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.
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.
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
. 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
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