Horsehair worm in pool

Long Worms in France

A reader wrote to us a while ago about a long worm he found in the garden of his rural home in France. The reader found the worm “wriggling through the long grass” and estimated it to be about as long as wholemeal spaghetti (or presumably any type of spaghetti, for that matter). The reader was wondering what this very long worm might be, and our best hypothesis is that it is a horsehair worm (Nematomorpha), which are also known as Gordian worms (after the legend of the Gordian knot). Although a long worm that resembles spaghetti is very likely to be a horsehair worm, it is worth mentioning that it could be something even more common: a simple earthworm.

We should begin by pointing out, as we often do, that not all relevant information is in our possession. The most pressing question is whether the worm is also as skinny as piece of spaghetti. If it is, then we are close to certain that our reader found a horsehair worm. Not many other worms, and none that immediately come to mind, have body dimensions that are comparable to the size and shape of spaghetti. However, there are certainly worms that can be as long as spaghetti – and this is all the reader says – including earthworms, one of the most common creatures people find in their gardens. By chance, we actually recently wrote an article about (what we believed to be) a foot-long earthworm (or 30-centimeter-long earthworm), which is probably about the length of a piece of spaghetti, so such creatures are out there. (In fact, earthworms can get considerably longer, but that is neither here nor there.)

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It would also be helpful to know if the creature our reader found in his garden was near a body of water, even just a small one, like a puddle. To make a long story short, horsehair worms are parasitoids (basically, parasites that kill or sterilize their hosts) that slowly grow inside creatures like beetles and cockroaches. After maturing inside the host (which is commonly, but not always, an insect), they leave through an opening in the host’s body. However, this generally occurs only if the host is near water, where the horsehair worms will quickly wriggle to once it has emerged. For this reason, people generally find horsehair worms in water (reports of horsehair worms in pools are sent our way from time to time), although this isn’t universally the case. Perhaps the reader found a horsehair worm that just exited its host and so hadn’t found a puddle/pond/pool/etc. yet.

To conclude, we suspect (but are not certain) that our reader found either a horsehair worm or an earthworm, but which one? Any worm that calls to mind a piece of spaghetti is likely a horsehair worm. Moreover, most readers recognize earthworms, and so don’t write to us about them (although they do on occasion). On the other hand, the reader made no mention of water in his question, and an aquatic environment is definitely the most common place to find a horsehair worm. In our final analysis, we think our reader found a horsehair worm, but he should also look into other possibilities just to be thorough.

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