Silkworms

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Silkworms are easy, fun and educational to grow in a classroom or at home. They are caterpillars that spin a silk cocoon and change into moths while inside. After hatching from an egg, the worms take one month to grow large enough to spin the silk. They spend three weeks in the cocoon, then emerge as a moth to mate and lay eggs. The eggs hatch into worms in a few weeks, and then the cycle continues.

Silkworms go through four stages of development, as do most insects:  egg, larva, pupa and adult.  The adult (imago) stage is the silkworm moth. The larva is the silkworm caterpillar.  Since the silkworm grows so much, it must shed its skin four times while it is growing.  These stages-within-a-stage are called instars.  The Latin (scientific) name for the silkworm is bombyx mori, which means "silkworm of the black mulberry tree".

Eggs

Get eggs from a friend or you can order them from a variety of online sources.  Put the eggs in the refrigerator (NOT THE FREEZER!) until you are ready to use them.  To find out where to buy eggs, artificial food, and other equipment, check out the Links section.

Mulberry leaves

Silkworms only eat fresh mulberry leaves (or artificial food). In California, trees lose their leaves in October and leaf out around late March. Therefore, you cannot raise silkworms year-round. You will need to locate local mulberry trees (Latin name Morus alba). Make sure to get permission from the owners and verify that they don't spray their tree with insecticide. If possible, just pick leaves (don't break off branches), so leaves will grow back faster. Leaves will keep fresh in Ziplock bags in the refrigerator for about 5 days. Please don't strip a tree. If an owner will allow you to break off a small branch, put it in a vase of water and pick leaves as necessary. A branch will last a week this way. Don't put caterpillars on a branch in water, since they will drown.  You might get tired of picking leaves and decide to park the caterpillars on a mulberry tree.  Hungry birds may devour your entire brood!  Artificial silkworm food (22 pounds minimum!) can be ordered from  http://www.mulberryfarms.com.  It comes as a powder to which you add water.  It works well with certain varieties of silkworm - but they will get bigger faster if they eat leaves.  Pictures of mulberry leaves can be found at http://www.suekayton.com/silk/leaves.htm.

Container

Since silkworms don't drink water, they get their moisture from the leaves so they must be fresh, not dried-out. Unless you want to change leaves three times daily, you need a covered container to raise the worms in. It should be almost air-tight to prevent leaves from drying out, but have small air holes for ventilation. Try a transparent plastic cake cover with the handle removed to allow air in through the screw-holes. In California, cheap lids are available from Smart and Final Iris restaurant supply stores.  You'll need a tray or plate to put it on, too.

Newborn caterpillars

Once you have located a source of leaves, and have a container ready, take your eggs out of the refrigerator. They will hatch in 7-20 days, depending on how far developed they were when they were put into the refrigerator. Placing the eggs in direct sunlight seems to speed up the process. Once you can see a dark ring and clear center in the egg, it is almost ready to hatch. They usually hatch at dawn. Have a few leaves on hand since they must eat within a day of hatching. Newborn silkworms will barely nibble at the leaf. It will dry out long before they could possibly eat it up. Change leaves at least three times a day at this stage, so they will grow quickly. If you have access to a low-power microscope (about 30x), let the kids look at a tiny caterpillar. Instead of a tiny black string, they have incredible detail.

The Japanese call this stage "Kego", which means "hairy baby". If you examine the eggshell under a microscope, you can see the pores that let air inside while the caterpillar is developing. The edges of the hole where the caterpillar emerged are black.

The larva stage

Twice a day (three times a day if you have no lid), give the worms fresh leaves. Newborn silkworms look like small black strings this size __ . They are initially too weak to crawl from the old leaf to the new one. Either place the new leaf directly on top of the old leaf, or carefully hand-pick all of the silkworms onto the new leaf. Throw out the old, dried leaf. Put the new leaf with silkworms into the container and replace the lid. If any newborn worms are on the paper with the eggs, gently move them onto a leaf (or place a leaf directly on top of the paper).

After five days, the worms will have the strength to crawl from the old leaves to new ones by themselves, so you won't have to hand-pick them. Then you can just place new leaves in the container. Every two days, empty the container to prevent mold from forming. You'll need to increase the number of leaves as they get older. Caterpillar poop looks like a small black speck when they are little, and like miniature black corn cobs when larger. Make sure leaves do not have dew or water on the surface when feeding newborn silkworms, since they will drown in any small surface film of water.

Silkworms will need to go home with the teacher or a child over the weekend since they need fresh leaves, and leaves dry out very quickly. The silkworms will shed their skin three times while growing. The shed skins are beige and usually roll up into a round wad. Sometimes the tiny silkworms will eat their shed skin. The larger ones don't. Each stage the silkworm goes through is called an "instar". First instar caterpillars are black. The second and third instars are grayish-white with black heads. The fifth and final instar has a white head. In between each instar is a time of resting and molting. The Japanese say the silkworm is "sleeping".

The silkworm does 80% of its eating during the fifth instar. The silk glands now make up 25% of its body weight. The larva has increased its size 10,000 times since birth. If this happened to a six-pound human baby, it would weigh 60,000 pounds when it was grown! The final instar of the larval stage is 2-3/4 inches long.

In each instar, the caterpillar has six real legs (as do all insects). It also has five pairs of false legs on the rear of its body. The body is made up of thirteen segments, each of which has a black dot on the side. These dots are called spiracles, and the silkworm breathes through them.

Spinning silk

The caterpillars take about a month to get big enough to spin a cocoon. The caterpillars like to spin cocoons in toilet paper tubes (slice them in half like Life Savers), paper towel tubes (cut into six slices), or in egg carton bottoms. You can tell that they are ready to spin when they stop eating and turn yellowish. When they get finger-sized, put sliced toilet paper tubes or egg cartons in their container. You can make a "cocoon condo" by stacking toilet paper tubes in a pyramid.

The cocoon-spinning process takes about three days. When they are spinning, try not to disturb their threads or they will have to start all over again. Once the cocoons are all spun, remove dried-up leaves to prevent mold from forming.

The silk is actually hardened silkworm saliva. It comes out of the mouth, not out of the rear end like a spider. When the silkworm ate great quantities of mulberry leaves, they were digested and nutrients were sent into the bloodstream. The silk glands absorbed these nutrients. The larva has a small spinneret on its lip, through which the silk emerges. The single strand of silk that forms the cocoon is about one mile long!

The silkworm moves its head in figure 8 patterns as it spins the cocoon. When the cocoon is partially made, you can see the head moving around inside if you hold it up to the light.

The moth stage

Inside the cocoon, the silkworm sheds its skin and turns into a brown-shelled pupa.   Inside this shell, the pupa turns into a moth.  This process takes three weeks, and then the moth emerges from the cocoon.  They usually emerge at dawn. The adult moth has a special spit which is used to dissolve the silk so it can push its way out of the cocoon. Silkworm farmers kill the moths before they emerge and make holes in the silk thread. When they emerge, the wings are crumpled, but they get pumped full of fluid and harden it about an hour.

Moths cannot fly, and neither eat nor drink. They mate, lay eggs, and then die within five days. After the moths emerge from the cocoon, they look for an opposite-sex moth to mate with. Females are significantly larger than males. Females periodically extrude a scent gland through the hole in their abdomen.  Males have a flap of skin at the end of their abdomen and flutter their wings a lot. Each moth will "urinate" a reddish-brown fluid shortly after emerging from the cocoon.  It dries to look like blood.  Explain to the kids that this is the moth's "pee" that it saved up since it couldn't "go" while it was in the cocoon.

The moths stay mated for about a day. After separation, the female lays eggs and the male looks for another female. Sometimes another male grabs the female before she can lay eggs. Each female will lay between 200 - 500 eggs! Put paper on the bottom of the container and remove empty cocoons as the moths emerge. The moths will lay eggs on the paper. When the moths are dead, please DO NOT throw out the dead moths or cocoons, since I use them to make display cases.

It is interesting to note that one ounce of silkworm eggs contains 40,000 eggs (1,500 eggs per gram).  These worms will eat 3,500 pounds (1500 kilograms) of mulberry leaves, and will spin cocoons which will produce 18 pounds (8 kilograms) of silk thread. It takes 1700 to 2000 cocoons to make one silk dress!

Eggs

When first laid, all eggs are lemon-yellow. After three days, they will turn white if they are infertile, or turn black if they are fertile. Fertile eggs might hatch a week or two after being laid in the middle of the summer, but they usually won't hatch unless subjected to "winter" in your refrigerator for at least several weeks. Wait until the eggs turn black before putting them in the Ziplock bag in the refrigerator. Once you take eggs out of the fridge, they will hatch in 7-20 days, or maybe not at all. Direct sunlight in the morning for a few hours hastens hatching.  Eggs will remain viable in the refrigerator for about five years.  To find out where to buy eggs, artificial food, and other equipment, check out the Links section.

Silk thread and cloth

If the moths were allowed to emerge from the cocoons, they would make holes in the silk thread.  The silkworm farmers kill the pupas inside the cocoons by baking them in a hot oven.  Then they soak the cocoons in boiling water to loosen the threads.  A person finds the end of the thread and places it on a winding bobbin.  Then a machine unrolls the cocoon, winding the silk from five cocoons together to make one silk thread.   Then the thread is woven into cloth. 

Classroom activities

Try these teacher-tested ideas:

Silkworm math. Have the kids measure the length of the silkworms and graph them as they grow.

Rainfall: When the silkworms are large, take the lid off the container and have the children be extremely quiet. They will be able to hear the sound of the silkworms moving around! It sounds like a gentle rainfall. The sound is not chewing, but their little suction-cup feet lifting off the leaves and plopping back down again.

Silkworm pet. Give each kid a silkworm in a cut-down milk carton on their desk. Have them put in a fresh leaf twice a day, and empty the poop out. Put in a stick and they can see it crawl around. Wait until the caterpillars are two weeks old since there is a high mortality rate for the first few weeks.

Heartbeat. With a full-grown caterpillar, you can easily see the heart pumping blood through the translucent skin. The heart is located at the rear end of the caterpillar on the top. You can see it pulse. The main artery carrying the blood is where the backbone would be if it had one.

Egg laying. If a female moth happens to be laying eggs, have the children watch. You can actually see the yellow eggs emerge one at a time from her rear end! She feels around with her ovipositor ("egg-layer" in Latin) until she feels an empty place to put the egg.

Coarse thread . You can make silk thread without killing any of the pupas. When the cocoons are spun, there is a fair amount of loose silk on them. Have the children gently pull it off the cocoon, making sure not to crush it. They can then roll it between their fingers to make a coarse silk thread.

Fine thread. In order to unwind the cocoon, you must kill the pupa inside. Place the cocoons on a cookie sheet in 200 degree oven for 30 minutes. Then drop the cocoons in boiling water. After five minutes, you can reach in (wearing rubber dishwashing gloves), and begin to unwind the cocoon. Unwinding five at a time will make a fine, strong, thread.

Silk bookmarks.  You can cut out shapes from cardboard and stick it on a bottle. Then place the spinning worm on the top. The worm, not having a corner to spin it's cocoon, will criss cross over the top of the card, and around the edges. Once the worm became a pupa, take it off the card, take the silk off the card and have a silk woven shape like a heart or cross or star. Of course the worms don't care much for corners on shapes, so there will be rounded corners instead of sharp ones. You can put more than one worm on a shape to make it thicker. These silk shapes made great bookmarks!

History

The history of the silkworm, which is also the story of silk, goes back to ancient times in China. Some of the stories have been handed down through the generations and are probably based party on fact and partly on legend and myth. The tale which persists is that about 2,640 B.C. a Chinese empress, Si-Ling-Chi, was watching the glistening amber cocoons that little worms were spinning in the mulberry trees in the palace gardens. She unwound one of the threads on a cocoon and found that it was one, very long strand of shiny material. Fascinated, she pulled strands from several cocoons through her ring to form a thicker thread. Eventually, with the help of her ladies of the court, she spun the threads into a beautiful piece of cloth to make a robe for the emperor, Huang-Ti. This magnificent material, silk, became known at the "cloth of kings".

For thousand of years on the royal family of China had silk. The Chinese kept the secret of how silk was made for 2500 years. The material was sold to the rulers of the West, but the source of the shiny thread that made the material was not revealed. The penalty in China for telling that the silk came from the cocoons of the little silkworms was death! Some very strange ideas were formulated as to the origin of silk. Here are a few: Silk came from the colored petals of flowers in the Chinese desert, silk was made of wondrously soft soil, silk came from a spider-like animal that ate until it burst open and the silk threads were found inside its body, and silk came from the silky fuzz on special leaves. These ideas seem far-fetched today -- but in ancient times they were serious theories.

Legend has it that the Japanese carries off four Chinese maidens, who knew the secret of silk, along with mulberry shoots and silk moth eggs. Today Japan is the leading producer of silk! Another story is that a Chinese princess married an Indian prince. She carries silkworm eggs and mulberry shoots in her elaborate headdress and the secret of raising silkworms in her head. Now silk was grown and produced in India. Finally, two poor monks told Emperor Justinian of Constantinople that they had learned the secret of silk. Justinian send them back to China to get some eggs and mulberry shoots for him. They returned many years later with the eggs and shoots hidden inside their hollowed-out walking sticks. Since Justinian was the emperor of Constantinople, a crossroads city, the secret soon spread throughout Europe. There are many more interesting stories about the history of silk. Have older children do some research in the library and report to the class.

Today silk can be worn by anyone -- not just emperors and noblemen and their families. Silk is made into many lovely fabrics, such as satin, velvet, chiffon, crepe, brocade, taffeta, faille, and shantung. A good class project would be to see how many different kinds of silk cloth could be collected and put them on a chart for the kids to see and feel. The beautiful colors of silk would also make a nice chart.

Modern silkworm moths have been bred to have white silk instead of the amber-colored silk of their wild ancestors. They also have large, fat bodies and tiny wings, so they cannot fly. This makes it easier for silkworm farmers to raise them (and easier for teachers, too!). If you were to release a domesticated silkworm moth into the wild, it would not be able to survive or reproduce.

In the wild, silkworms are eaten by ants, spiders, birds and mosquitos. If you have to spray insecticide near the silkworms, move them away for a few days.

Display case

You can make a display case showing the kids each stage in the silkworm cycle.  Buy a clear plastic box frame 11"x14".  A box frame is about 1-1/4" deep so you can put three-dimensional things in it.  It comes filled with a tagboard box.  Cut out one of the 11"x14" sides of the tagboard box, making a frame. 

Put captions on an 11"x14" piece of paper and glue it onto the inside of the tagboard frame.  Now glue dead moths, cocoons, silk thread and silk cloth in the appropriate places.  Cut out leaves from green construction paper and glue them in, too.  Make newborn "silkworms" from pieces of black buttonhole thread.   Make older "silkworms" of various sizes from Play-dough.  Let them dry before gluing them down.  Eggs can be Play-dough, beads, sesame seeds, or dots of yellow dimensional fabric paint.   Tape the cardboard frame inside the plastic box and admire your work!  Use clear silicone glue for best results (it comes in small tubes like toothpaste). 

Silkworm Science

Silkworms are insects.  All insects have six legs in the adult stage.   Silkworm caterpillars have six real legs, plus five pairs of pseudopods (false legs) on the rear of the body.  The very rear is split and used for grasping twigs and leaves.  All insects have no backbone or skeleton, but instead have an exoskeleton (exterior shell).  Some insects like cockroaches have a hard, crunchy shell.  Silkworms and silkworm moths have a soft skin.  Silkworms shed their skins several times while growing.

The only warm-blooded animals are mammals and birds. All animals without backbones are cold-blooded, which includes silkworms and all other insects. However, while moving around, all animals' muscles generate heat. If you have a covered container with lots of big silkworms, when you take the lid off, you can feel the heat that was trapped in the container.

The difference between warm-blooded and cold-blooded:
A warm-blooded animal always has the interior of its body at the same temperature (98.6 degrees for a human) unless it is sick. If their interior temperature gets too high or too low, it will die. A cold-blooded animal's interior temperature varies widely and is usually within a few degrees of the air around it.  On a cold winter day, a cold-blooded animal's temperature may be around 40 degrees F, and on a hot day it may soar to 90 degrees F.  It doesn't bother the cold-blooded animal a bit.

On warm days, a cold-blooded animal's muscles will be warm, so it can move easily.   On a cold day, when its muscles are very cold, it will become lethargic and sluggish.  Rattlesnakes in cold areas actually hibernate during the winter since they become too sluggish to move.  Bees cannot fly when their muscles are too cold.   Bees in cold areas warm up their flight muscles by shivering until they are warm enough to sustain flight.

Scientists are still arguing about whether dinosaurs were warm- or cold-blooded.

How can you tell if a silkworm is male or female?

I don't know how to tell the caterpillars apart, but the moths are easy once you know what to look for.  The smallest caterpillars, which make the smallest cocoons, turn into males.  The big caterpillars turn into females.  The in-between ones can go either way.  Male moths are smaller, and have a flap of skin at the rear.   Females periodically extrude a scent gland out the rear.  Look at the pictures in Sylvia Johnson's book for close-up photos to help you tell them apart. 

It takes more energy to make eggs than to make sperm since the eggs are so much larger.   The larger caterpillars have more energy, so they become females.  The small caterpillars, in order to maximize their contribution to the gene pool, become males so they can (hopefully) impregnate lots of females. 

What is the Latin (scientific name) for the silkworm?

Bombyx mori.  I suspect that Mori means mulberry.

Why are some cocoons yellow while others are white?

It's genetic.  Some people have blue eyes and others have brown eyes.  Almost all commercial varieties of silkworms make white silk.  There are also silkworm varieties that make yellow, orange and pale-green silk.  When you cross-breed a "white-silk" silkworm moth with a "yellow-silk" silkworm moth, you get some yellow offspring and some white offspring.  When you have a mom with blue eyes and a dad with brown eyes, some of their kids will have brown eyes and others will be blue-eyed.  Same idea.  WIth people, brown eyes are dominant.  With silkworms, the colored silk is dominant over white. 

Wild silkworms all make yellow silk, to blend in with dead leaves.  Over the centuries, silkworm farmers selectively bred for whiter and whiter silk until they achieved the pure white we see today.  They like the pure white because it can be dyed any color without having to bleach it first.  Nowadays, with natural and organic products gaining in popularity, people are selectively breeding for colored silks.  I propagate eggs for white, yellow, orange and green silk.  Cotton farmers have recently begun to breed for colored cottons and have a wide variety of naturally-colored cottons, including yellow, pink, pale-green and orange.

Why are some silkworms striped and others solid white?
They are all the same species.  These minor color variations are like people having different color hair and eyes - but we're still people.  

What do silkworm moths eat?

They don't eat (or drink) anything.  They mate, the female lays eggs, and then they die within 3-6 days.  Many different types of insects follow this pattern.  The mayfly is another example.  Almost all adult butterflies and moths have digestive tracts adapted for sipping nectar from flowers.  However, silkworm moths do not feed.   They still have a rudimentary gut, since they're descended from an ancestor that fed in adulthood.

How far can silkworms smell?
Silkworms and female silkworm moths have no sense of smell.  Wild adult male silkmoths have an acute sense of smell, but I don't know how far they can smell. I've never seen it in a book or tested it. From personal experience I know that a domestic silkworm male can notice a female silkworm at the opposite end of a large gymnasium, but never have tested a larger distance. I suspect that 1/4 mile would be the limit for a wild silkworm moth (downwind), but I bet that domesticated silkworms have lost much of that sensitivity since they were bred for silk, and natural selection no longer requires them to have any sense of smell whatsoever.   

Silkworm poem

I saw a tiny silkworm.
It had a funny name.
My teacher called it larva,
But it wiggled all the same.

One day it changed from hairy to smooth --
From black to very white.
Its body was much bigger, too,
And it did it overnight.

It changed like this
Just three more times
And always in between
It Ate and ate and ate and ATE
Mulberry leaves so green.

One day it stopped,
And started to spin
A shiny silken thread.
Around and round in figure eights
It moved its little head.

It made a cocoon so snowy white,
Its neighbors made theirs yellow and bright.
And then inside where none could see,
A pupa formed, my teacher told me.

One day it pushed from its cocoon --
How different it looked now.
It fluttered about on week, little wings --
A silkworm moth --but how??

It laid so many golden eggs
Near its empty white cocoon.
I wondered when the larva would hatch.
Would it be very soon?

Egg to larva to pupa to adult --
Is a strange, strange way
To change from being born
Into a grown-up moth, I'd say!

Silkworm Song
by Elaine Wade
(tune: "I am a pizza")

I am a silk worm (x2)
brown and white (x2 etc....)
I like to eat leaves
day and night.

I spin a silk thread
into a cocoon. 
I am a silk worm
Please feed me soon!

I am a silkworm
I wiggle and squirm. 
Cocoons don't take long
as you will learn.

Now I make a hole
and climb right out
I was a silkworm
Now I'm a moth!

I am a moth
brown and white
I lay tiny eggs
their colors are bright.

I like to fly (flap) around.
Happiness I bring.
I am a silkworm.
See you next spring!

More information

School teachers can ask questions via email:  kayton@alum.mit.edu

The book Silkworms, by Sylvia Johnson, is excellent for children and adults alike. It has great photos. Most bookstores or amazon.com can order it. Paperback ISBN 0-3225-9557-5 ($6). Hardback library binding ISBN 0-8225-1478-8 ($16).

The book The Empress and the Silkworm by Lily Toy Hong tells the story of how silk was first discovered in China - part fact, part fable.  $16.95 ISBN# 0-8075-2009-8 hardback.

Internet links

General silkworm information:
A teacher in Pennsylvania, USA has information at http://www.thewildones.org/Curric/silkworm.html

General information on butterflies can be found at http://snapper.bio.umass.edu/kunkel/Moths/strecker_silk.html

A site with general butterfly information (but it loads VERY slowly) is http://butterflywebsite.com/awards/showawards.cfm

General silkworm educational site http://www.sericulum.com/

Silkworm-raising information in English http://insected.arizona.edu/silkrear.htm and Spanish http://insected.arizona.edu/espanol/sedainfo.htm

Silkworm eggs and supplies:
Eggs and artificial silkworm food can be ordered from http://www.mulberryfarms.com
Eggs and display cases from http://www.carolina.com or  http://www.sericulum.com/
Eggs http://www.nilesbio.com/