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Horsehair worm
© Sharon LaPlante

Untying the Knot

As the first rains of autumn arrive, one of the strangest animals in the world returns to the scene. Most often encountered along the vegetated margins of seasonal creeks and ponds, the horsehair worm both repulses and fascinates people. It confuses them, too.

Look at an adult horsehair worm, and the origins of its common name become obvious. A typical specimen is between 10 and 24 inches in length and roughly the same diameter as heavy thread. It resembles a single strand of hair fallen from a horse's mane or tail and come miraculously to life. Because the worm is often encountered in water troughs and pools where horses drink, the association seemed obvious to past observers. And though we now understand that spontaneous generation is totally fanciful, many aspects of the life histories of these worms remain shrouded in mystery.

One thing we know for sure is that horsehair worms are not closely related to any other group of animals. As such, they're in their own phylum, the Nematomorpha. Another thing we know is that horsehair worms are parasites of invertebrates, mostly insects and a few crustaceans. And we know that when an insect hosting a fully developed worm comes into contact with water, the worm emerges and begins its free-living aquatic stage. This is the adult phase, during which mating and egg laying take place.

Beyond these facts, our knowledge is much more speculative. Most documentation suggests that the adult worms lay eggs that remain on vegetation after the water in which they're living recedes. These eggs are then eaten by crickets or other insects. If eaten by the right species -- a suitable host -- the worms hatch from their eggs and develop further.

Recent research, however, points to an even more complicated life cycle. It now appears that an egg hatches and becomes a cyst that must be eaten by an intermediate host, usually some aquatic insect. When this insect dies, a scavenger such as a cricket or grasshopper eats the carcass and ingests the encysted horsehair worm larva. If the scavenger is the appropriate species -- again, a suitable host -- the worm will develop inside it.

The worm must remain in its host until the insect comes into contact with water (most adult horsehair worms can survive on their own only briefly out of water). The worm, it's now believed, induces a physiological change in the host that causes it to seek out a body of water. If the host were simply motivated by thirst -- a potential effect of having the large coiled worm in its body -- it could satisfy its needs by drinking dew or by eating foods with high moisture content. But this isn't the case. Instead, the host travels to a pond or stream or puddle and enters the water, whereupon the worm emerges and the host dies.

So if you're out for a walk and see the body of a Jerusalem Cricket or Field Cricket floating in a small pool of water, take a few seconds to scan the weeds near the water's edge. You might see an undulating thread there, living proof that the bizarre sequence of events just described has once again been played out to its biological conclusion. Still, wouldn't it be easier if these creatures simply appeared when a bit of hair from a horse landed in a puddle and came to life?