They look like a small piece of wire, about a millimeter in diameter. Sometimes they wiggle, sometimes they tie themselves in knots and sometimes they just lie there. If you’ve found one in standing water, your first thought is probably “Internal Parasites!” Visions of intestinal infection commence to abound.
They are parasites, but only of “insects, including crickets, cockroaches, beetles, mantids, and grasshoppers, but also spiders and woodlice (sowbugs)” according to Dr. J.L.Capinera in http://edis.ifas.ufl.edu/in274. The insects take in the tiny eggs while drinking and the worms develop inside the insect. Finding a horsehair worm means an insect host died in the water and the worm was released.
Article written by
Livestock and Natural Resources Agent
Jefferson County Extension Service