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Synths are self-replicating robots/biomechanical organisms that evolved, were created, or taken over by the Combine and enslaved through their conquests and wars. They are the backbone of the Combine's subsequent armies, and usually the most feared troops. They are constructed deep in the bowels of the Citadel.
The encountered Synths typically appear as average to large insect-like or arthropod-like creatures, usually with segmented legs or antennae-like sensory structures, recalling locusts, crabs, or similar beings.
It is unknown how many of the Synths presented in the series are 'true' to their initial species or forcefully evolved by the Combine, since the Combine modify their appearance by adding mechanical equipment and weaponry. It is also unclear whether they originally were fully organic or were already cyborg creatures when the Combine enslaved them, and if they all come from the same planet. The Strider, however, appears to have a brain, as seen in Episode Two (this is the only known Synth to possess any "organ" at all). It is also suggested that some were created from scratch by the Combine.
It is likely that Synths were the primary forces used by the Combine during the Seven Hour War as the human-derived Combine units, such as Overwatch Soldiers, Hunter-Choppers, and APCs, would have only started appearing after the Combine rule on Earth had been established.
Behind the scenes
According to Valve artist Dhabih Eng, when approaching the task of designing the Combine's larger creatures, it was necessary to keep in mind that these lumbering war machines were possibly once organic creatures that the Combine had enslaved and converted during previous invasions. Therefore, they tried to incorporate elements that were neither strictly organic nor purely mechanical. They didn't want to have giant alien creatures with armor attached or implants added, but instead tried to treat it as a forceful evolution of the characters imposed on them by the Combine rather than nature. In the end, the designs intentionally blurred the line between the machine and animal in their appearance and more importantly their motion and how they reacted to the world around them.
According to Valve artist Ted Backman, he was interested in the synthetic life-form research done in the Los Alamos National Laboratory where they have little insects, and instead of being hard-coded with instructions on how to deal with the environment, they learn, evolve, and try to serve their own ends, and in doing that, they get these emergent behaviors.
Backman thought it would be really cool if someone created a thing that, instead of worrying about having to repair, upgrade, or maintain itself, it made a new version of itself every five years. It could evolve on its own and turn into a synthetic life-form that is more useful and interesting to keep around in large numbers as an invasion force. They have been calling them Synths for a while.
His inspiration was from Bishop in Aliens. When the android character Bishop was torn apart at the end of the film, the viewer can see the entrails, but they were obviously corrugated pipes - all this fluid that came out that looked like blood or lymphatic fluid, but it was obviously some sort of oh hydraulic fluid. Backman took it from there and thought, "Well, what if these things were actually kind of growing themselves? Maybe the Combine doesn't even really know how to make them anymore, they don't even know where they came from. They can't even maintain; they just keep them as their own domesticated wildlife."
The Combine Advisor.
The Advisor pod.
The Combine Dropship.
The Combine Gunship.
The Crab Synth.
The Mortar Synth.
The Combine Guard (part-human, part-Synth).
The Combine Synth Elite Soldier (part-human, part-Synth).