 If the animal is allowed to survive after spinning its cocoon and through the pupal phase of its lifecycle, it releases proteolytic enzymes to make a hole in the cocoon so it can emerge as an adult moth.These enzymes are destructive to the silk and can cause the silk fibers to break down from over a mile in length to segments of random length, which seriously reduces the value of the silk threads, but not silk cocoons used as "stuffing" available in China and elsewhere for doonas, jackets, etc.To prevent this, silkworm cocoons are boiled.