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Mermithid nematodes (Nematoda: Mermithidae) parasitize larval, pupal, and adult black flies (Diptera: Simuliidae) and may result in the development of intersex adults, generally considered to be feminized males. Simuliid gonad morphology and meiotic chromosome condition were examined in both early-instar larvae and late-instar larvae (≈ pharate pupae) to determine the developmental stage at which mermithid-associated feminization was initiated. We found that feminization occurred in early-instar larvae prior to gill histoblast formation. Infected larvae were morphologically male (rounded testes present) or female (elongated ovaries present), with morphological males exhibiting either male (achiasmate) or female (chiasmate) meiotic chromosomes; morphological females only had chiasmate chromosomes. Mermithid infection can also inhibit simuliid gonad development completely.

This study reports alterations in the drift behaviour of mayfly nymphs (Baetis bicaudatus) infected with the mermithid nematode parasite Gasteromermis sp. with respect to (i) their exposure to drift-feeding trout predators and (ii) the efficiency with which they locate food. Experimental stream channels and benthic and drift samples were combined to investigate drift behaviour. The drift behaviour of earlier instars did not differ between parasitized and unparasitized nymphs. Infected late-instar nymphs, however, drifted less frequently, drifted higher in the water column, and swam more frequently while drifting. Neither drift distances nor body angle while drifting were altered. Parasitized nymphs showed a reduction in food-location success in heterogeneous stream channels. The drift alterations therefore reduce both the exposure of the host to fish predators and the efficiency with which it locates food. The mayfly population is effectively divided into two groups: unparasitized nymphs (in which drift behaviour is of consequence to mayfly fecundity) and parasitized nymphs (in which drift behaviour is of consequence to mermithid survival and fecundity). It is suggested that locating food may be less important to the parasite, while the risk of trout predation may be higher for drifting parasitized individuals. Both of these factors would favour the parasite influencing its host to adopt lower drift frequencies.

Host effects on mermithid nematodes (Romanomermis sp.) were studied in four populations of snowpool Aedes spp. mosquitoes (Diptera: Culicidae) in western Wyoming. Two factors associated with nematode fitness, survival and adult size, were examined in relation to species and parasite load of the hosts. The proportion of entering parasites that successfully emerged from hosts was relatively high in Aedes impiger, Aedes increpitus, Aedes pullatus, and Aedes punctor, none of which produced melanotic encapsulations of parasites. Emergence success at all localities was lowest in Aedes nevadensis and Aedes hexodontus, species that commonly encapsulated nematodes. Proportionately fewer parasites were encapsulated per host as parasite load increased, and encapsulation by Ae. nevadensis and Ae. hexodontus occurred less frequently in populations in which these species constituted most of the available hosts. Mean adult lengths varied between populations but did not differ between postparasites from different mosquito species within the same site. Total volume of nematodes per host was independent of parasite load but size of individual parasites decreased with increasing parasite load. This suggests intraspecific competition for finite resources in superparasitized hosts.

Parasitic stages of Hexamermis glossinae from Glossina palpalis s. 1. and G. pallicera, which were collected immediately prior to the rainy season near Bouaflé, Ivory Coast, are described and illustrated. Specimens have an onchiostylet and were similar to Coccinellimermis coccinellae exochomi (Rubtzov, 1971) Rubtzov, 1978. Adults of Coccinellimermis are unknown. Possession of an onchiostylet is probably a specific rather than a generic character and Coccinellimermis is therefore a subjective junior synonym of Hexamermis. Results from 200 G. palpalis collected in a localized area showed that 5% of the flies were infected and that 12.3% of the males were parasitized compared with 2.1% of the females. Four of the 10 flies harbouring nematodes also harboured trypanosomes. Such concurrent infections were more frequent than expected since fewer than 10% of the 200 flies harboured trypanosomes. No correlation exists between age of the female flies, estimated by ovarian ageing, and length of the parasites. Infected females had fed and ovulated, and two carried a developing larva in the uterus. The life cycle of this mermithid is unknown, but all stages of the life cycle may occur where female flies larviposit.

Intraspecific variability and the influence of environmental factors on the morphometrics of Romanomermis culicivorax Ross and Smith, 1976 were determined by rearing the progeny of a single mating pair under different parasite intensities, host species, and temperatures. Individual measurements were frequently affected by environment and discriminant analysis revealed differences in overall shape between sibling populations. These effects were not constant within or between sexes. The growth relationships between measurements were examined and because of allometry a number of ratios commonly used in mermithid taxonomy at the species level were rejected. This led to an evaluation of morphometrics in species diagnosis.

A taxonomic review of the Romanomermis spp. parasitizing mosquitoes is presented, with a description of Romanomermis communensis sp.n. from Aedes communis (Degeer) larvae collected at Goose Creek, Manitoba. The type locality for R. communensis is identical with that of Hydromermis churchillensis Welch, 1960; however, the latter species was not found in 1974, 1975, or 1976. Consideration is given to the status of H. churchillensis.

Parasitism by Romanomermis culicivorax caused symptoms typical of starvation in Aedes vexans larvae in both field and laboratory experiments. Infected larvae were reduced in size, moulting at each instar was delayed, there was a failure to develop significant fat body, and secondary pigmentation occurred in later instars. Excluding samples taken after the onset of pupation of uninfected larvae, mean percent infection ranged from 7.2 to 18.8, 12.4 to 24.8, 9.2 to 23.2, and 38.0 to 49.0% when preparasites were applied at 10 000, 20 000, 50 000, and 100 000 m−2 to artificial field pools, respectively. Of 2786 parasitic juvenile nematodes encountered in dissected larvae during the field trial, 2.0% were melanized.

The morphological characters lateral chord structure, head, tail, vagina and spicule shape, which are those most frequently used in the diagnosis of mermithid species, were highly variable within the progeny of a single mating pair of Romanomermis culicivorax Ross and Smith, 1976. In the light of this variability the usefulness of these characters in species diagnosis is discussed and it is concluded that a series of specimens at various developmental stages are required for reliable diagnosis.

Prosimulium mixtum/fuscum S. &D., Cnephia mutata (Malloch), C. dacotensis D. &S., Simulium aureum Fries, and S. gouldingi Stone are univoltine. S. venustum Say, S. vittatum Zett., S. tuberosum (Lundstrom), S. corbis Twinn, S. latipes (Mg.), and S. decorum Walker are multivoltine. Neomesomermis flumenalis (Welch), Gastromermis viridis Welch, and Isomermis wisconsinensis Welch are univoltine. The eggs of Prosimulium and Cnephia species hatched synchronously with a batch of the eggs of N. flumenalis. The eggs of the first generation of Simulium species hatched synchronously with the eggs of G. viridis and I. wisconsinensis, and the overwintered batch of eggs of N. flumenalis. The larvae of subsequent generations of Simulium species were not infected by mermithids. Mermithid infections occur during the hatching and buildup of blackfly larval population.Caudospora simulii Weiser and C. brevicauda Jamn. occur in Prosimulium and Cnephia species during the cold seasons. Thelohania bracteata (Strickland, 1913), Pleistophora simulii (Lutz and Splendor, 1904), P. multispora (Strickland, 1913), and Coelomycidium simulii Debaisieux occur in Simulium species during the warm seasons. G. viridis and I. wisconsinensis overwinters in the egg stage, and N. flumenalis overwinters partly in the egg stage and partly as parasites in P. mixtum/fuscum. S. venustum and P. mixtum/fuscum are the most prevalent and abundant of the simuliids. N. flumenalis is the most prevalent and abundant of the mermithids and P. multispora is the most abundant of the microsporidans. Aeshna interrupta Walker and Rhyacophila fuscula (Walker) prey on S. venustum larvae.

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.

Naidid species Chaetogaster diaphanus, Chaetogaster diastrophus, Chaetogaster limnaei limnaei, Dero digitata, Dero nivea, Nais communis, Nais simplex, Nais variabilis, Pristina leidyi, Slavina appendiculata, and Stylaria lacustris were observed as paratenic and possibly intermediate hosts for larval mermithid nematodes from field collections. Mermithids were generally found in the anterior, dorsal region of the oligochaetes. Individual naidids were at most infected with two nematodes. No seasonal prevalence patterns could be firmly established owing to the sporadic occurrence of mermithid infections in the naidids.

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.

The physiology of glucose and amino acid uptake by the parasitic larval stages of the entomophilic nematode Mermis nigrescens was investigated in vitro using radioisotope-labelled substrates. Both glucose and amino acid uptake are transcuticular processes that are saturable with increasing substrate concentration but are not coupled to co-transport of sodium ions and are not energy requiring. Amino acid transport is a stereospecific membrane transport system probably located in the cuticle of the nematode. The sites for glucose transport, however, are believed to be beneath the cuticle as glucose transport is only saturable in the thin-cuticled 14-day-old larvae whereas it is not in the thick-cuticled 21-day-old larvae. Glucose transported by M. nigrescens is not converted to trehalose, a common pathway in other invertebrates, nor is trehalose found within the nematode's tissues or body fluids.

A number of simuliid populations infected with mermithids were studied to elucidate the nematode's life history patterns and the effect of this on the infected host's distribution. Some of the mermithids were very host specific, while others infected all simuliid species exposed to some degree. An extensive study showed that mermithids infecting Prosimulium mixtum failed to infect Stegopterna mutata which may have been due to the preparasites inability to recognize St. mutata as a potential host, or due to host behaviour, rather than microdistribution or physiology.In mermithid species where the majority complete their parasitic phase and emerge from the larval simuliid, the heaviest infections occur towards the downstream end of the host distribution. On the other hand those mermithid species which emerge only from adult simuliids have a high infection rate in the larval simuliids at the upstream end of host distribution. Infection occurs in the early simuliid instars, primarily first and second, with perhaps some in the older larvae. The rate of infection and the number of larvae per host changed between the time of simuliid hatch and the final host instar at most sites. However, the infection pattern in late instar larvae in any one stream appeared to be relatively stable from year to year.In the present study all mermithid infections of adult hosts were confined to the female fly irrespective of the mermithids life history pattern.

A new mermithid, Hexamermis glossinae sp.n., is described as a parasite of tse-tse flies in West Africa. Nematode parasites were reared from adults of Glossina palpalis, G. p. pallicera, and G. n. nigrofusca from the region of Vavoua in the Ivory Coast. This is the first description of mermithids from these important hosts.

Hydromermis churchillensis n. sp. is described. It is distinguished from other species of Hydromermis mainly by the fused spicules and pointed termini of both sexes. One to six larval nematodes may be found in a larva of Aedes communis (DeG.). They emerge to become free-living, molt, mature, mate, and oviposit in 2 months. Host activity is hindered, pupation prevented, and the host usually killed upon emergence of the worm. Worms are found occasionally in adult mosquitoes. Host reaction as encapsulation sometimes occurs. Ectocommensal protozoa are more common on uninfected than on parasitized mosquito larvae. At Churchill, Manitoba, approximately 10% of the mosquito larvae were killed, but in individual pools the percentage reached 80%. Nematodes were irregularly distributed in the forest and transition zones, but no correlation was found between the physical features of the pools and this distribution.

Embryos of R. communensis completed their development to the late coil stage in 16.5 days at 20 °C, 22 days at 15 °C, and 32 days at 10 °C. Embryos failed to develop at 5 °C, although 10% did reach the two-cell stage by 32 days. About 5% of the eggs at 20 and 15 °C hatched after embryonation was completed. There was no further hatching during the 32-day period of observation. The nematode is probably univoltine in its natural habitat near Churchill, Manitoba.

A new genus and species of mermithid nematode parasitizing adult females of Anopheles funestus (Giles) in Upper Volta is described. The new genus Empidomermis is characterized by having six head papillae, eight hypodermal cords, cuticle without noticeable cross fibers, two short slightly curved spicules (equal to or less than body width at cloaca), a relatively short S-shaped vagina, small amphids, and postparasitic juveniles with a well-developed tail appendage.The nematodes could only be successfully reared to maturity if the hosts were held on glucose 3–4 days after the blood meal. Postparasitic juvenile mermithids molted to the adult stage about 17 days after leaving their mosquito hosts and mating and oviposition occurred immediately afterwards. The eggs hatched about 54 days after oviposition when mosquito larvae were added to the container. Parasitized adult female A. funestus were sterilized and died soon after the nematodes emerged.

Romanomermis Coman, 1961 is resurrected from the synonymy of Eurymermis Müller, 1931, the generic description is augmented, and three new species are described: Romanomermis kiktoreak, R. hermaphrodita, and R. culicivorax. R. culicivorax is a segregate of a complex known previously as Reesimermis nielseni Tsai and Grundmann, 1969. R. nielseni is restricted to a species known only from Wyoming. Reesimermis Tsai and Grundmann is suppressed and R. nielseni s.str. is transferred to Romanomermis. Reesimermis muspratti Obiamiwe and Macdonald is moved to Octomyomermis Johnson. Keys are provided for the world species of Romanomermis and for the described mermithids that infect mosquitoes in North America. A brief commentary is included on the taxonomic utility of length and degree of fusion of the spicules, body shape, cuticular fibres, and double molt to adult.