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Life Cycle of Mulberry Silkworm

  • Zoology

In this life cycle of mulberry silkworm article, we will discuss the biology of bombyx Mori and the life cycle of mulberry silkworm (mulberry silkworm).

Biology

Bombyx mori, the mulberry silk moth, is a member of the Bombycidae family. This moth is endemic to China, but it has been successfully domesticated and bred in India, Japan, Korea, Italy, France, and Russia. The mulberry moth is responsible for 92% of India’s silk production.

Mulberry worm refers to the worm or larva of this moth that only feeds on the leaves of the mulberry plant. B.mandarina, its wild ancestor, can still be found in isolated parts of the Himalayan foothills.

Systematic Position

Phylum: Arthropoda

Sub-Phylum: Mandibulata

Class: Insecta

Sub-class: Pterygota

Division: Endopterygota

Order: Lepidoptera

Sub-order: Macrolepidoptera/Heterocera

Super family: Bombycoideae

Family: Bombicidae

Genus: Bombyx

Species: mori

Life Cycle of Mulberry Silkworm

The Mulberry silkworm ia s completely domesticated insect. The silk moth is dioecious, i.e., the sexes are separate. Fertilization is internal, preceded by copulation. The life cycle of mulberry silkworm includes a complicated complete metamorphosis, consisting of four stages viz egg, larva, pupa, and adult.

Eggs

After fertilisation, each female moth lays about 300 to 400 eggs an initial stage of life cycle of mulberry silkworm. These eggs are placed in clusters on the leaves of mulberry tree. The female covers the eggs by a gelatinous secretion which glues them to the surface of the leaves. The eggs are small, oval and usually slightly yellowish in colour. The egg contains a good amount of yolk and is covered by a smooth hard chitinous shell.

After laying the eggs the female moth does not take any food and dies within 4-5 days. In the univoltine (a single brood per year) they may take months because overwintering takes place in this stage but the multivoltine broods come out after 10-12 days. From the egg hatches out a larva called the caterpillar.

Bombyx Mori Biology and Life Cycle (Mulberry Silkworm)

Life Cycle of Mulberry Silkworm (Mulberry Silkworm)

Larva

The larva of silkworm moth is called caterpillar larva in life cycle of mulberry silkworm. The newly hatched larva is about 4 to 6 mm in length. It has a rough, wrinkled, hairless and yellowish white or greyish worm-like body. The full-grown larva is about 6.00 to 8.00 cm in length. The body of larva is distinguishable into a prominent head, distinctly segmented thorax and an elongated abdomen. The head bears mandibulate mouth and three pairs of ocelli.

A distinct hook-like structure, the spinneret, is present for the extrusion of silk from the inner silk-gland. The thorax forms a hump and consists of three segments in life cycle of mulberry silkworm late stages. Each of the three thoracic segments bears pair of jointed true legs. The tip of each leg has a recurved hook for locomotion and ingestion of leaves.

The abdomen consists of ten segments of which first nine are clearly marked, while the tenth one is indistinct. The third, fourth, fifth, sixth and ninth abdominal segments bear ventrally a pair of un-jointed stumpy appendages each.

These are called pro-legs or pseudo-legs. Each leg is retractile and more or less cylindrical. The eighth segment carries a short dorsal anal horn. A series of respiratory spiracles or ostia are present on either lateral side of the abdomen.

The larva is a voracious eater and strongly gregarious. In the beginning chopped young mulberry leaves are given as food but with the advancement of age entire and matured leaves are provided as food. The caterpillar moves in a characteristic looping manner. The larval life lasts for 2-3 weeks. During this period the larva moults four times. The stage of the larva between the two successive moulting is called instar. Thus, normally five instars occur.

After each moult, the larva grows rapidly. A full-grown larva is about 8 cm long and becomes transparent and golden light brown in appearance. A pair of long sac-like silk-glands now develops into the lateral side of the body. These are modified salivary glands.

The full-grown larva now stops feeding and now begins to secrete the clear and sticky fluid of its salivary glands through a narrow pore called the spinneret situated on the hypo pharynx. The sticky substance turns into a fine, long and solid thread or filament of silk into the air.

Larva

The thread becomes wrapped around the body of the caterpillar larva forming a complete covering or pupal case called the cocoon. The cocoon formation takes about 3-4 days. The cocoon serves a comfortable house for the protection of the caterpillar larva for further development. The cocoon is a white or yellow, thick, oval capsule which is slightly narrow in the middle.

It is formed of a single long continuous thread. The outer threads, which are initial filaments of the cocoon, are irregular but the inner ones forming later the actual bed of the pupa, is one long continuous thread about 300 metres in length, wound round in concentric rings by constant motion of the head from one side to the other about 65 times per minute.

The irregular surface threads are secreted first and the inner continuous thread later. The silk thread is secreted at the rate of 150 mm per minute. Within a fortnight the caterpillar larva transforms into a conical brownish creature called the pupa or the chrysalis.

The pupa lies dormant, but undergoes very important active changes which are referred to as metamorphosis. The larval organs such as abdominal pro-legs, anal horn and mouth parts are lost. The adult organs such as antennae, wings and copulatory apparatus develop. The pupa finally metamorphoses into the imago or adult in about 2-3 weeks’ time.

Adult

The adult moth emerges out through an opening at the end of the cocoon in about 2 to 3 weeks’ time, if allowed to live. Immediately before emergence, the pupa secretes an alkaline fluid, that softens one end of the cocoon and after breaking its silk strands, a feeble crumpled adult squeezes its way out. Soon after emergence, the adult silk moths’ mate, lay eggs and die.

Summary

The life cycle of mulberry silkworm completes in 45-55 days, consists of stages egg, larva, pupa and moth. Egg stage is lasting for 9-10 days, larval stage which is 24-28 days, pupal stage 8-10 days and moth stages 3-4 days.

Further Readings