Horsehair Worms in the Swimming Pool

A reader from Ojai, California wrote to us recently about a long worm found in his swimming pool. Relative to its width, the worm is actually extremely long, and in fact you would have trouble identifying it as any sort of living creature unless you know about horsehair worms (a.k.a. “Gordian worms” because their coiled up bodies look like Gordian knots), which we are fairly certain is what our reader found. The reader only asked if he found some sort of worm, and since horsehair worms are of course worms, the technical answer to our reader’s question is a mere “yes.” But one-word answers maketh a good article not, so we’ll give you some information about horsehair worms, with which we have some familiarity thanks to their prevalence and the frequency with which we receive questions about horsehair worms.

Horsehair worms are definitely one of the more bizarre worms you can come across because of their extraordinary body dimensions. Horsehair worms can be very long, regularly reaching lengths of about a foot (and in rare cases they can be almost three times as long), yet they are extremely skinny, generally no wider than the lead in a mechanical pencil. Here is a nice photo (which even includes a ruler!) that the reader submitted along with his question:


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Horsehair worm in pool

Basically, horsehair worms look like the hairs that form horses’ tails. Long ago, people actually thought that horsehair worms were horse hairs that had come alive. This is of course not true, but it gives you a good idea of just how much these worms resemble horse hairs. (For another picture of these strange looking creatures, check out the reader-submitted picture in this article about horsehair worms.)

Horsehair worms are nematodes (which are also known as roundworms) that are parasitic. They prey on several different insects, like grasshoppers, crickets, beetles, and cockroaches. Horsehair worms are able to enter insects because they (the insects) consume the larvae of horsehair worms, which are hidden in small cysts on certain types of vegetation. An insect will eat this vegetation, as is their habit, and in so doing they ingest the larvae. Once inside the insect, the larvae slowly begin to grow by absorbing the nutrients that the host insect consumes. Horsehair worms will stay inside the insects they infect for varying lengths of time, generally around four to twenty weeks. After this period of growth, the horsehair worm will emerge from the insect, often through its mouth, but this only occurs if the insect is near water. This is why our reader found a horsehair worm in his swimming pool. Presumably, an insect with a horsehair worm was near the pool, and this prompted the horsehair worm to exit the insect’s body. Not surprisingly, insects that are afflicted with horsehair worms tend to die because of the parasitic worm’s presence, although insects can survive if a horsehair worm exits their body before fully maturing.

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Since our reader found a horsehair worm in his swimming pool, which people presumably swim in, we should note in closing that these worms are nothing to worry about. Horsehair worms infect neither humans nor pets, and our reader could safely scoop one out of his swimming pool with his bare hand. They are actually beneficial to have around if you don’t want to encounter as many household pests like cockroaches.

So, to conclude, our reader found a horsehair worm, and they can actually be more of a blessing than a curse if their strange appearance doesn’t weird you out too much.

1 thought on “Horsehair Worms in the Swimming Pool

  1. We recently moved from the big city to God’s country here in Northern Montana. Our only source of water so far is from a spring on Forest Service Land. While trying to put a lawn in this year, my husband was watering constantly. He took the screen out of the hose as clogs are the norm with this sandy soil, and a horsehair worm appeared…stuck half-way through the screen! Hubby pulled it out and brought it into the house for me to look up on the web. Found your site and lo & behold, there’s a picture of our worm…Love this site and will keep coming back as we are always finding “weird” things in our little forest. Thanks so much!

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