Hairworm, a long, slender worm found in many ponds or quiet streams. Hairworms are sometimes called horsehair worms and horsehair snakes because it was once thought they were worms or tiny snakes that developed from horsehairs that fell into water. Hairworms are about 1/10 inch (2.5 mm) in diameter and from 1/4 inch (6.4 mm) to more than 3 feet (90 cm) long. Most species are black or brown, but some are gray or yellow. There are more than 200 species widely distributed around the world. A few species live in the sea or on land; little is known of their habits.

Freshwater hairworms lay strings of eggs in water. The larvae hatch out and form cysts that cling to plants in the water. Insects that feed on the plants swallow the larvae. The larvae live as parasites in the insects' bodies. After the worms mature, they break through the body walls of the host and return to water. Some marine hairworms live as parasites in crabs' bodies.

Hairworms make up the phylum Nematomorpha.