The Nematomorpha are a group of parasitic worms that develop within their hosts (primarily terrestrial insects and other arthropods) but reproduce in aquatic environments. Around 300 species of nematomorphs (or "hairworms") have been described. Nematomorphs are dioecious (i.e., have separate sexes). Tiny larvae (about 100 µm long) hatch from eggs. These larvae are equipped with two or three rings of cuticular hooks and terminal stylets with which they are able to penetrate soft epithelia of their hosts. Terrestrial hosts may become infected by drinking water containing larvae or they may prey on a paratenic host (see below). Although adults of most nematomorph species live in freshwater, a few species live in damp soil and the five known species in the distinctive genus Nectonema are pelagic in coastal marine environments. Within the host, nematomorphs increase dramatically in size from about 100 µm to several centimeters (some may exceed two meters in length). The size of the mature worm substantially exceeds the length of the host. By the time the worm is mature, it fills most of the host cavity with the exception of the head and the legs. Worms are ready to emerge only once they reach this stage. (Schmidt-Rhaesa 2002 and references therein; Thomas et al. 2002; Brusca and Brusca 2003)
The life cycle of nematomorphs has 4 stages: the egg, the pre-parasitic larva that hatches from the egg, the parasitic larva that develops within an invertebrate host (the "definitive" or developmental host), and the free-living aquatic adult. Within the definitive host, worms complete development, but they do not mate and oviposit until they are free-living in aquatic environments. Many nematomorph species have another type of host as well, a "paratenic" or transport host. The pre-parasitic larva enters a paratenic host but does not develop further until the paratenic host is eaten by a scavenger or predator in which it can develop. Within paratenic hosts, larvae penetrate through the gut, secrete a cyst wall, fold up, and become cysts. For at least some nematomorph species, the nematomorphs may make the transition from water to land by forming cysts in aquatic insect larvae, with the cysts surviving the host's metamorphosis to an adult which can then convey the nematomorphs to land. The great majority of freshwater nematomorphs have been collected from beetles or orthopterans. Paratenic hosts run the gamut from trematode flatworms to vertebrates. (Poinar 2001; Hanelt and Janovy 2004)
Many types of parasites are known to modify the behavior of their host in ways that benefit the parasite. Based on anecdotal observations, it has long been suspected that at least some mature nematomorphs, which must reach water to mate and reproduce, manipulate the behavior of their terrestrial insect hosts, causing them to seek water and jump into it. Investigations by Thomas et al. (2002) found clear evidence of this phenomenon in 8 tettigoniid orthopterans infected by the nematomorph Spinochordodes tellinii, as well as in the gryllid cricket Nemobius sylvestris infected by the nematomorph Paragordius tricuspidatus. In experiments, however, they found no evidence that hosts actively seek out water; rather, they suggested, infected hosts seem to display erratic behavior that eventually brings them close to water, which they then enter. Consistent with the findings of Thomas et al. (2002), Sanchez et al. (2008) found that this behavioral manipulation has two phases, first causing the cricket to wander into atypical habitats and next causing it to commit suicide by entering water.