Gordian worms

Gordian worm (Chordodes sp.) with egg strings Female Gordian worm with eggs in gelatinous strings.

Beatogordius spp., Chordodes spp., Gordius spp., Gordionus spp.

Gordian or horse-hair worms belong to the Phylum Nematomorpha (Greek: nema = thread; morphe = shape). As juveniles they are parasites of land-living arthropods, particularly grasshoppers, mantidscrickets and cockroaches.

The name 'Gordian' is derived from the mythological knot of King Gordius and relates to the ability of these worms to tie themselves in knots when mating.

During spring and summer (September to February) Gordian worms may be found in town water supplies, public and private swimming pools, water tanks, dams and ornamental ponds and even the water bowls of pets! Concerned members of the public or local authorities often contact the Queensland Museum regarding these sightings but by all accounts the worms are harmless to humans, pets or livestock.

Biology and life cycle

Chordodes female posterior Scanning electron microscope image of rounded posterior of Chordodes sp. female, with cloaca at the end.

Adult worms are relatively long (some can be up to 100cm), thin (2-3mm) and rigid rather than limp. Most species of Gordian worms have some bumps or papillae (called areoles) on their surface (cuticle) which may also have projecting spines or filaments.

All nematomorph species have separate sexes, with males and females showing characteristic differences in form. Females have a rounded posterior with the cloaca (common reproductive and intestinal opening) at the end. While in males the cloaca is before the end and males of some species also have a forked posterior. 

Mature Gordian worms seek out mates in freshwater and form copulatory tangles. After mating the male dies and the female lays eggs in gelatinous strings. Larvae emerge from the eggs 1-2 weeks later and infective suitable arthropod hosts that ingest them while feeding or drinking. Each larva has a spiny proboscis to allow penetration through the gut wall into the blood system (haemocoel) of the host. It appears that the developing parasite absorbs nutrients directly through its cuticle since the digestive system seems to be non-functional.

After several moults within the host, 'pre-adult' worms may induce their host to move to water, where they emerge (in the process often killing their host) and seek a mate to complete their life cycle. 

Identification

SEM image of surface features (areoles) Chordodes sp.Scanning electron microscope image of the surface of a Gordian worm (genus Chordodes), showing areoles, some with protruding filaments.Beatogordius sp. maleScanning electron microscope image of forked posterior of Beatogordius sp. male, with cloaca before the end.

To distinguish between the genera of Nematomorpha a range of visible characters are used. These include:

  • colour markings
  • the shape of the anterior and posterior ends
  • the types and patterns of areoles.

The colour of the cuticle can vary from a light creamy brown through to almost black. Some genera (e.g. Chordodes) have a characteristic light and dark mottled appearance while other genera (e.g. Gordius, Gordionus and Beatogordius) have a dark collar with a white tip.

The areoles on the cuticle are an important character and are best observed using a microscope. They range in appearance from simple rounded structures to those with elaborate crowns of protruding filaments. The cuticle may also have tubercles, spines or bristle-fields which, along with the areoles, can be used to distinguish between species.

Other worms with similar lifestyles

Mermithids (Phylum Nematoda) are worms that are superficially similar to the nematomorphs. They also parasitise insects as juveniles and are free-living as adults. They can be distinguished from Gordian worms as female mermithids have no obvious cloaca at the posterior end and male mermithids have 1 or 2 projecting spines (spicules) at the posterior end.

Mermithids have smooth cuticles that lack areoles and are usually pale brown in colour. The cuticle is also much thinner than that of the Gordian worms, so much so that internal structures are visible under microscopic examination.

Fact sheet

Gordian worms fact sheet (315 KB) pdf document icon

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