Their carapace-like armor is largely immune to small arms fire (with the notable exceptions of the Hunter and Shield Scanner) and bullets striking it produce a metallic 'ping' noise. Synths are also never observed to be 'self-replicating' in-game, but seem to be raised and/or manufactured by the Combine. For instance, Stalkers can be seen working on Gunships in the lower levels. The City 17 Citadel features factories where Synths are manufactured. This is the only known Synth to possess any organ at all, however, other Synths likely possess one too. The Strider however, appears to have a brain, as seen in Episode Two. It is also unclear whether they all come from the same planet (mirroring the Xen creatures that were also brought from other worlds and, for some of them, also enslaved). 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. Synths always appear as average to large insect-like or arthropod-like creatures, usually with segmented legs or antennae-like sensory structures, resembling locusts, crabs or similar beings. Crab Synths and Mortar Synths being transported on a conveyor belt.
