A sample was received and examined. The long, thin worms described are not earthworms; rather, they are insect parasites known as horsehair worms. Horsehair worms, also called Gordian worms, belong to the group Nematomorpha. They are similar to nematodes but are much longer (4 inches or more) and very thin (1/80 to 1/10 inch in diameter).

Horsehair worms emerge from their hosts, which are usually crickets or grasshoppers, during wet weather or when the host falls into water, such as in a swimming pool. People once thought they transformed from a horse's hairs because they were sometimes found in horse watering troughs. Horsehair worms are harmless (unless you're a cricket) and are probably not the cause of the mounds in your yard. The earthworms included in the sample are more likely the cause of the mounds.