Silkworm Lifecycle

Silkworms are a species of insect that were originally from China: PhylumAnthropoda,

OrderLepidoptera, FamilyBombycidae, GenusBombyx, SpeciesB. 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 turns to spring.  Because of this, placing silkworm eggs in

cold storage through the winter period, ensures they remain in diapause until you are ready

for them to hatch out. 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. This is another good reason to

cold store eggs for a few months, as giving them an even cold temperature assists them to

hatch around the same time.

 

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, and they number 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).