Hairlike Worms? Horsehair Worms

Horsehair worm by nail
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A reader from Texas wrote to us recently about some “hairlike worms” he is finding on the sidewalk after it rains. The hairlike worms (or hair-like worms – both are correct) are six inches to nine inches in length, which is long given their incredibly thin bodies. The reader wants to know what the hairlike worms are, and we are virtually certain we know the answer: horsehair worms. We suppose we could stop here, having unraveled the hairlike worm mystery, but below we’ll provide some information about and pictures of horsehair worms to round out the article.

Although the reader didn’t send us a picture of what he found, we’ve received several questions about horsehair worms before, and so have plenty of pictures of them. Here is a particularly helpful picture because in it there is a ruler for scale:

Horsehair worm in pool

And here are two more images that depict horsehair worms in the their natural element (minus the human finger in the second image):

Horsehair Worm in Forest

AAW--Horsehair Worm from Reader

As you can see in all of these pictures, horsehair worms are indeed hairlike worms. In fact, the reason they are called horsehair worms is that people long ago believed that the worms were formally tail hairs of horses that changed into living beings. Thus, the worms were at one point not thought to be hairlike, but literally hair. (For the record, horsehair worms are actually not as thin as a strand of hair – they are about as wide as the lead in a mechanical pencil.)

Why are we so confident our reader found horsehair worms? For one, they are common. People write to us about them fairly frequently, so it clearly isn’t unusual to find them. Two, the reader’s description of the worms he found could essentially be nothing but a horsehair worms. Some worms are long and some worms are super skinny, but they aren’t long and super skinny worms apart from horsehair worms.

Finally, the reader is finding the worms after it rains, and horsehair worms need moisture to survive. Without getting too deep into the details, which are covered in our article Horsehair Worms and Their Victims, horsehair worms are parasites that live inside insects like crickets and grasshoppers. After a horsehair worm grows and matures over a period of about four to twenty weeks, it will exit its insect host, but generally only if there is water nearby. (It is for the this reason that people often find horsehair worms in the swimming pool. The worms leave the insect and then hurry to the pool.) The worms our reader found must have abandoned their insect hosts during or after the rain storm, when there was plenty of moisture around. (We have actually written an article before about horsehair worms after rain.)

So, every piece of information our reader sent us suggests he found horsehair worms. Indeed, even if the reader only said he found a hairlike worm and nothing else, we would still be very confident he found a horsehair worm. Worm identifiers such as ourselves often work under a cloud of uncertainty, but in this rare instance, we’re all but certain we’ve correctly named what our reader found.


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