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The subject matter of this article contains in-development information that was cut from the final version of an official and/or canonical source and appears in no other canonical source. It may also contain incomplete information since not all cut material is publicly known.
The Sacktick was to be a macro virus, about the size of a dog, created by the Combine, and resembling real-world antlions. It was to be delivered by missile-like pods in numbers, generally to soften an enemy position before an assault. Its mouthparts were to be able to splay open, as part of a threat display. In addition, it was to make a clicking buzzing noise while bobbing its body up and down against the surface it would be attached to. The sack (the black growths on its both sides) was to be kept curled under the tick's body. Once fed, the sack was to unfurl behind the legs and be dragged behind the tick. Its only known depiction shows the creature bearing markings similar to that of the Gunship, said to denote delivery vehicle and task group.
The Sacktick was to seek out organic hosts, on which it would feed and use as food for the next generation of ticks – multiple organisms of this species would leap onto a person and attach to them, draining them of blood until they died. In this way, a small number of ticks dropped into an enemy stronghold were to multiply and create a great number of individuals.
As heard in the playable Half-Life 2 leak sound files, Sackticks were originally to be launched at the Borealis. Sackticks were to be unable to resist low temperatures, and Odell was to explain to Gordon that he was able to defeat them by shutting off the Borealis' generator, and let the ship ice over.
Behind the scenes
As speculated by a Valve employee, the Sacktick was cut during the development of Half-Life 2 probably because of the change of the game's design, and the fact that the creature might have turned out not to be "fun enough to preserve". It is known that a model for it existed, and was used as a placeholder for crows during their development.