1. Aquatic mermithids

Mermithids that attack the aquatic stage of their hosts are similar, varying mostly in the length of time required to complete their life cycle. The infective stage (preparasite) hatches from the egg as a second- stage larva and in most species is free swimming in still-water environments, but species that develop in moving water environments may attach themselves to surface substrate (Molloy and Jamnback, 1975). The preparasite appears to be nondirectional in its swimming activity, leading several authors to suggest that host contact is accidental. Kurihara (1976) reported that the preparasites of R. culiciuorax (Reesimermis nielseni) did not aggregate in the vicinity of restrained hosts over time. In contrast, Poinar (1979) suggested that the large amphidial openings in the preparasitic stage of R. culicivorax were associated with host attraction. Behavioral characteristics unique to the parasite-host association may increase the chance of contact. Romanomermts culicivorax is positively thigmotactic and negatively geotactic, which greatly increases their chance of contact with a suitable mosquito host (Petersen, 1973a). The preparasite is generally short lived: Gastromermis viridis, a parasite of blackflies, at all temperatures becomes sluggish when 2 days old (Phelps and De Foliart, 1964); and 50% of the preparasites of R. culiciuorax survived for 2.0 days at 12'C and 0.9 days at 30'C (Brown and Platzer, 1977). However, Johnson (1955) reported that the infective stage of Hydromermis contorta, a parasite of chironomids, survived 12 days at 2'C and 6 days at 17'C.

Upon successful contact, the preparasite attaches to the host by means of a stylet and enters the hemocoel through a hole made in the host's cuticle. The preparasites of Mesomermisflumenalis attach themselves to the thoracic region of blackfly larvae, pierce the integument, and enter the host within a few minutes. During this period and for several minutes thereafter, the hosts appear to be paralyzed. Gradually normal movement and feeding resume (Molloy and Jamnback, 1975). Though cuticular penetration is the usual mode of entry for mermithids, ingestion of the eggs and penetration of the gut wall have been suggested for several blackfly mermithids and have been reported for Pheromermis pachysoma, a parasite of wasps (Poinar et al., 1976).

Mermithids species that develop in larval hosts usually begin maturing soon after entering the host. The parasitic juvenile derives its nourishment directly from the host's hemolymph by transcuticular uptake. Generally, there is a single molt inside the host. The two final molts occur simultaneously after the mermithid leaves the host and enters the environment (Poinar and Otieno, 1974). The parasitic stage of R. culicivorax grows slowly for the first 3-4 days and then rapidly increases in size, requiring a total of 7-8 days at 26'C (Gordon et al., 1974). Similar growth patterns have been reported for other- mermithid species. The developmental period varies greatly with the temperature of the aquatic environment and many mermithid species over-winter in parasitized hosts.

Some mermithid species develop only in larval hosts under normal environmental conditions, while others may complete development in any host stage usually depending on the age of the host at the time of penetration. Still others mature only in the adult host. In the parasitic stage, mermithids stop feedingjust prior to emergence and form a second cuticle (retaining the previous larval cuticle), permitting them to become tolerant to the external environment. The postparasitic stage emerges by rupturing a hole in the host's cuticle by simple mechanical pressure, usually through the intersegmental membranes. Death usually occurs in larval hosts at the time of emergence. Adult hosts occasionally survive parasite emergence and can occasionally develop a reduced batch of eggs (Petersen et al., 1967).

After emergence, the free-living, nonfeeding postparasites burrow in the habitat substrate where they molt to the adult stage, mate, and lay eggs. This period may be as short as 7-10 days in some species but may require up to a year in mermithids that parasitize univoltine hosts.

The eggs of aquatic mermithids are usually spherical and transparent and contain small amounts of gelatinous material on the surface. The eggs of most aquatic mermithids have little tolerance for desiccation. However, eggs of mermithids that parasitize floodwater mosquitoes are known to tolerate long periods in dry habitats. Egg maturation begins immediately following oviposition and maturation is temperature dependent (Thornton and Brust, 1979). The eggs hatch if temperatures are optimal and adequate moisture is available. Where free water is absent, eggs mature but generally do not hatch until the habitat is flooded. Thus, when habitats remain flooded for extended periods, there is a continuous production of mermithids at low levels, but when habitats dry out and are reflooded, synchronized hatching of the mermithid and host eggs occur, often resulting in high levels of parasitism (Petersen and Willis, 1971).

A number of other unknown factors may also influence egg hatch. Recently, Platzer (1982a) reported that the eggs of Octomyomermis muspratti hatched more readily if they were ingested and passed through the alimentary canal of a larval mosquito, an apparent survival mechanism that assures the presence of hosts at the time of egg hatch.

2. Terrestrial mermithids

The life cycle of this group of mermithids follows that of aquatic mermithids but is usually more protracted with modifications to overcome desiccation. Females of Mermis nigrescens migrate from the soil to vegetation and lay eggs during periods of high moisture. The eggs are later eaten by grasshoppers and hatch in the gut, and the preparasites penetrate through the gut wall into the hemocoel. However, the preparasites of most terrestrial mermithids hatch from eggs in the soil and migrate to the surface in search of a suitable host. This migration usually occurs at night or during periods of rain or heavy dew. The preparasites of Filipjevimermis leipsandra in soil remain infective for up to 50 days (Cuthbert, 1968).

The length of the parasitic phase varies with species. Filipjevimermis leipsandra required less than 7 days (Cuthbert, 1968); Hexamermis arvalis, 7-12 days (Poinar and Gyrisco, 1962); Agamermis decaudata, 1-3 months (Christie, 1936); and M. nigrescens males 4-6 weeks and females 10-12 weeks (Christie, 1937). The postparasites emerge, killing the host, and enter the soil to a depth of I- 15 cm determined by soil moisture. Maturation and the beginning of egg laying may take as little as 2 weeks for F. leipsandra or a year for A. decaudata. Mating is usually necessary but at least two species, F. leipsandra and M. nigrescens, can reproduce parthenogenetically. The eggs of most species are laid in the soil where they mature and remain until adequate temperature and moisture stimulate hatching. Females of A. decaudata are reported to lay up to 10,000 eggs over a 2-year period (Christie, 1936).

The most complex and unusual life cycle reported to date for a mermithid is that of Pheromermis pachysoma, a parasite of yellowjackets, Vespula pensylvanica. Pheromermis pachysoma utilizes a paratenic host in its life cycle, a behavior unique for the Mermithidae. The eggs are deposited in wet seepage adjacent to springs and hatch only when ingested by caddisflies or other aquatic insects. After hatching, the nematodes penetrate various host tissues and enter a quiescent phase. They are carried into the adult stage of the insect but develop no further unless ingested by V. pensylvanica. Wasp larvae are probably infected when they are fed paratenic hosts captured by worker yellowjackets. Postparasitic juveniles emerge from the adult host when the wasp visits wet sites (Poinar, 1976; Poinar et al., 1976).