Synth

The Synth is a "self-replicating robot that evolve" with an unknown origin. The synths have been enslaved (like Vortigaunts and Humans) by the Combine. They make up almost all of the Combine's armor support units.

Biology
It is unknown how much of the Synth presented in the Half-Life games is original to the species or added by the Combine (see the Raising the Bar section for more information), however what is known is that the Synths are mixtures of organic material and machine. Their flesh (aside from the Shield Scanner and Hunter) is immune to gunfire and hitting it produces a metal ping noise. It can be torn open with force, though, as Dog tears a Strider's head open in Half-Life 2: Episode Two. Evidently, the only thing that can kill them are explosives from the RPG, the secondary fire of the SMG1, and Energy balls (again, aside from Shield Scanners and Hunters).

The only known organ that Synths possess is the brain, and their blood is a reddish-purple color.

Types
Various types of Synth have been brought over to Earth by the Combine, but other types of Synth are likely to exist. The total number of different Synth is unknown. It is likely that Synths were the primary forces used by the Combine during the Seven Hour War (this is likely, due to the newspaper clippings of the war that show a blurry synth Gunship), as the human-derived Combine units, as well as technology such as hunter-choppers and APCs, would have only started appearing after the Combine's rule on Earth had been established.

Types of Synths include:


 * Striders
 * Dropships
 * Gunships
 * Mortar Synths
 * Crab Synths
 * Combat Pods
 * Shield Scanners
 * Hunters

Raising the Bar Information
In Half-Life 2: Raising the Bar, there is discussion of the "Synths" as results of enslaved races, and also mentions of the Combine as moving through the universe and enslaving and assimilating various races into their military forces and empire. This has led to the suggestion of two theories for the nature of the Synth: they appear to either be biomechanoids (living machines) genetically engineered by the Combine from the DNA of assimilated races, or they are fully-existing creatures in their own right that have been enslaved by the Combine and forced to become cyborg soldiers, as the Combine are doing with humans.

Trivia
The design of the Combine Synths has a prominent shellfish design. The Valve offices have an aquarium including a nautilus, perhaps inadvertently being a source of inspiration for the Synth art design.