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Venomous Headcrab subspecies
Poison bite (10 damage, drops the player's health to 1 instantly, which eventually regenerates)
The Poison Headcrab (also commonly known as the black headcrab, named "Venom Crab" by Alyx Vance) is a Headcrab subspecies previously unknown before the Combine invasion. Like its relatives, the Poison Headcrab is able to infest and mutate humans into Zombies.
The Poison Headcrab can be identified by its dark skin (or exoskeleton, judging by the shine it has and its high health). They have thicker legs than other Headcrabs, with bristle-like hairs. White bands circle its knee joints, where its legs bend inward. The creature's dorsal markings are similar to those of a species of orb-weaving spider (Araneus diadematus). Unlike other Headcrabs, it has only four fangs, which are uniquely curved back and hook-like. These are most likely what they use to inject their neurotoxin. It's also interesting to note how large their mouths are. They're much wider and have a pink, fleshy, rim surrounding it, unlike other Headcrab species.
Gordon's first encounter with this beast is in an abandoned house in Ravenholm, just before he flips a switch to deactivate an electrical fence in the cobblestone street below. They are rare, usually appearing only where a Headcrab Shell was previously seen, furthering the idea that they are genetically modified creatures.
 Behavior and skills
At walking pace, the Poison Headcrab is the slowest Headcrab, but panics and runs away if attacked. Poison Headcrabs move slowly and cautiously when maneuvering but leaps with incredible speed while releasing an angry squeal when a suitable host is in a clear line of sight. As an ambush tactic, it leaps at and poisons a viable host in an attempt to create a Poison Zombie. Poison Headcrabs make a sound similar to Rattlesnakes when searching for prey. Also, as another lure, the Poison Headcrab will make a whistling noise.
The Poison Headcrab delivers its extremely powerful neurotoxin via the four fangs on its "beak". In gameplay, the neurotoxic Headcrab venom reduces the player's health to 1% immediately regardless of the current health amount. After a period of time, the HEV Suit will provide an antidote to restore the lost health, with 10% of it being lost from the Headcrab's physical damage.
As of the 2010 update, it appears that Poison Headcrabs alone are indeed incapable of killing the player.
 Poison Zombie
Like all other Headcrabs, Poison Headcrabs can latch onto a human's head and take control of their nervous system, creating a mindless 'zombie' that submits to the will of the Headcrab. Poison Zombies have a number of key differences from Standard Zombies, including increased health and the ability to carry 3 other Poison Headcrabs on their back.
Poison Headcrabs can be difficult to deal with, as their bite can leave you vulnerable to other enemies. Therefore, it's best to focus on the Headcrabs first, before attacking other foes. If there are Barnacles around, you can lure Poison Headcrabs into their tongues to get a quick kill and conserve ammo. However, this should not be considered a tactic in itself as it can be risky. It's better to attack these vicious creatures directly.
Poison headcrabs can be stunned and then killed with 3 blasts from the Gravity Gun. If you are not confident enough to use this method, you must keep away from the Headcrab while firing into its main body. If you are bitten, run away from the crab(s) and let your health recover.
If you are careful and in a situation which involves small rooms and large amounts of enemies (such as in Ravenholm), you can use your Crowbar to quickly dispatch of headcrabs, and any weapon of choice to kill any zombies in the area.
- According to Half-Life 2: Raising The Bar, the development team noticed that upon hearing the Poison Headcrab's sound playtesters would frantically start looking around for the Poison Headcrab so they could destroy it, regardless of any other present dangers.
- There is an unused animation called "spitattack". This suggests the Poison Headcrab used to have a ranged attack at some point.
- The Poison Headcrab's second scream, "ph_scream2.wav", appears to be a sped-up version of the Fast Zombie's first scream ("fz_scream1.wav", the "Howie Scream").
- If severely hurt, the Poison Headcrab will attempt to hide or run away from the player, in hopes that it will not be killed. It is also interesting to note that on rare occasions, the Poison Headcrab will ignore the player and try to seek refuge in a corner or secluded area.
- Poison Headcrabs are extremely dangerous to NPCs, as they lack Gordon's HEV Suit and thus have no access to the antidote for the neurotoxin. As a result, a hit from a Poison Headcrab will permanently reduce an NPC's health to 1, causing them to be instantly killed the next time they receive damage. However, Alyx Vance and Barney Calhoun regenerate health while Father Grigori is immune to Poison attack, and thus the Poison Headcrab is less of a threat to them, provided they are not under attack by other enemies around the time of the bite. The Rebels, however, can be healed with Medkits to restore their health.
- In Half-Life 2, the Poison Headcrab has a minimum distance for an attack. If the player is touching it, then it will not attack but instead crawl away to reach the minimum distance.
- If one were to guide a Poison Headcrab into a Barnacle's tongue it will eat it, however after eating the headcrab, it will die probably due to its poison. This may sometimes not happen in Half-Life 2 until the 2010 update.
- The poison headcrab appears to be the most intelligent headcrab. This is because it will run away if attacked and attempt to hide if close to death. It is also one of the only headcrabs that actually "hunt" their prey. Other headcrabs will wait for their prey.
- Poison Headcrabs are the strongest headcrab and in some cases the strongest enemy in the game. It is the only enemy able to drop the players health down to critical levels in one shot, and accompanied by other enemies such as Fast Headcrabs, can instantly kill the player. This being the player getting bitten by the Poison Headcrab and simultaneously getting attacked by the Fast Headcrab, this will also work with any other enemy. However, fortunately, they are the rarest headcrab and give plenty of time for the player to move away before attacking.
- There is only one sighting of Poison Headcrabs emerging from Headcrab Shells. Although it is implied in areas such as an abandoned building along the coast road, the first and only instance of a Poison Headcrab emerging from a pod is inside a container during the strider battle in the end of Episode One.
- The Poison Headcrab appears to be less aggressive in Episode Two, which means when shot, he will almost always run. In Half-Life 2 and in Episode One, the Poison Headcrab will only run when attacked unaware.
Poison Headcrabs in the City 17 Underground.
 List of appearances
- Half-Life 2 (First appearance)
- Half-Life 2: Raising the Bar
- Half-Life 2: Lost Coast (Non-canonical appearance)
- Half-Life 2: Episode One
- Half-Life 2: Episode Two