Gordiaceae

Gordiaceae is a family of parasitic nematodes, commonly known as horsehair worms or gordian worms. These organisms belong to the phylum Nematomorpha, and are characterized by their extremely long, thin bodies, which can reach up to a meter in length. Their common name derives from their appearance, often found twisted together in coils, resembling horsehair.

Adult horsehair worms are free-living in freshwater environments such as ponds, streams, and ditches. They do not feed as adults and their primary purpose is reproduction. The life cycle of Gordiaceae involves an aquatic larval stage and a terrestrial or semi-aquatic adult stage, with an arthropod intermediate host. The larvae are typically ingested by aquatic insects, such as mayflies or caddisflies. Once inside the host, the larvae develop into juveniles.

When the infected insect emerges from the water as an adult, it may be consumed by a larger terrestrial invertebrate, such as a cricket or grasshopper, or even a vertebrate like a frog. In the final host, the juvenile Gordiaceae worms mature into adults. The parasite then manipulates the host's behavior, often causing it to seek out water before the worm emerges to reproduce. The mature worms leave the host, usually by emerging from the anus, and return to freshwater to mate. If the host does not reach water, the worm will die inside it. They are considered harmless to humans.