Just when you thought it was safe to go back into the water:

They're the horsehair worms of the phylum Nematomorpha, one of only four animal phyla known to be completely parasitic. They have been thought of as "enigmatic" because until last year, no one had a clue about their life cycle.

What has been known is as adults, the worms are free-living in streams and lakes, where they gather to mate in tight masses that are almost impossible to unravel.


That, he said, proves to be a fatal meal for the insect. The nematomorph larva, which had ceased development when it encysted, emerges from the cyst and begins to eat the host creature from the inside out. When the larva finishes its meal, it emerges from the dead host as an adult, mates and dies. While crickets and grasshoppers aren't normally found in water (a necessary condition for the adult worm to emege), Hanelt said the speculation is that as the larvae near maturity, the insects become thirsty as well as weak and this can cause them to fall in the water.

He fed some of the infected snails to crickets, waited 30 days and put the now-infected crickets in water.

"Within one or two seconds, the worms start emerging and in one case, nine worms 30 to 40 centimeters (12 to 16 inches) long and about 2 millimeters in thickness came out of one cricket," Hanelt said. "We weighed the worms and we weighed the cricket and they were about equal weight. So when the worms were done with the host, there wasn't much host tissue left."

"As urban sprawl increases, the chances for human-worm interaction increases and some people or their pets may get infected," Hanelt said. "It's really a pseudoinfection because humans, dogs and cats really don't get infected the way a grasshopper would. It sure would make you sick -- and there's reason to be concerned about that -- but it's never been reported to be life-threatening."


I wonder what the other three phyla of completely parasitic animals are?