Paragordius varius

Geographic Range

The distribution of Paragordius varius is throughout both North and South America. This horsehair worm has been reported from the northern-most portions of Canada throughout the United States most commonly along the Eastern coastline and near the Great Lakes Region. Twenty-six states (including Hawaii) have had confirmed sitings of P. varius. This species has also been observed in Cuba, in Central America ranging from Costa Rica through Guatemala, as well as in the South American countries of Brazil, Argentina, and Columbia. (Hanelt and Janovy, 2004)


Paragordius varius adults live in quiet freshwater habitats such as lakes and slow-moving streams. In some cases adults may even be found in puddles or other places where rainwater commonly collects. Adults are non-feeding and are most commonly found near the shoreline to facilitate mating. Females may go on land briefly to lay their eggs before returning to the water, but this is not necessary and it is not uncommon for an adult to never leave the shallows of their freshwater habitat.

Eggs can survive on land or water. If they are deposited on land it is usually near the shoreline and the string of eggs is wound around a blade of grass. Eggs laid in the water are deposited close to the shore and may or may not be anchored to some type of aquatic vegetation.

Larvae must develop within the body cavity of a member of their definitive hosts, the Orthoptera. (Gustav, 1920)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams
  • temporary pools

Physical Description

Paragordius varius is a horsehair worm with a slightly off-center mouth and distinct grooves running along the body both dorsally and ventrally. The worm has a cylindrical body shape, with a slight decrease in diameter towards the anterior and posterior portions of the body. This decrease is exaggerated more in the females than the males. In both males and females, the body is typically light brown with a darker brown ring surrounding the anterior-most portion of the body. The body itself is enclosed in a cuticle that is densely covered with closely-spaced rounded areoles which are covered with small bristles. The area between the areoles is charcterized by the presence of cuticular rims, giving the illusion that the cuticle is covered in stripes. The areoles themselves vary between 20-30 µm in size and the majority of them contain a rounded nodule known as a tubercle at their center. These tubercles project an average of 6 µm above the cuticle of the worm. (de Villalobus, et al., 2000)

Adult males reach an average of 170 mm in length but generally range from 120-290 mm. The posterior end of the body is bifurcating, meaning that it is divided into two elongated lobes averaging between 560-700 µm in length and between 140-150 µm in diameter. The cloaca is oval-shaped and is found between 170-210 µm from the point of bifucation of the lobes at the posterior end of the worm. Cone-shaped bristles approximately 8 µm in length are found on either side of the cloaca and form several lines at the ventral anterior end of the worm. At the posterior-most end of the worm the bristles are found on the inner ventral portions of the bifurcating lobes but do not occur at their tips. (de Villalobus, et al., 2000)

Adult females are slightly larger than males ranging from 120-310 mm in length but have a similar diameter. The posterior end of the adult females differs from the male in that it is divided into three lobes instead of two (trifurcating versus bifurcating). The lobes do not have either bristles or tubercles on their surfaces and are slightly larger than in males, varying in length from 790 to 820 µm and in width from 190 to 230 µm. The cloaca has a similar positioning to that found in males but is more rounded in nature. Paragordius varius larvae are pure white during the entirety of the developmental period. Pigmentation takes place only slightly before the adult emerges from its host and the color is never altered. A larva has two primary body regions separated by a septum. The anterior portion of the body has hooks and stylets (which are lost in the adult form) and the posterior portion includes a gland and another morphological form that may be the beginnings of the intestine. (Schmidt-Rhaesa, 2005; de Villalobus, et al., 2000)

  • Sexual Dimorphism
  • female larger
  • sexes shaped differently
  • Range length
    100 to 470 mm
    3.94 to 18.50 in


Paragordius varius typically completes larval development within the egg 10-12 days after being laid. The fully developed larva will remain in the egg for another 7-10 days before hatching. Larvae are free-swimming and must find an intermediate host to infect within a few days or they will not survive. This intermediate host is most commonly an aquatic insect that is also in its larval stage. Infection occurs when the P. varius larvae bore into the tissue of the host using the hooks located on the anterior portions of the body. Immediately following infection a larva will form a cyst, typically in the intestine or musculature region of the host and remain in this dormant stage throughout the winter. The cyst is unaffected by and has no effect on the various phases of metamorphosis the aquatic insect undergoes during the winter and early spring (Hanelt & Janovy, 2004). Once the intermediate host has completed metamorphosis it flies to land where it may encounter the definitive host (an orthopteran) of P. varius. Transition to the definitve host is necessary for a larvae to develop into an adult. Infection of the definitive host can occur via direct predation of the insect by the host or, more commonly, when the insect dies (from causes not related to the larva) and its carcus is consumed by the definitive host. Once inside, a larva moves to the body cavity of the definitive host where it will complete development. A larva grows from approximately 10 mm to over 120 mm in length and from 93 µm to 400 µm in diameter in under 30 days. The adult cuticle develops around 20 days after infection of the definitive host. The larva has a physiologically active epidermis and intestine at the start of development, but activity tapers off as the intestines begin to decrease in size and the larval epidermis is replaced by the adult cuticle (Schmidt-Rhaesa, 2005). This represents a shift in the organism from a parastic larvae to a non-feeding adult form. Total development is short, only taking around 30 days, which allows for P. varius to produce up to three generations within a single year (Thorpe & Covich, 1991). Once it has reached adulthood P. varius will emerge from the definitive host when the host is near its preferred water habitat, usually during the late spring or summer. Paragordius varius may be able to manipulate its host to encourage it to enter the shallows of a body of water, the preferred habitat for adult P. varius. This theory is still being investigated (Hanelt et al., 2005). (Hanelt and Janovy, 2004; Hanelt, et al., 2005; Schmidt-Rhaesa, 2005; Thorpe and Covich, 1991)


Adults typically mate one to two days after they have left their definitive hosts. They will spend their time swimming in the shallows and may or may not move up onto land near the edge of the water to mate. When a male and female initially encounter each other, the female will give a stimulus that signals the male that she is ready and willing to mate. During copulation there is no penetration and the male simply releases his sperm almost immediately after the female gives the stimulus. The male has little to no control over the direction of his sperm and many times the sperm does not land on the appropriate posterior area of the female and conception does not occur. If the sperm does manage to land on the females' posterior end, it forms a large circular glob. The glob gradually passes into the seminal receptacle of the female, normally within 24 hours after mating. (Gustav, 1920)

Eggs are typically deposited in strings varying from 0.2-0.4 mm in diameter. Up to 6 million eggs may be layed at once and a female may continue to deposit eggs continuously for a 24 hour period. If the female has mated on land she will most likely wind her string of eggs around a string of grass or anchor it to some other structure. Other females will lay their eggs freely in the aquatic environment. Males die almost immediately following copulation and females die one to two days after depositing eggs. (Gustav, 1920)

  • Breeding interval
    Paragordius varius breeds once in its lifetime.
  • Breeding season
    Breeding season is usually late spring, early summer.

Paragordius varius does not partake in any paternal behaviors. Adults are typically deceased before their offspring hatch. (Gustav, 1920)

  • Parental Investment
  • no parental involvement


Paragordius varius survives for around one year. The majority of that time is spent in a dormant cyst stage within the intermediate host. Adult P. varius survive only a matter of one or two weeks after exiting their definitive hosts if they do not copulate. If copulation does occur, males will die almost immediately thereafter and females will die one to two days after she finishes depositing her eggs. (Schmidt-Rhaesa, 2005)

  • Average lifespan
    Status: wild
    1 years


Adult P. varius are non-feeding and survive for a short period of time in the shallows of whatever water source they were deposited in by the definitive host. Most of this time is spent searching for a mate. The only other free-swimming stage in the life cycle of P. varius occurs immediately after they have hatched from their eggs. The larvae only have a time span of one to two days to locate an intermediate host. During this time, the larvae do not feed and do not exit the water. (Hanelt and Janovy, 2004)

Communication and Perception

Paragordius varius may communicate via chemosensory receptors located throughout their bodies that may be used in the free-swimming larval stage to locate their intermediate hosts. These receptors also have the ability to sense the presence of water when their definitive host is near their preferred aquatic environment. This ability helps the fully developed adult P. varius to know when to begin a series of manipulations on the host to get it to enter the water. Finally, these chemoreceptors may aid P. varius in locating a mate during its free-living adult form. (Hanelt and Janovy, 2004)

Food Habits

Adult P. varius worms are non-feeding. The larvae are parasites of their definitive host and can absorb nutrients from their hosts through the cuticle and into their gut (which becomes severely reduced and non-functioning in adults). This absorption of nutrients can result in some weakness in their definitive hosts but is typically not fatal. (Gustav, 1920)

  • Animal Foods
  • body fluids


No predators for P. varius are known. However, adult worms have been known to be accidentally ingested by numerous species such as dogs, cats, and even humans. (Ali-Kahn and Ali-Kahn, 1977)

Ecosystem Roles

Paragordius varius has no detrimental effect on its intermediate hosts (typically aquatic insect larvae) and does not significantly affect the population density of its definitive orthopteran hosts.

The only significant role that P. varius may have in an ecosystem is possibly altering the behavior of its definitive orthopteran host. Infection with the P. varius larvae may cause erratic behavior in the definitive host, bringing the host close to a body of water. The larvae sense the presence of their preferred aquatic environment and manipulate their host to enter the water, where the parasites emerge as adults. (Thomas, et al., 2002)

Species Used as Host

Economic Importance for Humans: Negative

At this time, P. varius is not known to have any steady, significant economic impact other than the medical costs that ensue on the rare occasion when a human is infected with the worm. (Ali-Kahn and Ali-Kahn, 1977)

Conservation Status

Paragordius varius is not considered a threatened or endangered species and therefore is not protected under conservation laws.


Carly Coats (author), University of Michigan-Ann Arbor, Heidi Liere (editor), University of Michigan-Ann Arbor, John Marino (editor), University of Michigan-Ann Arbor, Barry OConnor (editor), University of Michigan-Ann Arbor, Renee Mulcrone (editor), Special Projects.



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map


living in the southern part of the New World. In other words, Central and South America.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


uses smells or other chemicals to communicate


a period of time when growth or development is suspended in insects and other invertebrates, it can usually only be ended the appropriate environmental stimulus.


union of egg and spermatozoan


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.


mainly lives in water that is not salty.

internal fertilization

fertilization takes place within the female's body


marshes are wetland areas often dominated by grasses and reeds.


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


Having one mate at a time.


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death


chemicals released into air or water that are detected by and responded to by other animals of the same species


"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.


rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.


Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).


offspring are all produced in a single group (litter, clutch, etc.), after which the parent usually dies. Semelparous organisms often only live through a single season/year (or other periodic change in conditions) but may live for many seasons. In both cases reproduction occurs as a single investment of energy in offspring, with no future chance for investment in reproduction.


lives alone


a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


Living on the ground.


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.


A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5? N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.


Ali-Kahn, T., Z. Ali-Kahn. 1977. Paragordius varius (Leidy) (Nematomorpha) infection in an: a case report from Quebec, Canada. Journal of Parasitology, 63/1: 174-190.

Gustav, H. 1920. Contributions to the life histories of Gordius robustus (Leidy) and Paragordius varius (Leidy). Illinois: University of Illinois.

Hanelt, B., J. Janovy. 2004. Life cycle and paratenesis of American gordiids (Nematomorpha: Gordiida). Journal of Parasitology, 90/2: 240-244.

Hanelt, B., F. Thomas, A. Schmidt-Rhaesa. 2005. Biology of the Phylum Nematomorpha. Advances in Parasitology, 59: 243-305.

Schmidt-Rhaesa, A. 2005. Morphogenesis of Paragordius varius (Nematomorpha) during the parasitic phase. Zoomorphology, 124/1: 33-46.

Thomas, F., A. Schmidt-Rhaesa, G. Martin, C. Manu, P. Durand, F. Renaud. 2002. Do hairworms (Nematomorpha) manipulate the water seeking behavior of their terrestrial hosts?. Journal of Evolutionary Biology, 15/3: 356-361.

Thorpe, J., A. Covich. 1991. Ecology and classification of North American freshwater invertebrates. San Diego, CA: Academic.

de Villalobus, C., G. Ramirez, S. Chavarria, F. Zanca. 2000. Cuticular ultrastructure of Paragordius esavianus Carvalho and P. varius. Proceedings of the Academy of Natural Sciences of Philadelphia, 150: 135-150.