ABSTRACT
The mermithids constitute a very important group of obligately parasitic invertebrate nematodes. They are widely distributed in both aquatic and terrestrial habitats and attack a wide range of invertebrates. Almost every mermithid is a potential biological control agent. However, most mermithids have never been reared completely through their life cycle in the laboratory. The mermithid life cycle can be somewhat complex when a second paratenic host is involved. After entering the host's hemocoel, the nematode initiates development, generally resting freely in the body cavity but sometimes entering nerve tissue for an initial period of adjustment. This chapter discusses individual mermithid species under the group of insects they parasitize. It presents an up-to-date key to point out important taxonomic differences between the known mermithid genera. Over 200 species of mermithids have been described and many more still await characterization.
