A day or two after they come out of their hosts, the adult worms mate in the water, and then around mid-August or Mid-October, they lay their eggs in the water. It takes about a month for the larvae to develop inside the egg. Once the larvae hatch, they somehow get into gastropods (like snails and slugs), insects, or earthworms, which live in moist habitats. There they build a cyst, and grow. The animal they grow inside is called an interim (temporary) host, because a final host is still needed.