Mermithidae

Mermithidae is a family of nematodes that are obligate parasites of arthropods, especially insects. Members are endoparasites in which free-living adults occur outside hosts, and juvenile stages enter hosts to obtain nutrients. The family is cosmopolitan, occurring in soil, freshwater, and semi-aquatic habitats. Genera include Romanomermis, Mermis, Heteromermis, and Hexamermis, among others. Morphologically, mermithids are slender, threadlike nematodes. They are characterized by a long, typically smooth body and a simple feeding apparatus in the later juvenile or adult forms, though external features vary by genus.

Life cycle: Eggs hatch into infective juveniles (IJs) that seek out suitable arthropod hosts. The IJ penetrates the host cuticle or gut and migrates through the hemocoel to reach the body cavity, where it grows and consumes host tissues. After maturation, the parasite emerges from the host, often killing it, and enters the free-living, adult phase in soil or water. The adults reproduce, and eggs or juvenile stages begin the cycle anew.

Hosts and ecological role: Mermithids infect a wide range of arthropods, most notably insect larvae such as mosquitoes, crane flies, moths, beetles, and other invertebrates. They can influence insect populations and have been explored for biological control of pests. A well-known example is Romanomermis culicivorax, used in some programs to reduce mosquito populations.

Human relevance: These nematodes do not infect humans and pose minimal risk to non-target vertebrates. They are of interest mainly to ecologists and biological-control researchers.