Years ago a child brought me a worm like this, strained in her toy net from the swimming hole. She asked what it was and the all-knowing adult in me answered "Its a worm." No, that didn't impress her either. After Thursday's Stream Team training, I can answer her better although she might not believe the answer or at the least would say "Yeeew!"
Horse hair worms (Nematomorpha) are aquatic parasitoids with a complicated life cycle. A parasite generally requires the resources of the host but doesn't routinely reduce its chances or reproducing. A parasitoid on the other hand is like an ungrateful parasite in that it either kills or sterilizes its host.
These tiny threadlike worms reach 20-32 inches in length. They lack an intestinal tract and absorb nutrients directly through their skin. When a male and female find each other they form a complex tight knot, leading to another name, the Gordian Worms. Occasionally, clusters of up to 100 will form a complex ball in puddles or watering troughs, leading to the legend that these are the long hairs of horses come to life.
The worm lays thousands of tiny eggs which develop into 0.01 inch long larvae. Some of these larvae collect on vegetation that is then exposed when water levels go down. Some species will be eaten by grasshoppers or katydids, developing in their hosts until they are ready to spring forth.
It gets really weird at this stage as they seem to take over their host's brain not unlike the
Cordyceps fungus affects an ant. The insect is unnaturally drawn to water, be it puddle. pond or even moist earth where the adult worm crawls out leaving its disabled host to die on its own. This process is shown in this
Youtube video.
Addendum April 7, 2013. Video taken of a horsehair worm on the Master Naturalist field trip is on line at this link. Gordius robustus
For closeup pictures of worms and their claspers, see marietta.edu.
