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Horse hair (or Gordian) worms (Nematomorpha)

Taxonomic group

Phylum: Nematomorpha

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.


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

  • Hard bottom: 3
  • Soft bottom: 4.3

The tolerance values (ranging from 0 to 10) give an indication of which are the sensitive taxa (values of 8 or more) and which are the tolerant taxa (values of 3 or less). For more information see: Indicator species

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).