Sericulture: Silk Production and Silkworm Rearing

• Silk is a fibrous protein of animal origin
• In Latin, 'sericum' refers to silk
• Silk producing organisms are commonly referred to as sericigenous animals
• Silk has various purposes, including entangling prey by spiders, anchorage by mussels, and forming protective sheaths like Lepidopteran cocoons
• Silk can be classified into Insect silk and Non-insect silk based on origin

Sericulture

• Sericulture is the practice of rearing silkworms for silk production, also known as silk farming
• Sericulture components include cultivation of food plants for silkworms, rearing silkworms, cocoon harvest, and reeling and spinning silk

Types of Sericulture

• Mulberry sericulture exists
• Non-mulberry sericulture also exists
• Tasar, eri, and muga are the major non-mulberry silks
• Forest/wild sericulture is also known as Vanya silk

Silkworm Species

• Bombyx mori family Bombycidae food plant is Morus alba (Mulberry)
• Samia ricini family Saturniidae food plant is Ricinus communis (Castor)
• Antheraea assamensis family Saturniidae, food plants are Litsea polyantha (Soalu), Machilus bombycina (Som)
• Antheraea mylitta family Saturniidae, food plants are Shorea robusta (Sal), Terminalia sp., Zizyphus jujuba

Mulberry Silkworm (Bombyx mori)

• Entirely domesticated and cannot be found in the wild
• Originates from Northern China
• Undergoes complete metamorphosis through four morphological stages: egg, larva, pupa, and adult
• Numerous races exist, categorized by voltinism, moultinism, and geographical distribution

Bombyx mori Systematic Position

• Class: Insecta
• Order: Lepidoptera
• Superfamily: Bombycoidea
• Family: Bombycidae
• Genus: Bombyx
• Species: Bombyx mori

Eggs

• Silkworm eggs are tiny, small, and elliptical, with one flat side
• The flat side is called a dimple
• Newly laid eggs range from pale to dark yellow
• Eggs darken over time
• Egg period lasts approximately 10 days

Larva

• Newly hatched larvae are black or dark brown, measuring about 3mm long
• Commonly referred to as ants or kego
• Larvae possess a large head and a body densely covered with bristles
• As the larva grows, it becomes smoother and lighter due to the stretching of the cuticular skin
• The last instar larva reaches about 10 cm long

Larval Growth and Markings

• Larval growth is marked by four moultings and five instars
• Larval markings include a large 'crescent' marking (lunule) on the second abdominal segment
• Also a pair of small 'star' markings on the fifth segment
• There is a projection called the 'caudal horn' on the dorsum of the eighth segment

Sexual Dimorphism

• Abdominal segments have develop sexual markings distinctly in the fourth and fifth instars
• Herold's gland is a single median opening on the ventral side between the 8th & 9th abdominal segments in males
• Ishiwatas gland has 2 pairs on the 8th & 9th abdominal segments in females

Silk Glands

• Full-grown caterpillars develop a pair of sericteries or silk glands
• Silk glands are modified labial glands ventral to the alimentary canal
• The glands account for about 50% of the larva's weight
• These glands are divided into anterior, middle, and posterior segments

Gland Makeup

• Lyonet's gland, or filippi's gland, joins the ducts of the silk glands and lubricates the passage through which silk passes
• Silk is made up of an inner layer of fibroin (75%) and an outer layer of sericin (25%)

Pupa

• Fully grown larvae spin a cocoon around themselves and transform into a pupa
• Cocoon spinning takes about 2 days, and transformation into a pupa takes another 2-3 days
• During this time, the larval body and internal organs undergo metamorphosis to become an adult moth
• The pupal stage lasts for 10-13 days

Pupal Stage Sexual Dimorphism

• Sexual markings are prominent and easy to identify
• Female pupae have a broader abdomen, while male pupae have a narrower abdomen
• Female pupa has a fine transverse line (X mark) on the ventral side of the eighth abdominal segment
• Male pupa has a small round spot on the ventral side of the ninth segment

Adult

• Adult moths emerging from the pupa are incapable of flight
• The adult body surface is covered with scales
• Antennae are conspicuous, large, and bi-pectinate
• Adult moths have vestigial mouthparts and do not feed during their short life

Adult Characteristics

• Female activity level is less active, male activity level is more active
• Female antennae are small, male are large
• Female body sizes are large, male are small
• Female abdomens are large and flat with seven visible segments, male are long and narrow with eight visible segments
• Female external genitalia have a median knob-like projection with secondary hairs, protruding and retracting to expel pheromones, males have secondary hooks known as harpes to helps with copulation

Adult Life Cycle

• Females begin laying eggs just after copulation, which is completed within 24 hours
• Females lay 400-500 eggs
• Eggs are laid in clusters and are covered with a gelatinous secretion by the female moth
• Adult moths typically live for 4-5 days

Eri Silkworm (Samia ricini)

• Family: Saturniidae
• It is domesticated
• It is commonly reared in Assam and West Bengal
• Suitable for high rainfall regions
• The host plant is castor
• Adult moths have wings that are dark brownish and white
• Caterpillars have body is white to greenish white, with brown heads, and tubercles all over
• The cocoon is loosely spun and is white or brick red, not as glossy as mulberry silk worm

Muga Silkworm (Antheraea assamensis)

• Family: Saturniidae
• Restricted to North eastern states, specifically Assam
• It is semi-domesticated and multivoltine
• Hosts include Som (Machilus bombycina) and Soalu (Litsea polyantha)
• Adult males are reddish pink, females are lighter
• Caterpillars are yellow on hatching and become translucent green
• Cocoons are built on the tree trunk and are easily collected
• Silk is golden yellow
• India has a monopoly on muga silk production

Tropical Tasar Silkworm (Antheraea mylitta)

• Family: Saturniidae
• It is considered wild and not domesticated
• Hosts include Shorea robusta, Zizyphus mauritiana, and Terminalia spp.
• Caterpillars are stout green with red spiracles
• Cocoons are oval, about the size of a hen's egg, and hang from terminal branches by a 5 cm long pedunculate attachment
• Cocoons can be collected from the jungle
• Cocoons range from light to deep brown in colour