Page last updated Wed 05 Feb 2020

Phylum Nematomorpha (horsehair worms, gordian worms)

Members of the Phylum Nematomorpha -- also known as horsehair worms or gordian worms -- are larval parasites on terrestrial insects or, in the case of one genus, a marine crab. Eighteen species are currently recognized from the freshwaters of North America, although the actual species richness is estimated to be 10x that (Poinar, 2008).

Adult gordian worms superficially resemble very long (10-30 cm) nematodes, but nematomorphs are morphologically distinct. For example, these worms have a highly reduced gut with no mouth! Sexes are generally separate.


  • Zhang, Z.-Q. 2011. Animal biodiversity: An introduction to higher-level classification and taxonomic richness. Zootaxa 3148: 7-12. [Available online]
  • Poinar, G.O., jr. 2010. Nematoda and Nematomorpha. [in] J.H. Thorp & A.P. Covich (eds.). Ecology and Classification of North American Freshwater Invertebrates, 3rd edition: 237-276.
  • Poinar, G., jr. 2008. Global diversity of hairworms (Nematomorpha: Gordiaceae) in freshwater. Hydrobiologia 595: 79-83. [Available online]
  • Brusca, R.C. & G.J. Brusca. 2003. Invertebrates, 2nd edition. Sinauer Associates, Inc., Sunderland, Massachusetts. 936 pp.
  • Bolek, M.G. & J.R. Coggins. 2002. Seasonal occurrence, morphology, and observations on the life history of Gordius difficilis (Nematomorpha: Gordioidea) from southeastern Wisconsin, United States. Journal of Parasitology 88: 287-294. [Available online]
  • Smith, D.G. 2001. Nematomorpha (Horsehair worms, gordian worms). Pennak's Freshwater Invertebrates of the United States, 4th edition: 217-226.
  • Watermolen, D.J. & G.L. Haen. 1994. Horsehair worms (Phylum Nematomorpha) in Wisconsin, with notes on their occurrence in the Great Lakes. Journal of Freshwater Ecology 9(1): 7-11. [Available online]
  • Chitwood, B.G. 1959. Gordiida. [in] W.T. Edmondson (ed.). Ward & Whipple Fresh-Water Biology, 2nd edition: 402-405.

Student Essays

Web Pages

Classification of Freshwater Taxa

Only taxa found in North America are listed. Formatting indicates if they are known (to us) from North America, the midwestern states or Wisconsin.

Kingdom Animalia [details]
Subkingdom Eumetazoa [details]
clade Bilateria [details]
clade Protostomia [details]
Superphylum Ecdysozoa [details]
Phylum Nematomorpha (horsehair worms, gordian worms) [details]
  .   Class Gordioida [details]
  .    .   Order Chordodea [details]
  .    .    .   Family Chordodidae (+ Spinochordodidae, + Chordodiolinae) [details]
  .    .    .    .   Genus Chordodes [details]
  .    .    .    .    .   Species C. morgani [detail]
  .    .    .    .   Genus Euchordodes, Neochordodes, Pantachordodes, Pseudochordodes
  .    .    .   Family Paragordiidae [details]
  .    .    .    .   Genus Paragordius [details]
  .    .    .    .    .   Species P. varius [detail]
  .    .    .   Family Parachordodidae [details]
  .    .    .    .   Genus Gordionus, Parachordodes
  .    .   Order Gordea [details]
  .    .    .   Family Gordiidae [details]
  .    .    .    .   Genus Gordius [details]
  .    .    .    .    .   Species G. difficilis, G. robustus

Site managed by Daniel L. Graf @ University of Wisconsin-Stevens Point