Silkworms ,sericulture,life cycle,fertilizaion
Sericulture is an applied branch of biology. It deals with the systematic study of silkworm rearing for the production of commercial silk.
Types of silk
Silk is produced by the caterpillar of insect called silk worm. There are different species of silkworms. However only a few of these are used for obtaining silk. Among them commonly used species is Bombyx mori. This larvae mainly feeds on the mulberry leaves (Morus alba).
The silk produced by this species is known as Mulberry silk.
The silk produced by other species is known as
Non mulberry silk
Types silk Name of worm distribution Plants
1. Mulberry silk Bombyx mori All over India Morus alba
2. Muga silk Antheraea asama Assam Michilus bombycina
3. Tasar silk Antheraea mylitta Bihar, orissa & M.P Terminalia tomentosa
4. Eri silk Philosamia ricins North India Ricinus communis
Life cycle of the mulberry silk moth
The mulberry silkworm is Bombyx mori. It belongs to Family BOMBYCIDAE and Order Lepidoptera, Class insecta. Its natural food is leaves of mulberry plant Morus alba.
The life history of Bombyx mori shows four stages namely EGG, LARVA, PUPA and ADULT.
1.The Adult silk moth :It is a creamy white moth and its body is fat. Two pairs of wings are present. These wings are weak. It does not fly and lives only for 4 to 5 days. In this duration the male and female mate. Both sexes bear feather like antennal on their heads and a pair of compound eyes.
Differences between adult male and female silk moth
1. Smaller in size Larger in size
2. Abdomen narrow and long Abdomen short and broad
3. Antennae larger Antennae are smaller
4. Dark in colour Comparatively lighter
5. Life span very short and dies Lives longer in order to lay eggs
soon after copulation
6. More active Less active
Soon after emergence, a male silkmoth copulates with a female silkmoth for nearly 3 hours and dies after copulation. Fertilization is internal. Female moth dies after laying the eggs.
12 hours after mating, the female lays around 400 to 500 eggs. The eggs are laid in clusters on the dorsal surface of mulberry leaves. They are covered by a gelatinous secretion of the female.
The eggs are centrolecithal. There is an opening called micropile at one end of the egg. The plasma membrane of the egg is surrounded by a primary membrane called viteline membrane. It is surrounded by secondary membrane called chorion.
After about 10 days of incubation the eggs hatch into tiny.
Larvae called caterpillars.
The young ones that hatch out of the eggs are called larvae. These larvae are also called the silk worms. These are voracious eaters and feed on mulberry leaves. They undergo ecdysis or moulting four times in as month. The larval stage between two multings is called instar. Thus, there are four moultings and five instars. After the 4th moult, that is the 5th instar larva stops feeding and enters into cocoon formation.
Structure of mature caterpillar
1. The 5th instar larva is a mature called caterpillar or cruciform larva.
2. Body is divided into head, thorax and abdomen.
3. head bears mandibulate mount, parts, spinneret and tense organs.
4. Mouth parts are adapted for cutting and chewing the mulberry leaves.
5. Mouth parts consists of a pair lips, a pair of mandibles and a pair of maxillae.
6. Below the lower lip in the hypopharynx there is a small tube like structure called
Spinneret through this the liquid silk is extruded during cocoon formation.
7. There are 3 pairs of simple eyes or ocellic.
8. A pair of antennae are present.
9. The thorax is made up of 3 segments. Each segment has a
pair of short, conical , joined
appendages called truelegs. Each leg consists of 3 segments and the last segment has a sharp claw.
10. The first thoracic segment bears a pair of spiracles.
11. Abdomen is made up of 9 segments.
12. Each of the 3rd, 4th, 5th, 6th and 9th abdominal segments. Contains one pair abdominal
appendages called prolegs or pseudolegs. There are short, unjoined and used for walking.
13. The 9th segment is formed by the fusion of 9th 10th & 11th segments.
14. On the dorsal side of the 5th abdominal segment these is dorsal horn.
15. Each of first 8 abdominal segments has a pair of lateral spiracles.
1. The total larval period lasts for 40 to 45 days.
2. The 5th instar caterpillar feeds for about 6 to 7 days and then stops feeding to become a pupa.
3. The larva secrets a sticky fluid from the silk glands.
4. The secretion comes through a small spinneret in the hypotharynx and form a long thread
of silk. It hardens on exposure to air.
5. Each larva produces about 400 to 500 meters of silk thread. It is wrapped around the body of the caterpillar in the form of a covering called cocoon or pupal case.
6. Cocoon formation takes about 3 to 4 days.
7. The caterpillar inside the cocoon undergoes changes and become a pupa or chrysalis.
8. Silk thread is composed of two types of proteins. The inner core consists of a protein called Fibroin. The outer enveloping layer is made up of the protein sericin.
Pupa or chrysalis
1. Pupa is a non – motile and non – feeding stage occurs within the cocoon.
2. Body consists of head, thorax and abdomen.
3. Head is small bears a pair of compound eyes, a pair of antennae and non functional mouthparts.
4. Thorax is large and prominent. It bears a pair of wings & 3 pairs of legs.
5. Abdomen is made up of segments but only a segments are visible.
6. Pupal period lasts for about 10 to 12 days.
Due to active metamorphosis in pupa the pseudolegs disappear and thorax develops 2 pairs of wings and 3 pours of legs. All internal organs ans systems including the reproductive system develop. Thus the pupa develops into a baby moth called imago.
The fully grown imago secretes an alkaline fluid containing an enzyme called cocoonase. It softens and dissolves the silk thread at one end of the coccon to form a hole. Finally the adult comes out of this hole. Soon after emergence, male and female moths copulate. The male dies after copulation. The female starts laying egg 12 hours after copulation and then dies.
Economic importance of silk
1. Silk is used in the manufacture of silk fabrics. These fabrics are used for the manufacture of garments, parachutes, fishing lines.
2. Silk thread is used in insulation coils in telephone and wireless receivers.
3. Silk is used in the manufacture of knitted material. It
is used to make gloves, socks,
Uses of silk industry byproducts
1. Feces of silkworms is used along with cow dung for the production biogas.
2. Feces of silkworms is used as an organic manure.
3. Silkworm feces is used in the production of fish food.
4. Oil is extracted from the dead pupae. This oil is used manufacture of soaps.
5. Cocoons of silkmoth are used in handicrafts for the manufacture of fancy items like garlands.
6. The residue mulberry leaves are fed to milching cows. It increases the production of milk.
7. Mulberry stems are used for making farm tools in villages.
8. Mulberry trees are used as fire wood.
9. Stems of mature trees are used in making cricket bats.
10. Silk is used in the many facture of surgical sutures.
11. Pupae are used in the manufacture of vitamin B2.
Decreasing or increasing the number of generations per year is voltinism.
(1) Univoltines - Have only one generation in a year.
(2) Bivoltines - Have two generations in a year.
(3) Multivoltines - Have four to five generations in a year.
12. Wastesilk – Two types of waste are produced during reeling
(a) Cooker’s waste : It is outer mart part of the coccons.
(b) Reeling waste : It is the strands of silk separated during reeling.
NOTE : - Mulberry cultivation is called moriculture.
Diseases of silk worms
Disease pathogen symptoms
Pebrine desease Nosema bombycis Larva turn brown & dies
Flacharie Virus Inactive & diarrhoea
Murcardina Fungal Calcifecation & hardens
Maggot disease Tricholyga sp (fly) Maggots feeds on caterpillar