Since horsehair worms are parasitic, they are assumed to be beneficial in the control of certain insects. Occasionally, horsehair worms are found after a cricket or cockroach is crushed, or when the host hops into a container of water, and the worm exits out of the insect’s body. Emergence from the host occurs only when the host is near water. Beetles, cockroaches, crickets or grasshoppers are the most common hosts in urban areas. One species of horsehair worm lives in salt water and parasitizes crabs. Immature stages are parasites on insects or crustaceans living in or near water, or in moist soil. They commonly swim or crawl about with a whip-like motion. Adults live in all types of fresh-water habitats and can be found in both temperate and tropical regions. Adults, the stage most commonly seen, live in water or very moist soil. Not much is known about the life of horsehair worms. They are creamy to blackish in color, and frequently are twisted and coiled like a discared thread. The body diameter is about the width of a pencil lead. Lengths of a foot or more are not common. The body of the horsehair worms is extremely long and thread-like.

One of the most common species in the United States in Gordius robustus. Horsehair worms are insect parasites that belong to the phylum Nematomorpha. Horsehair worms which emerge from parasitized insects were seen swimming in water troughs and supposed to have spontaneously transformed from the long horse hairs hence the term “horsehair worm”. Coincidentially, insects (including those parasitized by horsehair worms) also frequently fall into the water of horse troughs and die. Horse hairs frequently drop into watering troughs where they can accumulate. The long, thin structure of these worms is so similar to that of a hair that it was formerly thought that they were transformed from the tail hair of horses. Despite their sometime frightening appearance, these creatures are not harmful and have no economic importance. They are commonly found in puddles of water, on damp sidewalks and patios, or as they emerge from bodies of their insect hosts.

Horsehair worms are parasites of certain insects, especially crickets and grasshoppers.

Horsehair worm (left) and the cricket from which it emerged.

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