Guide to New Zealand's freshwater invertebrates

Horse hair (or Gordian) worms ( Nematomorpha : Nematomorpha )

Y-shaped tail end of a horse hair worm

Diagnostic features

At first glance horse hair worms could be mistaken for long strands of plant root or even copper wire, until they are observed moving. They can be many centimeters long, somewhat coiled, and typically they are red-brown in colour. The tail end of the body may be Y-shaped, while the head end is simply a rounded tip.

Typical habitats

We have found horse hair worms in bush covered and farmland, stony and gravelly streams.

Feeding

The larval feeding stage parasitises terrestrial arthropod hosts including crickets, wetas and millipedes. When mature the worm affects the brain of the host, forcing them to enter the water where the worm emerges for the aquatic breeding stage.

Indicator value

While most of our records are from streams with good water quality, horse hair worms also occur in sites with poor water quality and therefore they have relatively low tolerance values of 3 (hard bottom sites) and 4.3 (soft bottom sites).