Aliens are depicted as eusocial life-forms with a defined caste system which is ruled by a queen. Their life cycle comprises several distinct stages: they begin their lives as an egg, which hatches a parasitic larval form known as a facehugger, which then attaches itself to a living host by, as its name suggests, latching onto its face. The facehugger then "impregnates" the host with an embryo known as a chestburster, which, after gestation, erupts violently from the host's chest, killing it. The chestburster then matures to an adult phase within a few hours. The alien takes on some of the attributes of the host from which it was born.
Queen
Queen aliens are significantly larger and stronger than the warriors, approximately 4.5 meters tall.Their body structure differs also, having two pairs of arms, one large and one small. The Queen's head is larger than other adult aliens and is protected by a large flat crest above their heads. Unlike other aliens, the Queen also has high heel protrusions from her feet. The Queen also seems to have increased intelligence compared to the other lifecycle stages, as the Queen on LV-426 was able to learn to operate an elevator. Pregnant alien queens possess an immense ovipositor on their lower torso, similar to a queen termite's, which is responsible for creating facehugger eggs. The queen is able to detach from the ovipositor. When attached to her ovipositor, the queen is supported by a "biomechanical throne". that consists of a lattice of struts resembling massive insect legs. Unlike insect queens, there appears to be no need for drones to fertilize an alien queen's eggs.
Egg
The eggs laid by the queen are large, ellipsoidal leathery objects about one meter high with a four-lobed opening at the top. As a potential host approaches, the egg's lobes unfold like flower petals, and the parasitic facehugger explodes from within.
Facehugger
The facehugger is the second stage in the alien's life-cycle. Its eight long, bony, finger-like legs allow it to crawl rapidly, and its long tail can launch it in great leaps. These particular appendages give them an appearance somewhat comparable to chelicerate arthropods such as arachnids and horseshoe crabs.
The facehugger is a parasitoid; its only purpose is to make contact with the host's mouth for the implantation process, by gripping its legs around the victim's head and wrapping its tail around the host's neck. Upon making contact, the facehugger tightens its tail around the host's neck in order to render it unconscious through oxygen deprivation. The facehugger then inserts a proboscis down the host's throat, supplying it with oxygen whilst simultaneously implanting an embryo.
Chestburster
After implantation, facehuggers die and the embryo's host wakes up afterwards showing no considerable outward negative symptoms. Symptoms build acutely after detachment of the facehugger, the most common being sore throat, slight nausea, increased congestion and moderate to extreme hunger.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 inflicted either in lack of chest space due to the chestburster's presence, or even premature attempts to escape the host. The incubating embryo may take on some of the host's DNA or traits, such as bipedalism, quadrupedalism or possessing the mandibles of a Predator and other body structure changes. Over the course of 1-24 hours (indeterminable in some cases, and sometimes up to a week, in the case of some queens), the embryo develops into a chestburster, at which point it emerges, violently ripping open the chest of the host, killing it.
