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Nature Notebook – Caterpillar Silk

Why do caterpillars of silk moths produce their famous threads? Silk is produced and used by caterpillars of moths and butterflies as a safety device and to form their pupal homes.

Most caterpillars consume their leafy diet high in the tree canopy. The suction cup-like prolegs hold the caterpillar in place but, occasionally they fail. Or, the caterpillar purposely falls off a leaf to avoid a predator.

A caterpillar’s mouth is equipped with spinnerets which shoot out a lifeline of silk. One end sticks to a leaf or branch and the other end is the mouth. When the coast is clear, the caterpillar uses its thoracic legs (which are usually used to hold onto food) and side-to-side body movements to climb back up to its eating area.

The tubular spinnerets are attached to silk glands which are modified salivary glands. Like a spider’s, when the caterpillar’s liquid silk is exposed to air it solidifies.

Caterpillars also use silk to attach their bodies to a structure for pupation. Butterfly caterpillars form a sticky silk pad, insert their last pair of caterpillar legs into it and begin their last molt. Silk moth caterpillars cover themselves with a silky cocoon before undergoing their last molt.