Horsehair worms are very long, slender parasites of insects that are found in moist places. In New Hampshire they are mostly found during the late spring and summer. They have turned up in bird baths, water troughs, wet sidewalks, toilets, and crops in moist soil. They do not harm people, pets, or plants.
These animals constitute the Phylum Nematomorpha. The marine forms are in the Class Nectonematoida, while terrestrial and fresh water forms are in the Class Gordioida. Unlike earthworms, their bodies are not segmented. Typically they are 20 to 30 centimeters long (8-12 inches) but only 1 millimeter wide. Most are off-white in color. In some species the rear end of the body ends in two or three lobes. There is no distinct head. They are often found slowly writhing in a tangled knot (named after the Gordian knot of ancient Greek legend).
The worms deposit their eggs in strings in the water, often on water plants. In some species, insects become infected when they eat plants with attached eggs. In others, the eggs hatch in the water and the tiny larvae enter the body of an arthropod (insects and their relatives) living in the water or along the water’s edge. Beetles, crickets, grasshoppers, centipedes, millipedes and cockroaches are common hosts. The worms complete their development inside the body of the host. When mature, they exit the host when it is near water and the host dies. Horsehair worms are unable to parasitize humans.
Routinely rinse and flush water vessels to keep them clean. Excluding insects from water containers can also help. In some situations, controlling spillage will greatly reduce the problem. Horsehair worms do not survive long in dry environments.