Hairworms are parasites of crickets and other arthropods that are found all over the world. There are two classes within the Phylum Nematomorpha that include the freshwater gordiids and the marine group known as the nectonematids. To date, over 360 species of freshwater hairworms and 5 marine species have been described.
Aquatic Life Cycle
Hairworms spend a portion of their life as a parasitic juvenile in an insect host. When they reach maturity, they demonstrate a fascinating behavior in which they manipulate their host to find water. At this time, their insect host, such as a cricket, begins wandering aimlessly in search of water. When the cricket encounters water such as a stream or pond, the cricket will jump in and the worm will expel itself from the cricket. On occasion, infected crickets may find themselves inside our homes and utilize pet water bowls for their hairworm's exit.
After emerging from their hosts, hairworms have no eyes and do not feed. In fact, their only purpose is to find a mate and reproduce. Females lay millions of eggs in the water; freely or attached to rocks and/or sticks, depending on the species. Eggs hatch in 30-120 days (depending on the species) and a small hairworm larvae (30-60 microns) will fall to the bottom of the water. Here, they are ingested by invertebrates that feed at the bottom of the water column. Inside the invertebrate, the hairworm larvae forms a cyst, creating a protective shell or layer around itself. However, the hairworm larvae that will make it to adulthood, are those that find themselves inside an appropriate paratenic (transport) host. The paratenic host, in this case, bridges the ecological gap between the aquatic and terrestrial system. For example, a midge larvae lives in the water and might have consumed a hairworm larva that formed a cyst. When the midge larva molts to an adult, it leaves the aquatic system and flies off to the terrestrial systems with the hairworm cyst still inside its gut. When the midge dies, it could be eaten by a beetle or cricket (or other appropriate terrestrial host) and then the larva comes out of its cyst stage and burrows into the hemocoel of the host; now it is a juvenile parasite feeding on the fat of its host. After it has grown, the hairworm larva stops feeding and begins manipulating the host to find water. The beetle or cricket will then jump into the water and the hairworm senses the water and then emerges as a free-living adult. Recently, researchers found the first parthenogenetic species (females reproduce without a male). They named this species, Paragordius obamai, because it was found in an African village of President Obamai's grandparents.
Terrestrial Life Cycle
Recently, I discovered a new life cycle strategy in Oklahoma while working on my Ph.D. at Oklahoma State University. In this case, we don't know what arthropod acts as the final host but it is a species that lives in the soil. Each winter, after heavy rainfall, hairworms emerge from the lawns by the hundreds. They congregate on the sidewalk, streets, and lawns looking for a mate. I found that after mating, females go back to the soil and lay their eggs. These eggs and/or hatched larvae are then eaten by earthworms that then harbor hairworm cysts. We hypothesize that the arthropod host then eats earthworms and becomes infected.
Credit: Ben Hanelt and Nematomorpha.net
Hanelt B, Bolek MG, Schmidt-Rhaesa A (2012) Going Solo: Discovery of the First Parthenogenetic Gordiid (Nematomorpha: Gordiida). PLoS ONE 7(4): e34472. doi:10.1371/journal.pone.0034472
Hanelt, B., F Thomas, A Schmidt-Rhaesa. 2005. Biology of the phylum Nematomorpha. Advances in Parasitology. 59: 243-305