Xenomorph spikes

"@ S.M., Giger had nothing to do with the design for the Alien in Alien 3, as ADI developed their designs for the Bambi and Runner Alien without his input, even going so far to ignore any and all of Giger's suggestions. Though ironically Giger did create separately a similar design with the Cougar Alien. Also as depicted in Necronomicon IV, the dorsal tubes were Giger's idea..."

The tubes were included at Ridley's insistence. Giger was doing designs for the film without them. Necronom IV was merely the starting point.

As for Alien3 - he was contracted to provide designs for the film, which he did, which look a lot like what ended up on screen and was ultimately credited as such - despite ADI providing final design and execution.

Some interesting ideas and explanations here...

But i guess as Phallic Jaw mentions they are ultimately part of the Aesthetic HR Giger design which we have seen similar appendages in other pieces of art and only HR Giger would know if they was just a design Aesthetic or if he visioned they had any purpose.

It would be interesting if we are shown what their purpose is down the line even if its a Alien Anatomy Book released at some point in the future, but its fun to speculate their purposes.

@Parkerparrot

Indeed the Queen has spikes but it does not have the Tubes and so if they was to have a purpose i dont think they have the same purpose.

I would think the Queens Spikes are for attaching to a surface to help provide support while the Queen has her Egg Sack or some purpose to counter balance her for this purpose.

Some other theories are these could have been Wings or could change to Wings..

"Giger designed the tubes AND neck spike. They belong there. It IS part of the original Xeno. What they're for? Who knows?

ANY Alien movie depicting a "classic" Xeno without said tubes and spike is an abomination and one that I do not consider "canon", and neither would RS or Giger, the HORRIFIC CGI apparently birthed by a kindergartner graphic design student in Alien 3 notwithstanding."

Not following the logic.  The Alien 3 creature was apparently "birthed by a kindergartner graphic design student" (which is a new one since it's generally either a dog or ox), yet it's much the same as the creature Giger ultimately designed.

So Giger's design is at a level of a kindergartner graphic design student?

And the Alien wasn't CGI.

S.M.'s right the scenes in Alien 3 which are commonly misinterpreted as CGI were in actuality created using Rod Puppetry (same technique was used to create the Deacon in Prometheus) and Blue Screen, and unlike most movies in which a second unit director is usually in charge of such visual effect shots, these scenes were directed by David Fincher himself.

The scenes in question are the Aliens original birth from an Ox, the infamous scene depicted on most posters and DVD covers, and finally when the creature is running around the corridors of the Furnace towards the movie's end. The movement and speed of the creature in these shots are unnervingly alien and a welcome addition to the franchise and IMO should have been used throughout the movie, but unfortunately, Tom Woodruff Jr. insisted on suiting up for the other scenes - almost creating the impression that there are two Aliens in the movie. 

The ox birth burster was going to be a live action puppet before it was dropped and ended up as CG on the restored Assembly.

The puppet suffered from dodgy compositing and wouldn't have worked for close ups.

A larger scale live action rod puppet would have been more than suitable for the close ups, as James Cameron proved in Aliens with the Queen.

The Queen was a combination of effects - one of which was guys in a suit like Woodruff. I don't believe any were rod puppetry.

The Queen Alien was suspended from a crane with two suit actors portraying the Queens for arms, and extra technicians off camera operating the feet via rods and the mouth via animatronics. A similar, though simpler technique could have been used to realize the Runner alien in the mid to close up shots for Alien 3, but because of the film's many production woes and Woodruff's insistence to don the suit, it is no surprise that this never happened.

There was actually a Alien queen puppet in Aliens. The fightscene between Ripley and the Queen in the cargobay was shot extensively with puppets/models.

          " Bees have hives, man " 

"A similar, though simpler technique could have been used to realize the Runner alien in the mid to close up shots for Alien 3, but because of the film's many production woes and Woodruff's insistence to don the suit, it is no surprise that this never happened."

It's nothing to do with production woes; it's what was planned from the outset.  A puppet might've worked for close ups, but since you never see the full scale Alien below the chest, you'd have to wonder what's the point?  It doesn't make much sense economically to make a puppet where you have to construct animatronic arms and hands when you can have someone n a suit to achieve the same affect.