Nematomorpha (Horsehair Worms)


The phylum Nematomorpha, the hairworms, occurs worldwide. All members are developmental parasites of aquatic or terrestrial invertebrates. One genus, Nectonema, is marine, whereas the remaining species occur in freshwater as free‐living stages or inside terrestrial hosts. Hairworms have two or more hosts in their life cycle. The first paratenic host is just a carrier to the second developmental host. In the developmental host, the hairworm grows to maturity and often kills the host upon emergence. There is no direct economic or medical importance of hairworms and while they can reach up to a foot in length, they are rarely seen. A few species may effect populations of herbivorous insects, but most are not important as biological control agents. Most freshwater hairworms are threatened from polluted waters, habitat displacement and reduced host populations. Many could be considered as candidates for the Red Data Book of rare and endangered species. Hairworms are a relict group that is not closely related to any other living phylum. The preparasitic larvae are quite different from the adults since they possess spines for entering the body cavity of paratenic hosts. Once inside the paratenic host, the larva is surrounded by host tissue and is known as an ‘encysted’ larva. It remains in the cyst until the paratenic host is ingested by a suitable developmental host. If the second host is not suitable for development, the hairworm larvae can encyst again in the new paratenic host.

Key Concepts:

  • Hairworms are becoming endangered in many areas of the world.

  • Hairworms have attracted the attention of behaviourists who study how these parasites control the movements of their hosts.

  • Hairworms have no direct economic or medical importance.

  • Hairworms have the ability to arrest development in nondevelopmental hosts.

  • Hairworms are an enigmatic group that are not closely related to any other invertebrates.

  • Hairworms are an ancient group, with fossils dating back to the Early Cretaceous.

Keywords: nematomorpha; gordiaceae; hairworm; horsehair worm; marine hairworm; Nectonema; freshwater hairworm

Figure 1.

The preparasitic larva of the freshwater hairworm, Neochordodes occidentalis.

Figure 2.

An adult freshwater hairworm emerging from its orthopteran host. Note the dark colour of the parasite.

Figure 3.

Giant cells in the anterior end of Nectonema zealandica.

Figure 4.

Encysted larva of Neochordodes occidentalis in a paratenic host.



Poinar GO Jr (1999) Paleochordodes praecox n.g., n. sp. (Chordodidae: Nematomorpha), parasites of a fossil cockroach, with a critical examination of other fossil hairworms and helminths of extant cockroaches (Blattaria: Insecta). Invertebrate Biology 118: 109–115.

Poinar GO Jr (2008) Global diversity of hairworms (Nematomorpha: Gordiaceae) in freshwater. Hydrobiologia. 595: 79–83.

Poinar GO Jr and Brockerhoff AM (2001) Nectonema zealandica n. sp. (Nematomorpha: Nectonematoidea) parasitizing the purple rock crab Hemigrapsus edwardsi (Brachura: Decapoda) in New Zealand, with notes on the prevalence of infection and host defence reactions. Systematic Parasitology 50: 149–157.

Poinar GO Jr and Buckley R (2006) Nematode (Nematoda: Mermithidae) and hairworm (Nematomorpha: Chordodidae) parasites in Early Cretaceous amber. Journal of Invertebrate Pathology 93: 36–41.

Further Reading

Biron DG, Ponton F, Marché L et al. (2006) ‘Suicide’ of crickets harbouring hairworms: a proteomics investigation. Insect Molecular Biology 15: 731–742.

Bresciani J (1991) Nematomorpha. In: Harrison FW and Ruppert EE (eds) Microscopic Anatomy of Invertebrates, pp. 197–218. New York: Wiley–Liss.

Chandler CM and Wells MR (1989) Cuticular features of Chordodes morgani (Nematomorpha) using scanning electron microscopy. Transactions of the American Microscopical Society 108: 152–158.

Eakin RM and Brandenburger JL (1974) Ultrastructural features of a Gordian worm (Nematomorpha). Journal of Ultrastructure Research 46: 351–374.

Poinar GO Jr (1991) Hairworm (Nematomorpha: Gordioidea) parasites of New Zealand wetas (Orthoptera: Stenopelmatidae). Canadian Journal of Zoology 69: 1592–1599.

Poinar GO Jr (2004) Nematomorpha In: Yule CM and Sen YH (eds) Freshwater Invertebrates of the Malaysian Region, pp. 157–161. Malaysia: Academi Sains.

Poinar GO Jr (2010) Nematoda and Nematomorpha. In: Thorp JH and Covich AP (eds) Ecology and Classification of North American Freshwater Invertebrates, 3rd edn, pp. 237–276. Amsterdam: Elsevier.

Poinar GO Jr (2010) Phylum Nematomorpha, horsehair worms, Gordian worms. The New Zealand Inventory of Biodiversity, Vol. 2. In: Gordon DP (ed.) Kingdom Animalia. Chaetognatha, Ecdysozoa, and Ichnofossils, pp. 491–497. Christchurch: Canterbury University Press.

Poinar GO Jr and Chandler CM (2004) Synopsis and identification of North American hairworms (Gordioidea: Nematomorpha). Journal of the Tennessee Academy of Science 79: 1–7.

Schmidt‐Rhaesa A (1997) Nematomorpha. In: Schwoerbel J and Zwick P (eds) Süsswasserfauna von Mitteleuropa, vol. 4 (4), pp. 128. Stuttgart: Gustav Fisher Verlag.

Schmidt‐Rhaesa A, Hanelt B and Reeves W (2003) Redescription and compilation of Nearctic freshwater Nematomorpha (Gordiida), with the description of two new species. Proceedings of the Academy of Natural Sciences of Philadelphia 153: 77–117.

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How to Cite close
Poinar, George(Jun 2012) Nematomorpha (Horsehair Worms). In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001594.pub3]