Xenomorphs begin life as bulbous eggs consisting of slick, leathery flesh. These eggs are laid by a queen and will eventually hatch after absorbing enough nutrients from the ground.
HatchlingEditXenomorph hatchlings - also called "facehuggers" - are similar to arthropods, with long, boney appendages and a spindly tail. As soon as they hatch, they will seek out hosts in which to implant their parasitic embryos. When a suitable host is found, the facehugger will leap on to its victim's head, completely covering the face and anchoring itself in place with its legs. It supplies the victim with oxygen at the same time that its proboscis inserts into the victim's mouth, implanting the embryo. After the embryo is implanted, the hatchling will let go of its own accord and die.
Removing a facehugger from a victim seems to be impossible. Trying to pull it off causes its tail to tighten around the victim's throat, cutting off their air supply; even if the hatchling could be removed in such a way, it grips the victim's head so tightly that pulling it off would result in pulling off the victim's face as well. Trying to remove a facehugger surgically is equally foolhardy as puncturing its skin will cause its acidic blood to dissolve whatever instruments penetrate the flesh. Of course, killing a facehugger while it's attached to a victim may be preferable to the alternative of letting it implant its embryo, which matures rapidly and can threaten multiple people at its adult phase.
InfantEditAfter an embryo has been implanted, the facehugger dies and detaches from the host. The host regains consciousness and becomes ravenously hungry, and will eat large amounts of food to accomodate the growth of the xenomorph infant inside it. After only a few hours, the alien will chew its way out of its host's chest, painfully killing the host. This has earned the infant xeno the nickname of "chestburster".
A chestburster has a distinctly serpentine appearance, with a long sinuous body and no limbs. Despite its shape, it is a quick and stealthy creature and will speedily seek out a dark, enclosed area to hide while it grows to its adult phase. The xenomorph's transition from infant to adult has never been witnessed so it cannot be described with any degree of precision, but the time between birth and adulthood is only a matter of hours. Once it reaches its adult state, it becomes a horrifying killing machine of great strength, stealth and ferocity.
The exact shape of an adult xenomorph varies depending upon what kind of host it spawned from as it develops certain characteristics of the host species. Xenos born from human hosts grow into a semi-humanoid shape with two arms and two legs, with an often varying number of digits on the end of each limb. They are capable of moving on their two legs or on all fours and their claws allow them to easily grip surfaces, meaning that they can scale walls and ceilings with ease. Their long tails end with a wickedly sharp bladed tip, providing them an excellent weapon for slashing and stabbing prey. The creatures' heads appear to be cylindrical, with a smooth carapace covering the front portion. One of the xenomorph's most disturbing features is its tongue, which can extend from the mouth. The tongue has a mouth of its own and can easily snap through human bone. It is unclear how exactly xenomorphs track prey as they have no visible eyes or noses.
Adult xenomorphs all seem to have a distinct bio-mechanical appearance about them, even though they are entirely organic. Their biology is silicon-based and their chitinous bodies are extremely resistant to any environmental hazards and cannot be easily penetrated. Only the most powerful firearms will suffice in combatting xenomorphs, but even the most highly trained and equipped soldiers would find battling these creatures to be a suicidal gesture. Xenomorphs not only reproduce rapidly and hunt in packs, but they have a truly unique defence mechanism; their blood is a highly toxic and corrosive form of acid that can quickly dissolve through any known material (except for the xenomorphs' own flesh). Xenomorph bodies have a highly pressurised circulatory system, so if they are wounded, their blood will spray from the wound and likely douse the attacker if he is foolish enough to engage them close-up. Killing xenomorphs is an extremely hazardous course of action as wounding them causes their corrosive blood to spray and eat through the surrounding environment. If xenos were loose on a space station or ship, it's possible that their blood could eat through the hull and cause tremendous damage, even breach the outer hull and trigger an explosive decompression.