Many female aquatic insects return to water to lay eggs, a journey that can be as hazardous as was the exit. Exiting the water comes with its own hazards, for both insect and passenger hungry fish may well dine on the paired travelers, at which point the horsehair worm cyst is just a fish food morsel. Aquatic insects emerge from water to reproduce, and horsehair worm cysts stow away for the trip. Should a cyst survive, the next leg of the journey takes it out of the water. Second, the cyst must survive the host insect’s immune defenses some hosts wrap the cysts in compounds that can kill them. First, the host must be in an insect if the cyst ends up in a fish, snail, crayfish, or some other non-insect being, it is doomed. Two stars must align for success of the cyst’s next life stage. Once inside this first host, the larva burrows through the digestive tract into the host organism’s body and forms a resting cyst. While creatures from fish to stoneflies accidentally ingest these larvae, not all hosts are equal when it comes to horsehair worm survival. After hatching, the larvae, scarcely as large around as the thickness of a page, remain on pond and river floors until they are eaten by something else. Horsehair worms begin life as strands of gelatinous eggs in aquatic habitats including streams, lakes, puddles – and, yes, horse troughs.

The improbable details of the horsehair worm’s life cycles are as convoluted as they are gruesome.

Many insects might well be very pleased if this were true. These curious creatures earned their name because they are sometimes found in watering troughs and were once believed to be horsehairs that became animated. I can’t say, however, that I know of a single children’s book describing the impressive hero’s journey of Nematomorpha, commonly known as horsehair worms. From The Ugly Duckling to The Very Hungry Caterpillar to A Seed is Sleeping, these stories have brought the miracles of growth and maturation to life for generations of readers. Some of my favorite children’s books describe life cycles as heroic tales of persistence and redemption.

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