The Husks are a parasitic life form that enable the Skins to survive on earth. They are genetically contrived to resemble humans and are identifiable by the flap of skin in their backs that conceals the suit’s sealant. While Husks can deflect most threats and even survive being shot at, when the seal on a Husk’s back is hit or damaged the Husk falls to dust.
They mature over 20 years and once grown can survive for up to 50 years on earth. As a Husk nears the end of its lifespan it begins to shed layers of skin, a process that can be slowed by ensuring the Husk is kept moisturised.
The relatively short existence of the Husks on earth means that they must be cultivated and harvested by Skins with some frequency. We see this process beginning in series 2 episode The Harvest, and learn in the process that when a Husk is pre-emptively destroyed its corresponding Skin can sense this happening.