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Everybody knows the cocoon has two main parts-the shell and the pupa, but actually the cocoon consists of 5 main parts, they are cocoon clothing, cocoon shell, pupa lining, pupa and molting. The cocoon clothing is the outside layer delicate and messy silk filament, it can't be reeled to silk, but it can be used for spun silk, about 2% of cocoon weight; The cocoon shell which can be reeled to silk filament, about 50% cocoon weight; The pupa lining, which about 2.5% cocoon weight, is same to cocoon clothing, also can't be reeled to silk filament; The pupa is about 45% cocoon weight; The molt almost weight nothing.

Now you know only cocoon shell which about 50% cocoon weight can be reeled to silk filament, but is all silk shell can be reeled to silk filament? the answer is no, actually the silk shell consists of silk filament and other material, Other material include fats, salts, and wax.

Generally, one cocoon produces between 1,000 and 2,000 feet of silk filament, made essentially of two material. The fiber, called fibroin, makes up between 75 and 90%, and sericin, the gum secreted by the caterpillar to glue the fiber into a cocoon, comprises about 10-25% of silk. The finished silk fabric need to remove some percent of silk sericin, so the silk fabric becomes soft and shining.

The leftover silk may include cocoon clothing, pupa lining, the brushed ends or broken cocoons. This shorter staple silk may be used for spinning silk. The quality of spun silk is slightly inferior to reeled silk in that it is a bit weaker and it tends to become fuzzy. The waste material from the spun silk can also be used for making "waste silk" or "silk noil."