Horsehair worms (Fig. 1) are usually found moving about in puddles of water, on damp sidewalks and patios or they may come wiggling out of insects. These creatures have no economic importance to man other than being parasitic to some insects. Horsehai r worms belong to the phyllum Nematomorpha. One of the most common species in the United States is Gordius robustus (Pennak 1953).
The body of horsehair worms is extremely long and thread-like, and lengths of a foot or more are not atypical. The diameter is usually not much more than a pencil lead. The color is usually creamy to blackish. When found, they are usually twisted and coiled much like a piece of discarded thread. The hair-like nature of these worms is so striking that it was formerly thought that they arose spontaneously from the hairs of a horse's tail.
When water troughs were common in cities, hairs from horses would collect in the water. As a coincidence, insects parasitized by horsehair worms would also fall into the water and die. The worms would then emerge from the insect and begin to swim around. It appeared that the real horse hairs had become living worms. Hence, the term horsehair worm.
Not much is known about the life of a hairworm. Adults, the stage most commonly seen, live in water or very moist soil. The immature stages are all parasitic in insects and crustaceans. The adults live in all types of fresh-water habitats in temperate and tropical regions of the world. They commonly swim or crawl about by whip- like motion. The immature stages are parasitic to insects living in or near water or moist soil. Beetles, cockroaches, crickets or grasshoppers are the most common host insects. One species lives in salt water and parasitizes hermit crabs and true crabs. Emergence from the host occurs only when the host is near water. Occasionally, they are found after a cricket or cockroach is killed by someone crushing the insect, at which time the worm begins to wiggle out of the insect's body.
Since the worm is parasitic, it is assumed to be beneficial in helping control certain insects. Its value as a parasite is questioned because the worm does not kill the host until the host reaches maturity. Horsehair worms are not parasitic to man or his pets. Therefore, the creature is of importance only because it is a parasite and is one of nature's oddities. If they bother you, like they would if they were swimming around in your pool or bath, the best control is their removal with a net.
Horsehair worms can be confused with other parasitic worms of the order Nematoda. Parasitic nematodes can be separated from horsehair worms by the structure of the (posterior) end of the body. The "tail" of parasitic nematodes is hooked and the anal op ening occurs before the body's end. In contrast, the end of Gordius horsehair worms have a cleft (Fig. 2).