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.
verb (used with object) 1. to draw tight or taut, especially to the utmost tension; stretch to the full: to strain a rope. 2. to exert to the utmost: to strain one's ears to catch a sound. to impair, injure, or weaken (a muscle, tendon, etc.) by stretching or overexertion. to cause mechanical deformation in (a body or structure) as the result of stress. to stretch beyond the proper point or limit: to strain the meaning of a word. to make excessive demands upon: to strain one's luck; to strain one's resources. to pour (liquid containing solid matter) through a filter, sieve, or the like in order to hold back the denser solid constituents: to strain gravy.