The Gordian Worm

The Gordian Worm youtube video

The Gordian Worm is a very long, very slender insect parasite that gets its common name because it is sometimes found tangled up in what almost seems to be a living gordian knot. It is also known as a Horsehair Worm because of the typically ill-informed folklore that "the long, thin hairs of a horse's mane or tail [can fall] into the water trough as a horse [drinks] and later [come] to life."

The scientific name for this animal is Nematomorphora. In its larval stage it is a parasite of grasshoppers and crickets. What happens is that the eggs hatch into tiny larvae which are then encysted on submerged grass or other plants. When these are eaten by grasshoppers or crickets, the Nematomorphora larvae grow inside the host until they eventually break free into the adult worms. Exactly how these worms grow as long as they obviously do is not explained in my source material.

Interestingly there are occasional instances of Gordian worms infecting humans--though these seem to occur only in the Far East and are apparently instances of pseudoparasitism. I.e., a person accidentally or intentionally eats an infected grasshopper or cricket. This introduces the worm into the human, but the worm is incapable of thriving in its new host.

For a truly fascinating introduction to horsehair worms and similar parasites, watch this YouTube video: Horse Hair Worms: http://youtu.be/so8ScD6m1MI
Sources:
http://ohioline.osu.edu/hyg-fact/2000/2012.html
http://www.nematophora.net