Nematomorpha
historyNematomorpha (sometimes called Gordiacea, and commonly known as Horsehair worms or Gordian worms) are a phylum of parasitic animals that are morphologically and ecologically similar to nematode worms, hence the name. They range in size from long, and in diameter. Horsehair worms can be discovered in damp areas such as watering troughs, streams, puddles, and cisterns. The adult worms are free living, but the larvae are parasitic on beetles, cockroaches, Orthoptera, and crustaceans. About 326 species are known and a conservative estimate suggests that there may be about 2000 species worldwide. The name "Gordian" stems from the legendary Gordian knot. This relates to the fact that nematomorpha often tie themselves in knots.Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
Description and biology
Nematomorphs possess an external cuticle without cilia. Internally, they have only longitudinal muscle and a non-functional gut, with no excretory, respiratory or circulatory systems. The nervous system consists of a nerve ring near the anterior end of the animal, and a ventral nerve cord running along the body.
Reproductively, they are dioecious, with the internal fertilization of eggs that are then laid in gelatinous strings. Adults have cylindrical gonads, opening into the cloaca. The larvae have rings of cuticular hooks and terminal stylets that are believed to be used to enter the hosts. Once inside the host, the larvae live inside the haemocoel and absorb nutrients directly through their skin. Development into the adult form takes weeks or months, and the larva moults several times as it grows in size.
The adults are mostly free living in freshwater or marine environments, and males and females aggregate into tight balls (Gordian knots) during mating.
and its katydid host]]
In Spinochordodes tellinii, which has orthopterans as its vector, the infection acts on the insect's brain and causes it to seek water and drown itself, thus returning the nematomorph to water. |url=http://www.erin.utoronto.ca/~w3gwynne/BIO418/Nemato.pdf | doi = 10.1046/j.1420-9101.2002.00410.x}} — according to Thomas et al., the "infected insects may first display an erratic behaviour which brings them sooner or later close to a stream and then a behavioural change that makes them enter the water", rather than seeking out water over long distances. They are also remarkably able to survive the predation of their host, being able to wriggle out of the predator that has eaten the host.
The phylum is placed along with the Ecdysozoa clade of moulting organisms that include the Arthropoda. Fossilized worms have been reported from Early Cretaceous Burmese amber dated to 100–110 million years.Poinar, George; Buckley Ron (2006) Nematode (Nematoda: Mermithidae) and hairworm (Nematomorpha: Chordodidae) parasites in Early Cretaceous amber. Journal of invertebrate pathology 93(1):36–41
Relationships within the phylum are still somewhat unclear, but two classes are recognised:
Taxonomy
Nematomorphs can be confused with nematodes, particularly mermithid worms. Unlike nematomorphs, mermithids do not have a terminal cloaca. Male mermithids have one or two spicules just before the end apart from having a thinner, smoother cuticle, without areoles and a paler brown colour.Malcolm S. Bryant, Robert D. Adlard & Lester R.G. Cannon 2006. Gordian Worms: Factsheet. Queensland Museum. »http://www.qm.qld.gov.au/inquiry/factsheets/Gordian_Worms_20070605.pdf