Nematomorpha (Horsehair Worms)


The worldwide phylum Nematomorpha, that parasitise aquatic and terrestrial invertebrates, is a relict group unrelated to any other living or extinct phylum. One genus, Nectonema, is marine, while the remaining genera occur in freshwater or inside terrestrial hosts. Hairworms have two or more hosts in their life cycle. Upon hatching, the microscopic free‐living larvae are passively ingested by and encyst inside paratenic invertebrate hosts without further development. When the paratenic host is eaten by a larger invertebrate, it leaves its cyst and enters the body cavity of the new host where it may grow to an adult. If the second host is not suitable for development, the hairworm larva can encyst again and wait to be ingested by a final host. There is no direct economic or medical importance of hairworms and the long, slender adults are rarely noticed except when emerging from their final host.

  • Hairworms (Nematomorpha) are a unique group of invertebrate parasites.
  • Hairworm adults are free‐living and occur in fresh or saltwater.
  • After hatching, the microscopic larvae enter and encyst in invertebrates and vertebrates.
  • Final development occurs in large invertebrates that consume the paratenic hosts.
  • Most final hosts are killed when the hairworms emerge.
  • Hairworms are an ancient group, with fossils dating back to the mid‐Cretaceous.

Keywords: gordian worms; hairworms; nematomorpha; nectonema; amber fossils

Figure 1. White egg strand produced from an aquatic female of Neochordodes occidentalis. Insert shows several similar eggs strands after deposit in a water‐filled trench.
Figure 2. Cluster of eggs with a hatchling larva of Neochordodes occidentalis in the upper right corner.
Figure 3. Larva of Neochordodes occidentalis penetrating through the peritrophic membrane of a mosquito larva (Culex pipiens) paratenic host. It will encyst in the gut cells or body cavity.
Figure 4. Three larvae of Neochordodes occidentalis encysted in the hemocoel of a larva of the mosquito, Culex pipiens. Note variation in thickness of cysts.
Figure 5. Two adult Neochordodes occidentalis emerging from a Jerusalem cricket developmental host, Stenopelmatus sp. (Orthoptera).
Figure 6. A fossil hairworm, Paleochordodes protus, emerging from a cockroach in 20–40 mya Dominican amber.
Figure 7. The oldest known fossil hairworm, Cretachordodes burmitis, in mid‐Cretaceous Burmese amber.


Dorier A (1930) Recherches biologiques et systematiques sur les Gordiaces. Travaux de la laboratorie de Hydrobiologie Piscic de Grenoble 22: 1–184.

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 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.

Poinar GO Jr (2008) Global diversity of hairworms (Nematomorpha: Gordiaceae) in freshwater. In: Balian EV, Lévêque C, Segers H and Martens K (eds) Freshwater Animal Diversity Assessment, vol. 595, pp 79–83. Hydrobiologia: New York.

Yamada M, Tegoshi T, Abe N and Urabe M (2012) Two human cases infected by the horsehair worm, Parachordodes sp. (Nematomorpha: Chordodidae) in Japan. Korean Journal of Parasitology 50: 263–267.

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. Wiley–Liss: New York.

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. Academi Sains: Malaysia.

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.

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. Elsevier: Amsterdam.

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

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

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.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Poinar, George(Aug 2020) Nematomorpha (Horsehair Worms). In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0029151]