Long, thin black worms are known as horsehair worms, and as the name implies, they resemble hair from the tail of a horse. Horsehair worms are parasitic and are usually found in ponds, pools, water troughs and other watery places.
The parasitic nature of horsehair worms is exhibited as the worm's larvae enters a host species, usually an insect such as a cricket, beetle or grasshopper. The worm feeds on its host until it grows into an adult. The worm exits the host at a time when the insect seeks water, which causes the host to die. The worms are usually found entangled into a ball of worms or alone. Upon leaving a host, horsetail worms are white, but soon turn a brownish black color. While the worm's parasitic nature is discomforting for some, horsetail worms do not grow inside people, birds or plants.
A long-held superstitious belief is that horsetail worms develop from a horse's tail, dropping into the water as a horse drinks from a water trough. However, this is just a myth, as the worms are naturally attracted to small bodies of water. Homeowners may find horsehair worms in pet dishes, toilets or a backyard birdbath. The worms can be removed by hand, net or flushing.