Paragordius varius

P. varius have a slightly off-center mouth and distinct grooves running along its cylindrical body. Male worms can grow to be 12–29 cm in length whereas females are generally longer and can grow to be 12–31 cm in length. This parasite is a pseudocoelomate whose body consists of an areole containing cuticle layer, monolayer hypoderm, muscular layer, ventral nerve cord and a digestive tract.[4]

When the adult worm is in the water, it leaves their definitive host and spends their time swimming in the shallows in search for a mate. Upon initial encounter with each other, the female gives a signal to the male that she is ready and willing to mate.[5] No penetration is necessary during copulation and the male releases his sperm immediately. However, if the sperm does not land on the appropriate posterior area of the female, conception does not occur.[5] When conception does occur, P. varius lay eggs, which comes out in a long, white, spaghetti-like string.[2] These eggs are hatched into free-swimming larvae and must find an intermediate host, such as freshwater snails, mosquito larvae, or other small aquatic critters, to infect within a few days or they will die. Upon infection of one of these hosts, the larva takes about 5–14 days to develop into a cyst. At this stage it is ready to infect its definitive host where it can mature into an adult.[2]

The life cycle of nematomorphs has 4 stages: First is the egg, which takes about 10–12 days to complete its larval development after being laid. Second is the pre-parasitic larva that hatches from the egg; the fully developed larva remains in the egg 7–10 days before hatching. Third is the parasitic larva that develops within an invertebrate host; larvae swim about freely in the water after hatching, and are ingested with water when insects drink. If the larvae does not find a host within a few days, they will die.[2] The larvae uses hooks on the anterior portions of the body to infect its host. Once inside a host insect, the larva penetrate the insect's gut and enter its body cavity and forms a cyst in the muscular or intestine region.[2] They mature and escape from the host in two to three months. When the intermediate host completes metamorphosis it comes across a definitive host and evolves into an adult. The final stage of the nematomorphs is the free-living aquatic adult; the development only takes about thirty days where the P. varius is able to produce three generations in one year. They break through the body wall of the host and become free-living. Eventually the P. varius will leave the definitive host when the host is near water. This is done by manipulating its host to migrate to a shallow body of water.[2]

Adult hair worms that live in insects emerge from their hosts in late spring or summer. They typically start off swimming in shallow waters and move up onto land at the edge of the water to mate one to two days after they have left their definitive hosts.[2] Females will send stimulus signals to the male when she is ready and willing to mate. No penetration is done, the male simply releases its sperm and lands on the appropriate posterior area of the female.[2] Once the sperm lands, a large circular glob forms which passes into the seminal receptacle of the female within twenty four hours of mating. After mating, females lay as many as six million eggs and then die.

This parasite is known to infect insect species in the order orthoptera. Amongst the host species are grasshoppers, crickets, cockroaches, snails, mosquito larvae, or other small aquatic animals and insects. Mammals are not capable of being infected. However, dogs, cats, and even humans have been known to swallow adult worms accidentally.[2]

Paragordius varius has been found all across North and South America ranging as far north as Canada and as far south as Argentina.[2] It prefers environments containing bodies of water needed for reproduction. P. varius has also been found to be able to withstand temperatures as cold as -70 °C. It can be frozen at this temperature for weeks and when it thaws out, it is still completely capable of infecting its next host.[6]

P. varius alters the behavior of its definitive host, resulting in the host traveling to or near a body of water. The host is then manipulated into entering the body of water where the larvae can develop into an adult and exit its host.[2]