These are silkworm (Bombyx mori) cocoons. While I always knew that silk was made from silkworms, and that the cocoons were boiled and unraveled to make a single strand of silk, I did not know that the cocoon could be unraveled to 1 Km (~ 1,100 yards).
Silkworms eat the leaves of mulberry trees and are native to northern china. They are considered domesticated, and have been raised by humans for over 5,000 years. They are considered unable to survive in the wild at this point and depend on human intervention to complete their breeding. (Supposedly, because they have lost the ability to fly) The silkworm pupa is also considered a delicacy and is eaten after the cocoon itself has been boiled to soften the silk strands.