No common name

Mermis nigrescens

ORDER Stichosomida

FAMILY

Mermithidae

TAXONOMY

Mermis nigrescens Dujardin, 1842.

OTHER COMMON NAMES

None known.

PHYSICAL CHARACTERISTICS

A free-living adult and parasitic larvae species that infects the body cavity of grasshoppers. They are an unusually long and slender nematode, with a length of 1.97-7.87 in (5-20 cm), with females longer than males. Adult females are colored reddish brown at the anterior extremity, supposedly as an aid for light sensitivity. The terminal mouth has two closely associated lip papillae and four cephalic papillae placed further back. The crisscross fibers are distinct and the lateral chords are wide. The thick-walled brownish-colored eggs are small, divided in half by a distinct equatorial groove, and possessing prominent byssi (filamentous branches). The size of the eggs (larvae) is about 0.00276 in (70 pm) in length by 0.00118 in (30 pm) in width. When the grasshopper host ingests the eggs found on vegetation, which is usually grasses and plants, the juveniles will break open the egg at the equatorial groove and grow to a length of 3.9 in (10 cm) in a period lasting 1-3 months.

DISTRIBUTION

Commonly occur in the British Isles, Europe, and North America. (Specific distribution map not available.)

HABITAT

During their stage as developing larvae, they live primarily within locusts and grasshoppers, but may also infect other insect species. They may be located anywhere within the host's hemocoel (the body cavity where blood circulates). Once juveniles burrow out of a host, they will dig 6-8 in (15-20 cm)

into the soil where they molt into adults. They are presumed to live in temperate forests and grasslands, rainforests, and mountains.

BEHAVIOR

Adults are very agile and readily climb plants, especially during rainy seasons. During their adult stage while in the soil, there is no social interaction, except possibly to mate. Most nema-todes will move away from light, but this is usually not the case for this species. Females may remain in the soil for several years before emerging pregnant into a lighted, moist environment in order to find a site on grasses and plants to lay eggs. They usually stay in the soil during the cold winter, sheltering under debris, and then emerging in late spring, during periods of overcast, humid weather. It is believed that they are able to discriminate between light levels with the use of melanin, which is a binary system (either off or on). Even though they sometimes will move toward light, they will die from continued exposure of direct sunlight.

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 (most nema-todes do not infect their hosts by depositing eggs on plant foliage that will be eaten). Extending from the sides of the eggs are long, filamentous brances, called byssi, which become entangled with the plant, thus holding them in place. After infecting a host, juveniles penetrate through the gut wall into the hemocoel. Once located within the hemocoel, the parasites consume the hemolymph. Many worms may infect the same host. By late summer, the abdomen of many grasshoppers will be packed with these parasites. Such infections seriously stress and sterilize infected grasshoppers. After this period of infestation, the grasshopper will usually die, upon which the species exits the body of its dead host. The remainder of life is largely spent in the soil, except when adult females emerge for egg deposition aboveground.

FEEDING ECOLOGY AND DIET

Developing juveniles consume large amounts of amino acids, lipids, and carbohydrates from the hemolymph of the host. The free-living adults do not eat, so they must gain all of their nutrients while in the insect host.

REPRODUCTIVE BIOLOGY

During their post-parasitic stage, they live in the soil where they reach sexual maturity. Females may mate in the soil, but males are not necessarily needed for egg production. During or just after spring, females climb out of the soil onto vegetation where they deposit their eggs to await ingestion. Eggs can survive on foliage throughout the summer. They have an unusually large free-living stage when compared to other nematodes. A complete life cycle usually comprises of 2-3 years.

CONSERVATION STATUS

Not listed by the IUCN. The parasitic species is common and widespread.

SIGNIFICANCE TO HUMANS

Since the larvae kill the host grasshopper upon emerging, they have the potential to be used as a biological control agent if their rate of survival within the soil should be extended and if their numbers could be supplemented when released into the soil. However, nematode ecology is poorly understood, especially in nature, and such control practices are still far off into the future. ♦

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