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Life cycle of Silkworm

The farmers generally buy silkworm eggs (DFLs) from grainages, hatch and rear in their dwelling houses or in the silkworm rearing houses constructed separately.

Egg is the first stage of a silkworm’s life cycle. After mating, each female moth lays around 300 to 400 eggs. Silkworm eggs are flat, ellipsoid or oval in shape and have a tiny pore known as micropyle at the anterior end. They are hard and measure 1.3 – 1.4 mm in length, 0.87- 1.27 mm in widthand around 0.6 mm in thickness. Each egg weighs around 0.5-0.6 mg. Colourvaries in different breeds. Newly laid eggs are pale to dark yellow and the diapausing eggs turn to purplish gray often with green or pink tinge in 24 hours. The embryo develops inside the egg and transforms into larva which normally hatches in about 10 days from the day of laying.

a) Egg

After mating, each female moth lays around 400 eggs. Silkworm eggs are flat,ellipsoid or oval in shape and have a tiny pore known as micropyle at the anteriorend. They are hard and measure 1.3-1.4 mm in length, 0.87- 1.27 mm in widthand around 0.6 mm in thickness. Each egg weighs around 0.5-0.6 mg. Colourvaries in different breeds. Newly laid eggs are pale to dark yellow and the diapausing eggs turn to purplish gray often with green or pink tinge in 24 hours.

The embryo develops inside the egg and transforms into larva which normallyhatches in about 10 days from the day of laying. However, the eggs of temperatebreeds enter into a resting stage called hibernation (winter sleep) or diapause toescape the severe winter during which the mulberry plant also sheds its leaves andenters into a stage of resting known as dormancy. The diapause can be terminatedartificially.

b) Larva

Newly hatched larvae are brown or reddish brown in colour and look like antsto the naked eye. They measure about 3 mm long. They feed on the mulberryleaves and as they grow, the colour gradually turns into greenish white. The larvasheds its skin (integument) four times and this process is called ecdysis or moulting.

During this process, the larvae stops feeding and rest. The period between hatchingand moulting or between two moults or moulting and spinning stage is called aninstar. Thus, the larva passes through five instars in about 25 days and grows by10,000 times in size. The fully grown larva is cylindrical with a convex dorsal sideand a slightly flat ventral side. The body is divided into three regions, head, Thorax comprising of three segments and abdomenconsisting of 9 segmentsThe males are identified by the presence of milky white pear shaped Herold’s glandson the mid-posterior part of the eighth segment on the ventral side. The females are identified by the presence of two pairs of milky dottedstructures, one pair situated in the lateral side of the eighth segment of the ventralside and the other on the ninth segment. They are called Ishiwata’s glands.

c) Pupa

The fully grown larva spins a cocoon around it and transforms into a pupa, aresting stage. It takes about two days to spin the cocoon and another 2-3 daysfor transformation into pupa. The female pupa can be identified by its broaderabdomen, stout tip and an X- shaped mark on the ventral side of the eighthsegment. The male has a narrow abdomen, sharp tip and a dark spot on theventral side of the ninth segment. Though the pupa looks inactive, rapid internal modifications occur resulting in the development of various organs and thusmetamorphose into adult and the moth emerges from the cocoon.

d) Adult (Moth)

The silk moth emerges in 10-12 days from the day of spinning the cocoon. Themoth secretes an alkaline fluid from its mouth which softens the cocoon shellthrough which it pierces and emerges from the cocoon. The moths look likebutterflies, but the wings spread slightly downwards unlike the butterflies. Themoths also do not have any mouthparts and their only purpose is to mate and layeggs to continue their generation. They mate immediately and female lays around400 eggs in about a day. The moths die in about 4-5 days. The eggs hatch andcontinue the generation.