NC State Extension Publications

Description and Biology

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Horsehair worms, Phylum Nematomorpha, are nematode-like parasites that develop as larvae inside insects. Once the larval worm matures, it waits for the insect to die or fall into water so that the worm can emerge to complete its life cycle in water. Adult horsehair worms are aquatic and lay millions of eggs in a stringy, gelatinous mass (some strings are more than 7 feet long!). The tiny, newly hatched larvae swim about for a day or so and then are thought to encyst on vegetation where they are accidently consumed by crickets, grasshoppers, beetles or some other insects or the larvae may be ingested as its host insect drinks infested water. The worms develop inside the host insect and eventually kill it. Some horsehair worms grow almost 28 inches long (!) but all of them are extremely slender. They get their name from the mysterious appearance of the worms in watering troughs where people evidently imagined that the hairs from the horses fell into the trough and "came alive." Evidently infected grasshoppers or other insects fall into the water, rot, and release the horsehair worms. The newly released horsehair worms are white or pale, but soon darken to brown or almost black. As the worms wriggle about in water, they become tangled up into a loose knot, hence the other common name, Gordian worms (after the famous Gordian Knot).

injured cockroach from which a horsehair worm is emerging

The top view of an injured cockroach from which a horsehair worm is emerging.

An injured cockroach from which a horsehair worm is emerging

The underside of an injured cockroach from which a horsehair worm has just emerged.

Habitats and Hosts

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Horsehair worms are sometimes found in streams and ponds, but are usually noticed in watering troughs, bird baths, swimming pools, pet dishes, sinks, and bathtubs. Horsehair worms develop as parasites inside crickets, grasshoppers, katydids, beetles, cockroaches and even sowbugs and spiders. They cannot infect people, animals or plants.

Residential Recommendation

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No control seems necessary. It would probably be a good idea to cook them before eating them.


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For assistance with a specific problem, contact your local N.C. Cooperative Extension Center

This Factsheet has not been peer reviewed.


Professor Emeritus
Entomology and Plant Pathology

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Publication date: Nov. 20, 2012
Revised: Sept. 24, 2019

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