Horsehair worms - Phylum Nematomorpha
 
The Nematomorpha is a poorly studied phylum of maybe 2,000 species worldwide.  In their youth they are parasitoids on larger insects and some crustacea, living as adults in freshwater or very wet semi-terrestrial habitats. Adults up to 10cm long (sometimes much longer), but only 0.5 to 1mm wide, resembling thick horsehair.  When found adults are usually tightly coiled in knots, and another name for the group "Gordian worms" is based on the legend of Alexander the Great cutting the Gordian knot.
 
Species in Britain and Ireland
 
In Britain and Ireland we have seven species of nematomorph worms in 4 genera (Gordionus, Gordius, Nectonema and Parachordodes).  They are all rather similar and very hard to determine to genus, let alone to species level.  The best known species is probably Gordius aquaticus which parasitises large diving beetles such as Dytiscus marginalis.
 
 
 
Left: Nematomorph Gordius aquaticus, match head for scale. Right: unidentified nematomorph found in a puddle with the corpse of its host,  Roesel's bush-cricket Metrioptera roeselii.
 
Biology
Horsehair worms have a nematode-like cuticle and muscle system, but have no gut, respiratory or circulatory systems. They have separate sexes and eggs are fertilised internally, with males and females ecstatically knotted together in balls.
 
They can be confused with nematodes of the genus Mermis which also are parasitoids of large insect, but they are shorter and paler, with tapering bodies.
 
 
Life Cycle
Adult females lay their eggs in strings on water plants.  Somehow, the hatched larva finds a suitable insect or crustacean host, and burrows into its body cavity using hooks and spikes.  Insect hosts are typically crickets, cockroaches, grasshoppers and beetles.  The larvae live for months in the host, absorbing food through their cuticle, and moulting as they grow. Eventually they fill the host's body cavity and kill it as they bore out to freedom.  Some species have a macabre ability to make their cricket or grasshopper hosts head for and jump into water, neatly returning the adult to its proper environment.  The worms don't feed as adults, but can survive for long periods curled up among vegetation.
 
Role in gardens
Horsehair worms have little ecological significance in gardens, where they are very poorly known. You are most likely to find them in spring or early summer in the pond or in a puddle after they have broken out of their host.
 
Other sources of information
 
Website
Field Studies Council factsheet 
 
Books and papers
 
Hanelt, B.Thomas, F. & Schmidt-Rhaesa, A. (2005) Biology of the Phylum Nematomorpha.  Advances in Parasitology 59:243-305
 
 
Page drafted and compiled by Steve Head
 
 
Horsehair worms - Phylum Nematomorpha
 
The Nematomorpha is a poorly studied phylum of maybe 2,000 species worldwide.  In their youth they are parasitoids on larger insects and some crustacea, living as adults in freshwater or very wet semi-terrestrial habitats. Adults up to 10cm long (sometimes much longer), but only 0.5 to 1mm wide, resembling thick horsehair.  When found adults are usually tightly coiled in knots, and another name for the group "Gordian worms" is based on the legend of Alexander the Great cutting the Gordian knot.
 
Species in Britain and Ireland
 
In Britain and Ireland we have seven species of nematomorph worms in 4 genera (Gordionus, Gordius, Nectonema and Parachordodes).  They are all rather similar and very hard to determine to genus, let alone to species level.  The best known species is probably Gordius aquaticus which parasitises large diving beetles such as Dytiscus marginalis.
 
 
 
Left: Nematomorph Gordius aquaticus, match head for scale. Right: unidentified nematomorph found in a puddle with the corpse of its host,  Roesel's bush-cricket Metrioptera roeselii.
 
Biology
Horsehair worms have a nematode-like cuticle and muscle system, but have no gut, respiratory or circulatory systems. They have separate sexes and eggs are fertilised internally, with males and females ecstatically knotted together in balls.
 
Life Cycle
Adult females lay their eggs in strings on water plants.  Somehow, the hatched larva finds a suitable insect or crustacean host, and burrows into its body cavity using hooks and spikes.  Insect hosts are typically crickets, cockroaches, grasshoppers and beetles.  The larvae live for months in the host, absorbing food through their cuticle, and moulting as they grow. Eventually they fill the host's body cavity and kill it as they bore out to freedom.  Some species have a macabre ability to make their cricket or grasshopper hosts head for and jump into water, neatly returning the adult to its proper environment.  The worms don't feed as adults, but can survive for long periods curled up among vegetation.
 
Role in gardens
Horsehair worms have little ecological significance in gardens, where they are very poorly known. You are most likely to find them in spring or early summer in the pond, in a puddle or in a coiled mass on a flower head after they have broken out of their host.
 
Other sources of information
 
Website
Field Studies Council factsheet 
 
Books and papers
 
Hanelt, B.Thomas, F. & Schmidt-Rhaesa, A. (2005) Biology of the Phylum Nematomorpha.  Advances in Parasitology 59:243-305
 
 
Page drafted and compiled by Steve Head
 
 
Left: Nematomorph Gordius aquaticus, match head for scale. Right: unidentified nematomorph found in a puddle with the corpse of its host,  Roesel's bush-cricket Metrioptera roeselii.
 
Biology
Horsehair worms have a nematode-like cuticle and muscle system, but have no gut, respiratory or circulatory systems. They have separate sexes and eggs are fertilised internally, with males and females ecstatically knotted together in balls.
 
They can be confused with nematodes of the genus Mermis which also are parasitoids of large insect, but they are shorter and paler, with tapering bodies.
 
Life Cycle
Adult females lay their eggs in strings on water plants.  Somehow, the hatched larva finds a suitable insect or crustacean host, and burrows into its body cavity using hooks and spikes.  Insect hosts are typically crickets, cockroaches, grasshoppers and beetles.  The larvae live for months in the host, absorbing food through their cuticle, and moulting as they grow. Eventually they fill the host's body cavity and kill it as they bore out to freedom.  Some species have a macabre ability to make their cricket or grasshopper hosts head for and jump into water, neatly returning the adult to its proper environment.  The worms don't feed as adults, but can survive for long periods curled up among vegetation.
 
Role in gardens
Horsehair worms have little ecological significance in gardens, where they are very poorly known. You are most likely to find them in spring or early summer in the pond or in a puddle after they have broken out of their host.
 
Other sources of information
 
Website
Field Studies Council factsheet 
 
Books and papers
 
Hanelt, B.Thomas, F. & Schmidt-Rhaesa, A. (2005) Biology of the Phylum Nematomorpha.  Advances in Parasitology 59:243-305
 
 
Page drafted and compiled by Steve Head
 
 
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