A reader in Desoto, Missouri sent us a question the other day via the All About Worms Facebook page about a long, thin worm that he found on a wet concrete driveway. The way the worm moves its head is reminiscent of a snake, and the reader emphasized how skinny the worm is: about one millimeter across, and 15 centimeters long. So, it is 150 times longer than it is wide. These sound like the dimensions of a horsehair worm, which are also called Gordian worms, so we’ll explore this possibility in a little more detail below.
We should begin by noting that without a picture, it is very difficult to state with confidence what our reader found. (Even with a picture, this can be hard.) However, we know of only one worm that is both long and exceedingly skinny, and that is the horsehair worm, so we think there is good chance this is what our reader found.
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Horsehair worms are so called because long ago people thought that they were once the tail hairs of horses. Through some sort of mystical transformation, these long hairs would transform into living worms. Obviously, this isn’t true, but this old origin story captures the rather extreme body dimensions of horsehair worms.
Like so many other common worm names, “horsehair worm” can refer to thousands of different species of worm. These worms all belong to the Nematomorpha phylum. Horsehair worms are parasitoids, which basically means they are a particularly brutal parasite – they not only live off a host, but ultimately kill it. Horsehair worm victims primarily consist of insects like crickets, grasshoppers, beetles, and cockroaches, but they occasionally infect crustaceans as well. Horsehair worms grow and live in their hosts, and then will exit their body when it is near water. For this reason, people often find them in swimming pools or in puddles after it rains. The reader only mentioned that he found the worm on an overcast day, but perhaps it rained earlier, or maybe the wet concrete had some small pockets of water that drew the horsehair worm out. As for the worm moving like a snake, this is also consistent with it being a horsehair worm, as they move in a whip-like motion.
So, we can’t be certain our reader found horsehair worms, especially in the absence of a picture, but there are good reasons for supposing this is what he found.
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