source:
http://aliens.wikia.com/wiki/Xenomorph[More on this character soon]
The adult Xenomorph is a living weapon, noted for its ferocity and deadliness in any condition. Once fully matured, they have great physical strength and agility. They are masters of stealth; a favored method of acquiring prey is to wait in a dormant state until an appropriate victim strays near, and then drop down silently from behind.[5] The prey is generally blind to the fact that a xenomorph is present, due to its propensity to camouflage itself within its nest walls or the surrounding artificial environment given it's biomechanical appearance.[5][1][6]
Adult xenomorphs are quick and agile, and can run along ceilings and walls,[1][6] a skill they exhibit freely whether they are evading others, attacking, or hunting. They can survive in extreme temperatures, are well-adapted to swimming,[3] can respirate in harsh atmospheres, and can survive in vacuum for short lengths of time.[5] Their movement tends to be silent, and they do not radiate heat as their exoskeletal temperature matches the ambient temperature.[1][4] They also salivate profusely. This saliva is not acidic, though some xenomorphs do have the ability to spit acid, which may come from their stomachs or a special gland somewhere in their throats. This can be used to blind victims, much like a spitting cobra.[6][3]
The creatures possess no visible eyes; Giger mandated this in his original design, because he felt that it made the creatures much more frightening if one could not tell they were looking at them.[7] In the original Alien film, the top of the creature's head was semi-transparent, with empty eye sockets of human appearance visible within. This element was dropped in later movies, but reused for the Predalien design, on the front portion of its skull.
In Aliens, the adult creatures are lacking the smooth carapace covering their heads. 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 xenomorph. This maturation process involves the now adult xenomorph shedding the smooth dome-like plate covering its head, leaving the skin underneith to be exposed. In Alien³, a fisheye lens (which creates a perspective similar to that of a peephole) was used to depict the Xenomorph's sight. Whether this can be considered sight or just a filmed representation of the creatures' sensory perception is unknown.
However, in the novelization of the movie Alien, the creature is held mesmerized by a spinning green light for several minutes. Due to the absence of clearly visible eyes, it is possible that the creature uses echolocation to see its environment, much like bats. It may be for this reason that Xenomorphs hiss almost constantly. Though it may have the ability to see through an unknown structure, as they evolved differently from life on Earth. In the 2010 Alien vs. Predator game, it is mentioned on one of the Marine audio tapes that Xenomorphs have vision.
Xenomorphs may be able to detect their 'prey' through electro-reception. This is the method sharks use to detect even the most hidden prey. Creatures with the electro-reception sense can detect the electromagnetic field all animals produce allowing them to detect the creatures heart beat. This would explain how xenomorphs always know where the humans are.
