Why Do Caterpillars Spin Silk?

Most caterpillars destined to become butterflies don't have the ability to spin silk, while a typical moth caterpillar does. Butterfly caterpillars don't create cocoons for themselves. Instead, when they are ready to pupate, they hang upside down and create a tough outer shell that resembles a leaf hanging off a branch. Most moth caterpillars spin thick silk cocoons.

If moth caterpillars only spin silk once in their lives, it's to create a cocoon. When a caterpillar reaches full size, he grabs onto a branch or leaf with his lower legs and holds tightly. Then it uses silk to help attach his body to the branch and to spin an intricate cocoon of silk around his body. Each species makes a slightly different cocoon, but it takes a lot of silk to complete one. Silkworms, for example, use a single thread nearly 3,000 feet long for their cocoons.

Tent caterpillars come by their names naturally. They survive by creating tents of silk in the forks of tree branches. Most stay in the tents during the day, when most of their predators hunt, and venture out at night to eat. Some enclose tree leaves in the tents for food, enlarging the silk tents to encompass more leaves rather than leaving the safety of their silk homes. Tent caterpillars spin individual cocoons, separate from the tent, when it's time to pupate.

Most caterpillars draw silk from a spinneret, typically located on their lower lips. The caterpillars have modified salivary glands that make liquid silk, which goes through a tube to the outer edge of the spinneret. As the caterpillar draws out the silk, it dries immediately upon contact with air.