Groups of mating horsehair worms form tight knots. Females deposit eggs in water in long strings, sometimes up to 8 feet long, each containing up to several million eggs. Sometimes these are squeezed into the water in loose, small sections, like funnel cake batter; sometimes they are adhered onto a stick or other surface in a compact undulating pattern; sometimes they are extruded in free-floating threadlike strands. Males die after mating; females die after egg-laying.
Upon hatching, the rather cylindrical larvae enter their hosts (usually insects). They can do this directly, with the help of barb-like hooks and knife-like stylets, or they can encyst on nearby aquatic vegetation and enter the host’s body by being eaten. Or they can enter an intermediate host (such as the larval form of a aquatic insect) and travel with that insect as it pupates, flies over land, and dies; a cricket, being an omnivore, may eat the dead “transport host,” ingesting the encysted horsehair worm that way.
After ingestion, the cyst “awakens” and the larval worm bores through the gut of the host and enters the body cavity. There, the parasite lives inside the host, absorbing nutrients, molting as it grows.
Once reaching adult size, weeks or months later, it breaks through the body of the host and starts the cycle again.
Some horsehair worms, as they reach adult size, chemically infect the brain of their host (such as a cricket), compelling it to seek water to drink, wet, swim, and/or drown itself: the perfect environment for the adult horsehair worm.