Horsehair Worms

My son took a horse’s hair and placed it in a jar of water and the hair looks like it is swimming around like a worm. What would cause this?

Your son found a ‘horsehair worm’. These roundworms were named horsehair worms based on the fact they closely resemble the hair of a horse’s tail and on a superstition that a horse’s hair that falls into water may spontaneously generate a worm. They are often seen in watering troughs.

These are fascinating worms that are free-living adults and parasitic in arthropods as juveniles. They have a worldwide distribution and may be found in any aquatic habitat, whether the water is running or standing.

The adult female lays eggs in water in long strings around vegetation. After hatching in the fall, the larvae of genus Gordius (named for a king of Phrygia who tied an intricate knot to connect the yoke with the pole of the chariot) encyst on vegetation that is eaten by grasshoppers and other arthropods in the spring. Hatching takes up to 80 days depending on the species and the juveniles are free swimming with digestive systems. If the worm is of a genus that does not encyst on vegetation the larva enter a host when the host drinks or by penetrating the host’s body wall.

Freshwater hosts include grasshoppers, beetles, crickets, centipedes, millipedes, cockroaches, and leeches. There may be only one genus of saltwater horsehair worm with crabs serving as hosts. After larvae are taken into the digestive systems of their arthropod hosts they migrate to the hemocoel space to complete development (they penetrate the gut and enter the body cavity). The larval worms absorb nutrients directly through their body wall.

When the larva has completed development into an adult worm it will emerge from the host’s body when it senses the host is in or near water. If the host is a terrestrial insect, it is stimulated by an unknown signal from the worm to seek water. The worm may emerge by passing through the host’s digestive tract; other possible mechanisms of exit are not known.

Adult males are more active than females and may be seen ‘whipping’ around in the water. The adult digestive tract is vestigial so they survive on stored nutrients. Length ranges from 30 to 40 cm, but one variety is known to reach 120 cm. Diameter is usually about 1 to 3 mm.

They are in phylum Nematomorpha and are close relatives of the roundworms in phylum Nematoda. Phylum Nematomorpha has only about 230 to 320 species all called horsehair worms. They are not widespread parasites of arthropods, though in some locales infestation has been recorded in the high 90 percent range. Infected arthropods may not reproduce while hosting the worm.

They are a fascinating curiosity and are not harmful to humans. To see a photo of an adult horsehair worm, visit the Ohio State site linked above and scroll down to “Parasites Listed Alphabetically”, then choose “I-P”. Finally, click on Nematomorpha (Horsehair Worms).

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