Silkworm, voltinism and biology of silkworm.

Silk has been under use by human beings for various purposes since ancient times. Pure silk is
one of the finest and most beautiful natural fibres of the world and is said to be “the queen of
fibres. One of the methods was the rearing of silkworms on large scale with great care in natural
and controlled conditions. Different rearing techniques are applied in different parts of the world
for large scale production of silk threads of fine quality. This is known as sericulture.
Types of Silk: Moths belonging to families Satumidae and Bombycidae of order lepidoptera and
class Insecta produce silk of commerce. There are many species of silk-moth which can produce
the silk of commerce, but only few have been exploited by man for the purpose. Mainly four
types of silk have been recognised which are secreted by different species of silk worms.

(i) Mulberry Silk: This silk is supposed to be superior in quality to the other types due to its
shining and creamy white colour. It is secreted by the caterpillar of Bombyx mori which feeds on
mulberry leaves.

(ii) Tasar Silk: It is secreted by caterpillars of Antheraea mylitta, A. paphia, A. royeli, A. pernyi,
A. proyeli etc. This silk is of coppery colour. They feed on the leaves of Arjun, Asan, Sal, Oak
and various other secondary food plants.

(iii) Eri Silk: It is produced by caterpillars of Attacus ricini which feed on castor leaves. Its
colour is also creamy white like mulberry silk, but is less shining than the latter.

(iv) Munga Silk: It is obtained from caterpillars of Antheraea assama which feeds on Som,
Champa and Moyankuri.

Voltinism: Voltinism refers to the number of breeds raised per year. Voltinism is a genetically
determined heritable character under hormonal control. Based on voltinism B.moriis divided
into three type of races: univoltines, bivoltines, and poly or multi-voltines.

1. Univoltine races produce only one generation per year. The eggs laid remain in a
diapausing (quiet) condition till the next spring. Larvae of univoltines are very sensitive
to temperature and other environmental conditions. They are unsuitable for summer and
autumn rearing by artificial breaking of egg diapause. The larval period is very long. All
European races are Univoltines. The cocoons produced are commercially very superior.

2. Bivoltine races have two generations per year, the first generation adults developing from
eggs hatched in spring lay non dipausing eggs. The second generation adults developing
from these eggs lay eggs which remain in the dormant state till next spring. The larval
duration is as long as univoltines. Larvae are robust and tolerate environmental
fluctuations. They can be used for 'Summer and autumn rearing and three crops can be
raised per year. The cocoons are commercially superior. Japanese and Chinese races have
both uni and bivoltine varieties.
 3. Multi or polyvoltines have more than three generations per year. The larval duration is
short, and larvae are resistant to high temperature and high humidity. Larvae and cocoons
are small in size. Commercially cocoons are of poor quality. The adults lay non-
diapausing eggs.

Life cycle of tasar silkmoth

They are bivoltine i.e.; two crops in a year, one from August-October and other from
October-December. It is from August to December that the tasar insects are active and for the
rest of the year they are inactive i.e.; under dipause. The active and inactive phases of life of
tasar insects is controlled by environmental and hormonal factors.

Biology of Tasar silk moth

Systemic position

Phylum : Arthropoda

Class : Insecta

Order : Lepidoptera

Family : Saturniidae

Genus : Antheraea

Species : mylitta (Drury)

The adult :

The tasar insect is a very large moth exhibiting sexual dimorphism. The females are
bigger with a wing-span of 18 cms. The wings are grey or yellow or light brown with patterns of
red, with and black (brown) lines. Each wing has an eyespot or a circular ocellus in the centre.
The male are smaller with brown wing and wing span of 16 cms. The males have a narrow
abdomen with broad antennae when compared with the females.

The egg :

The egg is oval, dorsomventrally flat and bilaterally symmetrical aliong the
anteroposterior axis. It is 3 mm long and 2.5 mm in diameter. Althouhg the egg is pale white, it
appears brown due to a gummy coating. Two brown parallel lines are present along the
equatorial plane.

The larva (Caterpillar) :

The larva is cruciform or polypod type, with biting and chewing type mouth parts. It is
dull brown-yellowish with black-head about 7 mm long weighing 8 mg. the body turns green in
2 days. It grows and undergoes four moults and the fifth inserar larva is large 13 cms long and
weights 50 gms. Shining white lareral spots appear on 3 rd instar larva. The thorax is three
segmented and bears 3 pairs of walking legs, one pair per segment. The terminal segment bears
a anal flap and a pair of claspers. The fifth instar larva spins a cocoon.

The cocoon :

The cocoon is single-shelled, yellow or grey pendent, oval, closed and reelable with a
hard non-flossy grainy shell. At the anterior end there is a well formed dark brown penduncle
with a ring. Female spins larger cocoon than the male. The larva transforms into the pupa
inside the cocoon.

The Pupa :

The pupa is abtect, a dectious with well defined segmental body, the dark brown pupa
measures about 4.5 X 2.3 cm and weight 10.3 gm.

Bombyx mori

Phylum : Arthropoda

Class : Insecta

Order : Lepidoptera

Family : Bombaycidae

Genus : Bombyx

Species : mori

Life history of mulberry silk worm, Bombyx mori (L) :

The adult of Bombyx mori is about 2.5 cm in length and pale creamy white in colour.The
entire body is covered with scales. The males have longer antennae and narrow abdomen while
the female has small antennae, large and flat abdomen and is less active than the male. Due to
heavy body and feeble wings, flight is not possible by the moth. This moth is does not feed
during its very short life period of 2-3 days.

Fertilization :

Fertilization is is internal preceded by copulation. Just after emergence male moth

Egg laying :
 Just after fertilization, female starts egg laying which is completed in 1-24 hours. One
moth lays 400-500 eggs depending upon the climatic conditions and the supply of food material
to the caterpillar from which the female moth is obtained. The egg is laid in form of clusters and
covered with gelatinous secretion of the female noth which helps them in proper attachment.

Egg :

The eggs laid by the female moth are rounded and white in colour. The weght of the
newly laid 2,000 eggs comes to about 1 gm. With the increase in time after laying. Eggs become
darker. Two types of eggs are generally found viz., Diapause type and Non-Diapause type.

Hatching :

The eggs after ten days of incubation hatch into a larva called as caterpillar. Hatching is
the most important phase of silk moth life. After hatching caterpillars need continuous supply
of food because they are voracious feeders. It proper supply of mulberry leaf is not possible the
development of caterpillar would not be in a proper curse. Sometimes, due to lack of food
material, young caterpillars die causing great loss to the sericulture industry.

Caterpillar :

The newly hatched caterpillar is about 1/8th of an inch in length and is pale, yellowish-
white in colour. The caterpillars are provided with well developed mandibulate biting and
chewing type of mouth-parts adapted to feed easily on the mulberry leaves. The caterpillar is
twelve segmented and the abdominal region has ten segments having five pairs of pseudo-legs.
It is also provided with a small dorsal horn on very soft leaves of mulberry plants. As they are
voracious feeders, they grow rapidly which is marked by four moultings. After 1 st, 2nd,3rd and
4thmoulting caterpillars get changed into 2nd, 3rd, and 4th, 5th instars respectively. It takes about
21 to 25 days after hatching. The full grown caterpillar is 7.5 cm in length. It develops salivary
glands, stops feeding and undergoes pupation. The time taken for the full growth of the
caterpillar from young to the well grown stage varies with regard to the temperature, humidity,
food supply and type of race. The weight of the full grown caterpillar varies from 4 to 6 gm.

Pupa :

The caterpillars stop feeding and move towards corner among the leaves and secrete a
sticky fluid through silk gland. The secreted fluid comes out through spinneret (a narrow pore
situated on the hypopharynx) and takes the form of long fine thread of silk which hardens on
exposure to the air and wrapped around the body of the caterpillar in the form of covering
called as Cocoon.

Cocoon :

Cocoon is the white coloured bed of the jpujpa whose outer threads are irregular while
the inner threads are regular. The length of continuous thread secreted by a pupa for the
formation of cocoon is about 1000-1200 metres which requires 3 days to complete. The thread
is wound around the cocoon in concentric manner. The binding of threads round the cocoon is
achieved by the constant round motion of the head of te pupa from one side to the other at the
rate of 65 times per minute. The weight of one cocoon is about 1.8 to 2.2 gm and the weight of
the cocoon shell only is 0.45 gm. The size of the thread is 2.0 to 2.8 denier. The pupal period
lasts for 10 to 12 days and the pupae cut the cocoon and emerge into adult moths.

Emergence of imago :

Due to active metamorphic changes during pupation period the abdominal pseudo-legs
disappear and two pairs of wings develop. The silk worm within the cocoon secretes an alkaline
fluid to moisten its one end. The moistened end becomes soft where the threads are cut open
by the silk.