Mermithidae is a family of nematode worms that are endoparasites in arthropods. As early as 1877, Mermithidae was listed as one of nine subdivisions of the Nematoidea. Mermithidae are confusable with the horsehair worms of the phylum Nematomorpha that have a similar life history and appearance.Mermithids are parasites, mainly of arthropods. Most are known from insects, but some are recorded from spiders, scorpions and crustaceans.