Horsehair or gordian worms are unusual invertebrates in the phylum Nematomorpha. These very long, thin creatures have a superficial resemblance to animated horse hairs, hence their common name. Often, horsehair worms occur in seemingly inextricable tangles of two or more individuals, especially during the breeding season, which is generally in the springtime. The second common name of these animals originates with these breeding aggregations and refers to the legendary "Gordian knot." This was a very complicated knot devised by King Gordius of Phrygia, that could not be solved and untied, and eventually had to be cut with a sword.
Mature horsehair worms are typically 4-28 in (10-70 cm) long. However, they are only 0.01-0.1 in (0.3-2.5 mm) in diameter, a characteristic that changes little over the length of their body. The mouth is at one end, and the cloacal aperture or vent is at or near the other end.
Adult horsehair worms live in fresh waters of all types. Immature stages are parasites of various types of terrestrial insects, most commonly crickets, grasshoppers, and beetles, and sometimes aquatic insects. The adult animals move about by slow undulations and tangles, which is not a very efficient means of locomotion. Consequently, these animals tend to live in static or slow-moving waters and are not generally found in more energetic aquatic habitats.
Male horsehair worms die soon after they impregnate a female during the breeding season. The female lays a stringy egg mass that can contain several million ova, and she then dies. It has not yet been discovered how the larvae manage to parasitize their host insects. It is thought that the larvae may encyst on vegetation or organic debris in shallow water, which becomes exposed later in the growing season, when water levels drop. Presumably, insects ingest these tiny cysts when feeding, and if the insect is an appropriate host for that species of horsehair worm, it is thereby parasitized by the larva. The life cycle is completed if the host insect falls into water, so the adult horsehair worm can emerge into an appropriate habitat. Adults that emerge into a terrestrial habitat are unable to survive for long.
Horsehair worms are widely distributed, occurring from the tropics to the tundra. About 110 species of horsehair worms are known, of which only one species is marine, being a parasite of small crustaceans. All of the other specis occur in freshwater habitats.