Silkworm larvae feeding on mulberry leaves (click to enlarge)
Silkworm pupae in cocoons on a frame. The cocoon is spun from silk threads. (click to enlarge)
"After they have molted four times (i.e., in the fifth instar), their bodies turn slightly yellow and their skin becomes tighter. The larvae enclose themselves in a Cocoon of raw silk produced in the salivary glands that provides protection during the vulnerable, almost motionless pupal state. ...
The cocoon is made of a thread of raw silk from 300 to about 900 meters (1,000 to 3,000 feet) long. The fibers are very fine and lustrous, about 10 micrometers (1/2,500th of an inch) in diameter. About 2,000 to 3,000 cocoons are required to make a pound of silk. Based on 1 kilometer (about 1,100 yards) per cocoon, ten unraveled cocoons could theoretically extend vertically to the height of Mount Everest. At least 70 million pounds of raw silk are produced each year, requiring nearly 10 billion pounds of mulberry leaves. According to E. L. Palmer one pound of silk represents about 1,000 miles of filament. The annual world production represents 70 billion miles of silk filament, a distance well over 300 round trips to the sun."