As a larva/caterpillar the Cecropia Moth, one of our giant silk moths, usually has five larval stages, or instars, as it matures. At the end of the fourth instar the caterpillar begins spinning a silk cocoon within which it will overwinter as a pupa. The gland which produces the silk is located below the caterpillar's mouth.
There are three layers to the cocoon: the outer layer (stiffened and waterproofed with a liquid secreted by the caterpillar), a middle insulating layer of very soft silk strands and the innermost layer which consists of a case that encloses the pupa.
After about ten days inside the cocoon it spins in August, the caterpillar sheds its skin one last time and emerges as a pupa. For the next ten months the pupa remains encased in the cocoon as it undergoes the transformation into an adult moth. The moth will emerge next June and live only about ten days, just long enough to reproduce. (FYI, the outer layer of the cocoon was constructed within 24 hours.)
(Thanks to Jim Canfield, discoverer of spinning Cecropia caterpillar; dissected cocoon was one in which the pupa never metamorphosed into an adult)
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