A few butterflies and many moth species spin a silk case or cocoon prior to pupating, while others do not, instead going underground. A butterfly pupa, called a chrysalis, has a hard skin, usually with no cocoon.
4. tension - (physics) a stress that produces an elongation of an elastic physical body; the direction of maximum tension moves asymptotically toward the direction of the shear. natural philosophy, physics-the science of matter and energy and their interactions; his favorite subject was physics.
Each end of a string or rod under tension will pull on the object it is attached to, to restore the string/rod to its relaxed length. Tension is the opposite of compression. In physics, although tension is not a force, it does have the units of force and can be measured in newtons (or sometimes pounds-force).