Read this article to learn about the Lifecycle of Silkworm !
Species: B. mori
The Silkworm, Bombyx mori produces the silk of commercial importance. It is the caterpillar of a moth whose cocoon is used to make silk. This insect is also called the silkworm-moth and the mulberry silkworm (fig. 8.12). Male and female moths are flightless and lack functional mouth parts. The moths differ in morphological features. The female has a larger abdomen whereas the male has a much larger pair of antennae.
The life cycle of Bombyx mori demonstrates the most advanced form of metamorphosis. The serial progressions of four distinct stages of development complete one generation of the species; egg (ova), larvae, pupa and imago.
Egg is the first stage of a silkworm’s life cycle. The female moth lays an egg about the size of an ink dot during summer or the early fall. The egg remains in dormant stage until spring arrives. The warmth of the spring stimulates the egg to hatch. The egg of Bombyx mori is a very small and hard structure; about the size of a pin head and resembling a poppy seed (fig. 8.13). The egg shell provides a protective covering for embryonic development. When first laid, an egg light yellow. A fertile ovum darkens to a blue-gray within a few days.
The larva is the vegetative stage where growth takes place. The larva of Bombyx mori, commonly called a silkworm, is host specific to mulberry. During growth, the larva molts 4 times. The period between successive molts is called an instar. The silk worm, upon hatching, is about 1/8th of an inch and extremely hairy.
Young silkworms can only feed on tender mulberry leaves. However, during the growth phase they can eat tougher mulberry leaves as well. The larval stage lasts for about 27 days and the silkworm goes through five growth stages called instars, during this time. During the first molting, the silkworm sheds all its hair and gains a smooth skin.
As the silkworm prepares to pupate, it spins a protective cocoon. About the size and color of a cotton ball, the cocoon is constructed from one continuous strand of silk, perhaps 1.5 km long (nearly a mile). The silk cocoon serves as protection for the pupa. Cocoons are shades of white, cream and yellow depending on silkworm genetics. After a final molt inside the cocoon, the larva develops into the brown, chitin covered structure called the pupa. Metamorphic changes of the pupa result in an emerging moth (fig. 8.14).
If the silkworms are allowed to mature and break through the cocoon, the silk would be rendered useless for commercial purposes. So the encased insect is plunged into boiling water to kill the inhabitant and dissolve the glue holding the cocoon together. The end of the silk is then located and the cocoon unwound onto a spindle to be made into thread.
Cocoon is the stage in which the larva spins silk threads around it, to protect itself from its predators. The larva traps itself inside the cocoon in order to pupate. The color of the cocoon varies, depending upon what the silkworm eats. It can range from white to golden yellow. The second molting occurs inside the cocoon, when the larva turns into a brown pupa. It takes about 2-3 weeks for the pupa to metamorphose into an adult moth.
The adult stage completes the life cycle of Bombyx mori. It is the reproductive stage where adults mate and females lay eggs. Moths are flightless and lack functional mouth parts, so are unable to consume the food/nutrition.
Once the adult moth comes out of its cocoon, its only purpose is to find a member of the opposite sex, and mate. Males are larger than females and more active. They flap their wings rapidly to attract the females. Within 24 hours of mating, the male moth dies, while the female lays abundant eggs, after which it dies as well. There on, a new silkworm life cycle begins.
Commercial silk is made by boiling the intact cocoons and unwinding the single silk strand onto reels. Only a few moths are allowed to emerge to continue the population of silkworms. The rest are killed by being boiled in their cocoons. “Peace silk”, also known as “vegetarian silk” is raised and processed differently. The moths are allowed to emerge from their cocoons to live out their full life cycle. The silk is degummed and spun like other fiber, instead of being reeled. The resulting yam is soft, fluffy, and light like a cloud. This is the best silk for warmth and therapeutic use.