This website provides lots of scientific background on horsehair worms, including links to scientific literature and tips on where to find them.
The Nematomorpha are a group of parasitic worms that develop within their hosts (primarily terrestrial insects and other arthropods) but reproduce in aquatic environments. Around 300 species of nematomorphs (or "hairworms") have been described. Nematomorphs are dioecious (i.e., have separate sexes). Tiny larvae (about 100 µm long) hatch from eggs. These larvae are equipped with two or three rings of cuticular hooks and terminal stylets with which they are able to penetrate soft epithelia of their hosts. Terrestrial hosts may become infected by drinking water containing larvae or they may prey on a paratenic host (see below). Although adults of most nematomorph species live in freshwater, a few species live in damp soil and the five known species in the distinctive genus Nectonema are pelagic in coastal marine environments. Within the host, nematomorphs increase dramatically in size from about 100 µm to several centimeters (some may exceed two meters in length). The size of the mature worm substantially exceeds the length of the host. By the time the worm is mature, it fills most of the host cavity with the exception of the head and the legs. Worms are ready to emerge only once they reach this stage. (Schmidt-Rhaesa 2002 and references therein; Thomas et al. 2002; Brusca and Brusca 2003)
The life cycle of nematomorphs has 4 stages: the egg, the pre-parasitic larva that hatches from the egg, the parasitic larva that develops within an invertebrate host (the "definitive" or developmental host), and the free-living aquatic adult. Within the definitive host, worms complete development, but they do not mate and oviposit until they are free-living in aquatic environments. Many nematomorph species have another type of host as well, a "paratenic" or transport host. The pre-parasitic larva enters a paratenic host but does not develop further until the paratenic host is eaten by a scavenger or predator in which it can develop. Within paratenic hosts, larvae penetrate through the gut, secrete a cyst wall, fold up, and become cysts. For at least some nematomorph species, the nematomorphs may make the transition from water to land by forming cysts in aquatic insect larvae, with the cysts surviving the host's metamorphosis to an adult which can then convey the nematomorphs to land. The great majority of freshwater nematomorphs have been collected from beetles or orthopterans. Paratenic hosts run the gamut from trematode flatworms to vertebrates. (Poinar 2001; Hanelt and Janovy 2004)
Many types of parasites are known to modify the behavior of their host in ways that benefit the parasite. Based on anecdotal observations, it has long been suspected that at least some mature nematomorphs, which must reach water to mate and reproduce, manipulate the behavior of their terrestrial insect hosts, causing them to seek water and jump into it. Investigations by Thomas et al. (2002) found clear evidence of this phenomenon in 8 tettigoniid orthopterans infected by the nematomorph Spinochordodes tellinii, as well as in the gryllid cricket Nemobius sylvestris infected by the nematomorph Paragordius tricuspidatus. In experiments, however, they found no evidence that hosts actively seek out water; rather, they suggested, infected hosts seem to display erratic behavior that eventually brings them close to water, which they then enter. Consistent with the findings of Thomas et al. (2002), Sanchez et al. (2008) found that this behavioral manipulation has two phases, first causing the cricket to wander into atypical habitats and next causing it to commit suicide by entering water.
The Nematomorpha is a monophyletic taxon of parasitic worms that develop within their hosts (primarily terrestrial insects and other arthropods) but reproduce in aquatic environments. Around 300 species of nematomorphs (or "hairworms") have been described. Nematomorphs are dioecious (i.e., have separate sexes). Tiny larvae (about 100 µm long) hatch from eggs. These larvae are equipped with two or three rings of cuticular hooks and terminal stylets with which they are able to penetrate soft epithelia of their hosts. Terrestrial hosts may become infected by drinking water containing larvae or they may prey on a paratenic host (see below). Although adults of most nematomorph species live in freshwater, a few species live in damp soil and the five known species in the distinctive genus Nectonema are pelagic in coastal marine environments. For Nectonema, only hosts containing juveniles are known and these are all decapod crustaceans—either pelagic shrimps such as Pandalus or benthic crabs such as Cancer, Munida or Pagurus. Because nematomorph copulation takes place close to the surface, nematomorph larvae may infect crustaceans while the latter are still planktonic larvae. Within the host, nematomorphs increase dramatically in size from about 100 µm to several centimeters (some may exceed two meters in length). The size of the mature worm substantially exceeds the length of the host. By the time the worm is mature, it fills most of the host cavity with the exception of the head and the legs. Worms are ready to emerge only once they reach this stage. (Schmidt-Rhaesa 2002 and references therein; Thomas et al. 2002; Brusca and Brusca 2003)
The life cycle of nematomorphs has 4 stages: the egg, the pre-parasitic larva that hatches from the egg, the parasitic larva that develops within an invertebrate host (the "definitive" or developmental host), and the free-living aquatic adult. Within the definitive host, worms complete development, but they do not mate and oviposit until they are free-living in aquatic environments. Many nematomorph species have another type of host as well, a "paratenic" or transport host. The pre-parasitic larva enters a paratenic host but does not develop further until the paratenic host is eaten by a scavenger or predator in which it can develop. Within paratenic hosts, larvae penetrate through the gut, secrete a cyst wall, fold up, and become cysts. For at least some nematomorph species, the nematomorphs may make the transition from water to land by forming cysts in aquatic insect larvae, with the cysts surviving the host's metamorphosis to an adult which can then convey the nematomorphs to land. The great majority of freshwater nematomorphs have been collected from beetles or orthopterans. Poinar (2001) includes a table listing the the approximately three dozen invertebrate families that have been recorded as definitive hosts for developing nematomorphs. Paratenic hosts run the gamut from trematode flatworms to vertebrates. (Poinar 2001; Hanelt and Janovy 2004)
Originally, nematomorphs were classified as nematodes, but they have been recognized as a separate taxon since the name Nematomorpha was introduced in 1886. The traditional view, based on both morphological and molecular characters, considers Nematoda and Nematomorpha to be sister taxa, together forming a clade that has sometimes been termed Nematoida. (Bleidorn et al. 2002 and references therein; Bourlat et al. 2008 and references therein). However, Sørensen et al. (2008) argue that based on their analyses of 18S rRNA and histone 3 sequences, and consistent with some key morphological features, the Nematomorpha are actually sister to the Loricifera, not the Nematoda.
Many types of parasites are known to modify the behavior of their host in ways that benefit the parasite. Based on anecdotal observations, it has long been suspected that at least some mature nematomorphs, which must reach water to mate and reproduce, manipulate the behavior of their terrestrial insect hosts, causing them to seek water and jump into it. Thomas et al. (2002) found clear evidence of this phenomenon in 8 tettigoniid orthopterans infected by the nematomorph Spinochordodes tellinii, as well as in the gryllid cricket Nemobius sylvestris infected by the nematomorph Paragordius tricuspidatus. In experiments, however, they found no evidence that hosts actively seek out water; rather, they suggested, infected hosts seem to display erratic behavior that eventually brings them close to water, which they then enter. This behavior was investigated experimentally with the woodland cricket N. sylvestris infected by P. tricuspidatus nematomorphs. Consistent with the findings of Thomas et al. (2002), Sanchez et al. (2008) found that this behavioral manipulation has two phases, first causing the cricket to wander into atypical habitats and next causing it to commit suicide by entering water. The erratic behavior was induced before the worms were sexually mature, timing which the authors suggested may be an adaptation to increase the likelihood that the host would be near water when it was an optimal time (from the parasite's perspective) for the cricket host to enter water and drown. Biron et al. (2006) have taken intriguing first steps at identifying, at a molecular level, the mechanisms by which this parasite manipulates its host's behavior.
Based on several species that have been investigated, nematomorphs appear not to have an associated symbiotic or non-symbiotic bacterial flora, in striking contrast to groups such as the arthropods (Hudson and Floate 2009 and references therein).
Poinar (2008) reviewed the global diversity of freshwater nematomorphs, which he estimated includes over 300 described species and an actual species richness of around 2000 species. Poinar (2001) provides a dichotomous key to the known genera of nematomorphs; Smith (2001) includes a dichotomous key to genera known from the United States. Schmidt-Rhaesa et al. (2003) review the 17 species of freshwater nematomorphs they recognize from North America, including their known geographic occurrences and a dichotomous identification key. Poinar and Chandler (2004, cited in Hudson and Floate 2009) also provide a synopsis of North American nematomorphs.
The biology of the Nematomorpha has been reviewed by Poinar (2001) and Hanelt et al. (2005). Poinar (2001) also includes a discussion of the freshwater Mermithidae, a family of nematodes that parasitize invertebrates and are superficially similar to nematomorphs (a key to North American mermithid genera is included as well).
Nematomorpha (sometimes called Gordiacea, and commonly known as horsehair worms, hairsnakes,[1][2][3] or Gordian worms) are a phylum of parasitoid animals superficially similar to nematode worms in morphology, hence the name. Most species range in size from 50 to 100 millimetres (2.0 to 3.9 in) long, reaching 2 metres 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 are parasitic on arthropods, such as beetles, cockroaches, mantises, orthopterans, and crustaceans.[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. This relates to the fact that nematomorphs often coil themselves in tight balls that resemble knots.[7]
Nematomorphs possess an external cuticle without cilia. Internally, they have only longitudinal muscle and a non-functional gut, with no excretory, respiratory or circulatory systems. The nervous system consists of a nerve ring near the anterior end of the animal, and a ventral nerve cord running along the body.[8]
Reproductively, they have two distinct sexes, with the internal fertilization of eggs that are then laid in gelatinous strings. Adults have cylindrical gonads, opening into the 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 and absorb nutrients directly through their skin. Development into the adult form takes weeks or months, and the larva moults several times as it grows in size.[8]
The adults are mostly free-living in freshwater or marine environments, and males and females aggregate into tight balls (Gordian knots) during mating.[9][10]
In Spinochordodes tellinii and Paragordius tricuspidatus, which have grasshoppers and crickets as their hosts, the infection acts on the infected host's brain.[11] This causes the host insect to seek water and drown itself, thus returning the nematomorph to water.[9] P. 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.[12] The nematomorpha parasite affects host Hierodula patellifera's light-interpreting organs so the host is attracted to horizontally polarized light. Thus the host goes into water and the parasite's lifecycle completes.[13]
There are a few cases of accidental parasitism in vertebrate hosts, including dogs[14] and humans. Several cases involving Parachordodes, Paragordius, or Gordius have been recorded in human hosts in Japan and China.[15][16]
Owing to their use of orthopterans as hosts, nematomorphs can be significant factors in shaping community ecology. One study conducted in a Japanese riparian ecosystem showed that nematomorphs can cause orthopterans to become 20 times more likely to enter water than non-infected orthopterans; these orthopterans constituted up to 60% of the annual energy intake for the Kirikuchi char. Absence of nematomorphs from riparian communities can thus lead to char predating more heavily on other aquatic invertebrates, potentially causing more widespread physiological effects.[17]
Nematomorphs can be confused with nematodes, particularly mermithid 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 a paler brown colour.[18]
The phylum is placed along with the Ecdysozoa clade of moulting organisms that include the Arthropoda. Their closest relatives are the nematodes. The two phyla make up the group Nematoida in the clade Cycloneuralia. During the larval stage, the animals show a resemblance to adult kinorhyncha and some species of Loricifera and Priapulida, all members of the group Scalidophora.[19] The earliest Nematomorph could be Maotianshania, from the Lower Cambrian; this organism is, however, very different from extant species;[20] fossilized worms resembling the modern forms have been reported from mid Cretaceous Burmese amber dated to 100 million years ago.[21]
Relationships within the phylum are still somewhat unclear, but two classes are recognised. The five marine species of nematomorph are contained in Nectonematoida.[22] This order is monotypic containing the genus Nectonema Verrill, 1879: adults are planktonic and the larvae parasitise decapod crustaceans, especially crabs.[22] They are characterized by a double row of natotory setae along each side of the body, dorsal and ventral longitudinal epidermal cords, a spacious and fluid-filled blastocoelom and singular gonads.
The approximately 320 remaining species are distributed between two families,[23] within the monotypic class Gordioida. Gordioidean adults are free-living in freshwater or semiterrestrial habitats and larvae parasitise insects, primarily orthopterans.[22] Unlike nectonematiodeans, gordioideans lack lateral rows of setae, have a single, ventral epidermal cord and their blastocoels are filled with mesenchyme in young animals but become spacious in older individuals.
{{cite book}}: CS1 maint: multiple names: authors list (link) {{cite web}}: CS1 maint: archived copy as title (link) Nematomorpha (sometimes called Gordiacea, and commonly known as horsehair worms, hairsnakes, or Gordian worms) are a phylum of parasitoid animals superficially similar to nematode worms in morphology, hence the name. Most species range in size from 50 to 100 millimetres (2.0 to 3.9 in) long, reaching 2 metres 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 are parasitic on arthropods, such as beetles, cockroaches, mantises, orthopterans, and crustaceans. About 351 freshwater species are known and a conservative estimate suggests that there may be about 2000 freshwater species worldwide. The name "Gordian" stems from the legendary Gordian knot. This relates to the fact that nematomorphs often coil themselves in tight balls that resemble knots.
