by Whit Gibbons

October 7, 2007

I recently ran across an email of an environmental question and answer from a decade ago that was of particular interest because I learned some biology myself. A "mystery animal" had been discovered after heavy rains.

The "mystery animal" was described in the following way: "A young boy discovered a specimen that our neighborhood is unfamiliar with. It is 18 to 24 inches in length and has the diameter of a toothpick from head to tail. It 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 may be?"

I asked if she could mail it 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 co-worker Sarah Collie sat with a jar in her hands and a bemused look on her face. "Your worm arrived," she allowed.

So there it was, and exactly as described--two feet of slender worm with a forked tail and looking like a tangled coil of thin, copper wire. I had never seen a live one before, and neither had most of the other ecologists I checked with. However, as biologists, we had all read about such animals in textbooks. I emailed my new pen pal that the animal was a horsehair worm, belonging to a poorly understood group, the hairworms, with more than 300 species. In fact hairworms are in their own separate phylum whose biological relationship to other worms is unknown. The forked tail on this one meant it was a mature male.

This adult stage, which can reach a length of three feet, is a free living form, meaning it is not parasitic. One feature of hairworms is the paradox that they 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, but 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 process again.

Invertebrates that serve as hosts for hairworm parasites include millipedes, spiders, and insects, such as aquatic beetles, mosquitoes, crickets, and praying mantises. Fortunately, none of these hairworm parasites seems to have any use for humans. However, 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 three-foot long worm inside of a grasshopper!

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 part of the explanation. In laboratory studies that examine proteins inside of cells, including 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. Although the exact mechanisms are unknown, the chemical alteration of a cricket's or grasshopper's brain makes it want to jump in the water and drown. Clearly such abnormal behavior for an insect works to the advantage of the parasite. Brainwashing at its best.

The next mystery to be solved about these strange creatures is whether the worms produce chemicals that make an insect want to drink from the water where the eggs are laid. How else would an egg end up in a cricket?

The final email from the woman with the horsehair worm showed the kind of enthusiasm about the living world and nature that I like to see in people. "Thank you so much for all of your attention to this effort. I learn something new every day!" So do I.

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