Description and Life Cycle
The adults are free-living but the juveniles are parasitic. The preparasitic worms are minute, and infect their host when they are accidentally ingested. They cannot penetrate hosts from the outside. Nematomorphs infect insects, including crickets, cockroaches, beetles, mantids, and grasshoppers, but also spiders and woodlice (sowbugs).
The species most commonly encountered, and perhaps the best studied species, is Gordius robustus. It occasionally attains high densities in its tettigoniid host Anabrus simplex Haldeman, also known as the Mormon cricket. The following information is based on G. robustus.
Egg: The minute, ovate eggs are only about 40 microns wide and 50 microns long. They are deposited in water, in slender strips held together with gelatinous material. The number of eggs produced per female is enormous; Thorne (1940) estimated 27 million eggs form a single female. The eggs are deposited in the spring after the adults move from their overwintering sites in soil, under rocks, and in organic debris. Shallow water, including swamps, pools and streams, are acceptable oviposition sites.
Larvae: Apparently larvae swim about freely in the water after hatching, and are ingested with water when insects drink. Once inside a host insect, the larva penetrate the insect's gut and enter its body cavity. They mature and escape from the host in two to three months. Thorne (1940) suggests that emergence from the adult occurs only in the presence of water, though the mechanism for detection of water is unknown. Thomas et al. (2002) showed that infected hosts are more likely to enter water than non-infected hosts.
Adults: These worms resemble nematodes, but are very long, usually 30 to 40 cm but sometimes 120 cm in length, and quite narrow in diameter, often only 1 mm. The adults are featureless, with a blunt head and slightly swollen tail. The color is usually mauve-brown to black. The sexes are separate.
Importance
The impact of horsehair worms is highly variable, but usually slight. Because insects must drink
from free water in order to be infected, few seem to acquire these parasites. Also, they must be
long-lived in order for the worms to complete their development. However, Thorne (1940)
reported an infection level of 99% at one site, so the potential effect is great. Insects infected by
nematomorphs rarely produce eggs. Even a single worm is large relative to hosts, so sexual
development is eliminated. Horsehair worms are mostly a curiosity and come to people's
attention because of their large size and sinister appearance. Their common name, horsehair
worm, is derived both from their long thin appearance and the fact that they sometimes occur in
livestock watering tanks, where they emerge from drowned grasshoppers that carelessly landed
upon water.
Author: John L. Capinera, University of Florida
Photograph: Sharon LaPlante
Project Coordinator: Thomas R. Fasulo, University of Florida
Publication Number : EENY-117
Publication Date: October 1999. Latest revision: December 2004.
Copyright 1999-2004 University of Florida
Featured Creatures
Department of Entomology and Nematology
Division of Plant Industry
Electronic Data Information Source
adult