Mermis nigrescens is reported from many locations in North America and Europe. Related species are known from Africa.

Biogeographic Regions: nearctic (Native ); palearctic (Native )

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.

Range length: 40 to 200 mm.

Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry ; polymorphic

Sexual Dimorphism: female larger; sexes colored or patterned differently; sexes shaped differently

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.

Habitat Regions: temperate ; terrestrial

Terrestrial Biomes: savanna or grassland ; chaparral ; forest

Other Habitat Features: suburban ; agricultural

Larva feed on amino acids, lipids, and carbohydrates from the hemolymph of the host. The free-living adults do not eat.

Animal Foods: body fluids

Primary Diet: carnivore (Eats body fluids)

Animal / parasite / endoparasite
larva of Mermis nigrescens endoparasitises body cavity of Schistocerca gregaria

Animal / parasite / endoparasite
larva of Mermis nigrescens endoparasitises body cavity of Locusta migratoria

Animal / parasite / endoparasite
larva of Mermis nigrescens endoparasitises body cavity of Acrididae

Animal / parasite / endoparasite
larva of Mermis nigrescens endoparasitises body cavity of Dermaptera

Larva of this nematode are parasites of orthopteran insects, and can sometimes have substantial effects on host populations.

Ecosystem Impact: parasite

Species Used as Host:

• Orthoptera

The predators of this species are unknown, but likely include predators of soil invertebrates, such as carabid beetles, centipedes, and shrews.

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.

Communication Channels: tactile ; chemical

Other Communication Modes: pheromones

Perception Channels: visual ; tactile ; chemical

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.

Development - Life Cycle: diapause

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.

Range lifespan

Status: wild: 3 (high) years.

Typical lifespan

Status: wild: 1 (high) hours.

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.

Mating System: polygynous

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.

Breeding interval: Breed once yearly

Breeding season: ummer

Range number of offspring: 2000 (low) .

Average age at sexual or reproductive maturity (female): 1 years.

Average age at sexual or reproductive maturity (male): 1 years.

Key Reproductive Features: semelparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); parthenogenic ; sexual ; fertilization (Internal ); oviparous

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.

Parental Investment: pre-fertilization (Protecting: Female)

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.

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

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.

These worms could potentially be used as control agents for pest grasshoppers. However, nematode ecology is poorly understood, especially in nature.