Silk is a special natural fiber. It is lustrous, smooth, drapes well and is incredibly strong. Silk takes dye easily and can be made in many colors. Silk is historically important and has been in use for thousands of years.
Most commercial silk is called mulberry silk. Also known as cultivated silk or bombyx silk, mulberry silk is white, 10-14 microns in diameter and round in a cross-section. It is produced from cocoons of the mulberry silkworm, a caterpillar that eats mulberry leaves.
Silk is a fibroin protein that is produced on demand from glands of several groups of invertebrate animals, including silk moths. It is extruded as a liquid through openings called spinnerets. Spiders use this silk to spin a web and silkworms spin a protective cocoon for their pupa.
The silk is produced as a thick, sticky liquid through the openings of two spinnerets under the mouth. It solidifies in the air to form twin filaments of silk that become glued together into a single thread. Up to 400 yards of this silk fiber may be produced daily. However, these fibers are extremely thin and 400 yards of fiber does not translate to a significant amount of usable fiber. To produce 1 pound of silk, 100 pounds of mulberry leaves must be eaten by 3000 silkworms.
Silk's absorbency makes it comfortable to wear in warm weather and while active. It keeps warm air close to the skin during cold weather. Due to its strength and density, silk is excellent for clothing intended to protect from biting insects that would ordinarily pierce regular clothing. Silk usage has declined significantly due to the increased use of synthetic fibers.