Mermis nigrescens

Mermis nigrescens is reported from many locations in North America and Europe. Related species are known from Africa. (Christie, 1937; Webster and Thong, 1984)

Adults of Mermis nigrescens live buried in the soil, while larva live as parasites inside grasshoppers. The species is more frequently found in grasslands, meadows, clearings, and other well-vegetated but not tree-covered habitats. (Christie, 1937)

This is a large species of nematode. Adult males range from 40 to 50 mm long, while the females range from 80 to 200 mm long. The body of the worm tapers in the front, with the head slightly offset and rounded in the front. Larvae and adult males are generally cream colored, as are females if their uterus is empty. If the uterus contains eggs, the female will appear brownish or black because of the coloration of the eggs she contains. Males have two spicules, one of which is used in reproduction. Both sexes possess two laterally-placed anterior chemoreceptors called amphids, but they lack posterior phasmids. Both sexes also possess a terminal mouth with two lateral papillae close to it and four head papillae further back.

In adult females, there is an orange/red area behind the head, which is associated with a unique light sensitive organ. Mohammed et al (2007) give details of the structure and function of this organ. It serves to allow the female to know where to go when laying eggs. (Christie, 1937; Mohammed, et al., 2007; Webster and Thong, 1984)

Embryos begin to develop in the eggs while still in the female. By the time the eggs are laid (in late spring or early summer) the offspring are already stage 2 larvae. If an egg is ingested by a grasshopper or other orthopteran, the outer layer of the egg rubs off and within minutes the larva begins to move out of the inner shell.

Once hatched, the larva pierces the gut wall of the host and enters the body cavity. Larval worms feed on host body fluids and tissues and grow. They emerge from the host in approximately 3-6 weeks. The worms develop faster in warmer temperatures. Size and species of hosts affects developmental time, and larval worms stay longer and grow larger in mature adult hosts than in nymph or young adult hosts. Female worms tend to remain in the host longer than males, and are larger at emergence than males. Worms puncture the host body wall and emerge as 4th stage larvae (the host and any remaining worms die). They crawl 15-20 cm into the soil where they molt, become mature adults and mate. Males die after mating. Females spend the winter in diapause in the soil and emerge on rainy or dew mornings in the following spring/summer to climb up on to plants and lay their eggs. (Christie, 1937; Webster and Thong, 1984)

Males mate with many females, while the female will only mate with one male. Males have a flexible tail including one of the spicules which they use to wrap around the female and hold the female in place during copulation. (Christie, 1937)

After spending the winter in the soil, emerge in the late spring or early summer on damp or rainy mornings, and climb on the leaves of plants, usually 30-60 cm above soil level and lay thousands of dark brown eggs on the leaves. The eggs are attached with threads of adhesive.

Eggs from unmated females have been found to be viable, so apparently parthenogenesis is possible in this species. (Christie, 1937; Webster and Thong, 1984)

No parental care is provided after eggs are laid. Females provision the eggs with a coating that protects them from sunlight damage, and glue to hold them onto leaves. (Christie, 1937; Webster and Thong, 1984)

Most Mermis nigrescens complete their lifecycle in one year, but in the absence of rains or other moisture to trigger egg-laying, females can persist in the soil for 2-3 years. Females die shortly after climbing out of the soil to lay eggs. (Chapman, et al., 1990; Christie, 1937; Webster and Thong, 1984)

The larvae of M. nigrescens are endoparasitic. During their larva stages they infect the body cavity of grasshoppers. They may be located anywhere within the host's hemocoel. Once juveniles burrow out of a host, they will dig into the soil where they molt into adults. They are dormant in the winter.

The action of females crawling on plants in order to deposit their eggs on vegetation above ground is considered an important behavior modification for insect parasitism, and is an unusual behavior in nematodes. Most nematodes will move away from light, but adult females of this species are positively phototaxic when laying eggs. (Atkins, 2004; Belovsky, et al., 2000; Mohammed, et al., 2007)

This species does not have a home range.

Little is known about communication in this species. It is likely to be based on touch and chemo-sensing. These worms can sense chemicals, touch, and moisture, and adult females have an unusual (for nematodes) light-sensing structure. (Christie, 1937; Mohammed, et al., 2007; Webster and Thong, 1984)

Larva feed on amino acids, lipids, and carbohydrates from the hemolymph of the host. The free-living adults do not eat. (Atkins, 2004; Belovsky, et al., 2000)

The predators of this species are unknown, but likely include predators of soil invertebrates, such as carabid beetles, centipedes, and shrews. (Belovsky, et al., 2000)

Larva of this nematode are parasites of orthopteran insects, and can sometimes have substantial effects on host populations. (Atkins, 2004; Belovsky, et al., 2000; Webster and Thong, 1984)

These worms could potentially be used as control agents for pest grasshoppers. However, nematode ecology is poorly understood, especially in nature. (Belovsky, et al., 2000; Chapman, et al., 1990; Webster and Thong, 1984)

Normally these worms have no known adverse effects on humans. There are a very small number of cases of M. nigrescens finding its way into humans, but this is extremely rare. Poinar and Hoberg (1988) describe a case where a female was found in an infant's mouth, and give references to the few similar cases. (Poinar, Jr. and Hoberg, 1988)

The status of populations of this species are unknown, but it is not believed to require special conservation. The species has not been rated by the IUCN, or other agencies.