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.
Also termed tonus; the normal state of balanced tension in the tissues of the body, especially the muscles. Mentioned in: Contractures. syn tonus (1) in skeletal muscle, a state of tension that is maintained continuously-minimally even when relaxed-and which increases in resistance to passive stretch.