There are at least eighteen species known from North America, with eight or more species found in Arkansas.įreshwater habitats include watering troughs, puddles, creeks, rivers, and subterranean (cavernicolous) streams. 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. The marine genus Nectonema has been reported from the northern Atlantic, the Mediterranean, Indonesia, Japan, Norway, and possibly New Zealand.

However, current estimates suggest that only eighteen percent of horsehair worm species have been described worldwide. There are over 350 species, with the majority from freshwater and about five known species of the same genus ( Nectonema) occurring in marine environs. The Nematomorpha are considered the sister phylum to the roundworms, phylum Nematoda.

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.

Among the latter group, four families and at least twenty-one genera (including two fossil genera) have been recognized. Nematomorphs have been placed in two taxa, the marine Nectonematoidea and the freshwater Gordiida (=Gordioidea). The first published report of a horsehair worm from Arkansas was in 1907. 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. 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. These worms are sometimes found in coiled clusters termed “Gordian knots” from the intricate legendary knot of Greek mythology. As adults, however, they are free-living in aquatic environments. Horsehair worms belong to the phylum Nematomorpha and are typically obligate parasites of terrestrial arthropods (e.g., beetles, crickets, cockroaches, locusts, grasshoppers, and mantids).

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