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Gastromermis cordobensis n. sp (Nematoda: Mermithidae) a parasite of larvae of the blackfly Simulium lahillei Paterson & Shannon (Diptera: Simuliidae) in Argentina, is described. Diagnostic characters of this species include a mouth ventrlly shifted; six cephalic papillae; eigh hypodermal chords; small and pear shaped amphids; a long and S-shaped vagina; a singl spicule, which is long, has non-uniform walls, and a tip with sculpture; three rows of genital papillae, the middle one with 18 pre-anal and 10 post-anal papillae, the lateral rows have 36 papillae each; oval eggs; and post-parasitic juveniles with long thin tails. Pre-parasitic and parasitic juveniles are included in the description.

Two new species of the Mermithidae (Nematoda) are described from the Japanese blackfly Simulium japonicum Matsumura. They are the first representatives of Gastromermis and Isomermis from Japan. Gastromermis mesostoma n. sp. is characterized by a terminal or near terminal mouth, 6 hypodermal cords and the absence of cloacal or vulvar lips. Isomermis bipapillatus n. sp. is characterized by the small size of the spicules, their curved shape and by a pair of terminal hypodermal protuberances in the tail.

From material collected by R. Le Berre in 1972 and by J. J. Petersen and B. Mondet in 1975, two new species of Mermithidae are described from blackfly larvae, including Simulium damnosum s.l., for the first time in West Africa. These species, Gastromermis philipponi n.sp. and Gastromermis leberrei n.sp. possess a rudimentary excretory gland and the preparasitic larvae of the former contain a characteristic caudal appendage which is lost during penetration into the simuliid larvae. Therefore, these two species are the only ones of Gastromermis known to have six longitudinal cords.

Twenty-four described mermithids are listed as known members of the Canadian fauna. Five of these are redescribed, and eight new species are described; namely Capitomermis micropeos, Gastromermis plethelos, Gastromermis leptopeos, Gastromermis pachopeos, Hydromermis macrospiculatus, Neomesomermis leptoderos, Neomesomermis welchi, and Neomesomermis yukonensis. A taxonomic key is provided to separate the 10 genera to which the 24 species belong. Keys identify the species of the genera Gastromermis, Hydromermis, and Neomesomermis. Morphology of the male head and genitalia are critically analysed and consistent male characteristics incorporated into four keys.

Gastromermis metae sp.n., a parasite of Simulium equinum and S. ornatum (Diptera: Simuliidae) is described and illustrated. The morphological variation in this species led to an evaluation of diagnostic characters currently used in mermithid taxonomy and an assessment of species in the genus Gastromermis. Ninety-five out of 99 published descriptions were examined: 25 were adequate, the remaining 70 are species inquirendae.

One female of Eusimulium rendalense Golini and three of E. fallisi Golini, among nearly 470 simuliids captured from Anas boschas L. in Norway, were found naturally infected with mermithid nematodes Two of these infected flies had fully fed on avian blood. The nematodes may be a species of the Gastromermis boophthorae complex Welch and Rubzov.

Different methods of killing, fixing and processing to mounting medium for adult males of Romanomermis culicivorax and Gastromermis sp. were tested and compared. Qualitative characters used in species diagnosis, particularly head shape and cuticle thickness, and quantitative characters were differentially altered by the methods tested. No method produced specimens with all the characters of live individuals. The best results were obtained after killing by immersion in water at 65° for 3s, fixing in TAF for 1 week and processing to glycerol by Seinhorst's method. Romanomermis culicivorax is currently distinguished from R. iyengari by fixation artifacts and is a species inquirenda. Interbreeding experiments are required to determine the taxonomic status of R. culiciaorax.

>Gastromermis boophthorae is distinguished from related species by its longer spicule, its characteristic lateral cord, numerous and small cells in the head region, and longer proximal reflex of the vagina. This species is divided into six varieties that are based mainly on differences occurring in the free-living postparasitic stages. It is recognized that the term “variety” is seldom used in nematology and is unprotected nomenclatorially, but its use here seems justified to designate infraspecific populations that appear at present to be sympatric and anatomically discontinuous, and that can only be recognized as immature nematodes. Six species of blackflies were found to be hosts and infection is reported from May until August in the Leningrad region. Percent infection ranged from 1 to 10 but reached 100% in one sample. Synchronization of host and parasite development is probably important in determining incidence of parasitism.

The life cycles of three mermithid species were synchronized with those of the chironomid hosts. Hydromermis palustris overwintered as an advanced parasite in Cladotanytarsus sp., emerged in early May, and infected new hosts throughout the summer. Gastromermis deltensis overwintered as an early parasite, mainly in Harnischia sp., emerged in late May, and also infected new hosts throughout the summer. Hydromermis conopophaga probably overwintered as an egg, infected new hosts in spring, and emerged from Polypedilum simulans and Harnischia sp. in late June. It had only one generation per year. The chironomids overwintered as third instars. Cladotanytarsus sp. was the first to molt to the fourth instar in the spring, Harnischia sp. was second, and P. simulans last. Both Cladotanytarsus sp. and Harnischia sp. emerged throughout the summer; P. simulans had a limited period of emergence in the summer. The sex of the mermithids was correlated with their number per host. Intersexes in parasitized adult Harnischia sp. are described and illustrated.

Gastromermis deltensis n. sp., Hydromermis conopophaga Poinar, 1968, and Hydromermis palustris n. sp., parasites of chironomids (Diptera) in the Delta Marsh, Manitoba, Canada, are described and illustrated. The three species showed patterns of morphological variation, the most common being variation in ventral displacement of the mouth. This led to a reconsideration of the genera Gastromermis and Hydromermis. Spicule length is proposed as the differential characteristic between the two genera. Species placed to date in Gastromermis are reassessed on this basis.