Nematomorphs are a group (phylum) of parasites.  They are long and thin worms (up to several meters (yards) long) and grow to this size within their hosts.  The Nematomorpha consists of two groups (classes).  The nectonematids are parasites of marine crustaceans such as crabs and shrimp.  The gordiids are usually parasites of terrestrial arthropods such as crickets, grasshoppers, beetles, mantids and cockroaches.  Based on molecular evidence, the phylum Nematomorpha has been shown to be the sister group to Nematodes. 

Fossil gordiids have been found emerging from a cockroach trapped in fossilized amber dated at 15-45 million years ago.  However, it has been suggested that this group might date back to the Carboniferous.

Compared with most animal phyla is a relatively understudied metazoan phylum. Although nematomorphs make up only 1 of 3 animal phyla specializing solely on a parasitic life style, little attention has been focused on this enigmatic group scientifically.

Gordiids

Freshwater nematomorphs are known as gordiids.  They are also known as Gordian worms, wire worms, horsehair worms, and water snakes.  In German, they are commonly referred to as "Saitenwurm" or violin string worm.

Adult worms.  Each worm roughly measured 25 inches.

Why are they called Gordian Worms?  

According to legend, during the fourth century, a Phrygian farmer named Gordius was chosen as an oracle and was given a small kingdom to rule. In gratitude to the Olympian Gods, Gordius hung one of his plows in the temple of Jupiter with a knot so cunning it could not be untied. The oracles predicted that whoever untied the plow would become ruler of Asia. All who made an attempt at the knot failed. Years passed, before a man named Alexander solved the puzzle by taking his sword and with one confident stroke loosened the plow. This hero was duly honored with the prize as well as the title Alexander the Great.

  
Life Cycle

Gordiids spend only a part of their lives as parasites, usually crickets. Adults are free-living in aquatic environments such as lakes, rivers or streams.  Adults are up to a yard long but only a fraction of an inch thick.  Adults mate in aquatic environments and a single female may lay as many as 10 million eggs here!  Often worms are found in a large knot of worms, known as a Gordian knot.

A gordiid egg.  Paragordius n. sp. To see more eggs, click here.

 
Eggs will develop into larvae within about 2-4 weeks.  These larvae will hatch, and move about the water in slow, creeping motions.

Larvae are heavy, cannot swim and will stay at the bottom of the water column, and will thus look for hosts in the bottom of rivers and streams.  Gordiid larvae will now have to find a way to get into the body of the cricket.  Crickets are not found in the same environment as the gordiid larvae, so the larvae must use a transport host, in order to be transported to the cricket.  Gordiid larvae do this by inching along the bottom of the stream or lake, and sooner or later will be ingested (swallowed) by another animal.

Once the gordiid larvae are inside most any aquatic animal, they encyst.

These cysts are extremely resistant and can stay alive within the transport host for at lest one year.  Many animals such as fish, snails, small crustaceans and many kinds of small insect larvae can get infected with gordiid cysts.  Most of these hosts likely represent a "dead end" host for gordiid cysts, since the diet of crickets does not consist of snails and fish, since these animals are restricted to the aquatic environment.

So how do gordiid cysts get into crickets?  They likely use aquatic insects.  Aquatic insects, as larvae look like small "worms" and live under water.  However, when they metamorphose they turn into the flying insects, which we are used to seeing.  This also means that the cysts formed within these insects can be carried from the water (in the insect larvae) to the land (in the insect flies).  The flies can then carry these cysts to where the crickets live.  Many studies have shown that crickets are omnivores- this means they eat anything and up to one-third of what they eat is dead insects; thus, also gordiid cysts!!!

Once inside of the cricket, gordiid larvae excyst and penetrate through the gut and into the body cavity of the cricket.  Here the worms grow up to adults.  This growth period takes from 4-20 weeks.   Interestingly enough, the worms can only feed while inside their cricket hosts.  So, all of the energy the worms need for living in the water, mating, etc., is acquired while inside the cricket.

As a final act, worms manipulate the behavior of the cricket, and it commits 'suicide' by jumping into water.  Once the worms sense the presence of liquid (water in this case) they start actively exiting the host. A process which can take a few seconds to a few minutes. To watch a video of this process, click here.

A stated above, the nectonematids are parasites of marine crustaceans such as crabs and shrimp. These worms are very rarely seen, and only a few dozen reports exist, documenting them.  All nectonematids fall in the genus Nectonema.

The marine genus Nectonema is known from several locations worldwide (including the Northern Atlantic, the Mediterranean, Indonesia, and Japan), with the latest report being from New Zealand. With two exceptions, reports are usually made of single worms.  The exceptions have been from the Bay of Fundy and some fjords in Norway. In most cases, Nectonema was found inside its hosts and rarely free-living.