Silkworm forms a quality of fibre called silk. From the business point of view, mainly 2 kinds of silkworms are found in Nepal. They are:
1. Eri silkworm (Attacus recinii) feeds on castor leaves
2. Siri silkworm (Bombyx mori) feeds on mulberry leaves
Super family: Bombycoidea
History of Silk Production:
The silkworm has long ago been bred for its silk. According to legend, silk was discovered in China, at around 2700 B.C. The emperor’s mulberry trees were being damagedby silkworms. His wife was picking a cocoon off the tree, but dropped it into hot water by mistake. Afterward she discovered that the cocoon could be unwound to produce a lustrous fabric. A fabric we now know as silk. At the time the secret of silk was closely guarded and reserved only to China’s emperors and other important people. As time passed the secret to the creation of silk spread to India and Europe. Now the whole world knows about silk. This fiber has remained in high production and remains on top of many new fibers.
External Structure of Silkworm:
The silkworm is creamy white in colour, 2-3 cm long and shining in nature. Regions of the body are divided into three principal parts, namely the head the thorax and the abdomen. The head has two sets of six oceli which are situated just behind and downward on the left and right face. Antennae are situated a little under the ocelli, the antenna is composed of three short segments. The mouth parts are located downward and in front of the face and are composed of a pair of mandibles and maxillae with labrum and labium. The mandibles are used for mastication and consist of two hard pieces. The maxilla consists of a single maxillary lobe and the palpi, which is made up of three segments. The labrum hangs down from the frontal portion to from the flap of the mouth.
The labium is situated on the ventral region of the had and has a pair of labial palps which form a sensory organ. A spinneret is situated at the proximal position between the two labial palpi.
The thorax bears three pairs of legs and two pairs of wings. Thorax has three parts; namely prothorax, meso-thorax, and meta-thorax. The three pairs of legs develops each from prothorax, meta-thorax, and meso-thorax and the two pairs of wings develops each from meta-thorax, and meso-thorax. The wings at 25 mm in length.
The abdomen which is hairy consists of 10 segments. The body of a female silk moth is wider than the male silk moth due to a large number of it contains. The entire body is covered with minute scales. Adults can’t fly and they do not feed.
Life cycle of silkworm:
Silkworms are unisexual. The adult moths seldom meet and are primarly concerned with reproduction. The male and female silk moths mate in the tail to tail position and fertilization is internal. After mating the male dies. The female is a oviparous and lays eggs. The female survives for about 4 to 5 days only after mating. The life cycle of a silkworm completes in about 45 to 50 days.
Immediately after mating, the female lays about 300 to 400 eggs in cluster upon mulberry leaves which completes in about 1-24 hours. They stick to the leaves with gelatinous secretion. Eggs are whitish, small pin-head sized and later they change into grey as development proceeds. The eggs weight about 2000 eggs to a gram. They measure 1 to 1.3 mm in lengthy and 0.9 to 1.2 mm in width.
When mulberry leaves are not available, the eggs are stored in cold places between 2-4°C to avoid where hatching. When the eggs are placed at a temperature of about 18-25°C, the larva imerges out of the eggs within 10 to 12 days.
The larva just after hatching has many setae on the body surface and the body color is black in normal strains. The young larva is about 3 mm in length. At about two days after hatching, the setae on the body surface become less conspicuous. After one day, the body length reaches about 7 mm and the surface of the skin becomes glossy. The larva stops eating for 24 h (moulting). During this time, the larva produces a new cuticle and sheds the old one. This phenomenon of shedding the old skin is called ecolysis. Since molting is repeated 4 times during the larval period, there are 5 feeding periods or instars. The duration of molting is rather constant among different strains, while the length of each feeding period varies among different instar and also different strains.
I instar → 3 – 3 ½ days - 1st moult 24 hours
II iinstar→ 2 ½ – 3 days - 2nd moult 20 hours
III instar → 3 - 3 ½ days - 3rd moult 24 hours
IV instar → 4 - 4 ½ days - 4th moult 30 hours
V instar → 6 – 7 days – Spinning
In view of the four intervening moults, the larval life is divided into the five distinct stages or instars which are referred as five different stages. The first three as “young ages” and the rest as “lato ages”. The silkworm which hatches from egg is known as the caterpillar. It is a tiny creature about 6 mm long and move about in a characteristic looping manner. At this time they are look like black or brown coloured ones. As the larvae grow, it becomes smoother and lighter in colour due to the rapid stretching of the cuticular skin during the different instars of the larval stages.
After the 4th moult, a pair of long sac-like silk glands develop on the lateral sides of the body of the 5th Instar. The silk glands security kind of sticky fluid which on coming in contact with air, it becomes hard and fine thread. Then the larva caterpillar gates in closed in this thread which actually forms a case around its body. This case is called cocoon. The cocoon is white or yellow, thick over capsule. The Caterpillar larva changes into crysalis,the larval stage of silkworms completes in about 25 to 32 days.
In the 5th instar, the larvae attain a maximum length of about 70 mm and feed voraciously, when the larvae develop fully and stop eating, the larval skin becomes transparent. These mature larvae are usually placed in a spinning nest, this process being called “mounting”. After about 2 days, the larvae-pupal ecolysis is called the pharate pupa and the duration is about 24 h through the steps of the pupal stage for 3 days and pharate adult stage for 5 days, the adult moth emerges by pupal-adult ecolysis.
The pupa is the inactive stage in which its body protected in a cocoon undergoes very important active changes called metamorphosis. It now stops feeding but shows internal development. The pupa stage of silkworms complete in about 12 to 14 days. Silk fibre is obtained from the cocoon of silkworms.
For obtaining silk the cocoons are kept in boiling water or in a hot open to destroy the glue of the cocoons. Then the Silk fibre is unwound from them. The length of thread used in the formation of one cocoon is about 1000 metres.
The adult of Bombyx mori is about 2.5 cm in length and pale creamy white. After emergence the adult is incapable of flight because of its feeble wings and heavy body. It does not feed during its short adult life. The body of moth has general plan of insect body organization .The ocelli are absent. The antennae are conspicuous, large and bipectinate. The meso- and meta-thorax bear a pair of wings. The front pair overlapp the hind pair when the moth is at rest.
The moth is unisexual and shows sexual dimorphism. In male eight abdominal segments are visible; while in female, seven. The female has comparatively smaller antennae. Its body and the abdomen are stouter and larger, and it is generally less active than male. The male moth possesses a pair of hooks known as harpes at its caudal end; while the female has a knob like projection with sensory hair. Just after emergence, male moths copulate with female for about 2-3 hours, and die after that. The female starts laying eggs just after copulation, which is completed within 24 hours. A female lays 400-500 eggs. The eggs are laid in clusters and are covered with gelatinous secretion of the female moth.
After active metamorfosis the pupa changes into an adult silkworm. Imago of breaks the cocoon to come out from it. It cannot fly. It survives for about 5-7 days after its emergence from the cocoon. The opportunity adult female Silk moth lays eggs after its maturity. In this way silkworm completes its life cycle in 45 to 50 days.
Alimentary Canal (The Gut) (*Only for knowledge not for SEE)
The mid-gut is the largest organ, extending over 8 of the 13 body segments. The mid-gut tissue comprises goblet, cells, columnar cells, regenerative cells and a basement membrane histological and cytological observations of the mid-gut have been carried out in detail by (Sujita, 1943).
The hind-gut is composed of epithelial cells forming numerous projections. Each of which is made up of two or three large cuneiform cells. Three regions are recognizable in this part: the small intestine, the colon and the rectum.
Silk Gland (*Only for knowledge not for SEE)
The silkgland is the second largest organ in the silkworm body. It develops from the paired invagination in the labial segments. In the full-grown larva it occupies most of the ventro-lateral side of the body from the 4th to the 8th segment. It is divided into three distinct parts, anterior, middle and posterior The middle part is further divided into three parts named fore, mid and hind parts.
The number of cells in the silk gland varies according to the strain and voltinsm, but does not differ between the right and the left glands, sexes, post embryonic development stages, or conditions of incubation during embryonic stages (Ono, 1942).
How silkworm forms silk?
The larval growth is marked by four moultings and five instar stages. The full-grown caterpillar develops a pair of sericteries or silk glands. Sericteries or silk glands are modified labial glands. These glands are cylindrical and divided into three segments: Anterior-, middle- and posterior-segments. The inner lining cells are characterized by the presence of large and branched nucleus. These glands secrete silk which consists of an inner tough protein, fibroin, enclosed by a water soluble gelatinous protein, sericin. In Bombyx, the fibrinogen which on extrusion is denatured to fibroin is secreted in the posterior segment of the gland and form the core of the silk filament in the form of two very thin fibres called brins. The sericin, a hot water soluble protein, secreted by middlesegment of the gland, holds the brins together and covers them. The duct from another small gland called Lyonnet’s gland, that lubricates the tube through which the silk passes, joins the ducts of the silk glands. Finally, the silk is moulded to a thread as it passes through the silk press or spinneret.
Important Terms (Only Definitions are Important for SEE)
The process of killing pupa inside cocoon is termed as stifling. Good-sized cocoon 8-10 days old are selected for further processing. Stifling is done by subjecting cocoon to hot water, steam, dry heat, sun exposure or fumigation.
The process of removing the threads from killed cocoon is called reeling. The cocoons are cooked first in hot water at 95-97°C for 10-15 minutes to soften the adhesion of silk threads among themselves, loosening of the threads to separate freely, and to facilitate the unbinding of silk threads. This process is called cooking. Cooking enables the sericin protein to get softened and make unwinding easy without breaks. The cocoons are then reeled in hot water with the help of a suitable machine. Four or five free ends of thethreads of cocoon are passed through eyelets and guides to twist into one thread and wound round a large wheel. The twisting is done with the help of croissure.
The silk istransferred finally to spools, and silk obtained on the spool is called the Raw Silk or Reeled Silk. The Raw silk is further boiled, stretched and purified by acid or by fermentation and is carefully washed again and again to bring the luster. Raw Silk or Reeled Silk is finished in the form of skein and book for trading.
The waste outer layer or damaged cocoons and threads are separated, teased and then the filaments are spun. This is called Spun Silk.
Importance of Silk:
· Garments in various weaves like plain, crepe, georgette and velvet.
· Knitted goods such as vests, gloves, socks, stockings.
· Silk is dyed and printed to prepare ornamented fabrics for saries, ghagras,
· lehengas and dupattas.
· Jackets, shawls and wrappers, Caps, handkerchiefs, scarves, dhotis, turbans.
· Quilts, bedcovers, cushions, table-cloths and curtains generally from Eri-
· silk or spun silk.
· Parachutes and parachute cords.
· Sieve for flour mills.
· Insulation coil for electric and telephone wire.
· Tyres of racing cars.
· Artillery gunpowder
· Preparation of surgical strings, fishing rods from the gut. For this purpose the intestines of Silk worms are extracted made into strings,dried,treated and packed.
Characteristic of Silk:
· Natural colour of Mulberry silk is white , yellow or yellowish green; and of Eri, brick red or creamy white or light brown.
· Silk has all desirable qualities of textile fibres, viz. strength, elasticity, softness, coolness, and affinity to dyes.
· The silk fibre is exceptionally strong having a breaking strength of 65,000-lbs/sq. Inch.
· Silk fibre can elongate 20% of original length before breaking.
· Density is 1.3-1.37g/cm3
· Natural silk is hygroscopic and gains moisture up to 11%.
· On burning it produces a deadly hydrocyanic gas.
· Silk is poor conductor of heat and electricity. However, under friction, it produces static electricity. Silk is sensitive to light and UV- rays.
· Silk fibre can be heated to higher temperature without damage. It becomes pale yellow at 110°C in 15 minutes and disintegrates at 165°C.