An example of a horsehair worm life cycle. After hatching, the larva is eaten by an aquatic insect (such as a mayfly nymph). Inside the mayfly, the larva encysts and does not begin to develop until the mayfly is eaten by a host (such as a mantid) that the horsehair worm can parasitize. Once in the mantid, the horsehair worm grows to adult and emerges when the mantid seeks out water. Many variations on this cycle occur.

Illustration by Christine M. Dewees.