Xenomorph

The Alien (also known as a Xenomorph XX121 or Internecivus raptus) is a fictional endoparasitoid extraterrestrial species that is the antagonist of the Alien film series. The species made its debut in the film Alien (1979) and reappeared in the sequels Aliens (1986), Alien 3 (1992), and Alien Resurrection (1997). It also featured in the crossover films Alien vs. Predator (2004) and Aliens vs. Predator: Requiem (2007), with the skull of one of the creatures appearing briefly in Predator 2 (1990). Similar creatures of slightly different designs such as the "Deacon" make a brief appearance in the Ridley Scott film Prometheus (2012) along with the "Neomorph" and a variation of the Xenomorph known as a "Praetomorph" which appears in the sequel Alien: Covenant (2017). In addition, the Alien appears in various literature and video game spin-offs from the franchises.

The Alien design is credited to Swiss surrealist and artist H. R. Giger, originating in a lithograph titled Necronom IV and refined for the series's first film, Alien. The practical effects for the Alien's head were designed and constructed by Italian special effects designer Carlo Rambaldi. The species's design and life cycle have been extensively augmented, sometimes inconsistently, throughout each film.

Unlike many other extraterrestrial races in science fiction (such as the Daleks and Cybermen in Doctor Who, or the Klingons and Borg in Star Trek), the Aliens are not sapient tool-makers: They lack a technological civilization of any kind, and are instead primal, predatory creatures with no higher goals than the propagation and self-preservation of their own species, including the elimination of other lifeforms that may pose a threat to their existence. Like wasps or termites, Aliens are eusocial, with a single fertile queen breeding a caste of warriors, workers, or other specialist strains. The Aliens' biological life cycle involves traumatic implantation of endoparasitoid larvae inside living hosts; these "chestburster" larvae erupt from the host's body after a short incubation period, rapidly mature from juvenile into adulthood within hours, and seek out more hosts for implantation.

Name
This creature has no specific name; it was called an alien and an organism in the first film. It has also been referred to as a creature, a serpent, a beast, a dragon, a monster, a nasty, or simply, a thing. The term xenomorph (lit. "alien form"—from Greek xeno-, which translates as either "other" or "strange", and -morph, which denotes shape) was first used by the character Lieutenant Gorman in Aliens with reference to generic extraterrestrial life. The term was erroneously assumed by some fans to refer specifically to this creature, and the word was used by the producers of some merchandise.

The species' binomial names are given in Latin as either Internecivus raptus (meant as "murderous thief") in the Alien Quadrilogy DVD or Lingua foeda acheronsis (meant as "foul tongue from Acheron") in some comic books. The main Alien from Alien vs. Predator is listed in the credits as "Grid", after a grid-like wound received during the film from a Predator's razor net. Alien: Covenant actually credits the Alien as Xenomorph, while also listing a different variety of the creature as the Neomorph. In The Weyland-Yutani Report, the Alien encountered by the Nostromo was specifically referred to as "Xenomorph XX121".

Characteristics
As the film series has progressed, the creature's design has been modified in many ways, including differing numbers of fingers and limb joints and variations in the design of the Alien's head.

Appearance
When standing upright, the Aliens are bipedal in form, though depending on their host species, they will either adopt a more hunched stance, or remain quadrupedal when walking or sprinting. Their overall stance and general behavior is a result of the mixture of the DNA of the embryo and the host. They have a skeletal, biomechanical appearance and are usually colored in muted shades of black, gray, blue or bronze. Their body heat matches the ambient temperature of the environment in which they are found, so they do not radiate heat, making them indistinguishable from their surroundings through thermal imaging. In most of the films, adult Aliens are capable of running and crawling along ceilings and walls. They have great physical strength, having been shown to be capable of breaking through welded steel doors in numbers.

Aliens have segmented, blade-tipped tails. The sharp tip was initially a small, scorpion-like barb, but from Aliens onwards the blade design increased in size and changed in appearance to more closely resemble a slashing weapon. From Alien: Resurrection onwards, the tails have a flat ridge of spines at the base of the blade. This was introduced to help them swim convincingly, and was left intact in the subsequent crossovers. The original shooting script for Aliens and the novelization both featured a scene in which Lieutenant Gorman is "stung" by the barb tail and rendered unconscious; in the final cut of the movie, Gorman is knocked out by falling crates. As a weapon, the strength of the tail is very effective, having been shown to be strong enough to impale and lift a Predator with seemingly little effort.

They have elongated, cylindrical skulls, with eyes underneath the "visor". In the novelization of Alien, the character Ash speculates that the Xenomorphs "see" by way of electrical impulse, similar to a shark's lateral line. This method is illustrated in the original Alien vs Predator PC game and reused for the "Predalien" 28 years later. The Alien's inner set of jaws are powerful enough to smash through bone and metal. How the creatures see is uncertain; in Alien 3, a spherical lens was used to illustrate the Alien's point of view, so, when the film was projected anamorphically, the image exhibited severe distortion. In the novelization of the movie Alien, the creature is held mesmerized by a spinning green light for several minutes.

In Aliens, the adult creatures have a more textured head rather than a smooth carapace. In the commentary for Aliens, it was speculated that this was part of the maturation of the creatures, as they had been alive far longer than the original Alien, although James Cameron stated that he simply left the carapace off because he liked them better that way. The smooth design of the carapace would be used again in Alien 3 and Alien: Resurrection, although made narrower with a longer muzzle and more prominent chin. This design would be kept in Alien versus Predator, and abandoned in Aliens vs. Predator: Requiem in favor of the ribbed design.

Throughout their appearances, human-spawned Aliens have been shown to have different numbers of fingers. In Alien, the creature has webbed, six-fingered hands. In Aliens, the number of fingers is reduced to three (two "paired" and a single, opposable thumb), and they are shown to be much longer and more skeletal. In Alien: Resurrection, the number of digits is increased to four, with two long middle fingers and a pair of thumbs. This design is kept in the Alien vs. Predator films, though the hands were made bulkier in order to make the Aliens seem more formidable against the Predators.

Aliens have been alternatively portrayed as both plantigrade and digitigrade organisms, usually relative to their host. Human-spawned Aliens were usually portrayed as having humanoid hind limbs, while in Alien 3, the featured Alien sported double-jointed legs due to its quadrupedal host. This characteristic would be continued in Alien: Resurrection for the human-spawned Aliens. Tom Woodruff, who had previously played the "dog-alien" in Alien 3, described the human-spawned Aliens in Resurrection as feeling more like a dog than the previous creature, despite having been born from human hosts. The human-spawned Alien warriors would revert to a plantigrade posture in Alien vs. Predator.

Physiology
Alien blood is an extremely potent acid and is capable of corroding almost any substance on contact with alarming speed. It is dull yellow in color, and appears to be pressurized inside the body so that it spurts out with great force when the creature is wounded. Ron Cobb suggested the idea of the Alien having acid blood as a plausible means to make the creature "unkillable"; if one were to use traditional firearms or explosives to attack it, its blood would eat through the hull of the ship. The Alien novelization suggests that, at least at the "facehugger" stage, the acid is not blood but a fluid maintained under pressure between a double layer of skin. In the Aliens vs. Predator: Requiem documentary: "Science of the Alien", it is hypothesized that the Aliens' acid blood could be some type of "hydrosulphuric acid" compound due to its corrosiveness and the conspicuously toxic effects on living human tissue. The documentary also speculates that Aliens are immune to their own acidic and toxic liquids due to an endobiological build-up, similar to the human stomach's ability to protect itself from its own digestive fluids. The documentary takes this hypothesis one step further and speculates that the Alien organism's protection system against its own toxic hydrosulphuric acid is a bio-organically produced Teflon insulation. In the original Alien, the facehugger is shown to be able to "spit" acid, dissolving the faceplate of Kane's helmet and allowing the creature immediate access inside. This ability is also exhibited by adult Aliens in Alien 3 and Alien: Resurrection; much like a spitting cobra, they use it to blind their victims.

Aliens can produce a thick, strong resin that they use to build their hives and to cocoon their victims, and they can use the walls of their hives as camouflage. Aliens also salivate heavily in the form of a sticky, clear slime; while not a toxic substance in and of itself, it is common for the Alien films to use it as a suspense-building device, wherein a character will notice the falling saliva before noticing its source lying in wait above them.

Intelligence
During various events in Aliens on the LV-426 colony Hadley's Hope, in Alien: Resurrection on the USM Auriga, in the crossover movie Alien vs Predator, and in Alien 3 when they are trying to trap the Alien in the lead works the species displayed observational learning and problem-solving skills, and in the former two cases the Aliens learned how to operate machinery at a very basic level. On LV-426, they were able to cut power in a section of the complex to gain access to the humans, and the Alien queen learns to board an elevator by observing Ripley and Newt escaping in the one beside it. The novelization of the film includes a scene where Bishop speculates that the queen establishing her "nest" at the base's main power plant could have been chosen either for the feral, animal reason of the warmth that it would provide or for the rational reason of selecting a location where any attackers would be unable to destroy her without destroying the entire facility. In the director's commentary for Aliens, James Cameron noted that the creatures in Aliens had been alive for far longer than the Alien in the original, and had more time to learn about their environment. On the USM Auriga in Alien: Resurrection, the Aliens kill one of their own, using its blood to melt through their enclosure and escape (according to the novelization, it was inspired to do so from genetic memories inherited from the original Ripley); in Alien vs. Predator, they use a similar strategy to free the queen from her chains. An Alien also uses acid spurting from its severed tail as an improvised weapon by flicking it, indicating they are fully aware of the effects of their acid blood. Finally, in Alien 3, the inmates and Ripley try to lure the Alien into the lead works. It becomes increasingly apparent that the Alien realizes it is a trap. At one point, it hesitates to enter the lead works and at other points, it stops all prey just before going into the lead works. All of this indicates it recognized danger and sought to avoid it.

Life cycle
Aliens are eusocial life-forms with a caste system ruled over by a queen. Their life cycle comprises several distinct stages: they begin their lives as an egg, which hatches a parasitoid larval form known as a facehugger, which then attaches itself to a living host by, as its name suggests, latching onto its face. In the Alien 3 novelization, Ripley commented that this parasitoid would probably be able to use any host from as small as a cat, to as large as an Asian Elephant.

The facehugger then "impregnates" the host with an embryo, known as a "chestburster", which, after a period of gestation, erupts violently from the host's chest, resulting in the death of the host.

The chestburster then matures to an adult phase, shedding its skin and replacing its cells with polarized silicon. Due to horizontal gene transfer during the gestation period, the Alien also takes on some of the basic physical attributes of the host from which it was born (something noticed by Ripley in Alien 3, when the Xenomorph plaguing the complex, having gestated within a Quadruped; a dog in the theatrical release, and an ox in the director's cut; whereas all the others she had previously seen had gestated within Humans/Bipeds), allowing the individual alien to adapt to the host's environment (breathe the air, etc.). This is also shown in the two live-action crossover films, Alien vs. Predator (2004) and Aliens vs. Predator: Requiem (2007), where an embryo, having gestated within a Predator/Yautja, displayed Predator/Yautja physical traits (arthropod-like mandibles) from eruption-onwards. This process of horizontal gene transfer is also shown to be two-way; in Alien Resurrection (film & novelization), Ellen Ripley's clone, Ripley-8, is shown exhibiting numerous Xenomorph characteristics, physical and behavioural; this is touched more upon in the novelization (chapter 4), where it's described that, when a host is infested with an Xenomorph embryo, it doesn't just infest the host like a parasite, but also like a virus, "a major breakthrough in adaptive evolution ... a way to guarantee that any host, any host at all, would provide whatever it was the developing embryo needed, even if/when the host's body was inadequate."

The adult phase of the Alien is known by various different names. The adult Aliens have been referred to as "drones", "warriors", "workers", and sometimes "soldiers", similar to the way ants have been defined. The names of the adult phase have also been used to name different types of adult phases of the Alien in numerous sources, including video games, comic books, novels, and the films, but only in the commentaries by the team who created the films. No official name has been given to the adult stage of the Alien in the films themselves.