Nematomorphs are often referred to as "horsehair worms" as these worms are very long and thin without a distinct head. Until the late 1800's it was believed that these worms were shed into the water from horse's manes and tails. In reality their life cycles are much more interesting. Hair worms are not closely related to any other group of known invertebrates. They derived from an ancient worm-like body plan very early in evolution. Their closest known relatives are the nematodes and the rotifers. The characteristics that link these three groups are the presence of a pseudocoel, as well as their general body plans and musculature.
Nematomorphs are only a millimeter or two in diameter but are usually 10 to 100 centimeters in length. They lack a distinct head, but possess a thick iridescent cuticle that is made up of cris-crossed fibers for added strength.
This group has only longitudinal muscles, but they are thick and run the entire length of the body. Adults achieve locomotion by contracting these muscles in whipping undulations. Juveniles have no need to move because they are parasitic.
Osmoregulation and oxygen requirements are unknown for this group of organisms and circulation is limited to the movement of fluid through the large pseudocoel.
Nematomorphs lack excretory organs, although the vestigial midgut may be used as a kidney. However, it does not filter waste and reabsorb the usable contents as do more complex kidneys. Instead, wastes are collected, concentrated, and secreted into the midgut as a holding tank. As adult horsehair worms don't live very long, the limited holding capacity is not a concern.
All nematomorphs species reproduce sexually. The male wraps himself around the female in many coils. Depending on species the sperm is placed directly inside the cloaca of the female or a packaged spermatophore is attached near her cloaca and the sperm swim inside.
Horsehair worms have 4 life stages: egg, pre-parasitic larva, parasitic larva, and free-living adult. Details of their life histories vary in three ways.
- In some species the egg hatches in the water and the pre-parasitic larva is ingested by the proper host. Here it changes from pre-parasitic to parasitic stage to adult without ever having to transfer hosts.
- In other species, typically found in temporary ponds, the pre-parasitic larva hatches from an egg, and as the pond begins to dry up it encysts on plant matter. When a definitive host arrives and feeds on the plant matter, ingesting the cysts accidentally, the parasitic larva emerges from the cyst and infects it.
- Finally, a pre-parasitic larva hatches and does not soon encounter a definitive host. It either dies due to lack of nourishment or uses an alternate, though less suitable, host in which it encysts but cannot feed. These temporary hosts can be just about any type of vertebrate or invertebrate, even a human. These larvae burrow into the flesh or are ingested by the host and encyst in the body tissues. If this host host is eaten by a definitive host (such as when a praying mantis eats an infected mayfly) the parasitic larva emerges and enters the gut of its final host. Cysts can survive even in if a temporary host dies, and if scavengers feed on the dead tissues, the cysts can be passed into the new host by that route.
It is not known how selective the larvae are about hosts. It is also unknown for how long the pre-parasitic forms can encyst before they exhaust their metabolic reserves and die.
As soon as horsehair worms transform from larvae into adults, their pharynx (muscular feeding structure at the mouth) becomes non-functional, and their digestive tract degenerates. As a results the adults never feed. The sole purpose of the adult form is to mate.
All nematomorphs have parasitic larvae. The goal of a larva is to be ingested by an adult insect such as Orthoptera (e.g. grasshopper) and Coleoptera (e.g. giant water beetle). Once inside the digestive system of their host they burrow into its gut lining and feed on the nutritious haemolymph.
All horsehair worms, excepting the genus Nectonema, occur in freshwater. They are found in both temporary ponds and flowing waters.
When a large number of adult nematomorphs are crowded into a small area, they wind themselves together into a giant mass resembling a Gordian knot. In Greek mythology, the king Gordius, fastened his wagon to his horse's yoke with a knot that was impossible to untie. An oracle announced that whoever released the knot would be the next ruler of Asia. Alexander the Great cut the Gordian knot with his sword, and went on to conquer Asia.