Adult females of some Mermis species
deposit eggs on vegetation from which they are ingested by herbivorous
insects; others deposit eggs in soil from where hatched larvae migrate
to the surface and penetrate available hosts. After ingestion, juvenile
nematodes (about 750 um long), penetrate the gut wall and enter the body
cavity of the host insect.
Infection by Mermis nigrescens, for
example, inhibits development of the ovaries in grasshoppers, and the
host may die before or after the nematodes emerge from its body.
After emerging from the host, the nematodes enter
the soil where they grow to the adult stage in 2-4 months and become
sexually mature. They may remain in the soil for up to 3 years although
a two-year life cycle is usual. Adults of M. nigrescens may
either mate, or the females reproduce the parthenogenetically.
Adult males of M. nigrescens are about 40-60
mm long; females 60-160 mm. Males are not as long-lived as are the
females. Females with fully developed eggs migrate to
the soil surface and deposit eggs on vegetation only after rainfall,
often in the morning hours before the sun dries the foliage. Up to
14,000 eggs may be produced per female. When laden with eggs, the
tan-colored females appear to have a dark stripe running the length of
the body, but this dark coloration disappears as the eggs are deposited.
The brown or reddish-brown eggs are about 50 um in diameter. They have a
cluster of branched filaments at each pole. As the leaf or soil surface
dries, the filaments adhere to the substrate and the eggs are anchored.
Since egg deposition occurs in moist conditions,
parasitism of grasshoppers and other hosts can be enhanced irrigation.