the mess hall for a meal and a cup of coffee before getting ready to continue the ship’s mission. Mid-way through breakfast you feel an aching in your chest. Is this a heart attack? Some cryo-sleep side effect? Suddenly the pain becomes too intense. You fall back on the table and in one searing burst of pain before everything goes black, a creature tears violently from your chest, looks at the people gathered around you, and rushes out of the room...
The scene described above is adapted from Ridley Scott’s 1979 science fiction-horror film Alien, which chronicles the fate of the crew aboard the USCSS Nostromo after they inadvertently expose themselves to a deadly alien parasite. The movie earned critical acclaim and box office success at its release and has subsequently remained an important part of science fiction and popular culture . Alien has spawned several successful sequels, novels, video games, and an ongoing prequel series, and the film was even selected for preservation in the United States National Film registry by the Library of Congress in 2002 .
Arguably what makes the franchise so impactful is how utterly terrifying the titular Aliens are. These creatures are so grotesque and foreign to audiences, particularly in their reproductive strategy, that fans are simultaneously horrified and fascinated by what is on screen. Perhaps unknown to many viewers, the lifecycle of Xenomorph XX121 from the films is not so unlike that of many common parasites found on our planet. The rest of this essay will compare the biology of the Xenomorphs to parasitoid wasps and nematomorph worms from Earth to illustrate how close to reality the biology of these aliens is and to discuss this exceptional instance of science inspiring artists.
The lifecycle of Xenomorph XX121 shares a striking resemblance to the reproductive strategy of some wasp species from the insect family Apocrita. Females of these species parasitize other small arthropods or arthropod eggs by ovipositing their own eggs into live hosts (Image 2) . The wasp eggs develop and hatch into larvae within the arthropod, and then the larvae consume the still-living host from the inside before emerging from its body to continue development (Video 1). These wasps are known as parasitoids because infection is immediately lethal for the host, in contrast with conventional parasites that chronically feed on their hosts without causing sudden mortality.
Compare this to the Xenomorph lifecycle in which the animal begins inside an egg which hatches into its human-infectious stage. This form uses its finger-like appendages to tightly grasp the face of a human victim while positing an embryo down the throat of the host and into his or her chest, similarly to how the female parasitoid wasp oviposits her clutch into her victim. The newly implanted embryo gestates within the host for several hours before violently bursting through the host’s chest to continue development, just as in the case of the young parasitoid wasps (Video 2). When the Xenomorph larva implants an embryo in a human, the host is fated to die within a short period of time as a result of the parasite growing inside of him or her. Because the host dies suddenly from the parasitic relationship, Xenomorph XX121 would be also categorized as a parasitoid.
Another earthly parasitoid with similarity to Xenomorph XX121 is the species Paragordius tricuspidatus from the phylum of parasitoid worms, Nematomorpha. The P. tricuspidatus larvae infect crickets living near streams by crawling into their bodies and then developing inside their insect hosts. These worms can grow up to 6 inches in length and fill the entire body cavity of their hosts at maturity . At the end of its developmental process, P. tricuspidatus sends a neurochemical signal to its host that compels the cricket to jump directly into a body of water. The nematomorph is able to detect the host’s contact with water and subsequently bursts from its host, killing it in the process (Video 3). The now free-living, non-parasitic adult nematomorph does not feed (they don’t even have mouths!) but instead lives off nutrient reserves stockpiled from when it was within its host and remains by the stream to reproduce (Image 3). While the Xenomorphs from the original Alien movies lack mind-controlling behavior, they are very similar to the nematomorphs in that they hatch from eggs, seek suitable hosts for development, and subsequently kill their hosts as they emerge as a free-living lifeforms.
The science fiction of the Alien franchise is a famous example of art imitating life. These films present audiences with a terrifying monster with an unspeakably awful method of reproduction, which happens to closely resemble the lifecycles of scores of invertebrate parasitoid species on earth. The Alien movies are uniquely powerful in their ability to take real world biology and present it in the unfamiliar context of space to entertain and terrify audiences.
Interestingly, the film is frightening both before and after studying parasitoids from Earth. The Xenomorphs may seem uniquely cruel and terrifying before learning that many real species also reproduce by violently killing their hosts. However, after knowing how common parasitoid species are in nature and how closely the Xenomorph lifecycle resembles them, maybe it’s even more frightening to consider how lucky we are as a species to not be parasitized by such creatures.
Alex Sercel (@asercel)
Contributing Author, Signal to Noise Magazine
PhD Student, UCLA Molecular Biology Institute
 Alien (1979) Box Office Summary. http://www.the-numbers.com/movie/Alien#tab=summary
 Library of Congress. Complete National Film Registry Listing. https://www.loc.gov/programs/national-film-preservation-board/film-registry/complete-national-film-registry-listing/
 Pennacchio, F. & Strand, M. R. Evolution of Developmental Strategies in Parasitic Hymenoptera. Annu. Rev. Entomol. 51, 233-58 (2006).
 Thomas, F. et al. Biochemical and histological changes in the brain of the cricket Nemobius sylvestris infected by the manipulative parasite Paragordius tricuspidatus (Nematomorpha). Int. Journal for Parasitology. 33, 435-443 (2003).