The Headcrab Canisters are a direct example of the Combine's attempt to
A closed Headcrab Canister, just fired into Resistance territory
harness the Xen wildlife released on Earth to control the opposing forces on Earth. They appear in the form of big rocket shells, which are fired by the Combine to get to grips with areas dominated by The Resistance. The canisters contain around 4 to 6 headcrabs each and open on impact, releasing the headcrabs.
An empty Headcrab Canister
The direct effects of the canisters can be witnessed in the town of Ravenholm, a former safe haven for The Resistance, which is now infested by a wide variety of Headcrab Zombies as a result of the Headcrab Canisters.
The Canisters mostly contain either the Classic or Fast Headcrab variety, although it is to be assumed there are also canisters containing Poison Headcrabs judging from the Poison Zombies encountered in Ravenholm.
The canisters make their first appearance at the end of the Route Kanal chapter, and on his way through Ravenholm and City 17, Gordon Freeman will be reacquainted with them first hand. Also, when the Black Mesa East compound is discovered by the Combine, Headcrab Canisters can be seen flying over and into Dog's courtyard.
Trivia
The Headcrab Canisters show interesting similarities to medieval biological warfare. During the Middle Ages, an army attacking a castle could resort to catapulting Plague-infested corpses over the castle walls to infect the castle's inhabitants, to eventually force a surrender.
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Headcrab Canisters
