The horsehair worm is swallowed in larval form by an insect host. As the worm grows and matures inside the insect, the insect host seems to surrender its will to the horsehair worm.
Three months after the horsehair worm parasitizes the insect, this host—almost zombie-like—is compelled to seek out water. When the insect enters the water, the mature worm emerges from the host's body wall—-as a writhing, knotting, mass of hair-like worm.