Silkworms are a species of insect that were originally from China: Phylum – Anthropoda,
Order – Lepidoptera, Family – Bombycidae, Genus – Bombyx, Species – B. mori.
Silkworms are in fact not worms at all, but caterpillars of the silk moth. The Bombyx mori
silkworm has been domesticated for approximately 5000 years. Subsequently the
species is dependent on humans for survival, and no longer occurs in the wild. Through
selective breeding the bombyx mori silkworms have branched into races that differ in
larval (catepillar) markings, cocoon size, silk colour, and silk quality. Silkworms complete
their life-cycle in approximately 6-8 weeks. In ideal conditions the larval (worm stage)
can be as short as 21 days. Ideal conditions include a temperature of 27 degrees C and
constant access to high quality food, whether fresh mulberry leaf or artificial diet (chow).
A silkworm cocoon is one continuous strand of silk approximately 300m – 900m in
length. The production of silk is called Sericulture and is an important industry in many
countries of the world. It takes approximately 3,000 cocoons to produce half a kilo of
silk. Please see the different stages of the silkworms life-cycle below for more detailed
information.
Eggs
Silkworms eggs bear a strong likeness to poppy seeds. When the silk moth first lays her eggs,
they are yellow in colour. The fertile eggs slowly change to a blue/grey colour over a
number of days. The eggs change once again, to a lighter blue-grey colour approximately 2-4
days prior to the emergence of the first instar larva. The eggs of the silkworm remain in a state
called DIAPAUSE until they have been through a period of cold (winter). When the temperature
rises in spring it stimulates the development of the larva within the eggs. Once stimulated, eggs
take approximately 10 days to hatch. Silkworms hatch naturally in Australia from Early July
depending on the region. Whilst silkworms usually (amazingly) coincide their hatching with the
first emergence of leaves on the local mulberry trees, occasionally they emerge too early due to
temperature fluctuations as winter transitions to spring. Because of this, placing silkworm eggs in
cold storage through winter period ensures the eggs remain in diapause until you are ready
for them to hatch. Furthermore, silkworm hatch-lings do not always emerge all at the same time.
A batch of eggs can hatch continuously over a 6 week (or longer) period. Cold storage
provides the eggs with an even temperature and ensures all eggs hatch within a few
days of each other.
Larval (Catepillar) Stages
Newly hatched silkworms are dark, furry and black in colour, and about 3 – 5 millimeters in
length. This is the first instar of the larval (caterpillar) stage. There are 5 instars separated by
four moults of the silkworms skin. Once the silkworms shed their first black furry skin, they
become smooth. Some silkworms are white (plain silkworms) and others have black striping
(zebra silkworms). It is during this stage of the silkworms life-cycle that they eat copious
amounts of food, growing from a 5 millimeter worm to a 5-6 centimeter worm over just a few
weeks. The silkworms amazing rate of growth has been likened to a human baby growing to the
size of an adult elephant in 6 weeks! Because silkworms are domesticated, they remain in the
breeding container you provide for them and do not attempt to escape. However, occasionally
silkworms may walk some distance in search of food if none is available, and this behaviour is
most often seen in new hungry hatch-lings. Silkworms are difficult to handle when very small
and can be easily crushed. It is therefore helpful to use specialized tweezers for lifting silkworms
in the first instars, to avoid damaging their delicate bodies. Unlike butterfly caterpillars, they do
not hold on strongly, and it is possible to lift them without tearing their feet. Although silkworms
can tolerate gentle handling, they are very susceptible to bacterial and viral infections. Because
of this it is vitally important to wash and dry hands thoroughly prior to handling silkworms. After
the fourth and last moult of their skins, the silkworms will eat over half the food they will
consume in their lifetimes. Once they reach a critical size they will empty their alimentary system
completely in preparation for spinning their cocoon. At this point the colour of the worm develops
a slight orange tinge, and the skin appears more translucent. The body of the worm will also
appear shorter with intersegmental ridges of skin on the silkworms head and body. The
silkworms behaviour will also change when cocooning is imminent. The silkworm will hold its
upper body off the surface and wave its head to and fro in search of an ideal cocooning space.
Cocoons
Once the silkworm has selected a space for cocooning, it begins to spin a network of support
threads. Within and central of the supporting threads the silkworms will construct its cocoon. The
silkworm moves its head in a figure of 8 pattern, slowly building the walls of the cocoon around
itself for over 48 hours using one continuous strand of silk up to 900m long. A silkworm cocoon
is ovate in shape and constructed of a non woven, completely closed wall of silk fibres bound
together with sericin proteins, or ‘wormspit’. The wormspit varies in colour from white through
gold to green and even pink tinged. After spinning the inner softest layer of the cocoon, the
silkworm changes into a pupa by shedding its worm skin completely. The silkworm pupa remains
in its cocoon for 10-14 days. Silkworms can do unusual things like spin cocoons in pairs, and
can even walk up to 10 meters to find a ideal place to cocoon. If silkworms are disturbed after
they form the support threads for their cocoon, they may become confused and spin random silk
structures such as flat mats of silk and may even cease to spin at all, and pupate with no cocoon
formed around them. This is the stage in the silkworm industry that the cocoons are baked or
boiled to kill the pupa, then each cocoon is REELED. This is when one continuous strand of
silk is unwound from each cocoon, either by hand or by a machine. 6-8 of these strands are
twisted together to create one thread of silk. Often the cooked PUPAS are eaten and in some
countries are a valuable source of protein.
Silk Moths
At peaceful silkworms every moth is allowed to hatch from its cocoon and complete its life-cycle.
Each Silkworm stays in its cocoon for approximately 10 – 14 days, then hatches inside the
cocoon from its pupa skin. The moth then secretes a clear protease enzyme saliva which
dissolves the protein sericin that binds the silk of the cocoon together. Once the sericin dissolves
the moth can push itself out. Silk moths are creamy white, with darker wing veins, black eyes
and dark antennae. The males have larger antennae and are generally a smaller size, and their
wings are more perfectly formed than the wings of the females. Females have large abdomens
full of unfertilized eggs and their wings often appear slightly twisted. Additionally the behaviour of
male and female silk moths is different. The males will flutter and scoot across the surface once
they sense the pheromones of a female, and they also turn their abdomens around to the side in
classical silk moth mating behaviour. The females will often pull themselves slowly to the side of
a breeding container to pump out their wings and may sit with their abdomens raised and and
yellow genitalia visible. Females sometimes show a ‘flutter’ like behavior, usually displayed
whilst sitting in one place. The flutter is generally rhythmic with both wings in sync. Often all the
females will display this behavior at the same time. Mating takes between 12 – 24 hours. After
mating is completed, the female lays her eggs, between 200 – 500 in total. As
the silk moths do not eat or drink, they die naturally soon after laying their eggs, the males may
mate with more than one female before they die. Silk moths live for approximately 3 days, but
can survive up to two weeks. Silk moths generally do not fly and will usually stay in the breeding
box (Male moths may fly briefly or leave the breeding container in search of a female).
