If you just look at a picture of one there are no eyes readily apparent but they clearly know where they are going. Could their screeching be some form of echo-location?
The xenomorph entry at aliens.wikia discusses possible theories for the senses (including vision) of the xenomorphs. Echolocation and some form of vision (through the carapace) both get mentioned, indeed.
What seems to indicate and support that they do have eyes, imo, are the following:
- Various computer game adaptations close to the canon provide ways for players to play with the xenomorphs. Of course, what they display as the xenomorph's vision may be just a translation necessitated by game design requirements (it would be hard to sell a game in which the player has to navigate levels through sounds only. :)
- The newborn alien has eye sockets and eyes. True, this creature is much closer to humans than the standard xenomorph emerging from a human.
- Several "canonish" images - drawings by Giger, photos of Giger's sculptures - show that beneath the carapace, the xenomorph's head is human-like (at least in the case of those xenomorphs that had humans as their "base creature"), with eye sockets. Considering the extremely tight and practical "design" of the creatures (as bio-engineered weapons, most likely developed by the Engineers, the race of the Space Jockey), it is highly unlikely that they would have / develop useless biological features - which, in turn, seems to indicate that they do have eyes in those eye sockets that can see through their own protective carapace. What their vision could be like is a good question, though.
Wikipedia's Alien entry has the following on the subject:
Giger conceived the Alien as being vaguely human but a human in full armor, protected from all outside forces. He mandated that the creature have no eyes, because he felt that it made them much more frightening if you could not tell they were looking at you.
They have elongated, cylindrical skulls, but possess no visible eyes, though in the original Alien film, the top of the creature's head was translucent, with empty, human-looking eye sockets within. This element was re-used for the "Predalien" 28 years later. How the creatures see is uncertain. In Alien 3, a fisheye lens was used to illustrate the Alien's point of view. In the novelization of the movie Alien, the creature is held mesmerized by a spinning green light for several minutes.
Do xenomorphs "see"?
I have to re-watch the movies, but my recollection is that aliens move about as if they relied on non-visual senses. First off, they live in very dark environments (there's no lighting in the xenomorph hives), and even when they leave they mostly come out to hunt at night, mostly.
I also recall scenes in which Ripley or others are very close to a xenomorph and can clearly see it, but the xenomorph seems uncertain as to their precise location (though it clearly detects their presence and is actively looking for them). Likewise, when a colonial marine pops his head up into the plenum space and shines a light at the advancing xenomorphs, they don't react to the light at all:
This, along with the strange vocalizations they make and the constant opening of the mouth suggests that they might be using a combination of echolocation and some kind of directional olfactory sense like many animals have.
That said, the alien queen, at least, appears to be able to see what Ripley is doing at a distance, e.g. when she threatens the eggs with the flamethrower. It's possible she has a rudimentary optical sense combined with an IR/heat-sensing ability like pit vipers:
Here we have a Sigorny Weaver in her natural habitat engaging in some napalm-assisted negotiations with a proud xenomorph mother.
Evolution of the eye
It's believed that eyes evolved originally from clusters of photosensitive cells on the skin surface. These rudimentary "eyespots" were only capable of detecting ambient light levels without directional sensitivity (like how we detect heat on our skin). Later, these cells formed into slight depressions on the skin, allowing limited directional sensitivity.
As the depression grew bigger and the opening got smaller, the eyespot turned into a water-filled chamber with only a tiny aperture for light to shine through—becoming, in effect, a pinhole camera capable of finer directional sensitivity and rudimentary imaging. And from there, it gradually evolved into the eyes that we see on animals today.
I would suggest that the large elongated dome carapace that also forms the xenomorph's head likely conceals echolocation organs, like the "melon" found in the "foreheads" of cetaceans that's used as an acoustic lens:
Additionally, the surface of the carapace could be covered with photosensitive cells giving xenomorphs a rudimentary visual sense that extends into the infra-red range of thermal radiation.
Of course this isn't canon, but you have to admit, there is a slight resemblance...
Taken from the accepted answer to the question "Can Aliens see cloaked predators?" over on Movies and TV:
Aliens don't have eyes. They sense their prey thru changes in air pressure and noise. How they manage this with such accuracy isn't known, but a Predator's cloak bends light so that they appear camouflaged by their surroundings. When a Predator moves when cloaked they still emit changes in air pressure.
Aliens are also effective hunters underwater and can sense their prey's electromagnetic field much like a shark, and Aliens also adapt the senses of the host who gave birth to them. So if they are grown in a Predator they will take on Predator traits.
Aliens are a scary bunch! :)
I always thought that the eyes were under a dome, and that at least human spawned xenomorphs would use sight as a primary or secondary sense. It is also possible that xenomorphs use electroreception to observe their surroundings. Electroreception would also mean that they could "see" equipment being used such as motion trackers smartgun displays anything electrical. If the electroreception was sensitive enough a xenomorph could see a single muscle twitch,and that means that it could use this sense as a sort of improvised sight when there is a lot of electrical activity in the area. it would also explain the power being cut out in aliens.
The games tend to point out that they use other senses, for example in AVP 2010 they make it very clear that they use pheromones, this may not be cannon but it is certain what ever sense they use the elongated head and brain presumably allows for extra computation using sensory input, making up for there lack of sight.
The carapace is transparent. This is not seen clearly in the films, much like someone wearing sunglasses, because of the lighting. In making of pictures and Giger's art eye sockets and human-like skull are clearly visible under a transparent hood/dome.
See the barrel-eye fish, which sees in exactly this way. If you find a picture of one, the dark spots that look like eyes are actually nostrils, and it sees with green domes in the middle of it's head.
Yes its speculation though. we never actually see eyes, but since there are eye sockets and a transparent dome I bet this is the intention...along with all the other senses and probably extended spectrum. the extended spectrum like infra red might mean it's clear despite the murky dome over it.
Cave dwelling animals and deep sea animals are often blind, and can often be predators too.
The phronima, a tiny crustacean which is the inspiration partly for the alien (due to appearance and life cycle). Also has a kind of dome'd head...but it happens that this IS its eye...and its transparent...so that could also be true.
Sight is only what your brain presents to you with the input it gets from your eyes. Its is believed that bats perceive echo location as strongly as sight combined with touch as they can feel the air echoing back.
I imagine that GIANT head is filled with nerves and sensory organs. If they are a bio-weapon ( which we are led to believe ) then they can probably see you, smell you, taste you, see IR and UV, echo locate and feel your em patterns. They can probably feel you heart beat through the air!
To be fair, in every movie, they have proven over and over that they are expert killers. As Ash said, Their perfection is only matched by their hostility. The perfect survivor