These notes have been authorised by the Black Mesa Xenian Research and Development Team. As the chief xenologist operating within the R&D team, I have been selected to document and explain, in detail, the differing subspecies of the most common form of Xenian lifeform we have
identified thus far-the parasitic creature nicknamed the "headcrab".

Whilst the full extent of their capabilities has only somewhat been tested, enough rigorous lab testing has been conducted to provide a clear guide for how to identify different subspecies, their biology, and, should the situation arise, the best way to combat them.

(All images are courtesy of the HEV-XO-27 Automaton design, where headcrabs made temporarily docile by an energy supplement were photographed. The crowbar seen in them is the weapon possessed by the automaton.)

This species of headcrab is the most proilfic in the border world, and the most easily obtainable for research and development purposes. Typically, it possesses a weaker hide than their more biologically evolved compatriots, making them much less of a threat. They attack, like all other sub-species of headcrab, by flinging their bodies at their chosen host, intending to latch onto their head and take control of the host's nervous system from there.

Their claws are sharp, and can cause significant bodily harm if wounds are not addressed quickly. Besides that, they have few other unique characteristics. A good hit with one of our lab-tested crowbars was enough to incapacitate the creature. A testing automaton was used to evaluate their effectiveness in swarms, and results proved conclusive that they exhibit no form of hive mentality and instead seek out individual hosts at random, sometimes colliding with one another in mid-air when attacking.

(*CLASSIFIED NOTES* DR WALLACE BREEN: These "headcrabs", as they have been so deemed, would prove a valuable biological weapon should a form of transport for them be perfected. Rocket propelled canisters of some type could be of considerable worth on the international weapons market.)

This type of headcrab possesses more locomotive capability than their standard cousins. The absence of approximately 24% of the fatty tissue usually present on other species of headcrabs allows them to be considerably more nimble, as well as the presence of addtional muscle ligaments which allow them to propel themselves at hosts with more force and speed. Initial dissections also seem to indicate the presence of some form of alkaline liquid which neutralises lactic acid, reducing stress on muscles.

Due to their speed, they are much more unpredictable, and tests with both personnel and automatons were unable to keep track of their rapid movements in swarms. They possess a greener tinge than their standard counterparts, which is theorised to be due to their limited capability to photosynthesise in order to provide energy for their speedier movements.

(*CLASSIFIED NOTES* DR WALLACE BREEN: Extraction of the lactic acid neutralizer could prove lucrative for international militaries. As before, weaponisation of the organism could also prove valuable, although more caution should be taken, given their greater danger.)

The Fire Headcrab, as the name suggests, possesses an exterior body temperature in excess of 750 degrees fahrenheit, being the minimum temperature necessary for combustion of some gases. The reason more precise information is not known is that, due to their heat, readings cannot be safely obtained.

They possess a redder tinge, and the few successful dissections conducted (therefore not providing entirely viable results due to a lack of sufficient repetition) seem to indicate that this is due to a higher blood pressure causing the higher body temperature. Respiration is thought to, by alteration due to some unknown chemical, be exothermic in nature as well, explaining their outwardly temperature.

It is unknown how they function at such high bodily temperatures. However, theories suggest another as-of-yet unknown chemical stabilises body cells. This has been put forward due to the headcrab's small combustion upon the ceasing of brain activity.

(*CLASSIFIED NOTES* DR WALLACE BREEN: An interesting sub-species, possessing of great monetary value, both for weaponisation as organic, breedable bombs and for their ability to radiate heat naturally. Extraction of chemicals could also prove very valuable.)

Just like their fiery cousins, these headcrabs are so named due to their extremely low bodily temperature-at least below 32 degrees fahrenheit, as water freezes in contact with the headcrab. Their extremely low temperature is passed on through an endothermic reaction to their hosts, providing sluggish movement and quickly causing an onset of hypothermia and, presumably, frostbite after prolonged exposure.

Upon death, their internal fluids freeze rapidly into ice structures, suggesting that some unknown keeps the fluids from freezing whilst the creature is alive. They are one of the only species to demonstrate any higher form of thinking, deliberately slowing hosts for other headcrabs to take over more easily. They are light blue with distinctive ice-blue veins, carrying some form of anti-freeze fluid.

(*CLASSIFIED NOTES* DR WALLACE BREEN: Admittedly, I see less capability of weaponisation for this species, as their effects are less immediate and effective than the others. However, there may be some value in exporting them with the intent of cold-environment warfare.)

This variety of Xenian parasite possesses natural toxins in its body, which it regularly exhumes in a vapour which diffuses in a small radius around the headcrab. It is also highly radioactive, suggesting some isotope or chemical in the creature is responsible for the toxins produced. Prolonged exposure produces a similar effect to that of gamma ray exposure, although it is key to note that the toxins are so lethal that cells are not given the chance to mutate into cancer before total cell death.

Due to their nature, only testing with automatons and non-human life has been conducted. The hosts of such headcrabs (as tested on other bipedal beings taken from the border world) develop rashes and exhude the toxin in liquid form as opposed to gaseous. They are a dark brown colour, indicative of the poison they contain.

(*CLASSIFIED NOTES* DR WALLACE BREEN: The implications of extracting the toxin responsible for bio-chemical warfare are obvious. They would be of great value to any military seeking to weaponise them. They could be shielded in lead containers for transport if necessary.)

The Electric Headcrab functions very similarly to an electrical eel, and is the most well-documented of the uncommon headcrab species. By reversing electrical polarity very quickly through a signal sent by the brain to electrocytes in the body, a current is generated which can then be directed by the headcrab at their potential host.

The voltage of this burst is significantly higher than that of an electric eel, however: a shock of around 2000 volts can be generated for approximately three miliseconds. Such a shock can temporarily paralyse the motor functions of their host, allowing them to attach themselves quickly and effectively. They are a light blue colour, similarly to the ice headcrabs, although without the much more distinct ice-blue veins.

(*CLASSIFIED NOTES* DR WALLACE BREEN: Weaponisation of this species could indeed prove useful, but I possess greater interest in the possibility of inducing electrical discharges from them at will. Such an action may be of great value as they could be used as organic batteries.)

The researcher who tracked this elusive headcrab named it "Ethereal" as it seems to pass in and out of reality on a consistent basis. Whilst it maintains a constant presence in this world, it seems to be in flux due to the transportation from Xen. It is unknown whether it naturally occurs there, but approximately one out of every thousand standard headcrabs transported from the border world exhibits this effect.

As a result, the creature has the curious ability to often phase right through objects of its choosing-meaning that the only way to contain it for long periods of time was to surround it with a field designed to strengthen the transportation signal made for moving materials to and from the border world. Their phasing nature makes them partially transparent.

(*CLASSIFIED NOTES* DR WALLACE BREEN: I do not believe weaponising or attempting to make use of this creatures capabilities would be possible. Containment is difficult enough under highly controlled conditions, and, at any rate, their rate of occurrence is simply too rare to make them efficient to breed.

This headcrab posssesses limited psionic ability, much like the Vortigants we have previously detained from the border world. With it, it is able to disorient and confuse hosts prior to attempts to latch onto them, improving its chances of success. Automatons were also affected, suggesting they are capable of a form of EMP as well.

They are coloured purple, and have been seen to give off small lilac discharges, possessing of a similar signature to that given off by Vortigants. However, their psionic abilities pale in comparison and have thus far proven to be capable of very little other than that previously mentioned.

(*CLASSIFIED NOTES* DR WALLACE BREEN: This species of headcrab promises, perhaps, the most potential. Similarly to the Vortigants, their ability to tap into psionic energy and mental powers could make them a valuable asset to other nations and to my employer.

In this small guide, I intended to provide the information necessary for future researchers of the creature to gain more insight into their complex biology. Additionally, as performed with all Lambda projects, this information should prove useful should an emergency occur whereby such lifeforms would have to be fought.

For now, this is all the information readily available by science teams.

Dr. Aaron Mosley

((Having found this gem of a HL1 mod, I decided to do a little lore exercise as a result of there being very few guides for this game. For newcomers to the game, there's probably something of interest in here in learning some of the base mechanics of how the game functions. I, however, am also in love with the lore of the HL universe and thought I'd try my best to expand on it a little with this. I hope you enjoyed reading through it! Let me know if there's any lore points you want me to elaborate on.))