Illinois Natural History Survey - University of Illinois

Horsehair Worms in Illinois

The phylum Nematomorpha (from the Greek nema, "thread," and morphe, "shape"), consists of a group of invertebrates, commonly called horsehair or gordian worms, that has no close relationships with any other living organisms. The name horsehair is derived from the worm's hair or threadlike appearance in its adult stage. The first known fossil record of this group dates from the Eocene (40 - 70 million years ago), but experts suggest it may have its roots in the lower Paleozoic Era (over 500 million years old).

As larvae, horsehair worms are parasites of insects and other aquatic and terrestrial invertebrates, most notably grasshoppers, crickets, locusts, katydids, and beetles. Other hosts include caddisflies, dragonflies, spiders, millipedes, centipedes, crustaceans, and leeches. Host specificity has not yet been well documented. Other hosts include representatives of vertebrate groups.

Most horsehair worms are observed in their adult, free-living stage among the vegetation near edges of ponds and streams, although a few semiterrestrial species occur in damp soil. A small group of marine species, parasitic on crustaceans, have been collected from coastal environments.

Nematomorphs measure up to 1 meter in length and from less than 1 to 3 millimeters in diameter; diameter is uniform from one end to the other. Color usually ranges between light tan to dark brown, although yellowish and black individuals have been observed. Size and coloration vary considerably, even within the same species. Males are smaller than females, and the two sexes may be found in equal and unequal numbers in the same population. Locomotion of adult horsehair worms is at most a very slow whipping action; males seem to be much more active than females.

Horsehair worms reproduce sexually, in spring, early summer, or autumn. Eggs, often numbering in the millions, are laid in long gelatinous strings. For aquatic forms, the incubation period may range between 15 and 80 days, depending upon water temperature.

The mode of development of nematomorphs after hatching has been the subject of much debate. Some experts suggest that the larvae encyst on vegetation or other substrates along the water's edge soon after hatching. Eventually, some of these cysts are ingested by hosts feeding on these substrates. The cyst rapidly degenerates in the digestive tract of the new host. The larva then burrows its way through the intestinal wall into the host's body cavity, continuing its development. When ingested by inappropriate hosts, the cyst may degenerate and then reencyst in the tissues of the host. If this inappropriate host is then ingested by one of its predators (an appropriate host for the nematomorph). The cyst may again disintegrate and continue its life cycle in this new host. Other researchers have suggested that after the nematomorph larva emerges from the egg, it will penetrate the body wall of just about any animal it happens to encounter, although again, normal development will occur only in hosts. This mode of development may occur in the semiterrestrial species of horsehair worms.

A Horsehair Worm Tied up in a Proverbial Knot.

After entering the body cavity of an appropriate host, the larva grows to a juvenile stage, then emerges from the host to mature. During the larval stage of development, the horsehair worm digests and absorbs surrounding tissue. This period of metamorphosis occurs over a period of several weeks to several months; eventually the larval form develops into a tightly coiled mass in the host. One to several horsehair worms may occur in a single host. The parasite uses the important nutrients of the host, probably impairing its reproductive system. It has been suggested that hosts seek water when the horsehair worm is ready to emerge, perhaps even being driven to water by some physiological cue. Once the host enters the water, the horsehair worm breaks through the body wall of the host. Newly released horsehair worms soon die if they do not have access to water.

Several stories are associated with Nematomorpha. A common name of nematomorphs is gordian worm, which originated from its similarity in appearance to a knot, specifically one created by Gordius, king of Phrygia around 330 B.C. As the mythical story goes, Gordius used this knot to bind a chariot to a pole. He declared that whoever could undo the knot would be ruler of all Asia. The name horsehair worm might also originate from what ancient observers perceived as the spontaneous transformation of hairs from horses that, having fallen into watering troughs, developed into living worms.

Over 230 species of nematomorphs have been described worldwide. Unfortunately, taxonomic and ecological studies of this group in North America are limited compared to those of other invertebrate groups. At least 4 and possibly as many as 16 genera occur in North America. Identification of species has been difficult in many cases because characteristic features have been limited to microscopic surface patterns and the sculpturing of the cuticle. Recent light and scanning electron microscopical studies by several researchers have contributed important systematic information to the study and classification of this group. To date, only two species of horsehair worms are known to occur in Illinois: Gordius robustus and Paragordius varius ; a third species, Chordodes morgani, known to occur in states surrounding Illinois (Iowa, Wisconsin, Michigan, Tennessee, and Ohio), most likely occurs here, also. Additional collecting and study of specimens currently being conducted by the authors of this article undoubtedly will add to the number of species that occur in Illinois.


Mark J. Wetzel, INHS Center for Biodiversity, and Dreux J. Watermolen, Wisconsin Department of Natural Resources, Madison.

Please report any problems with or suggestions about this page to: 
Subject: INHSPUB-2147
Last Modified 3/19/96

Illinois Natural History Survey

1816 South Oak Street, MC 652
Champaign, IL 61820

Terms of use. Email the Web Administrator with questions or comments.

© 2019 University of Illinois Board of Trustees. All rights reserved.
For permissions information, contact the Illinois Natural History Survey.

Staff Intranet