by Whit Gibbons

October 26, 2014

With Halloween on the horizon, I am reminded of a “mystery animal” someone once sent me.

The enigmatic creature was described like this in an email: “A young boy discovered a specimen that our neighborhood is unfamiliar with. It is about 24 inches long, has the diameter of a toothpick, is brown in color, and has a flat nose and forked tail. It was found in a dirty puddle after we had excessive rains. We live in a South Carolina farming community. Do you have any idea what this might be?”

I asked the questioner if she could mail the specimen to me in a jar with tiny air holes in the top and damp paper towels inside.

When I arrived at work two days later, my coworker Sarah Collie sat at her desk with a jar in her hands and a bemused look on her face. “Your worm has arrived,” she announced.

And indeed, there it was, exactly as described – 2 feet of slender worm that looked like a tangled coil of thin copper wire with a forked tail. I had never seen a live one before and neither had any of my ecologist colleagues I checked with, although we knew about such animals. The animal was a horsehair worm, belonging to a poorly understood group, the hairworms, with more than 300 species.

Hairworms are in their own separate phylum whose biological relationship to other worms is ambiguous. The scientific name of one hairworm is Gordium, after the Gordian knot, which could not be untied (a difficulty Alexander the Great reportedly overcame by slicing the knot in half with his sword).

After seeing the twisted knotty mess that a 2-foot-long worm the diameter of a piano wire could get itself in, I thought the name most appropriate. The forked tail on this one meant it was a male.

Adults, which can reach a length of 3 feet, are in a free-living form, meaning they are not parasitic. But the larvae are. Paradoxically, hairworms are special because they are so unspecialized. They have no digestive system, no respiratory system, and no circulatory system. Adult horsehair worms do not eat.

After leaving the insects they grew up in, the males and females mate and reproduce in water. The female lays eggs that float in the water. If an egg is eaten by an insect, the egg hatches and the tiny parasite larva drills its way out of the insect’s intestine and takes up residence in the body cavity. It feeds on the inside of the insect until it grows into a long worm ready to start the cycle again.

Invertebrates, including spiders and insects serve as hosts for hairworm parasites. In what might appear to be the inspiration for the movie “Alien,” the larvae grow to the size of giant worms inside the unfortunate invertebrate host before they emerge.

Imagine a 3-foot-long worm inside a grasshopper! Hairworm parasites have little use for people, although rare infections of humans in China, Japan, and Canada have been reported in medical and parasitological scientific journals.

How does the worm know it will end up in water so it can mate? A remarkable study by several French scientists on hairworms that infect crickets and grasshoppers may offer a partial explanation.

In laboratory studies that examined the brain of insects, the investigators found that the hairworm parasite actually alters the behavior of the insect by producing molecules that enter the insect’s central nervous system.

The exact mechanisms are unknown, but chemical alterations in the brain make the insect jump into water and drown. Such abnormal insect behavior puts the now-developed worm where it wants to be. Brainwashing at its most effective.

The final email from the woman with the horsehair worm displayed the kind of enthusiasm about the natural world that I like to see in people. “Thank you for this information. I learn something new every day!” That day, so did I.

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