A parasite is an organism that lives in or on another organism and gets it nutrients from it. It is estimated that 40 percent of all animal species are parasites, and that all wild animals have at least one parasite. The horsehair worm is a widespread one – there are 11 species in the United States – that you may be unaware of.

Horsehair worms are very thin (1/80th to 1/10th of an inch) and resemble a hair from the tail of a horse – hence its name. You can find these worms in wet areas such as ponds, puddles or pools of water. Horsehair worms may look sinister because of their large size (they can be 4 inches or more in length), but they are harmless to people and pets. They must be swallowed by a host, usually as larvae; they cannot enter a host from the outside.

A female lays a string of eggs (as many as 27 million!) in water. Each egg is microscopic, so you cannot see it. In a few weeks to months, depending on water temperature, larvae hatch and, within 24 hours, form a protective covering. If a suitable insect host – such as a grasshopper, cricket, praying mantis, pillbug or cockroach – swallows a larva, the protective covering dissolves in the host’s gut. The larva has hooks that it uses to bore through the wall of the host’s gut into its body cavity.

The worm has no mouth. It absorbs nutrients in the host’s body through its skin. The worm grows and grows for two to three months until it takes up most of the host’s body, except for its head and legs. Once the worm is mature, it is ready to leave the host, but it needs to get back to water to reproduce.

Usually hosts, such as crickets, avoid water – they get all the water they need through their food and drops of dew; a cricket infected by a horsehair worm heads straight toward water and jumps right in. How the worm commands the cricket is not completely understood, but the worm seems to send out neurotransmitters that compel the host to change its behavior.

Once the cricket jumps into water, the worm emerges quickly from the host’s abdomen. In the water, male and female worms find each other and pair up in tangled masses. The male passes his sperm to the female and dies. The female lays millions of eggs underwater on a stick or rock. Then she too dies, flattened like a squashed paper straw. After the eggs hatch, insects ingest the microscopic parasites, and the cycle begins anew.

Horsehair worms are sometimes called cabbageworms from their penchant for accumulating in pools of water on cabbage leaves. They are also called Gordian worms because of their tendency to look tangled and knotted up, like a Gordian knot.

Horsehair worms inducing hosts to jump into water and die are just one example of parasites that alter a host’s behavior for their own benefit.

Scientists discovered recently that horizontally polarized light is strongly reflected off the surface of water and that many insects read this type of light as a signal to seek or avoid water. Research indicates that horsehair worms can manipulate the perception of the host insect, making the host travel toward horizontally polarized light. In a Japanese study, infected crickets and grasshoppers were 20 times more likely to enter a stream than crickets and grasshoppers that were not infected by horsehair parasites.

You might think that if a cricket carries a horsehair worm in its body and a fish or frog eats the cricket, that that would be the end of the worm. You would be wrong – a horsehair worm can escape from both its cricket host and the digestive tract of the cricket predator.

In fact, this is relatively common. It takes a mature horsehair worm up to 10 minutes to emerge from the abdomen of its host. During this time, the cricket is floundering at the surface of the water, an attractive meal to fish and frogs. If a fish or frog eats the cricket, the horsehair worm emerges first from the cricket’s abdomen and then wriggles out of the mouth, nose or gills of the fish or frog. The entire double-escape process takes anywhere from eight to 28 minutes. 

The next time you encounter some standing water, take a closer look. You may be able to spot these fascinating worms for yourself.

Seen nearby

Tamara Saviatto spotted this wild turkey in her Framingham yard on July 1.

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Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.