The xenomorph's 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.

Unlike many other extraterrestrial races in film and television science fiction (such as the Daleks and Cybermen in Doctor Who, or the Klingons and Borg in Star Trek), the xenomorphs are not sapient toolmakers — they lack a technological civilization of any kind, and are instead primal, predatory creatures with no higher goal than the preservation and propagation of their own species by any means necessary, up to and including the elimination of other lifeforms that may pose a threat to their existence.

Prior to writing the script to Alien, O'Bannon had been working in France for Chilean cult director Alejandro Jodorowsky's planned adaptation of Frank Herbert's classic science-fiction novel Dune.

Giger initially offered to completely design the Alien from scratch, but Scott mandated that he base his work on Necronom IV, saying that to start over from the beginning would be too time-consuming.

[7] Giger also gave the Alien's mouth a second inner set of pharyngeal jaws located at the tip of a long, tongue-like proboscis which could extend rapidly for use as a weapon.

As critic Ximena Gallardo notes, the creature's combination of sexually evocative physical and behavioral characteristics creates "a nightmare vision of sex and death.

When standing upright, the Aliens are bipedal in form, though, depending on their host species, they will adopt either a hunched stance or remain fully erect when walking, sprinting, or in hotter environments.

The sharp tip was initially a small, scorpion-like barb,[12] but from Aliens onwards the blade design increased in size and changed in appearance to more closely resemble a slashing weapon.

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 acidic blood is a bio-organically produced Teflon-like insulation.

[25] 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.

The novel for the film Aliens includes a scene where Bishop speculates on the reason why the queen established her "nest" at the base's main power plant.

[34] Adult xenomorphs are capable of creating their own reproductive egg ('ovamorph') by embedding their prey into an organic substance that (in theory) metabolically reacts to merge host-parasite genetic material.

After impregnation, facehuggers die and the embryo's host wakes up afterward, showing no considerable outward negative symptoms and a degree of amnesia regarding events at the time of implantation.

Symptoms build acutely after detachment of the facehugger, the most common being sore throat, slight nausea, increased congestion, and moderate to extreme hunger.

[12] In later stages where the incubation period is extended in preparation of a queen birth, symptoms will include a shortness of breath, exhaustion, and hemorrhaging (detectable through biological scanners and present in nosebleeds or other seemingly random bleeding incidents), as well as chest pains caused by a lack of space due to the chestburster's presence or even premature attempts to escape the host.

The incubating embryo takes on some of the host's DNA or traits, such as bipedalism, quadrupedalism,[4] possessing the mandibles of a Predator,[44] and other structural changes that enable adaptation to its new environment.

According to Weyland-Yutani medical scientists in Aliens: Colonial Marines, the chestburster will draw nutrients from the host's body in order to develop a placenta as it grows, attaching itself to several major organs in the process.

However, it soon undergoes a dramatic growth spurt, reaching adult size in a matter of hours; in Alien, the chestburster had grown to 2 metres (6.6 ft) in height by the time the Nostromo crew located it again.

Scott drafted a series of alternative designs for the chestburster based on the philosophy of working "back [from the adult] to the child" and ultimately produced "something phallic".

The only differences behavior-wise was that this Alien behaved more like a dog or another quadrupedal animal that generally is prone to using its mouth instead of its front legs as its primary weapon to attack and maul its victims with its teeth.

This, however, changed towards the movie's climax, at which point the monster, after surviving a torrent of molten lead, burst from the liquid and went into a rampage, pursuing Ripley and presumably attempting to kill her until she destroyed it by showering it with freezing water, causing it to explode from thermal shock.

The cloned queen inherits a perversion of a human womb, and as a result, it ceases to lay eggs and gives birth to a humanoid mutant hybrid.

Physically, the human/Alien Newborn is very different from other alien young, being larger, with pale, translucent skin, a skull-shaped face with eyes, a human tongue, and a complete absence of a tail.

The Newborn creature was originally scripted by Joss Whedon as being an eyeless, ivory-white quadruped with red veins running along the sides of its head.

The Predalien shares many characteristics with its hosts, such as long hair-like appendages, mandibles, skin color, blood that glows in the dark (though still acidic), and similar vocalizations.

The Deacon is the result of a "Trilobite" (which takes its name from a group of extinct marine arthropods), a large facehugger-like creature, attacking and impregnating an Engineer.

This behavior is just one of several demonstrating the Neomorph's far more feral nature; they are voracious predators, often eating the corpses of their victims, and they appear to lack their xenomorph cousins' hive structure, possibly since they propagate through mutated animal life.

[16] The Offspring, featured in Alien: Romulus, is the result of pregnant character Kay injecting a serum derived from the Xenomorph's genome into her neck, leading to a rapid mutation of her unborn fetus.

It terrorizes the remaining crew of the Corbelan, damaging the android Andy and feeding off of its mother Kay before pursuing Rain, but is finally defeated by her jettisoning it into the planetary rings below.