Appearance. If you look into the water of a Minnesota lake during spring or late summer, you may see what looks like a long, black or brownish thread coiling and writhing about. This alien-looking worm is a member of the Nematomorpha phylum and is commonly called the horsehair or Gordian worm. Adults are about ¹⁄₁₀ inch in diameter and typically 6 to 10 inches long but can reach lengths of up to two feet.

Habitat and Name. Horsehair worms are found in aquatic habitats ranging from lakes and rivers to small puddles and water containers. Most known species reside in freshwater, though several marine species have been documented. The common name of the horsehair worm comes from its resemblance to the hair of a horse tail and its occurrence in livestock water troughs. The name Gordian worm comes from the similarity of entangled adults to the intricate mythological knot.

Life History. Adult female horsehair worms release millions of eggs into the water, which hatch into microscopic larvae. These larvae must parasitize an insect host by being inadvertently eaten or by burrowing into a host within a few weeks to complete their complex life cycle. If the host is a large aquatic insect, such as a water beetle, the larvae grow into their adult stage before killing the host as they burrow out of its exoskeleton. If the host is a small aquatic invertebrate, such as a midge, which completes its life cycle out of water, the larvae remain dormant. When the midge emerges as an adult and dies on land, it may be scavenged by a terrestrial insect, such as a cricket or a grasshopper. Inside the terrestrial host, a larva grows to its adult length. It remains coiled inside the terrestrial host until the unfortunate insect is driven to water by unknown processes. At the water, the adult horsehair worm emerges from its host and begins its aquatic phase.

Diet. Larval horsehair worms feed on the fluids of their host insects. Once horsehair worms emerge as adults, they do not feed.

Study and Status. Little is known about the distribution of horsehair worms in Minnesota, as this group is not well studied. Some research suggests there may be hundreds of species worldwide. About 18 known species are found in North America. Nematomorpha are a relict animal group with no clear relationship to other organisms. Their uncommon and unpredictable occurrence makes these parasitic organisms difficult to study. Horsehair worms, which do not parasitize humans and have little effect on insect populations, are simply one of many unique animals in Minnesota's abundant waters.