Most adult forms of Xenomorph have been seen to exhibit a series of tube-like dorsal spines from their backs; one notable exception is the Runner, which lacks dorsal spines altogether. In cases where they are present, there are typically four such spines arranged in pairs, with a larger pair close to the creature's head and a smaller pair behind. Their exact purpose is unknown, although studies conducted by Weyland-Yutani have concluded that they are part of the Xenomorph's respiratory system, acting as atmospheric filters and delivering the requisite elements to the Xenomorph's body, similar to spiracles found on Earth-based insects. Several terrestrial aquatic creatures likewise use spiracles to absorb oxygen without inhaling water, explaining the Xenomorph's proven ability to remain submerged for extended periods.
Backspines may be a physical marker of the strength and power of a Xenomorph. Stronger, smarter castes such as Praetorians and Queens have more numerous spines, while less intelligent and fragile castes like the Runner have no spines. In some Predaliens, this relation is explicitly demonstrated. Post evolution, the Predaliens on LV-742 would grow more numerous and longer dorsal spines every time they molted into more durable versions of themselves. The Ravager also develops more numerous and ornamental spines post-evolution, along with a more hardened chitinous body and the ability regenesis.
Queens and PraetoriansQueens have a variation of the dorsal spines. Instead of actual, tube-like structures, spines run down its back, numbering at six instead of four. Praetorians have a similar set-up along their spines, being genetically similar to a Queen Alien.
PredaliensSome Predaliens shown also lack dorsal spines, similar to the runner. Interestingly, the back spines of a Predalien resemble that of a Praetorian and are the only ones to do so. It is unknown if this is just an aesthetic similarity or if there is anything more behind it.
Runner aliens completely lack spines, instead having a smooth back. This is likely so that the creature, more tuned into its speed than its strength, is more aerodynamic, making it faster. However, this could be a side effect of a non-humanoid host.
CarriersCarrier Xenomorphs have specialized back spines, which have evolved to carry around the otherwise vulnerable Facehugger. Their spines shed nutritious material that Facehuggers consume while aboard them.
- The Xenomorph's creator, H. R. Giger, originally included the spines in the creature's design to balance out its elongated head.
- Alien writer Dan O'Bannon originally wanted the spines to extend from the Alien's chest, but director Ridley Scott ultimately chose to have them on the creature's back.
- In Alien: Isolation, the Alien's dorsal spines can be seen vibrating from time to time when it stands still and is searching for the player. This may indicate the spines are a sensory organ of some sort.
- Xenomorphs have been seen swimming in sand and water with their dorsal spines protruding from the surface, not unlike a shark. It is unclear why they do this.
- ↑ 1.0 1.1 1.2 Weyland-Yutani Archives (2008), 20th Century Fox [Blu-ray]
- ↑ 2.0 2.1 Aliens versus Predator: Extinction (2003), EA Games, Fox Interactive.
- ↑ 3.0 3.1 "Monster Legacy - Star Beast — Alien". Retrieved on 2017-02-22.
- ↑ Alien: Isolation (2014), Creative Assembly, SEGA [Microsoft Windows, PlayStation 3, PlayStation 4, Xbox 360, Xbox One].