The major adaptive feature of endoparasites is the presence of piercing mouthparts. Endoparasites are parasites that live inside the body of their host, and to survive, they need to feed on the host's tissues or blood. The piercing mouthparts allow them to attach themselves to the host's tissues and feed on their fluids. This feature is an adaptation that helps endoparasites to obtain their necessary nutrients from their host while avoiding detection and expulsion by the host's immune system.