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The moment a bagworm hatches and before it begins to eat, this caterpillar begins making is case or bag. Once this is complete the caterpillar will crawl inside the case and never emerge again until it changes into an adult moth. The cases appear to act as a protection against predators and are often well camouflaged. Each caterpillar of the species makes a very distinctive case, which has made it easier to identify the species than a study of the adult moths. The cases are fashioned out of silk and usually include some plant material, twigs, leaves and fibers that are cemented into the matrix. The size of these cases will range from 1\4 inch up to 6 inches in some species.

When the caterpillar finishes its bag and has crawled inside it can begin feeding. As it eats the head and thorax stick out from the case but it quickly retreats inside to rest after it has finished securing the bag to solid object with silken threads. The rear end of the bag is left open for the disposal of its excretions. As the bagworm begins to grow it simply adds more materials to the front of the bag to increase it in length and diameter. Like all caterpillars, the bagworm feeds voraciously on the leaves of some plants. They are considered to be pest of fruit trees, sometimes completely stripping the tree completely bare.

Once a bagworm has eaten its fill they pupate inside their bag. In the pupa stage the bagworm's body is completely reorganized as it is transformed into a moth. The male bagworms turn into adult moths that are strong fliers but with short lives and incompletely developed mouth parts so they cannot feed. The female bagworms in some cases stay inside their pupal shell inside their bag. Others develop much like the males but become degenerate being forced to subsist on food that was stored up when she was a caterpillar. The female's legs and wings are reduced and in some cases even missing leaving her to become little more than an egg laying machine. In some species the females are able to crawl completely out of the pupal case and the bag where they wait for the males to find them.

It is believed that the male bagworm is guided by scent that is emitted by the female. Once fertilization has taken place the female will crawl back to her bag, lay her eggs and die. When the female is unable to leave her bag the male will mate with her through the open end of the bag. The female will then die with the eggs still inside her body and the baby caterpillars make their way through her body to begin construction of their own bags. There are many species of animals besides the bagworm whose powers of movement are very limited. Even so these animals have been able to spread, in some cases enormous distances.

Scientists believe that the bagworm is able to move great distances due to other agents that carry them. Just as water animals get carried on mud on the feet of birds or mammals, many small animals are carried by the wind, being swept high enough to be carried across oceans to other continents. There is at least one way known that bagworms are spread. The eggs of this caterpillar are very hard shelled and when a bird eats the contents of a bag containing the body of the female and her eggs, the eggs are passed through the birds system unharmed to be dropped miles away.