The mesoskeleton is the name given to the outer skeletal surface of the species Xenomorph XX121. Uniquely, the Xenomorph possesses both an insect-like exoskeleton and a mammalian endoskeleton, the former of which has been dubbed a "mesoskeleton" following studies conducted by Weyland-Yutani scientists.
The Xenomorph mesoskeleton is a silicon-based shell that affords the creature great natural defense. As well as protecting the Xenomorph from an extraordinarily wide array of environmental extremes, including the vacuum of space (at least for short periods), it is incredibly resilient to physical damage. It is even capable of stopping some forms of small arms fire, although it can be readily penetrated by armor-piercing ammunition as well as many examples of Yautja weaponry. It also effectively insulates the Xenomorph, not only protecting it from extreme temperatures but also ensuring it gives off no discernible thermal signature. Much like many insect and reptile species on Earth, the rigidity of the mesoskeleton forces the Xenomorph to repeatedly shed its outer skin as it grows.
Despite the mesoskeleton's resistance to temperature fluctuations, rapid and extreme temperature shifts have been known to cause it to become brittle and shatter. Furthermore, while it is necessarily impervious to the effects of the Xenomorph's own highly corrosive acid blood, even long after the creature's death, it is known to be vulnerable to other forms of acid, as discovered by the crew of the Marion on LV-178.
- Various sources have described the Xenomorph's outer carapace as "chitinous" in nature, not silicon-based, although the intended scientific accuracy of this is debatable — the term is often used as a simple descriptive phrase in novels and comics.
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