Cocoon That Resembles a Bag

There is a caterpillar that makes a bag around itself and drags along, as it feeds and grows. It also pupates inside the bag and develops into an adult. It is the Bag Worm.

The bag worm, Thyridopteryx, is the caterpillar of black moth found in Pennsylvania.

Soon after hatching from the egg, between May and June, it builds a small bag around its body with silk and plant material. If the larva is on the Red cedar, it will strengthen the bag with redar scales. With most of its body hidden safe, the larva protrudes its head and legs from the opening of the bag. Feeding, growth and molting is completed by August. The bag also is simultaneously made large, to fit the size of the larva. The bag now measures two inches long and is cone shaped.

A fully grown larva moves with its bag to a convenient spot and secures itself. It closes the bag, reverses its position so that it is head down inside the bag. It becomes pupa. Now, the bag functions like a cocoon… The pupa stage lasts for about four weeks before developing into an adult moth.

If a male moth develops from the larva, it will have black body covered by dense hairs. It will have wings, with about one inch wing span. The male is an active flier and will leave the bag eventually.

If on the other hand, a female develops, it will remain a grub without wings, legs, antennae or eyes. Its body is cream colored and without hairs. It will remain in the bag.

During early October, the female releases chemical substances (pheromones) which attract males. On smelling the pheromones, the male leaves its bag and flies to the female bag. It inserts its abdomen through the opening of the female bag, and mates with the female. It dies after mating.

The fertilized female lays hundreds of eggs, drops from the bag and dies. The eggs remain in the bag, safely protected until May next year. Each generation takes one year.

The bag worm is a strange creature with a unique life cycle. Its bag is a well disguised cocoon protecting the insect right from the larval stage.

One annoying news about the bag worm is, it is a pest.