Caterpillars feed on mulberry leaves. After reaching the sufficient size the caterpillar spins a cocoon and spends its pupal stage inside. Cocoons need to be unravelled to get silk. After growing, the caterpillar which must have turned into a butterfly by undergoing metamorphosis must eat its way out the cocoon. So there is very little possibility of getting silk from such a cocoon. To get silk, the cocoons are put in boiling water during the process of which pupae die. Approximately 15 silkworms are killed to make a gram of silk thread, and 10,000 are killed to make a silksari.
Yes it is necessary to kill the pupa by boiling in water or drying it in the sun at the right stage as if the pupa break the coccon by itself and come out we cannot spin the silk into fibers as it is discontinous.
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