Dissect a Silkworm Cocoon

Most of us are familiar with silk as a fine fabric that is made into neckties and dresses. As a textile, silk is highly valued because it is shiny and smooth, strong yet lightweight, and it can be easily dyed or treated to take on the color or chemical properties of other substances. These well-known properties led scientists to develop new products from silk, including important medical materials such as replacement cartilage, surgical sutures, and small spheres that can be used to deliver drugs to certain parts of the body (not all of these are available yet on the market). What many people do not realize is that the small silk fibers that give silk its desirable characteristics come from the mouths of caterpillars called silkworms.

The silk used by humans comes from the domesticated silkworm, Bombyx mori. The silkworm is the caterpillar of a moth in Lepidoptera, the order of insects that includes moths and butterflies. Lepidoptera are holometabolous insects, which means that they undergo a complete metamorphosis during their lifetime. Just like butterflies, silkworm moths begin their life as an egg that then hatches into a growing, feeding caterpillar. When a silkworm has eaten enough, it constructs a cocoon made out of silk fibers, and inside that cocoon it turns into a pupa. After many days, a fully formed adult silkworm moth emerges through a spit-soaked opening in the bottom of a cocoon.

Dissecting a silkworm cocoon before the adult has emerged provides many clues about the transformation of a silkworm into an adult moth—a change that is normally impossible to view through the walls of a cocoon once it has been constructed. In this activity, you’ll learn about the insect origins of silk by dissecting a cocoon and “degumming” it to reveal the protein, called fibroin, that scientists use for constructing new materials.

Materials

Non-living, unemerged, silk cocoons – one or two per experimenter

Sharp scissors

Straight pins

4-6 inch square of cardboard or corkboard for pinning

Pencil

Paper for labeling

Beaker and hotplate, or saucepan and stove

Stirring rod or spoon

Plate or tray

Paper towels

Water

Powdered dishwasher detergent

Tweezers, fork, or tongs

Optional: camera

Optional: scale for measuring mass

Optional: silk cocoon dissection sheet

Cocoon Exterior – What is on the outside of a cocoon?

• Take a moment to make some observations about the color, shape, weight, size, and texture of the outside of the cocoon. Gently shake the cocoon and listen for anything inside. If you have a scale, measure the mass of the cocoon. Record your observations in a notebook or on the silk cocoon dissection sheet.

• Use your fingers to gently tug at the loose silk around the outside of the cocoon. You may find you are able to remove the exterior silk in one single sheet. Rub this silk between your fingers, describing the texture in your notes, then pin it to your pinboard and label it so that you can compare it to silk collected in later steps.
• Use your fingers to gently tug at the loose silk around the outside of the cocoon. You may find you are able to remove the exterior silk in one single sheet. Rub this silk between your fingers, describing the texture in your notes, then pin it to your pinboard and label it so that you can compare it to silk collected in later steps.
• Now, use a pin to snag some of the second layer of silk from the outside of the cocoon. Pull enough that you can feel it between your fingers and observe its texture, and record your observations. Pin this silk next to the silk you pulled off in the previous step, and label it.

A silkworm constructs its cocoon from the outside in, starting with a quick scaffold that it can easily rest upon while weaving and that will ultimately secure the cocoon to surrounding plants. The silk used to make the cocoon starts out thick but gets thinner and thinner as the silkworm works its way into the center of the cocoon.

Cocoon Interior – What’s inside of a cocoon?

• Using sharp scissors, nick the center of the cocoon, creating a hole just big enough to get the tip of your scissors into.
• Carefully, cut around the outside of the cocoon along the long axis. Stop periodically to peek into the cocoon and make sure that you are not cutting into anything that might be hidden inside. Cut until you have two halves.
• Once you’ve opened your cocoon, take a moment to sketch what you’ve discovered inside. If you have a scale, measure the mass of the empty cocoon and compare it to its original mass.
• Using pins, carefully pin the objects that you’ve found to your pin board, and try to identify them based on the information below. Pin one half of the cocoon to your pin board.

After constructing its cocoon, the silkworm caterpillar is fully encased inside. There, it will gradually stop moving and shed its skin-like caterpillar exoskeleton to emerge as a pupa, the intermediate phase between the caterpillar and the adult moth. The exoskeleton, called exuvia, is left in the bottom of the cocoon. A silkworm pupa already looks more like an adult than a caterpillar, with a shortened, segmented body, a clearly defined head, and the beginnings of what will be the adult legs. It takes the pupa approximately two weeks to develop into an adult moth.

Cocoon Interior – How is interior silk different from exterior silk?

• Take your finger and touch the inside of the unpinned half of your cocoon. Record your observations in your notebook or on your dissection sheet.
• Use a needle to scrape gently at the inside wall of the cocoon half until you are able to pull a layer of silk away. Feel the silk between your fingers and, in your notes, compare its color, texture, and flexibility to the silk from the exterior of the cocoon.

The silk used to construct a cocoon is actually a single, continuous thread that can be over 900m long. The silkworm weaves its cocoon from the outside in, laying down silk in figure-eight patterns. As it does this, the silk fiber gets thinner and thinner.

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Source: Science Friday

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