A horsehair worm leaves its home, the body of a mantis. It's an astonishingly long creature. How did it all fit up inside its host?
It's entirely possible that you have a horsehair worm in you, too. Do you feel a slight itching sensation? That could be the worm twisting and churning inside of you.
-via Gizmodo, which also provided the .gif
Horsehair worms have a relatively simple life cycle for a parasite. Their larvae form a cyst that sticks to plants. These are eaten by crickets or grasshoppers or similar insects. A mantis might pick one up by eating an infested cricket. Upon ingestion, the cyst releases the larva, which lives inside the insect and matures into an adult worm. It may be eating the internal organs of the host, or perhaps lives on nutrients in the host insect's blood. When mature, the worm somehow causes the host insect to seek out water. I recall a horrible news story about a swarm of crickets that jumped, lemming-like, into a swimming pool, releasing swimming worms en masse. At times, these worms may release themselves from their insect hosts on the smallest hint of free water, such as rain or even a mopped floor.
Oh, you don't have to worry about being infested with horsehair worms. They only infest insect hosts.