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Silk is a natural fiber of exceptional strength, texture, and
luster. When silk fibers are spun into thread and woven into fabrics,
the result is an exquisite commodity. Silk was first made in China,
and for centuries the methods of production were cloaked in secrecy,
so valuable was the technology to those who controlled the art
and industry of silk making. Eventually, however, the secret and
the organisms escaped the control of the Chinese, and thriving
silk industries were established in Japan, Arabia, and Spain.
Even today, with the vast array of synthetic fibers that rival
silk in many ways, the demand for the real thing is still high.
Although the larvae of most moths and butterflies produce silk,
that produced by Bombyx mori is the silk of commercial
importance. The silkworm moth lived in nature 4500 years ago when
the Chinese silk industry was in its infancy, but as years passed,
the insect became so domesticated that it can no longer fend for
itself in the wild. It can no longer fly, move more than a few
centimeters to find its food, or defend itself against predators.
As the silkworm prepares to pupate, it spins a protective cocoon.
About the size and color of a cotton ball, the cocoon is constructed
from one continuous strand of silk, perhaps 1.5 km long (nearly
a mile). If the silkworm were allowed to mature and break through
the cocoon, the silk would be rendered useless for commercial
purposes. So the encased insect is plunged into boiling water
to kill the inhabitant and dissolve the glue holding the cocoon
together. The end of the silk is then located and the cocoon unwound
onto a spindle to be made into thread.
Life cycle. A silkworm starts its life as a
tiny egg laid by the female moth. The egg is just about this size:
. The egg, laid in the summer or early fall, remains dormant until
the warmth of spring stimulates it to start developing. When silkworms
first hatch in the spring, they are tiny3 mm or so (about
1/8")and hairy. They require young tender mulberry
leaves during their first few days. As they grow, they can eat
tougher leaves, and late in their development they will eat any
mulberry leaf you can supply.
The larvae advance through five stages of growth, called instars.
The silkworm literally outgrows its skin five times, and molts
its outgrown skin. With the first molt the silkworm loses its
hairy exterior, and for the rest of its larval life its skin is
soft and smooth.
Silkworms grow rapidly, eventually reaching the size of your
ring finger. Then they spin beautiful oval white or yellow cocoons
in which they pupate. After 23 weeks the creamy-white adult
moths emerge from the cocoons. They clamber around, vibrate their
wings rapidly, and mate, but they don't fly or attempt to escape
from their container. During the adult phase of the life cycle,
the silkworm moths do not eat or drink. After mating, the female
lays a profusion of eggs, and the moths die.
Males and females look slightly different, and students will
be able to tell them apart with a little practice. The female
has a larger abdomen. The male has a much larger pair of antennae,
which look like long rakes or comb-shaped eyebrows, and vibrates
its wings rapidly to attract a female.
Silkworm Feeding Silkworms eat mulberry leaves; lots of them! But getting leaves in the late fall and winter months is nearly impossible, as the trees are deciduous. If you are doing the Silkworm Investigation in the winter, there is an alternative food. With every order of silkworm eggs you will be sent a half-pound of dry silkworm chow. Preparation requires hot tap water and a heat-source such as a microwave oven or stove-top. Water is mixed with the dry powder and then brought to a boil. The resulting mixture is poured onto a sheet of cling wrap, cooled, wrapped, and stored in the refrigerator. When firm, the silkworm chow can be sliced and fed to the hungry larvae.
The cooked Silkworm Chow can be stored in the refrigerator for a month or two if kept in an airtight container. Each bag of the dry powder comes with detailed instructions on the back of the package. Make sure your hands are clean when handling the cooked chow as the silkworms are susceptible to bacterial problems if their food is not kept sterile.
But remember, if you are raising silkworms in the spring, summer or early fall, fresh leaves are the best food source. Ask the Kindergarten teachers to plant a mulberry tree during their FOSS Tree module and you'll be set!
If you are using mulberry leaves, the first 10 days the larvae will need catkins or young tender leaves, but after that the larvae will eat any leaf you can provide. Keep leaves in the refrigerator. Feed the silkworms once or twice a day.
Think about the timing of the investigation.
The silkworm eggs must hatch when mulberry leaves and catkins (flowering portion of the mulberry tree) are available (see the above section for an alternate feeding possibility if mulberry leaves are not available). If you are not sure when mulberry trees begin budding in your area, ask a colleague or inquire at a nursery. See the background section for the Silkworms Investigation for more specific information.ursery. See the background section
for more specific information.
Obtain silkworm eggs. Eggs of the silkworm must
be obtained from a colleague who worked with silkworms last year,
or ordered from a biological supply company (see the Materials
folio for more information about obtaining insects). Order 50
eggs. If you purchased eggs from a biological supplier, plan to
conduct this part as soon as the eggs arrive, because they will
hatch 12 weeks after you receive them.
What to do when they arrive. Purchased silkworm
eggs usually arrive loose in a vial. working on a large piece
of white paper, use the little paintbrush to divide the eggs into
eight piles, and put one pile into each of eight vials. Cap the
vials. Keep them in a warm place out of direct sunlight until
you are ready to introduce them to students.
Eggs from a colleague may be stuck to paper. If this is the case,
cut or tear the paper so that each piece has 10–15 eggs,
and put the bits of paper into the vials.
Habitat. A shoe box is all that you need to
make a silkworm habitat. Choose a place in the room where the
silkworms will be warm but not in direct sunlight. Place the shoe
box in an open plastic bag, or drape a sheet of plastic over the
box. The idea is to reduce evaporation from the leaves a bit without
developing a humid environment.
If the eggs are scattered all over the box, that is OK, but the
larvae should be placed on a leaf. New larvae must be rounded
up each day and delivered to a fresh mulberry leaf.
Larva. Silkworm larva are delicate at first
and should not be handled for the first 2 weeks except with a
tiny paintbrush. By the time the larvae are 2 cm (1") long,
students can carefully pick up and gently hold them. The larvae
seem to survive better if they are kept together in a single culture
early in life—later they can be kept in pairs or small groups
on students' desks.
Plan for spinning. Get a medium-size corrugated
cardboard box and a couple of paper egg cartons. Open the egg
cartons and attach them to the inside walls of the box. The silkworms
will spin in the depressions in the egg cartons. The silkworms
must all be in this box for spinning their cocoons. The time for
this will be signaled by the first larva that starts to spin,
either in your class habitat or, more likely, in one of the group
Prepare for silkworm moths. Once the larvae
spin cocoons, they require no further care. The moths will emerge
in a couple of weeks and can be handled by students. They do not
eat or drinkthey mate, lay eggs, and die.
Prepare for mating and egg laying. Get a large
flat box, or cut a taller one down to about 10 cm (4"). Line
the bottom with paper. As the adults emerge, move them to this
new box. The moths will stay in the open box. The females will
lay eggs on the paper, making them easy to collect.
Collect eggs. The eggs will remain viable for
a year with minimal care. Seal them in a labeled zip bag and put
them in the refrigerator (not the freezer!) as soon as all the
moths have died. If you don't refrigerate the eggs, they will
still hatch, but over an extended period of time instead of all
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