An interdisciplinary look at life cycle of silk worms

The silkworm, which originated in China, has been cultivated for silk since prehistoric times. Until Roman times, the origin of silk was a closely guarded mystery. The emperor Justinian sent two monks to spy and discover the secret if silk. They smuggled eggs and mul­berry seeds back across the Silk Road, a journey of two years.
Where I teach in Rabat, Morocco, Bombyx mori silkworms hatch in mid-spring when the mulberry leaves reappear. For about six weeks, they eat only mulberry leaves, growing from barely visible to a few centime­ters long. As the caterpillars grow, some develop black stripes and others are white. Still other species of silk­worms, such as Antheraea pernyi, are green.
The caterpillar then spins a silk co­coon (it takes nearly 3,000 cocoons to make a pound of silk) and hatches about two weeks later as a moth. The moths mate, lay eggs, and die in a few days. Each female may lay hundreds of small eggs that remain dormant until the following spring. The eggs require a period of cool temperatures for incubation to begin. The complete life cycle is less than two months, and the eggs can be stored in a refrigerator for hatching the following spring.
Silkworms in the Classroom
For several years, I have been raising silkworms and observing their growth cycle in my classroom. Silkworms and mulberry trees can be found in many parts of the United States, but if they are not native to your area, these ac­tivities can also be used with other types of insects that have complete metamorphosis, such as moths, butterflies, beetles, or ants. Silkworm eggs and detailed instructions are available from biological supply com­panies, such as Carolina Biological.
Silkworms can be used in a variety of educational activities for every age group, related to science, mathemat­ics, ecology, history, social studies, language arts, art, and physical edu­cation. They are a live and growing bit of nature on the windowsill, a source of wonder and fascination. Silkworms are quite harmless and they don't make distracting noises or a big mess.
Silkworms eat only the leaves of the mulberry (Morus alba) and Persian mulberry (Morus nigra or Morus persica). A live and self-renewing source of leaves is necessary. Due to climate variations and differences in varieties of mulberry trees, you will probably need to modify this activity to fit your particular region. In some areas of the northern United States, the mulberry tree may not leaf out early enough to complete the silk­worm life cycle before the end of the school year. However, you may want to try using frozen mulberry leaves that are thawed and misted with wa­ter (Frye, 1992), or taking the silk­worms home and feeding them again in the evening to shorten their life cycle (Zimmerman, 1977).
The Cycle of Life
When the first mulberry leaves ap­pear in the spring, pick one and lay it over your silkworm eggs. Lift the leaf off at the end of the day and you will see tiny black worms clinging to the underside of the leaf.
If you want your students to inves­tigate the length of the silkworm life cycle, set each day's hatch in a different shoebox. Feed the silkworms twice a day if possible, even though they can live more than two days without leaves. The caterpillars require tiny quantities of leaf material when they first hatch, but as they grow, their demands increase enormously. At maximum growth, one caterpillar can eat nearly a whole mulberry leaf in a day. Mulberry leaves can be picked and kept wrapped in a damp cloth in a plastic bag on the bottom shelf of the refrigerator for more than a week.
The caterpillars move slowly and tend to stay near their food supply. They can be kept in a shoebox or a shoebox top near a window, but not in direct sunlight. If you think that little fingers might pinch and squash the caterpillars, keep silkworms in a glass-sided insect observation cage or terrarium. It is important to main­tain a mperature between 20°C and 30°C, with an optimal temperature of 29°C. If classroom temperatures fall below this range in the evenings or on weekends, an incubator should be used.
Students can build a "silkworm apartment complex" out of toilet pa­per rolls (see photograph, next page). When the silkworms are ready to weave cocoons, they will climb, searching for a corner or space to at­tach the initial web of silk within which they will weave the cocoon. This arrangement allows students to watch the caterpillar weaving its co­coon.
The moths eat their way out of one end of the cocoon and flutter around for a male. Bombyx mori silk moths do not fly and will stay in the area where they hatch. The eggs stick to glass, wood, cardboard, or paper, so it is best to put the moths on a paper or in a cardboard box wave the eggs can be stored over winter and sealed against insects and spiders. Though I have had success storing the eggs in a closet, you can better control the date of hatching in you keep the eggs on the lower shelf of a refrigerator until spring. They will hatch about 19 days after you remove them from the refrigerator.