Crickets infected with Horsehair Worms are forced to commit suicide by drowning so that the adult parasite can reproduce.

Nematomorpha

There are two groups of Horsehair Worms (Phylum Nematomorpha) - the Gordiids (over 200 species which parasitise land arthropods as larvae and live freely in freshwater as adults) and the Nectonematids (5 species described which parasitise marine arthropods as larvae and reproduce in the sea). Very little is known about the biology of either group.

Adult Worms

When horses were the main form of transport and horse-troughs were common it was quite normal to find horse hair in the water. Occasionally an infected insect would drown in these troughs and the emerging adult worms would be noticed. The adult worm looks very much like a long hair, so it was assumed that the horsehair had somehow ‘come alive’. A little more imagination turned these ‘Horsehair Worms’ into ‘Horsehair Snakes’. Their other name comes from the legend of the Gordian knot created by Gordius around 330 B.C. in ancient Phrygia because the adults writhe and tie themselves in complicated knots (Gordian Worms).

The adult worms do not feed, but simply mate in water and release their eggs. When the eggs hatch out the larvae either parasitise an arthropod immediately or form cysts on plants which will be later eaten by their host.

Parasitic Larvae

Once inside a suitable host arthropod, the larvae begin to grow. Eventually they occupy much of the space inside the poor creature, and when they finally emerge from its body (after the host's death) they are many times its length! If their host is eaten by another animal, such as a frog, the adult will simply wriggle out of its mouth and search for water.

Mind Control

Land arthropods do not normally leap into water, but an infection by Gordian Worms has a strange effect on behaviour (at least in grasshoppers and crickets). Infected crickets will seek out water when their parasite reaches maturity and obligingly commit suicide by drowning. It is thought that the larvae are able to produce chemicals that affect the host’s brain directly, and that it is possible that other parasitic species might be able to do the same.

Parasite Research

Studying the way that Gordian Worms are able to control the behaviour of their host might lead to a better understanding of parasite/host relationships in general, and if it turns out that human disease organisms behave in similar ways it might have far-reaching implications for control of these parasites (Malaria, Chaga’s Disease etc.). This is a good example of how the academic study of an obscure animal can lead to useful discoveries – if we can understand what makes Kissing Bugs bite humans we might be able to design chemicals to control their behaviour in the same way that the Horsehair Worms control their Grasshopper hosts. (See blog)

See also: Australian Museum Online

Read about Toxoplasma gondii - another parasite that controls host behaviour. and similarly Dicrocoelium dendriticum

Other articles by John Blatchford