Xenomorph

One of the few non-Covenant extraterrestrial species encountered by humanity, these creatures also hold the distinction of being the first, although rumours of their existence have never been common, and encounters with them on LV-426 in the Calpamos system, and on the classified Fiorina 161 penal colony, were classified at the highest levels to prevent a general panic. With the Human-Covenant War, the existence of these creatures has become a poorly kept secret among UNSC forces, and various factions have involved themselves with trying to study the elusive, dangerous, and extremely valuable creatures. Outbreak of Xenomorphs on UNSC colonies or human-occupied or claimed world constitutes a -grade threat assessment, at least four of which have been confirmed declared and contained during Insurrectionist flare-ups in the mid and latter 2550s, either accidentally or intentionally, as well as unconfirmed near-outbreak during Operation: VORAUSSICHT investigations into Project VENATOR.
==History==
===Prehistory===
Xenomorph prehistory, and especially their origin, is an enigma, one that has resulted in a number of competing theories among the limited scientific circle that has access to the classified materials pertaining to the species. One branch holds that the Xenomorphs must be an artificial species, engineered by an ancient, technologically advanced species for purposes that remain unclear. Within this branch, two schools of thought have emerged, in contrast to each other: the first claims that the Xenomorphs are meant to be the perfect weapon, intended to be targeted and deployed against enemy civilisations; the other holds that they are biological terraforming equipment, meant to sterilise a planet so that it can be converted into a usable platform. The second major branch rejects this interpretation, pointing that nature has produced its own unique horrors, though rarely to such a horrific and successful extent.
The Orion Nebula is technically classified by the Covenant and its successors, the New and True Covenant, as a holy site associated with their gods, the Forerunners, and limited archaeological evidence indicates that it was indeed a major hub of inhabitation, development and trade for the ancient species. The fact that the Xenomorph "homeworld" is located in this region of space is therefore suspicious - the Forerunners would surely never have tolerated the existence of such a threat, and at the same time the span between their disappearance and their discovery is far too short for such specialised biological weapons to have evolved unaided. The logical conclusions are thus, either that the Xenomorph species made their current home after the Forerunners left, indicating that at some point they were capable of space flight; or that they were intentionally transplanted there by the Forerunners, either as a biological preserve or as an imposed exile.
===First Encounters===
Xenomorphs are speculated to have originated on a planet within the Orion Nebula, at least 1344 lightyears from Sol, hypothetically designated "Xenomorph Prime" by ONI. Though human contact with the planet is documented, the location remains a closely guarded secret of the Weyland-Yutani corporation, even from ONI - the exact circumstances of the encounter are also classified, but are known to have involved a non-Xenomorph, non-Covenant extraterrestrial species. This planet serves as the Xenomorph homeworld, though whether they are truly indigenous to the planet or whether they were introduced there at some stage of their history remains impossible to prove. Regardless, "Xi-Prime" as it is known to ONI analysts is the centre of Xenomorph "culture", home to the great Queen Mothers that guide the species itself, of which the Queens are but smaller echoes. A number of different attempts were made, by both the Weyland-Yutani Corporation as well as the Colonial Military Administration, occasionally working together albeit uneasily, making the fifteen month long journeys in attempts to gather intelligence and sample specimens or material, or even to attempt to eradicate the potential threat the species presents. These attempts were classified by the CMA or buried under litigation and NDAs by Weyland-Yutani, and subsequent attempts to uncover information pertaining to them have had little success.
Covenant Missionary ships entering the Orion Nebula circa 442 CE recorded encountering monstrous entities. While no concrete description of them has ever surfaced, and many attributed their disappearance to one of many Flood-infested Forerunner installations left by their Gods, the fact that the Xenomorph homeworld lies in that region of space is suspicious. The missing ships were followed up by a small taskforce sent to cleanse whatever infestation they found, tracking the same radio signals - these too disappeared, though not before transmitting a desperate warning to the Covenant. While the Orion Nebula remained classified as a sacred site by the successors of the Covenant, such as the New Covenant, they, the Blood Covenant and the Sangheili Armed Forces would be hesitant to enter it unless pressed to by military factors. Several Jiralhanae and Sangheili ships would disappear during the War of Vengeance, all trace of them lost. The UNSC embassy requested Sangheili permission to attempt a salvage operation, and while permission was given it was done so grudgingly. The remains of the ships were never recovered, despite long-range high-resolution UNSC sensor sweeps of the region, though elements of the Office of Naval Intelligence remained long after the salvage operation was cancelled.
After their post-war declassification during the infamous leaking of Operation: VORAUSSICHT material in 2563, ONI would reveal three major incidents as part of its attempt at damage control - the original incident at LV223 in 2093, LV-426 in 2422 and another incident at the same planetoid in 2479, both in the Zeta2 Reticuli, and a related incident at Fiorina 161 in 2480, all perpetrated by the Weyland-Yutani Corporation. The second incident would lead to the deaths of an entire CMA platoon, the last survivors of which were killed on Fiorina while trying to return to UEG space. The incidents would become a rallying point around pro-ONI sentiment, and would eventually lead to Weyland-Yutani's declaration of bankruptcy and liquidation, its assets sold off to various other factions.
===Post-War Status===
With the conclusion of the Human-Covenant War, humanity would enter a period of peace, the first since the beginning of the Insurrection, almost six decades earlier, and this brought them a reprieve which they used to rebuild, rearm, and reorganise itself - but the UNSC was not the only human faction to benefit. Operating during the war, rogue elements of ONI who ignored the regular chain of command deemed study of Xenomorph material of paramount importance in the post-war reconstruction period - Earth and most of its colonies were safe, but war would still rage, and the need for new ideas and strategies to combat any threats Earth would face in the future was considered viable. Project VENATOR was one such faction - establishing a research facility, "Raptors Nest", at a small but well-guarded colony world, VENATOR was headed by a war criminal from Project CHISEL, codenamed OXFORD, and managed to cultivate a significant number of Xenomorph types, using these to experiment. Methods were sought to target the Xenomorphs, attempting to insert "programmed" behaviour patterns to stop human casualties - no efforts were made to exempt Sangheili from targeting - all of which failed, even using Queen-derived pheromones. A test-deployment against a Jiralhanae Blood Covenant colony would be deemed an unqualified success, slaughtering the local Jiralhanae, Unggoy, Kig-Yar and captive Sangheili inhabitants, and the planet was later devastated by nuclear and MAC bombardment. An accident would compromise the Raptors Nest facility shortly before a UNSC investigation strike team arrived, leading to a desperate battle for survival and containment of the Xenomorph threat in both the faciliy, and in orbit. The vast majority of known information is derived from recovered VENATOR material, despite the deaths of most of its scientific staff, and the disappearance and suspected death of project head Codename: OXFORD.
Other factions would also express great interest, especially Codename: DRACULA. Especially interested in the applications of the Xenomorph to his genetic augmentation projects, notably Project GREY, DRACULA went through considerable effort to procure samples, and his forces even participated in the battle for Raptors Nest, capturing Xenomorph material and specimens. Using this material, DRACULA would incorporate augmentations derived from Xenomorph and Flood DNA into his experimental supersoldiers, producing supersoldiers that were virtually unkillable, though dangerously unstable, but physically and psychologically, with a 78% risk of insanity or death caused from these augmentations. VORAUSSICHT personnel would spend years hunting DRACULA down, constantly eluded by the fugitive.
Others, such as OZYMANDIAS, would show exactly the opposite - from the leaked VORAUSSICHT materials, even OXYMANDIAS, the man who approved of CHISEL, found Xenomorph research to be inherently dangerous, far too dangerous to be worth any potential payoff. In hindsight, he would be proved all too accurate.
Presumably, Ξ Prime still exists, inhabited by its Queen Mothers and their derivative hives. Currently, the war effort against the Governors of Contrition prevent further efforts to investigate or eliminate this threat, with a moratoriam imposed by Secretary General Pierre Plantard for the duration of the crisis.
==Description==
Xenomorphs are decidedly extraterrestrial, and may be one of the most "alien" species encountered by humanity with the exception of the Flood. Most forms are vaguely humanoid in size and body shape, though both factors vary, and can usually be seen with an ebony black exoskeleton, though even this is not a hard rule. The only thing that can be said for Xenomorphs as a species is that they are adaptable, ferocious, and extremely difficult to kill. UNSC encounters with Xenomorphs have historically been hampered by sheer infamiliarity with their biology, leading to sometimes fatal mistakes made by the local chain of command. Various factors have attempted to rectify these deficiencies - the Weyland-Yutani colonisation/defence contractor, during the late 23rd to late 25th centuries, as well as various other military and civilian organisations. A subsection of the Office of Naval Intelligence's Section-III, Project: VENATOR, run by Codename: VENATOR, and investigated by Operation: VENATOR, would be deemed a disaster, leading to the loss of three corvettes, hundreds of personnel, and the near-escape of Xenomorph specimens into space. Project GREY would also use data produced by research on Xenomorphs. Given their virtually incomprehensible biology, virulent life cycle, and sheer ferocity, the spread of these creatures through inhabited space may be deemed a threat on a galactic scale. Though not as fast as Flood infestation, the results are similar - exponential growth and rapid expansion that could render the galaxy just as lifeless.
===Biology===
Xenomorphs, wherever they evolved, must have been apex predators. Every facet of their physiology is adapted for such a niche, making them the perfect hunters: a tough chitinous exoskeleton of an unknown material that render the creatures undetectable to thermal imaging and acidic bodily fluids to render any assault phyrric; a locomotive arrangement that is flexible and adaptable for most environments; and a mouth arrangement that includes an external set of jaws, as well as an internal set of jaws, with significant bite force and puncture capability. This has made them highly desirable for biological weapons research, but so far, thankfully, have produced few results.
====Environment====
Few clues exist in the Xenomorph's biology as to the precise nature of whatever homeworld they evolved on. Given their supreme durability against both extreme heat and cold, it may be inferred that their homeworld fluctuates between both extremes to a significant degree, and that this is an adaptation. Their resistance to most forms of radiation may also indicate that they are adapted to withstand high natural radiation. Their ability to rapidly adapt to changing situations within a few generations and life-dependent reproductive cycle indicates that their homeworld possesses native fauna with which the species continues its life cycle, and a number of environment types which require such adaptability. At the same time, however, all of these reasons have been proposed as reasons why the species must have been genetically engineered at some point in their past - the fact that they can operate in almost all environments is certainly quite a coincidence for a naturally evolved creature, though far from impossible.
Regardless, as experience has proven time and time again, Xenomorphs are capable of surviving in any environment, and have proven their robust natures throughout human contact with the species. Evidence exists that a small incursion on Earth in the early twentieth century was covered up by government officials for fear of causing a panic, and out of embarrasment at the ease with which the invaders slaughtered local military personnel. Evidence recovered from this time indicates that the Xenomorphs had no problems adapting to local conditions, either in the American mid-west or in polar regions. Engagements on Acheron, a moon orbiting the planet Calpamos (a ringed gas giant) in the Zeta 2 Reticuli system, also show that Xenomorphs exhibit great resistance to vacuum conditions, atmosphere that is barely breathable by humans, and is more than capable of blending into terrain. Partially, their adaptability is due to their ability to change environmental conditions - the few "hives" investigated have been reported to be near "tropical[ly]" hot, and it may be that Xenomorphs prefer this temperature. The presence of "eggs" might also be a factor, leading some to theorise that the temperature is an important factor in the egg laying and incubation process.
====Behaviour====
Xenomorphs exhibit a level of aggression that is unseen in any known sapient species, save for the Flood. Partially this is influenced by their rapid life cycle - forced to incubate their embryoes within still-living vertebrate species, the Xenomorph species must expand rapidly in order to reproduce, and for that they must subdue and capture populations of vertebrates in which to incubate their embryoes. The rapid maturation of these juveniles into larger, mature creatures further accelerates this process, producing a species that can rapidly spread through a population with increasing swiftness. No estimates have been made on the typical lifespan of individual creatures, though it is theorises that Queens and other members of the leadership caste have extremely long lifespans, far longer than the Drone or Warrior caste.
In encounters with humans, Xenomorphs have typically fared extremely well. A single individual drone managed to subdue almost the entire crew of a Weyland-Yutani tug vessel, and a hive of the creatures would overrun a human colony and slaughter a CMA Marine platoon, despite the Marines' use of advanced weaponry and equipment, effectively using superior tactics to flank chokepoints and overwhelm them at close range, where the effectiveness of their weapons would be reduced. While both engagements displayed their notable agility, speed and strength, it is their intelligence that is surprising, with a hive easily able to coordinate its members to ambush the Marine platoon, using camouflage and remaining unseen and unheard until provoked by the platoon. Moreso, their realisation that ventillation shafts would bypass machinegun posts shows a remarkable leap in thinking, and some biologists wonder whether even Drones and Warriors are sentient and self-aware to some degree.
Their interactions with other non-human species is poorly understood, though it would appear that they have a shared history with two separate species - a warrior race colloquially known as "Predators", and an ancient and as-yet unencountered race known only among research circles as "Space Jockeys". In the case of the Yautja, Xenomorphs have been a source of exploitation - hives have been intentionally seeded on worlds, allowed to build up, and then hunted for sport. At other times, clans have been able to domesticate Xenomorphs as attackers. The Space Jockeys have not yet been encountered by human or Covenant forces, the only contact being with a fossilised specimen, perhaps evidence that the race is extinct - one of perhaps many species to be extinguished by the Xenomorphs.
Xenomorphs display the ability to communicate with each other to an unusual extent. Physical gestures and audible sounds seem to play little part in the process, though pheromones are suspected for close-range communication between individuals. More unusual is the ability of Xenomorphs to coordinated for prolonged periods, and the ability of Queens to control their broods over extended distances. Telepathy has been proposed by some, but dismissed as unscientific by the majority, although no alternative remains. Nevertheless, attempts at keeping Xenomorphs isolated from the influence of a Queen failed spectacular with VENATOR, and although the filtration system prevented pheremone communication, coordination occured regardless. Xenomorphs also seem to possess some kind of genetic memory - experiences and memories from one generation are passed to the next, and some exchange from the host may occur. As with anything related to Xenomorph research, the resources simply do not exist to research it, nor have safe opportunities to do so presented themselves to UNSC researchers.
===Social Structure===
The Xenomorph species are eusocial, with hives possessing a highly regimented and sharply differentiated series of social castes, each with different tasks and functions. At the highest levels are the Queens, who lay hundreds of eggs in their lifetimes and guide the creatures of the hive. Further down in the hierarchy are the protectors of the hive, the Warriors and Praetorians, the former small and nimble soldiers and the latter large, devastating shock troops, equivalent more to tanks than to infantry. Beneath them are the worker caste, and at the few lowest levels are the first reproductive phase of the Xenomorph, the Facehugger. Individual Xenomorphs display overwhelming aggression, almost to the pointof insanity - in numbers, however, a hive can be devastatingly intelligent. The exact mechanism by which the Queen controls the hives members remains poorly documented, although pheromones are suspected to play at least a small part.
===Reproductive Cycle===
====Early Life====
=====Egg=====
Technically, the name "egg" is a misnomer, since they are more like shells or pouches than true eggs. Surprisingly soft and leathery, they are nevertheless durable, capable of withstanding centuries, perhaps millenia of neglect in a form of suspended animation with no apparent side-effects, carrying one or more Facehuggers contained within and protecting it/them until suitable host species encounters them. The "egg" seems to be an extension of the Facehugger itself - observors have reported that it seemed alive, and responded to both commands by higher caste members and to motion within a close proximity, indicating at least some motility. The top of the egg is able to open up, four "flaps" peeling away to allow the egress of the Facehugger contained within. The eventual fate of a "used" egg remains unclear, given that no conclusive observations have yet been conducted. They may be living entities in their own right, filling another niche within the hive - they may be recycled for nutrients, or even refilled by a Queen. Too little is known of Xenomorph biology to make a conclusive statement.
=====Facehugger=====
The organisms known colloquially as "Facehuggers" are the second stage of the Xenomorph species on their journey to maturity. Exactly how they fit with their later phases remains unclear - the Facehugger may be an entirely separate species, off of which the Xenomorph embryos cling to parasitically until they can be implanted within a host. Alternatively, the Facehuggers may still be a contiguous part of the process, inserting a small part of themselves to be allowed to develop to maturity, the rest of the organism dying after its usefulness has expired. Regardless, the lifespan of a Facehugger is not long - the implantation process takes several hours, and then the Facehugger them simply drops off. Some signs of life remain, but these are largely involuntary spasms or muscle memory. As far as the Facehugger is concerned, it has fulfilled its genetic imperative, and death soon follows.
The Facehugger itself is similar to forms of Terran arthropods. Eight spindly legs bear a flattened carapace protected by a chitinous exoskeleton. On the underside of the creature are its reproductive organs, used to implant the Xenomorph foetus within a host - moving with surprising speed for a creature of its size, the Facehugger can leap through the air at potential hosts, clamping onto their faces and immobilising their prey as the implantation process is begun, inserting a tube down the victim's throat to allow continued respiration, as well as to transfer the fetus. Aiding in this process is a long, prehensile tail - this appendage can serve as a stabiliser, used to guide the Facehugger during long leaps, but is also used to clamp itself to the victim, used on human victims to grip the neck.
Removal of a Facehugger on hosts has been attempted, but with few successes. Like all members of the Xenomorph life cycle, the Facehugger possesses internal reservoirs of extremely acidic liquid, capable of melting its way through three layers of Titanium-A battleplate, even in small amounts. Additionally, even if a base strong enough can be found to neutralise this acid, the Facehugger ties itself to its host with a thoroughness few other parasites can hope to match - neurologically, the Facehugger keeps its victim in a paralyzed coma, and physically, the creatures tail can be used to strangle the victim, used as a deterrent to prevent premature removal. A very small number of victims have been able to survive the implantation process, but they are of such rarity that replication is extremely difficult.
=====Chestburster=====
The embryo, once implanted, matures rapidly as it uses the host body for fuel, leeching nutrients to grow at a shockingly fast rate. The organism is usually incubated within the chest cavity within humans and humanoids, using the host's own ribcage for protection, making their way out after fully gestating by puncturing the ribs and literally bursting out in a shower of gore that earns it its distinctive moniker. Once free of the host, the Chestburster flees under its own power - at this stage of its development, it is still extremely vulnerable, and will seek dark, warm and crampt spaces to begin the molting process, shedding its skins as it rapidly grows. Exactly how the Chestburster can undergo such rapid maturation remains unclear - after implantation, the development of the embryo into a fully-grown chestburster can take a matter of hours, and the growth into a fully developed Xenomorph can be just as rapid.
The Chestburster itself displays classif pedomorphism, with rounded and smaller features that will later be developed in the adult. Those gestated within humanoids have been observed to be serpentine, with almost vestigial arms, though they retain the elongated head and internal second tongue/mouth of adults. Gestated within a quadruped, the chestburster appears to take on phenotypical traits of the original host, emerging and using spindly and underdeveloped legs to flee from any potential danger. Chestbursters that have been designated to develop into Queens, starting a new hive, already display the distinctive crest of the adult and the powerful arms and legs used for bipedal locomotion. It may be that each body type results in a unique type of Chestburster, and attempts to replicate the process by VENATOR did indeed result in significant variety. Naturally, all forms of Chestburster mature extremely quickly into mature specimens, and are battle-ready within hours, making the spread of the Xenomorph species a rapid process.
====Maturity====
There seems to be a considerable amount of overlap between mature Xenomorphs. Drones and Warriors share most phenotypic traits, though the Drone lacks the ridged crest of its counterpart. Likewise, the Runner fills a similar niche as the Warrior, but is radically different. Some metamorphosis seems to take place as well, mutating into higher caste members to fill niches that are vacant. It remains unclear whether these are all separate castes altogether, or whether they are simply one caste with considerable flexibility.
=====Drone=====
Drones are perhaps the most important of the Xenomorph's life stages, serving as a hive's basic worker caste. It is the Drones that construct the hive itself, secreting a resin that hardens quickly and serves as both camouflage and protection, and it is the Drones that tend to a hive's Queen and the eggs she lays. Drones are also the most common members of a hive, and some overlap exists between them and Warriors - both possess a similar body structure, and protect the hive from threats viciously, though the Drones lack the ridged cranial protrusion of their Warrior brethren, instead possessing a smoother cranial dome. The significance of this is unclear, but it serves as a visual identifier between the two different types.
Drones are bipedal, and are generally humanoid in shape. This is not to say that they are very similar to humanity - the elongated, phallic crest protrudes backwards a significant distance, and a powerful bladed prehensile tail serves to balance the creature. This tail can also be used as a weapon, used either in scorpion fashion to stab or impale a victim or to deflect a thrown punch or projectile. Like all stages, it also possesses a second internal mouth which possesses tremendous bite force, able to puncture dense materials, and can be used as a weapon in the event that its opponent is able to hold it off. This "tongue" can extend a significant distance, and penetrates deeply through flesh and bone.
Behaviourially, Drones seem less aggressive than Warriors, and prefer to use stealth and cunning to achieve their goals. Aboard a ship, or within a colony infrastructure, a Drone can use ventillation shafts, maintenance ducts, and other hard-to-reach areas to travel, moving rapidly. Though bipedal, a Drone can also move quadrupedally along flat surfaces, and is extremely agile. Rather than seeking to kill their victims, a Drone will instead attempt to incapacitate them. This may be instinctual, a genetic imperative intended to facilitate reproduction - once incapacitated, the victim(s) can be taken to a nearby hive and implanted by Facehuggers. Even when a hive is not nearby, a Drone will prefer to neutralise threats using surprise and shock, rather than brute force. In the absence of Warriors, Drones are also flexible enough to serve this role, and if there is no hive nearby at all, a Drone will undergo metamorphosis, rapidly mutating into a Queen itself to start a new hive.
=====Warrior=====
For all the subtlety of the Drone, the Warrior is the true enforcer of the hive, possessing extreme aggression and responding to threats posed to itself or the hive rapidly and violently. Overall, Warriors are identical to Drones in most physical respects, although perhaps a little less lean and with the aforementioned ridged crest. Behaviourally, though, Warriors are stronger, more agile, and much more violent, and are fielded in packs to overwhelm foes with strength and numbers, effectively using group tactics. They have shown considerable ability to learn and adapt, testing military defences and learning how to circumvent them, as well as some degree of coordination with the Queen despite the distances involved in expeditionary strikes. At the same time, Warriors have been shown to suppress their violent instincts - during raids within Xenomorph hives, the Warrior caste has allowed intruders to penetrate deeply enough so that, once they react, escape is next to impossible, all done to inflict high casualties.
Warriors have also shown considerable agility and flexibility, similar to Drones, used to great effect in battle. Able to leap significant distances in one bound, run along horizontal surfaces and even climb through tight spaces along the wall or ceiling, Warriors are capable of infiltrating virtually any structure unless specific precautions are taken. Warriors also seem to show no regard to the loss of fellow members of the hive - during engagements, some Warriors can be sacrificed to test machinegun emplacements or in probing strikes, and despite their loss the actions ensure higher chance of overall success.
=====Runner=====
Runners appear to be the result of gestating within quadrupedal hosts, and in hive environments may be employed as scouts or cavalry. They differ from the more standard Drone or Warrior in several key ways - while they retain the basic body profile, their skin is a lighter tone than previously encountered specimens. This may be an adaptation to the environment - the first Runner encountered, on Fiorina 161, was in a new environment, starkly contrasted from the dull, muted greys and blacks of a ships interior or a colony town's internal infrastructure. The lighter pigmentation may have been a camouflage mechanism. Regardless, colour is not the only difference the Runner has to its brethren - rather than the bipedal plantigrade locomotion of Drones and Warriors, Runners are digitigrade, possessing double hinged legs, and are quadrupedal. This gives them far improved mobility over other varieties, with higher top speed and even greater leap distance, but hinders dexterity and agility - with greater mass and higher speed, Runners have trouble with sharp turns and corners, and their hoof-like hindquarters reduce their ability to grip.
As with all members of the Xenomorph castes, the Runner has shown considerable intelligence, and has shown enough presence of mind to be selective in its targets. It is able to detect the presence of incubated Xenomorph embryos within hosts, and will avoid such targets if it can to ensure the expansion of the hive - on the other hand, it is able to "stage" kills in order to appear as accidents, hiding its own presence from unsuspecting investigators. Runners therefore can be considered scouts and infiltrators, staging first-strikes into enemy territory and keeping their presence unknown until the hives assault can follow up, using their speed and intelligence to further the hive's goals.
====Other====
Theoretically, there are as many Xenomorph variants as there are body types in the galaxy that are large enough to incubate a Xenomorph embryo. While the obove types are considered "baseline" variants, the most commonly encountered, UN, CMA and UNSC forces have also encountered a number of other, more unusual variants. Xenomorphs take on a rough size and body structure of whatever the host creature was.
File:XenoRhino.jpg|Experimentation with large Terran mammals produced what has been dubbed the "Rhino", a large, powerful but cumbersome behemoth.
File:XenoStalker.jpg|While similar in body plan to the Runner, the Stalker is larger and purely quadrupedal, bred using feline hosts.
File:XenoJockey.jpg|Encountered under exceptional circumstances, this form was incubated for millennia within the corpse of an unknown species.
File:XenoCroc.jpg|Incubated within Terran reptile lifeforms, the "Croc" is large, powerful and fast, as well as amphibious.
File:XenoCrusher.JPG|Although the conditions that produced this form remain unclear, the "Crusher" acts as a living battering ram, plowing through barricades and attackers with a thick protective shield.
File:XenoElite.jpg|Encountered during Operation VENATOR, this form was bred using captured Covenant prisoners of war, and bears traits of Sangheilian anatomy, despite the fact that not all the prisoners were Elites.
====Adulthood====
While the term "adulthood" may be a misnomer, members of the upper echelons of Xenomorph society are usually much larger than the lower-ranking castes, and much stronger but far less manoeuvrable than their smaller, nimbler counterparts. These parts of Xenomorph society are poorly documented, and more members than are currently known may exist.
=====Praetorian=====
While Warriors are protectors of the hive, it is the Praetorians that serve as the elite bodyguards of the Queen. Significantly larger than other Xenomorph castes, a Drone or Warrior can molt into a Praetorian to bolster the Queen's personal protection, growing enormously in size and with thicker, nigh impregnable exoskeleton. Some battlefield observers have claimed that Praetorians can mutate further, growing elongated claws that puncture metal, though these remain unconfirmed. Praetorians have also bee observed acting as battlefield commanders, with Warriors accepting and enacting orders and directions given by Praetorians. Researchers differ over the implications of the Praetorian caste - while some believe them to be a separate caste, perhaps a prototypical "King Alien" to the usual Queens, others disagree, conjecturing that they are themselves juvenile Queens, kept physiologically stunted to avoid internal rivalries within the hive, and able to assume the duties of the previous Queen in the event of its death.
Being larger and heavier than Drones or Warriors, Praetorians lack the speed or agility of the smaller castes, but compensate with sheer strength and durability. Their exoskeletons are virtually bulletproof, and with their sheer bulk Praetorians can be used as heavy muscle, battering down doors otherwise inaccessible to smaller Drones or Warriors. Despite such capability, however, Praetorians are usually kept in reserve by the Xenomorph hives, and serve as the protectors of the Queen within the hive itself. As such, Praetorians have been rarely encountered by UNSC Marine and Army forces except in expeditionary missions, which themselves are rare.
=====Queen=====
Queens are the leaders of the hives, and the most important members of known Xenomorph society. Every aspect of Xenomorph hive life is devoted to protecting and serving a Queen - the hives is constructed and warriors raised to protect her, Drones tend to her needs and care for the thousands of eggs she may lay during her lifetime, and expeditionary strikes are made to recover incapacitated hosts for implantation to expand the hive. As the leader of the hive, the Queen guides every movement it makes, inside and outside of the hive. Pheromones are suspected to play an active part in Xenomorph communication, though the Queens are still able to retain control over hive members over considerable distances and for extended periods through unknown means. Queens are also the most intelligent known members of the Xenomorph species, and have even been observed working through complex problems and using sophisticated machinery. Queens are extremely protective of their "children", and if threatened will take care to ensure their safety - if they are destroyed, the Queen will enter a rage, ruthlessly attacking their destroyer and going to extreme lengths to exact vengeance.
Queens are also the largest Xenomorph caste, towering above even Praetorians in size. In the few instances where Queens have taken to the field of battle, they have posed a significant threat to all involved participants, although extreme heavy machinery and explosive weapons have proven effective. Queens rarely participate so overtly, however - for the most part, they are sealed within the hive, attached to a large, swollen ovipositor, laying a nearly constant stream of eggs with which to expand the hive, implanting more victims to swell the hive's ranks. This eggsack is detachable, but this causes a Queen tremendous pain. Although Drones will pupate into a Queen in the absence of higher caste members, a form of "Royal Jelly" appears to be involved in the creation process of a successor Queen
======Empress======
When numerous hives exist on a planet, controlled by numerous Queens, rather than allow inter-hive competition one Queen will mutate into a larger, winged variant, termed an "Empress", serving as a central control and coordinator for the separate hives. This appears to be separate to the hypothetical Queen Mother caste.
=====Queen Mother=====
Entirely hypothetical, Queen Mothers are speculated to be the highest members of the Xenomorph caste structure, controlling the many hives throughout the galaxy from their homeworld through an unknown means. Controversially, some researchers that these Queen Mothers exist in a constant state of warfare against each other, with multiple factions waging war for supremacy of their homeworld, constantly kept in a state of equillibrium by the shifting balance of power. No concrete evidence for this exists, but if true it may be the one thing stopping the Xenomorph species advancing enough to leave their homeworld, ironically protecting the galaxy from themselves.
==Remarks==
*"Geez, these things almost make the Flood look tame. I mean, for the love of god. It...leaps out of an egg and impregnates you? And then it bursts out of your chest? And then turns into a jet black killing machine? I...what the hell!"
*"You hear rumours that UN Marines encountered something in deep space during the early days of interstellar travel. I never believed it myself until I saw Acheron, and what was there."
*"You ever see one of these things stand up against a Spartan? Neither did I. But I hear it was quite a fight."
*"Many of my colleagues refuse to believe these things could ever evolve naturally. I've seen theories about them being biological weapons, or even tools for terraforming, but I think they conveniently forget that nature has already produced life cycles that are just as horrific, if not quite as complicated."
==Gallery==
File:Aliens vs Halo by Rahll.jpg|Andrew-306 encounters a Facehugger and Drone during Operation: VENATOR
File:6688a5e72ceb8d753d4928cb1fa4679c.jpg
File:Queen AlienPress still01.jpg
File:ALIEN vs HALO redux by Apos.jpg