|Long, thin and worm-like|
|A straight gut which is non-functional|
|The cuticle and body organisation of adults similar to that of Nematoda|
|The adults free-living but short-lived|
|Separate sexes and internal fertilisation|
|The larval stages are parasitic on arthropods|
|Freshwater, terrestrial and marine|
|No circulation system|
|No respiration system|
|No excretory system|
Greek: nematos = thread, morphe = form
About 250 Nematomorpha species have been described so far, including four British freshwater species. Their common name comes from the superstition that the worms are born from horse hair falling into water.
They range in size from 10 cm to over 100 cm in length, but are always less than 3 mm in diameter, and are parasites of insects (often beetles or grasshoppers) and other arthropods. They are found world wide.
Locomotion is achieved by the same method as the Nematoda. The main difference between Nematomorpha adults and Nematoda adults is the degenerate gut in the Nematomorpha.
The role of the adults is not feeding, but reproduction and dispersal, and they have a featureless body, as the name, hair worm, suggests. They are free living, usually in freshwater or damp soil where the smaller male swims or wriggles towards the relatively inactive, larger female to mate.
The main feeding is done in the juvenile and larval stages (the drawing below shows the larval stage). These resemble the adult Kinorhynchans, some species of Priapidula, and Loriciferans. The larvae are equipped with eversible stylets that may be used in penetrating the host's tissues.
The female lays her eggs in long strings in water. If the host is a terrestrial insect it is stimulated by some unknown mechanism to find water when the parasitic larval hairworm is ready to emerge as an adult.