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Definition: nematomorph from The Macquarie Dictionary

a member of the phylum Nematomorpha comprising parasitic animals similar in shape and behaviour to nematode worms.

Summary Article: Nematomorpha (Horsehair Worms)
from Encyclopedia of Life Sciences

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.





horsehair worm

marine hairworm


freshwater hairworm


Members of the phylum Nematomorpha, the hairworms, represent a relict group that occurs worldwide and has no clear or close relationship with any other living phylum of invertebrates. Based on the number of described species, which is less than 400, nematomorphs can be considered as a small and little studied group (Poinar, 2008). Although one genus, Nectonema, is marine and develops inside crabs and shrimps, all of the remaining species occur in freshwater as adults, and as parasites of aquatic or terrestrial invertebrates, especially insects, during their developmental stage. Several myths surround hairworms (often called horsehair worms in rural areas); the most common being that they originated from horse mane hairs that came alive after falling into water. Even though Joseph Leidy disproved this 'theory' in 1870 after placing horse hairs in water without observing any 'vivification', there are some who still hold to this belief. See also Leidy, Joseph

Basic Design

There are two classes in the phylum Nematomorpha: the freshwater hairworms or Gordiaceae, comprising over a dozen genera and the marine hairworms or Nectonematoidea, with a single genus (Poinar and Brockerhoff, 2001). Freshwater hairworms are often encountered as free-living adults in sources such as ponds, streams, lakes and even man-made structures like watering troughs.

Hairworms are normally dark brown or black (sometimes yellowish) in colour, with the male often a slightly darker hue than the female. Externally, the body is covered with a hard opaque multilayered cuticle, which can be relatively smooth (as in Gordius of the family Gordiidae) or contain various projections (areoles), warts or bristles on the surface (as in members of the family Chordodidae). The sculpturing on the epicuticle furnishes important taxonomic characters. The anterior end of hairworms is usually attenuated whereas the posterior end may be rounded, bilobed or even trilobed. Beneath the cuticle are many layers of cylindrical collagenous fibres, spirally coiled along the length of the adult. Below the underlying epidermis are layers of longitudinal muscles. As with nematodes, circular muscles are absent in the Nematomorpha. The body cavity (usually termed a pseudocoelom) is filled with mesenchymal cells, which produce a foamy appearance. The mouth of the adult is located at the 'calotte', or anterior tip of the body, whereas the associated pharynx is thought to be nonfunctional and the one-celled thick intestine appears to be degenerate. There is no evidence that the modified alimentary tract is used to process food and in fact the intestine, which has separated from the remainder of the alimentary tract, is thought by some to function as an excretory organ. Host body fluids would then probably be absorbed through the body wall, much like the situation in mermithid nematodes, although in the latter group the modified intestine is a food storage organ or stichosome. The nervous system consists of a ganglionic mass of cells lying in the head region, which is connected to a ventral nerve cord. Other sensory structures may also occur in the head region and a large sac in the head of Paragordius was considered to function as a primitive 'eye' by one author. In both sexes of hairworms, the genital ducts empty into the intestine, forming a cloaca (common chamber receiving the ducts of both the reproductive and alimentary systems). Males have paired cylindrical testes, each of which connects with the cloaca via a separate sperm duct. Females possess paired ovaries and separate oviducts. As far as is known, amphimixis (bisexual reproduction) is the only type of reproduction in hairworms. See also Nematoda (Roundworms)

The eggs of hairworms are microscopic in size and deposited in water. The minute larval stage (Figure 1) that hatches from the egg is quite different from the adults (Figure 2) and is adapted to enter the tissues of potential hosts. The larva consists of an anterior presoma (preseptum) containing a proboscis armed with rows of spines (or hooks) used to pull the body into the tissues of invertebrate hosts, and a body (post-septum) containing the adult tissue primordia.

The preparasitic larva of the freshwater hairworm, Neochordodes occidentalis.

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

Marine hairworms of the genus Nectonema possess both a ventral and dorsal epidermal cord, in contrast to only a ventral cord in other members of the phylum. Nectonema also has a complete body cavity, unfilled by mesenchymal cells, extending the length of the organism. A rather unique character of marine hairworms is the presence of giant cells (usually four) in the anterior portion of the body (Figure 3). The function of these cells is not known but they appear glandular in nature.

Giant cells in the anterior end of Nectonema zealandica.

Diversity and Lifestyles

Diversity in hairworms comes, in part, from their ability to parasitise a wide range of invertebrate hosts. Classes of invertebrates that have been reported to serve as developmental hosts for freshwater hairworms are Hirudinea (leeches), Chilopoda (centipedes), Diplopoda (millipedes) and the Insecta. Concerning the latter class, some 12 orders and 26 families of insects serve as developmental hosts of freshwater hairworms, with the highest numbers in the orders Coleoptera (beetles) and Orthoptera (roaches, grasshoppers, crickets, etc.). Marine hairworms are only known to parasitise Crustacea (crabs and shrimps) and they have been recovered from tropical and temperate waters in both hemispheres. See also Crustacea (Crustaceans), Insecta (Insects), and Parasitism: The Variety of Parasites

The exact number of larval stages in the life history of hairworms is not known since the total number of moults has not yet been documented in a single species. Four stages commonly occur in freshwater hairworms: the egg, the preparasitic larva that hatches from the egg and enters an invertebrate paratenic host, the parasitic larva that develops within the invertebrate developmental host and the free-living aquatic adult. One or two moults apparently occur inside the developmental host. Invertebrates are the only known developmental hosts of hairworms. However, the life cycle often involves a paratenic host (one in which the hairworm larva enters but does not develop) that is later ingested by a developmental or final host (one in which the hairworm can complete its development). The paratenic host is usually an invertebrate, but can also be a vertebrate. Indeed, preparasitic larvae are known to bore into human tissue. Adult hairworms have even been recovered from the digestive and urogenital tracts of humans; however, there is no evidence that they developed in these sites.

In most freshwater hairworms, the paratenic host is the larval stage of an insect whose adult stage is terrestrial. When the preparasitic larva is ingested by a paratenic insect host, it passes into the midgut of the host and then, with the aid of its hook-armed proboscis, pulls or bores its way through the gut epithelial cells and enters the body cavity. After reaching the fat body or other internal organs, the larva encysts. The cyst (Figure 4) is formed by secretions released from glands located in the body portion of the larva. Afterwards, the cyst may be encapsulated by host blood cells as the invertebrate responds to the foreign intruder. There is no evidence of feeding or growth of the hairworm larva inside the paratenic host. If the resting larva is removed from its cyst, it will often attempt to re-encyst if provided with another paratenic host. These encysted larvae will normally remain alive as long as the host, although some may be killed by deposits of melanin produced by the host as a defence reaction. The encysted larvae pass unharmed through the pupal and into the adult stage of the paratenic host. In fact, it is often essential that the encysted larvae enter the adult stage of the paratenic host since that is often the stage eaten by a larger potential developmental host. When this occurs, the hairworm larvae leave their cysts, traverse the gut wall, and initiate development in the haemocoel of the developmental host.

The life cycle described above involving a paratenic host is the one most frequently encountered. However, some claim that the preparasitic larva can encyst on aquatic vegetation and that the developmental host is infected after ingesting infested plant material. Others report that the preparasitic larva is simply ingested directly by the developmental host without the intervention of a paratenic host. Most definitive hosts are large-bodied insects such as grasshoppers, gryllacridids, stenopelmatids, cockroaches and beetles. The final moult to the adult stage occurs inside the developmental host. See also Coevolution: Host-Parasite

Thus, hairworms are already adults when they emerge from their developmental host, which they normally do by boring through membranous tissue between the host's sclerites (Figure 2).

Encysted larva of Neochordodes occidentalis in a paratenic host.

One interesting feature of the life history of freshwater hairworms is how they are able to 'lead' the host to a water source when the parasite has finished its development and is ready to emerge. Since the life cycle of hairworms can only continue when the adults have reached fresh, and since many of the hosts are truly terrestrial and only sporadically visit standing water, it would appear that the cycle would not be able to continue. However, by some unknown process, the mature hairworm can change the behaviour of the host to make the latter enter a water source. This could be the result of a driving thirst as the hairworm takes up large amounts of liquid from the host's haemolymph, or it could be much more sophisticated and involve some hormonal change. Obviously, the hairworms are able to sense the moment when the developmental host enters water since they then leave their hosts. In most cases, the developmental host dies after the parasite leaves, however some large-bodied hosts can continue to survive for some time if the wound heals properly. In still water, freshwater hairworms swim by simple body undulations, while in moving water they are carried to quiet backwater areas by the current. Mating occurs as the male winds its body around that of the female. Although there is no intromittent organ in the male, some observations suggest that sperms are placed in the female cloaca, whereas other researchers observed spermatophores deposited in the vicinity of the female being taken up into her cloaca. The eggs may be deposited singly, in clusters or in elongate strings. Special adaptations occur with hairworms that live in fast-flowing streams. In this habitat, the females affix their eggs to rocks or other stationary objects in the stream bed so that the eggs are not washed away.

Species of Nectonema parasitise various crabs and shrimp, including both shore and pelagic forms. The adults possess external natatory bristles, which occur in rows along their body wall. They can often be seen swimming by undulatory body movements in harbours or bays. Their life cycle is unknown but it is assumed to include a paratenic host, possibly a member of the Annelida or Crustacea (Poinar and Brockerhoff, 2001).

Fossil History and Phylogeny

Although the Nematomorpha is undoubtedly an ancient group that has survived to the present and probably at least dates back to the Carboniferous, there are only two records of definite hairworm fossils. The first is two specimens in 20-30 million year (My) Dominican Republic amber. One is in the act of leaving the body of its cockroach developmental host, whereas the other has already emerged (Poinar, 1999). The second record is a single hairworm in 100 My Early Cretaceous Burmeseamber. This specimen is not associated with any host (Poinar and Buckley, 2006) See also Fossil Record

Various authors have attempted to show relationships between hairworms and several other 'pseudocoelomate' phyla such as the kinorhynchs, priapulids, rotifers and nematodes. However, it is difficult to establish lineages when many basic aspects of hairworm morphology and development have yet to be elucidated. Suffice to say that the body plans of both the preparasitic larva and the adult hairworm are unique in the animal kingdom. Even the sperms are considered aberrant and support the contention that the hairworms separated from the other members of the Aschelminthes quite early and evolved independently thereafter. See also Nematoda (Roundworms), Rotifera, Priapulida, and Kinorhyncha

  • 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.
  • Glossary

    Raised areas separated by grooves on the epicuticle of many adult freshwater hairworms.


    The posterior portion of the two-segmented preparasitic larva that contains the glands of encystment, also called as the postseptum.


    The anterior or head region of adult freshwater hairworms.


    Spherical deposit that encloses the hairworm larva in a paratenic host.

    Developmental host

    A host in which the parasite can complete its development. Sometimes called the final host.


    The outermost body layer of a hairworm adult, often sculptured and containing areoles, bristles and warts.

    Freshwater hairworms

    Members of the class Gordiaceae that occur in terrestrial or freshwater hosts.

    Giant cells

    Large, glandular appearing cells (usually four) in the anterior region of marine hairworms (Nectonema sp.).


    Stage of freshwater hairworm found in either an aquatic environment (preparasitic larva), a paratenic host (resting larva) or developmental host (parasitic larva).

    Marine hairworms

    Members of the class Nectonematoidea that occur in marine hosts.

    Paratenic host

    A host that carries a parasite internally but does not support its growth or reproduction.


    The anterior portion of the two-segmented preparasitic larva that contains the reversible proboscis, also called as the preseptum.

    George Poinar Jr
    Oregon State UniversityCorvallis, Oregon, USA
    Wiley ©2007

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