aka: Horsehair Worms
aka: Hairworms

Horsehair worms belong to the phylum Nematomorpha and are typically obligate parasites of terrestrial arthropods (e.g., beetles, crickets, cockroaches, locusts, grasshoppers, and mantids). As adults, however, they are free-living in aquatic environments. These worms are sometimes found in coiled clusters termed “Gordian knots” from the intricate legendary knot of Greek mythology. Another myth is related to the common name given these worms, “hairworms” or “horsehair worms,” originating from the idea that horse hairs that fell into water became worms. This belief was not disproved scientifically until American anatomist and paleontologist Joseph Leidy (1823–1891) noted in 1870 that horse hairs placed in water for many months did not come to life. The first published report of a horsehair worm from Arkansas was in 1907.

Nematomorphs have been placed in two taxa, the marine Nectonematoidea and the freshwater Gordiida (=Gordioidea). Among the latter group, four families and at least twenty-one genera (including two fossil genera) have been recognized. The Nematomorpha is an ancient phylum that extends back to at least the Carboniferous, with the oldest fossil dating from the early Cretaceous, or about 100 million years ago. The Nematomorpha are considered the sister phylum to the roundworms, phylum Nematoda.

There are over 350 species, with the majority from freshwater and about five known species of the same genus (Nectonema) occurring in marine environs. However, current estimates suggest that only eighteen percent of horsehair worm species have been described worldwide. The marine genus Nectonema has been reported from the northern Atlantic, the Mediterranean, Indonesia, Japan, Norway, and possibly New Zealand. Members of the freshwater genera Gordius and Paragordius are cosmopolitan, while other genera (Chordodes, Beatogordius, Gordionus, Neochordodes, Parachordodes, and Pseudochordodes) have a more restricted geographic distribution. There are at least eighteen species known from North America, with eight or more species found in Arkansas.

Freshwater habitats include watering troughs, puddles, creeks, rivers, and subterranean (cavernicolous) streams. There is no special collecting or sampling technique to gather adult specimens, as most can be netted or simply collected from water by hand, most often during late spring and summer. For accurate identification, both sexes should be collected; however, some males can be identified alone using appropriate taxonomic keys.

Historically, horsehair worms were first noted by St. Albertus Magnus (1193–1280) and studied by others in the seventeenth century in Europe. In the United States, Joseph Leidy was a pioneer of studies of nematomorphs, and, between 1898 and 1907, Thomas H. Montgomery Jr. contributed to early knowledge of the North American fauna. More recently, a great deal has been discovered about horsehair worms from the collaborative research of Matthew Bolek (Oklahoma State University), Ben Hanelt (University of New Mexico), John Janovy Jr. (University of Nebraska), and Andreas Schmidt-Rhaesa (Universität Hamburg).

The general life cycle of a gordiid includes juveniles as parasites of arthropods that live on land, whereas adults live freely (not as parasites) in aquatic environments. Worms mate in the water and oviposit egg strings that eventually produce infective stages. These develop in about one to two weeks into semi-sessile larvae that can survive between two weeks and two months. These larvae infect and encyst indiscriminately within a variety of aquatic vertebrate and invertebrate hosts. Two crucial transmissions must be made during their life cycle: (1) transition from terrestrial definitive host to water and, (2) transition from aquatic larvae to the definitive host. Interestingly, other hosts may be responsible for bridging the gap between the free-living aquatic environment and parasitic terrestrial environment.

During the worms’ parasitic stage, they modify host behavior. For example, the worms are well known for inducing water-seeking at the expense of the host, leading it to essentially commit suicide in delivering the parasite to the habitat for adult worm emergence. At the end of the worms’ parasitic stage, the infected host (a cricket, for instance) enters water, and the adult worms are released. One recent study has even shown that infection with nematomorphs negatively affects calling behavior of male crickets.

Interestingly, a new and unique species of horsehair worm—Paragordius obamai—was named in 2012 in honor of the forty-fourth president of the United States. This new worm was collected in a remote part of Kenya at Kasabong stream, which is eleven miles to the northwest of the town of Kogelo, where President Barack Obama’s father was born.

One gordiid, Paragordius varius, is likely the most common and widespread species in the New World; it is distributed throughout the contiguous United States and Canada and has also been reported from Hawaii and from throughout South America. Paragordius varius is the first widely domesticated gordiid species and is the easiest to maintain in the laboratory, where it produces equal number of sexes and females lay 500,000 to 8,000,000 eggs over their two-week life span.

Horsehair worms have no direct economic importance, although they can serve as natural biological control agents by reducing the number of plant-feeding grasshoppers and crickets. They also have no real medical importance but have been reported to be passed in the vomitus or feces of humans, perhaps from contaminated food or water. In addition, adult hairworms have been associated with the urogenital tract of humans, and larvae will burrow into human facial tissue, sometimes resulting in orbital tumors.

The first published report of a horsehair worm from Arkansas (unknown specific locality) was in 1907 by Thomas H. Montgomery Jr., who found Gordius villoti Rosa, which is now G. robustus Leidy. Undetermined gordiids were reported from caves in Izard and Stone counties in 1976. In addition, macroinvertebrate surveys by students at Arkansas State University in Jonesboro (Craighead County) during the period from 1983 to 2006 reported Gordius sp. and Paragordius sp. from the Hiatt Prairie, St. Francis Sunken Lands, Strawberry River system, and the Dale Bumpers White River National Wildlife Refuge. However, the most comprehensive study in Arkansas, to date, documented new state records in 2012 for Chordodes morgani and Paragordius varius, as well as county records for those in the Gordius sp. “complex.” Those horsehair worms in the latter complex include cryptic species thought to be consisting of at least eight distinct species. Molecular analysis will be needed to help unravel this complex of hairworms.

Identification of horsehair worms using traditional morphological techniques of larvae is problematic, so it is necessary to use molecular approaches to identify larvae in infected hosts. This helps facilitate linking of the juvenile and adult stages, essential to understanding each species’ ecology and life history. Researchers are just now beginning to help advance the body of knowledge of this poorly known phylum.

For additional information:
Barquin, A., B. McGehee, R. T. Sedam, W. L. Gordy, Ben Hanelt, and M. R. Wise de Valdez. “Calling Behavior of Male Acheta domesticus Crickets Infected with Paragordius varius (Nematomorpha: Gordiida).” Journal of Parasitology 101 (2015): 393–397.

Begay, Alyssa C., Andreas Schmidt-Rhaesa, Matthew G. Bolek, and Ben Hanelt. “Two New Gordionus species (Nematomorpha: Gordiida) from the Southern Rocky Mountains (USA).” Zootaxa 3406 (2012): 30–38.

Biron, David G., Fleur Ponton, Cécile Joly, Aurélie Menigoz, Ben Hanelt, and Frédéric Thomas. “Water Seeking Behaviour in Insects Harbouring Hairworms: Should the Host Collaborate with the Parasite?” Behavioral Ecology 16 (2005): 656–660.

Bolek, Matthew G., Erin Rogers, Cleo Szmygiel, Ryan P. Shannon, Whitney E. Doerfert-Schrader, Andreas Schmidt-Rhaesa, and Ben Hanelt. “Survival of Larval and Cyst Stages of Gordiids (Nematomorpha) after Exposure to Freezing.” Journal of Parasitology 99 (2013): 397–402.

Bolek, Matthew, G., Cleo Szmygiel, Austin Kubat, Andreas Schmidt-Rhaesa, and Ben Hanelt. “Novel Techniques for Biodiversity Studies of Gordiids and Description of a New Species of Chordodes (Gordiida, Nematomorpha) from Kenya, Africa.” Zootaxa 3717 (2013): 23–38.

Cochran, Phillip A., Jacob D. Zanon, and Ben Hanelt. “The Horsehair Worm Gordionus violaceus (Nematomorpha: Gordiida) in Minnesota.” Great Lakes Entomologist 46 (2013): 133–134.

Fair, Jeanne M., Ben Hanelt, and Kassidy Burnett. “Horsehair Worms (Gordius robustus) in Nests of the Western Bluebird (Sialia mexicana): Evidence for Anti-Predator Avoidance?” Journal of Parasitology 96 (2010): 429–430.

Hanelt, Ben. “An Anomaly against a Current Paradigm: Extremely Low Rates of Individual Fecundity Variability of the Gordian Worm (Nematomorpha: Gordiida).” Parasitology 135 (2009): 211–218.

———. “Hyperparasitism by Paragordius varius (Nematomorpha: Gordiida) Larvae of Monostome Redia (Trematoda: Digenea).” Journal of Parasitology 95 (2009): 242–243.

———. “Nematomorpha.” In Grzimek’s Animal Life Encyclopedia: Lower Metazoa, 2nd ed. Edited by M. Hutchins and D. Thoney. Chicago: Schlager Group Inc., 2003.

Hanelt, Ben, Matthew G. Bolek, and Andreas Schmidt-Rhaesa. “Cryptic Species of Hairworm Parasites Revealed by Molecular Data and Crowdsourcing of Specimen Collections.” Molecular Phylogenetics and Evolution 82 (2015): 211–218.

———. “Going Solo: Discovery of the First Parthenogenetic Gordiid (Nematomorpha: Gordiida).” PLoS ONE 7 (2012): e34472.

Hanelt, Ben, and John Janovy Jr. “Life Cycle and Paratenesis of American Gordiids (Nematomorpha: Gordiida).” Journal of Parasitology 90 (2004): 240–244.

———. “The Life Cycle of a Horsehair Worm, Gordius robustus (Gordioidea: Nematomorpha).” Journal of Parasitology 85 (1999): 139–141.

———. “Morphometric Analysis of Non-Adult Characters of Common Species of American Gordiids (Nematomorpha: Gordioidea).” Journal of Parasitology 88 (2002): 557–562.

———. “New Host and Distribution Record of Gordius difficilis (Nematomorpha: Gordioidea) from a Vivid Metallic Ground Beetle Chlaenius prasinus (Coleoptera: Carabidae) from Western Nebraska, USA.” Comparative Parasitology 67 (2000): 107–108.

———. “Spanning the Gap: Identification of Natural Paratenic Hosts of Horsehair Worms (Nematomorpha: Gordiida) by Experimental Determination of Paratenic Host Specificity.” Invertebrate Biology 122 (2003): 12–18.

———. “Untying a Gordian Knot: The Domestication and Laboratory Life Cycle of Paragordius varius (Nematomorpha: Gordiida).” Journal of Natural History 38 (2004): 939–950.

Hanelt, Ben, F. Thomas, and Andreas Schmidt-Rhaesa. “Biology of the Phylum Nematomorpha.” Advances in Parasitology 59 (2005): 243–305.

Looney, Chris, Ben Hanelt, and Richard S. Zack. “New Records of Nematomorph Parasites (Nematomorpha: Gordiida) of Ground Beetles (Coleoptera: Carabidae) and Camel Crickets (Orthoptera: Rhabidophoridae) in Washington State.” Journal of Parasitology 98 (2012): 1–6.

McAllister, Chris T., Matthew G. Bolek, and Ben Hanelt. “Horsehair Worm, Paragordius varius (Nematomorpha: Gordiida): New to the Fauna of Oklahoma.” Southwestern Naturalist 58 (2013): 249–250.

McAllister, Chris T., and Ben Hanelt. “New Host and Geographic Distribution Record for Chordodes morgani (Nematomorpha: Gordiida), from Oklahoma.” Proceedings of the Oklahoma Academy of Science 92 (2012): 75–76.

Montgomery, Thomas H., Jr. “The Distribution of the North American Gordiacea, with Description of a New Species.” Proceedings of the Academy of Natural Sciences of Philadelphia 59 (1907): 270–272.

Poinar, George O., Jr., and J. J. Doelman. “A Reexamination of Neochordodes occidentalis (Montg.) comb. N. (Chordodidae: Gordioidea): Larval Penetration and Defense Reaction in Culex pipiens L.” Journal of Parasitology 60 (1974): 327–335.

Robison, Henry W., Chris T. McAllister, and Ben Hanelt. “New Geographic Distribution Records for Horsehair Worms (Nematomorpha: Gordiida) in Arkansas, Including New State Records for Chordodes morgani and Paragordius varius.” Journal of the Arkansas Academy of Science 66 (2012): 197–199. Online at (accessed October 20, 2020).

Schmidt-Rhaesa, Andreas, Ben Hanelt, and Will Reeves. “Redescription and Compilation of North American freshwater Nematomorpha (Gordiida).” Proceedings of the Academy of Natural Sciences of Philadelphia 153 (2003): 77–117.

Smith, Darryl R., Dean D. Watson, Robert R. Clevenger, and R. Craig McBride. “Paragordius varius.” Archives of Pathology and Laboratory Medicine 114 (1990): 981–983.

Szmygiel, Cleo, Andreas Schmidt-Rhaesa, Ben Hanelt, and Matthew G. Bolek. “Comparative Descriptions of Non-Adult Stages of Four Genera of Gordiids (Phylum: Nematomorpha).” Zootaxa 3768 (2014): 101–118.

Chris T. McAllister
Eastern Oklahoma State College


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