Publisher Summary This chapter discusses Bombyx mori or silkworm, which is used for sericulture and is one of the most economically important insects in the world. The species of silkworm usually raised by sericulturists is B. mori (Lepidoptera: Bombycidae). Along with Drosophila melanogaster, B. mori larvae have been used as a model for various biological studies for many years. B. mori larvae have a unique metabolic system for producing a large amount of cocoon protein and efficiently using dietary nitrogen. Both male and female silkworms digest and absorb about two thirds of the nitrogen in the mulberry leaves they consume, and high percentages of the digested and absorbed nitrogen (66% in females and 70% in males) are utilized in the production of cocoon protein. During the last larval stage (fifth instar), the silk gland produces the silk for the cocoon from a pair of curved glands found on the ventral side of the digestive tube. The weight of this organ accounts for about 25% of the weight of larvae in the late fifth instar. The silk gland can produce massive amounts of fibroin and sericin, the proteins constituting silk. Sericin surrounds a fibroin core. Therefore, when cocoon threads are reeled, most of the sericin is removed, and the remaining raw silk is composed of fibroin alone.