# Nematomorpha

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Phylum of parasitoid animals; horsehair worms

Nematomorpha Temporal range: Cenomanian–Recent PreꞒ Ꞓ O S D C P T J K Pg N Possible Atdabanian Record Paragordius tricuspidatus Scientific classification Kingdom: Animalia Subkingdom: Eumetazoa Clade: ParaHoxozoa Clade: Bilateria Clade: Nephrozoa Clade: Protostomia Superphylum: Ecdysozoa Clade: Nematoida Phylum: Nematomorpha Vejdovsky, 1886 Orders and families Gordioidea Rauther, 1930 Chordodidae May, 1919 Gordiidae May, 1919 Nectonematoidea Rauther, 1930 Nectonemidae Ward, 1892

**Nematomorpha** (sometimes called **Gordiacea**, and commonly known as **horsehair worms**, **hairsnakes**,[1][2][3] or **Gordian worms**) are a [phylum](/source/Phylum) of [parasitoid](/source/Parasitoid) [animals](/source/Animal) similar to yet distinct from [nematode](/source/Nematode) [worms](/source/Worm) in [morphology](/source/Morphology_(biology)), hence the name. Most species range in size from 5 to 10 centimetres (2 to 4 in), reaching 2 metres (6 ft 7 in) in extreme cases, and 1 to 3 millimetres (0.039 to 0.118 in) in diameter. Horsehair worms can be discovered in damp areas, such as watering troughs, swimming pools, streams, puddles, and cisterns. The adult worms are free-living, but the [larvae](/source/Larva) are [parasitic](/source/Parasite) on [arthropods](/source/Arthropod), such as [beetles](/source/Beetle), [cockroaches](/source/Cockroach), [mantises](/source/Mantises), [orthopterans](/source/Orthoptera), and [crustaceans](/source/Crustacean).[4] About 351 freshwater species are known[5] and a conservative estimate suggests that there may be about 2000 freshwater species worldwide.[6] The name "Gordian" stems from the legendary [Gordian knot](/source/Gordian_knot). This relates to the fact that nematomorphs often coil themselves in tight balls that resemble knots.[7]

## Description and biology

Nematomorphs possess an external [cuticle](/source/Cuticle) without [cilia](/source/Cilia). Internally, they have only longitudinal muscle and a non-functional gut, with no [excretory](/source/Excretory_system), [respiratory](/source/Respiratory_system) or [circulatory systems](/source/Circulatory_system). The nervous system consists of a [nerve ring](/source/Nerve_ring) near the anterior end of the animal and a [ventral nerve cord](/source/Ventral_nerve_cord) running along the body.[8]

Reproductively, they have [two distinct sexes](/source/Dioecy), with the internal fertilization of eggs that are then laid in gelatinous strings. Adults have cylindrical [gonads](/source/Gonad), opening into the [cloaca](/source/Cloaca). The larvae have rings of cuticular hooks and terminal stylets that are believed to be used to enter the hosts. Once inside the host, the larvae live inside the [haemocoel](/source/Haemocoel) and absorb nutrients directly through their skin. Development into the adult form takes weeks or months, and the larva [moults](/source/Ecdysis) several times as it grows in size.[8] Depending on species, a mouth can be either absent or present in adults. Also the pharynx can be absent, cellular in structure or a cuticularized tube.[9]

The adults are mostly free-living in freshwater or marine environments, and males and females aggregate into tight balls (*Gordian knots*) during mating.[10][11]

*[Spinochordodes tellinii](/source/Spinochordodes_tellinii)* and its *[Meconema](/source/Meconema)* host

In *[Spinochordodes tellinii](/source/Spinochordodes_tellinii)* and *[Paragordius tricuspidatus](/source/Paragordius_tricuspidatus)*, which have [grasshoppers](/source/Grasshoppers) and [crickets](/source/Crickets) as their hosts, the infection acts on the infected host's brain.[12] This causes the host insect to seek water and drown itself, thus returning the nematomorph to water — according to Thomas *et al.*, the "infected insects may first display an erratic behaviour which brings them sooner or later close to a stream and then a behavioural change that makes them enter the water", rather than seeking out water over long distances.[10] *Paragordius tricuspidatus* is also remarkably able to survive the predation of their host, being able to wiggle out of the predator that has eaten the host.[13] Similarly, *[Chordodes](/source/Chordodes)* sp. nematomorphs affect the light-interpreting organs of *[Hierodula patellifera](/source/Hierodula_patellifera)* mantises so the host is attracted to horizontally [polarized light](/source/Polarization_(waves)). Thus the host goes into water and the parasite's lifecycle completes.[14] Many of the genes the parasites use for manipulating their host have been acquired through [horizontal gene transfer](/source/Horizontal_gene_transfer) from the host genome.[15]

There are a few cases of accidental parasitism in vertebrate hosts, including dogs,[16] cats,[17] and humans. Several cases involving *Parachordodes*, *Paragordius*, or *Gordius* have been recorded in human hosts in Japan.[18]

## Life cycle and development

The Nematomorpha life cycle consists of four stages: (1) the [egg](/source/Egg), (2) the pre-parasitic [larva](/source/Larva), (3) the parasitic larva, and (4) the free-living aquatic adult.[19] Throughout these four stages, the worms parasitize multiple [invertebrate](/source/Invertebrate) hosts, inhabiting different environments.[19]

Freshwater worms typically mate in the spring, depositing their strings of eggs in the water.[19] Within 3 to 4 weeks, pre-parasitic larvae emerge from these eggs.[19] These larvae must parasitize an invertebrate host — their primary hosts being larger [arthropod](/source/Arthropod) species, such as [mantids](/source/Mantidae), [beetles](/source/Beetle), [dragonflies](/source/Dragonfly), [crickets](/source/Cricket_(insect)), and [grasshoppers](/source/Grasshopper).[19][20] Some studies report that mantids are among the most important primary hosts of Nematomorpha in [tropical](/source/Tropics) and [subtropical](/source/Subtropics) regions.[20]

The Nematomorpha life cycle often includes multiple hosts.[20] If aquatic larvae are fortunate enough to be ingested by a primary host organism, they will then continue their development into adults.[19] However, it is much more common that they seek out an intermediate host, usually aquatic invertebrate [larvae](/source/Larva) (e.g. [mayfly](/source/Mayfly) larvae, [mosquito](/source/Mosquito) larvae, or [chironomid](/source/Chironomidae) larvae).[19][20] Once larvae find themselves in an intermediate host, they encyst themselves in the host's [body cavity](/source/Body_cavity).[19][20] These parasites leverage aquatic insects as vehicles to move from water to land, retaining viable [cysts](/source/Cyst) through [metamorphosis](/source/Metamorphosis).[21] Once conditions to continue development are met (i.e. the intermediate host is eaten by a primary host), they will reemerge from this cyst and infect the primary host.[19][20]

Some pre-parasitic freshwater Nematomorpha larvae are known to encyst themselves on debris or vegetation.[19] If they are ingested by a primary host organism (such as a [millipede](/source/Millipede)) in this state, they will then progress into the parasitic stage.[19]

## Community ecology

Mermithid worm (Phylum Nematoda), often confused with horsehair worms (Phylum Nematomorpha). Dark structures are eggs, not seen in horsehair worms. Germany 2021.

Owing to their use of [orthopterans](/source/Orthoptera) as hosts, nematomorphs can be significant factors in shaping community ecology. One study conducted in a Japanese [riparian](/source/Riparian) ecosystem showed that nematomorphs could cause orthopterans to become 20 times more likely to enter water than uninfected orthopterans; these orthopterans constituted up to 60% of the annual energy intake for the [Kirikuchi char](/source/Kirikuchi_char). The absence of nematomorphs from riparian communities can thus lead to char preying more heavily on other aquatic invertebrates, potentially causing more widespread physiological effects.[22]

## Taxonomy

See also: [List of bilaterial animal orders](/source/List_of_bilaterial_animal_orders)

Nematomorphs can be confused with nematodes, particularly [mermithid](/source/Mermithidae) worms. Unlike nematomorphs, mermithids do not have a terminal cloaca. Male mermithids have one or two spicules just before the end, apart from having a thinner, smoother cuticle without areoles and paler brown colour.[23]

The phylum is placed along with the [Ecdysozoa](/source/Ecdysozoa) clade of moulting organisms that include the [Arthropoda](/source/Arthropod). Their closest relatives are the nematodes. The two phyla make up the group [Nematoida](/source/Nematoida) in the clade [Cycloneuralia](/source/Cycloneuralia). During the larval stage, the animals show a resemblance to adult [kinorhyncha](/source/Kinorhyncha) and some species of [Loricifera](/source/Loricifera) and [Priapulida](/source/Priapulida), all members of the group [Scalidophora](/source/Scalidophora).[24] The earliest nematomorph could be *[Maotianshania](/source/Maotianshania)*, from the Lower Cambrian; this organism is, however, very different from extant species;[25] fossilized worms resembling the modern forms have been reported from mid [Cretaceous](/source/Cretaceous) [Burmese amber](/source/Burmese_amber) dated to 100 million years ago.[26]

Relationships within the phylum are still somewhat unclear, but two classes are recognised. The five marine species of nematomorph are contained in [Nectonematoida](/source/Nectonematoida).[27] This order is monotypic containing the genus *[Nectonema](/source/Nectonema)* Verrill, 1879: adults are planktonic and the larvae parasitise [decapod crustaceans](/source/Decapoda), especially crabs.[27] They are characterized by a double row of natotory [setae](/source/Seta) along each side of the body, dorsal and ventral longitudinal epidermal cords, a spacious and fluid-filled [blastocoelom](/source/Blastocoelom) and singular [gonads](/source/Gonad).

The approximately 320 remaining species are distributed between two families,[28] within the monotypic class [Gordioida](/source/Gordioida). Gordioidean adults are free-living in freshwater or semiterrestrial habitats and larvae parasitise insects, primarily [orthopterans](/source/Orthoptera).[27] Unlike nectonematiodeans, gordioideans lack lateral rows of setae, have a single, ventral epidermal cord and their blastocoels are filled with [mesenchyme](/source/Mesenchyme) in young animals but become spacious in older individuals.

## References

### Citations

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1. **[^](#cite_ref-itisGodioidea_28-0)** ["Gordioidea"](https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=699878). [Integrated Taxonomic Information System](/source/Integrated_Taxonomic_Information_System).

### General and cited references

- Pechenik, Jan A. (2010). "Four Phyla of Likely Nematode Relatives". *Biology of the Invertebrates* (6th International ed.). Singapore: Mc-Graw Hill Education (Asia). pp. 452–457. [ISBN](/source/ISBN_(identifier)) [978-0-07-127041-0](https://en.wikipedia.org/wiki/Special:BookSources/978-0-07-127041-0).

## Further reading

- Baker GL, Capinera JL (1997). "Nematodes and nematomorphs as control agents of grasshoppers and locusts". *Memoirs of the Entomological Society of Canada*. **171**: 157–211. [doi](/source/Doi_(identifier)):[10.4039/entm129171157-1](https://doi.org/10.4039%2Fentm129171157-1).

- Hanelt, B.; Thomas, F.; Schmidt-Rhaesa, A. (2005). "Biology of the phylum Nematomorpha". *[Advances in Parasitology](/source/Advances_in_Parasitology)*. Vol. 59. pp. 243–305. [doi](/source/Doi_(identifier)):[10.1016/S0065-308X(05)59004-3](https://doi.org/10.1016%2FS0065-308X%2805%2959004-3). [ISBN](/source/ISBN_(identifier)) [9780120317592](https://en.wikipedia.org/wiki/Special:BookSources/9780120317592). [PMID](/source/PMID_(identifier)) [16182867](https://pubmed.ncbi.nlm.nih.gov/16182867).

- Poinar GO Jr (1991). "Nematoda and Nematomorpha". In Thorp JH, Covich AP (eds.). *Ecology and Classification of North American Freshwater Invertebrates*. San Diego, CA: Academic Press. pp. 249–283.

- Thorne G (1940). "The hairworm, *Gordius robustus* Leidy, as a parasite of the Mormon cricket, *Anabrus simplex* Haldeman". *Journal of the Washington Academy of Sciences*. **30**: 219–231.

## External links

[Wikisource](/source/Wikisource) has the text of the [1911 *Encyclopædia Britannica*](/source/Encyclop%C3%A6dia_Britannica_Eleventh_Edition) article "[Nematomorpha](https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Nematomorpha)".

- Capinera, J. L. [Horsehair Worms, Hairworms, Gordian Worms, Nematomorphs, *Gordius* spp. (Nematomorpha: Gordioidea).](http://edis.ifas.ufl.edu/in274) University of Florida [IFAS](/source/Institute_of_Food_and_Agricultural_Sciences). Published 1999, revised 2005.

- [Nematomorph worm – Behavior modification of cricket by nematomorph worm.](https://www.youtube.com/watch?v=76BnVsaLiR0) YouTube.

- Gordian worms discussed on [RNZ](/source/Radio_New_Zealand) *[Critter of the Week](/source/Critter_of_the_Week)*, [6 November 2015](http://www.radionz.co.nz/national/programmes/afternoons/audio/201777689/critter-of-the-week)

v t e Extant animal phyla Domain Archaea Bacteria Eukaryota (major groups Metamonada Discoba Diaphoretickes Hacrobia Cryptista Rhizaria Alveolata Stramenopiles Plants Amorphea Amoebozoa Opisthokonta Animalia Fungi Mesomycetozoea) Animalia Porifera (sponges) Ctenophora (comb jellies) ParaHoxozoa (Planulozoa) Placozoa (Trichoplax and relatives) Cnidaria (jellyfish and relatives) Bilateria (Triploblasts) (see below↓) The phylogeny of the animal root is disputed; see also Eumetazoa Benthozoa Bilateria Bilateria Xenacoelomorpha (acoels and relatives) Chordata (vertebrates and relatives) Ambulacraria Echinodermata (starfish and relatives) Hemichordata (acorn worms and relatives) Protostomia Ecdysozoa Scalidophora Kinorhyncha (mud dragons) Priapulida (penis worms) Loricifera (corset animals) Cryptovermes Nematoida Nematoda (roundworms) Nematomorpha (horsehair worms) Panarthropoda Onychophora (velvet worms) Arthropoda (insects and relatives) Tardigrada (waterbears) Spiralia Gnathifera Chaetognatha (arrow worms) Gnathostomulida (jaw worms) M+R Micrognathozoa (Limnognathia) Rotifera (wheel animals inc. acanthocephalans) Platytrochozoa C+E Cycliophora (Symbion) Entoprocta or Kamptozoa Rouphozoa Platyhelminthes (flatworms) Gastrotricha (hairybacks) Mesozoa Orthonectida Dicyemida or Rhombozoa ?Monoblastozoa (Salinella) Lophotrochozoa Annelida (earth worms and relatives) Mollusca (snails and relatives) Nemertea (ribbon worms) Lophophorata Brachiopoda (lamp shells) B+P Bryozoa or Ectoprocta (moss animals) Phoronida (horseshoe worms) The phylogeny of Bilateria is disputed; see also Nephrozoa Deuterostomia Xenambulacraria Centroneuralia Major groups within phyla Sponges Demosponges Glass sponges Calcareous sponges Cnidarians Anthozoans inc. corals Medusozoans inc. jellyfish Myxozoans Chordates Lancelets Tunicates Vertebrates Echinoderms Sea lilies Asterozoans inc. starfish Echinozoans inc. sea urchins Hemichordates Acorn worms Pterobranchs Nematodes Chromadorea Enoplea Secernentea Arthropods Chelicerates inc. arachnids Myriapods Pancrustaceans inc. hexapods Rotifera Bdelloidea Monogononta Seisonidae Acanthocephala Platyhelminths Turbellaria Trematoda Monogenea Cestoda Ectoproctans Phylactolaemata Stenolaemata Gymnolaemata Annelids Polychaetes Clitellata Sipuncula Molluscs Gastropods Cephalopods Bivalves Chitons Tusk shells Phyla with ≥1000 extant species bolded Potentially dubious phyla †

v t e Extant life phyla/divisions by domain Bacteria Abditibacteriota Acidobacteriota Actinomycetota Aquificota Armatimonadota Atribacterota Bacillota Bacteroidota Balneolota Caldisericota Calditrichota Chlamydiota Chlorobiota Chloroflexota Chrysiogenota Coprothermobacterota Cyanobacteriota Deferribacterota Deinococcota Dictyoglomerota Elusimicrobiota Fibrobacterota Fidelibacterota Fusobacteriota Gemmatimonadota Kiritimatiellota Lentisphaerota Minisyncoccota Mycoplasmatota Nitrospinota Nitrospirota Planctomycetota Pseudomonadota Rhodothermota Spirochaetota Synergistota Thermodesulfobacteriota Thermomicrobiota Thermotogota Verrucomicrobiota Vulcanimicrobiota "Acetithermota" "Aerophobota" "Auribacterota" "Babelota" "Binatota" "Bipolaricaulota" "Caldipriscota" "Calescibacteriota" "Canglongiota" "Cloacimonadota" "Cosmopoliota" "Cryosericota" "Deferrimicrobiota" "Dormiibacterota" "Effluvivivacota" "Electryoneota" "Elulimicrobiota" "Fermentibacterota" "Fervidibacterota" "Goldiibacteriota" "Heilongiota" "Hinthialibacterota" "Hydrogenedentota" "Hydrothermota" "Kapaibacteriota" "Krumholzibacteriota" "Kryptoniota" "Latescibacterota" "Lernaellota" "Lithacetigenota" "Macinerneyibacteriota" "Margulisiibacteriota" "Methylomirabilota" "Moduliflexota" "Muiribacteriota" "Nitrosediminicolota" "Omnitrophota" "Parcunitrobacterota" "Peregrinibacteriota" "Qinglongiota" "Rifleibacteriota" "Ryujiniota" "Spongiamicota" "Sumerlaeota" "Sysuimicrobiota" "Tangaroaeota" "Tectimicrobiota" "Tianyaibacteriota" "Wirthibacterota" "Zhuqueibacterota" "Zhurongbacterota" Archaea Methanobacteriota Microcaldota Nanobdellota Promethearchaeota Thermoproteota "Aenigmatarchaeota" "Altarchaeota" "Augarchaeota" "Geoarchaeota" "Hadarchaeota" "Hadesarchaeota" "Huberarchaeota" "Hydrothermarchaeota" "Iainarchaeota" "Micrarchaeota" "Nanohalarchaeota" "Nezhaarchaeota" "Parvarchaeota" "Poseidoniota" "Undinarchaeota" Eukaryote Protist Acavomonidia Amoebozoa Anaeramoebae Apicomplexa Bigyra Bigyromonadea Caelestes Cercozoa Chlorophyta Chromerida Ciliophora Colponemidia Cryptista Dinoflagellata Euglenozoa Foraminifera Fornicata Glaucophyta Haptophyta Hemimastigophora Malawimonada Nebulidia Nibbleridia Ochrophyta Preaxostyla Heterolobosea Hyphochytriomycota Oomycota Parabasalia Perkinsozoa Picozoa Radiolaria Rhodelphidia Rhodophyta Telonemia Fungi Chytridiomycota Blastocladiomycota Neocallimastigomycota Glomeromycota Zygomycota Ascomycota Basidiomycota Land plant Bryophyta Marchantiophyta Anthocerotophyta Lycopodiophyta Pteridophyta Cycadophyta Ginkgophyta Pinophyta Gnetophyta Animal Porifera Ctenophora Placozoa Cnidaria Xenacoelomorpha Chordata Hemichordata Echinodermata Chaetognatha Kinorhyncha Loricifera Priapulida Nematoda Nematomorpha Onychophora Tardigrada Arthropoda Platyhelminthes Gastrotricha Orthonectida Dicyemida Rotifera Acanthocephala Gnathostomulida Micrognathozoa Cycliophora Nemertea Phoronida Bryozoa Entoprocta Brachiopoda Mollusca Annelida Incertae sedis Parakaryon Relate: Extraterrestrial life

Taxon identifiers Nematomorpha Wikidata: Q5186 Wikispecies: Nematomorpha ADW: Nematomorpha AFD: Nematomorpha BOLD: 261851 EoL: 1539 Fauna Europaea: 11802 Fauna Europaea (new): 73e74fb8-88f4-4a4a-b27d-edfc96ae15ed GBIF: 64 iNaturalist: 63142 IRMNG: 223 ITIS: 64183 NBN: NHMSYS0000841368 NCBI: 33310 NZOR: 77e832fd-e34c-4197-a049-baa2b0ae9022 Open Tree of Life: 189836 Paleobiology Database: 67132 WoRMS: 162564

Authority control databases International GND National Latvia Israel Other Yale LUX

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Adapted from the Wikipedia article [Nematomorpha](https://en.wikipedia.org/wiki/Nematomorpha) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Nematomorpha?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.