The Alien (sometimes referred to as a xenomorph) is a fictional endoparasitoid extraterrestrial species that is the primary antagonist of the Alien film series. The species made its debut in the 1979 film Alien, and reappeared in its sequels Aliens (1986), Alien 3 (1992), and Alien Resurrection (1997). It has also appeared in the two Crossovers Alien vs. Predator (2004) and Aliens vs. Predator: Requiem (2007), as well as the various literature and video game spin-offs from the series.
Unlike many other recurring enemy extraterrestrial races in science fiction, the Aliens are not an intelligent civilization, but predatory creatures with no higher goals than the propagation of their species and the destruction of life that could pose a threat. Like wasp or termite, Aliens are eusocial, with a single fertile breeding a caste of warriors. The Aliens' biological life cycle, in which their offspring are violently implanted inside living hosts before erupting from their chests, is in many ways their signature aspect. Their design deliberately evokes many sexual images, both male and female, to illustrate its blurring of human sexual dichotomy.
The Alien design is credited to Swiss surrealist and artist H. R. Giger, originating in a lithograph called Necronom IV and refined for the series' first film, Alien. The species' design and life cycle have been extensively added to throughout each film.
The script for the 1979 film Alien was initially drafted by Dan O'Bannon and Ronald Shusett. Dan O'Bannon drafted an opening in which the crew of a mining ship are sent to investigate a mysterious message on an alien planet. He eventually settled on the threat being an alien creature; however, he could not conceive of an interesting way for it to get onto the ship. Inspired after waking from a dream, Shusett said, "I have an idea: the monster screws one of them"; planting its seed in his body, and then bursting out of his chest. Both realized the idea had never been done before, and it subsequently became the core of the film. "This is a movie about alien interspecies rape," O'Bannon said on the documentary Alien Evolution, "That's scary because it hits all of our buttons." O'Bannon felt that the symbolism of "homosexual oral rape" was an effective means of discomforting male viewers.
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Official Site: 20th Century Fox
Links: Wikipedia: Xenomorphs Essays on the Xenomorph's biology 20th Century Fox's Official 'Alien' Site