Caterpillars and Silk

Caterpillars of both moths and butterflies have the ability to produce silk, much like a spider. They lose this capability once they transition to butterflies and moths, as it is no longer needed. As caterpillars, they use their silk in a variety of ways, some of which are truly startling.

How Caterpillars Produce Silk:
Like spiders, caterpillars produce silk through a “spinneret”. This tube-like structure is located on the lower side of the caterpillar’s mouth. Silk is generated as a liquid in the salivary glands and excreted through the spinneret; as it makes contact with air, it turns into a solid silk strand. Silk is one of the strongest natural fibers – it’s said that pound for pound, silk is stronger than steel.

A monarch caterpillar uses its spinneret to create silk.
Copyright © 2006 Hannah Nendick-Mason via BugGuide.net

How Caterpillars Use Silk:
Silk is important to caterpillars from the time they emerge from the egg to the time they enter their pupal state. Many kinds of caterpillars use a single strand of silk as a “safety line” to anchor them to the plant they are eating. If disturbed or dislodged, the caterpillar can drop down and hang from the silk line, almost seeming to rappel away from danger. When the danger has passed, the caterpillar draws the silk back in and makes its way back to the leaf to continue its meal.

Some caterpillars use silk to help create protective shelters. The “leaf-roller” caterpillars attach silk to both sides of a leaf and then draw the silk tight, pulling the leaf around them, as shown in the picture to the right of a canna skipper. This hides them from predators and creates a safe haven for dining in peace. Other silk shelters are more obvious, such as those created by tenting caterpillars. In that case, large groups of caterpillars create large protective “tents” of silk, in which all the caterpillars can live and eat in relative safety. (Learn more about tent caterpillars here.)

The most well-known use of caterpillar silk is, of course, in preparing to pupate. Moth caterpillars and butterfly caterpillars use the silk differently in this process. Moth caterpillars use silk extensively to spin a cocoon, a protective shelter in which they will shed their skin one final time and form a pupa. This process is especially obvious in the Silkmoth family, including the Polyphemus moths we raise here at BioWorks. These moth caterpillars spin hundreds of feet of unbroken silk, sometimes incorporating nearby leaves and twigs, in a process that can take hours or days. Click here to see time-lapse video of this process.

Butterfly caterpillars do not spin a protective cocoon. Instead, they use their spinnerets to create a small pad of silk to which they attach a hook at the end of their abdomen, known as a cremaster. Some caterpillars also produce a strand of silk about mid-way down their bodies, as an additional anchor to hold them in place while in chrysalis.

In the picture to the right, notice the Eastern Black Swallowtail caterpillar is attached at the end of the abdomen to the stick. (You can click the picture to enlarge it.) Though difficult to see, there is a pad of silk there. Note that the caterpillar has also slung a silken string around its midsection, holding it in place. The string remains even when the caterpillar pupates into chrysalis. If the tail end of the caterpillar should become detached, the center line will still be there to keep the chrysalis safe until the butterfly emerges.

One uncommon use of silk by caterpillars is for hunting. A select few caterpillars around the world are carnivores instead of herbivores, and use silk to catch their prey, much like spiders. One such species is Hyposmocoma molluscivora, a moth found in Hawaii. This caterpillar uses silk to trap snails, after which it climbs inside the shell, backs the snail into a corner, and eats it. Learn more about this unusual caterpillar here.

What About Silk Cloth?
Commercial silk is generally produced by the Domesticated Silkmoth caterpillar, Bombyx mori. This silkworm was domesticated thousands of years ago in Asia from the Wild Silkmoth, Bombyx mandarina.  B. mori does not live in the wild, and in fact could not survive in the wild because the adult moth is flightless. The caterpillars are raised in large factories where they are fed White Mulberry (Morus alba) leaves up to eight times a day. Once in cocoon, most of the pupae are not allowed to survive to adult moths, as they would destroy much of the silk when breaking out of the cocoon. The cocoons are generally boiled, which kills the caterpillar and loosens the silk for unwinding. To learn more about the process of producing silk commercially, click here.