Some may also infect snails and slugs. The worms we see in water are adults lucky enough to have made their way from their host to water. Those that can't reach water, like those living in grasshoppers perched on the ragweed on a cold September night, die quickly of dehydration. After leaving the host, the adults are no longer parasites. What happens next is the subject of some debate.
Some sources say the larvae hatch out and bore their way into aquatic insects like mayfly nymphs.
When the nymphs become adults, the mayflies fly to land, mate, and die. The flies are also land insects that become infected with the horsehair worm larvae. However they enter their host, it is certain that the larva often grows in these land insects into a sinuous, skinny parasite that saps its host's vitality and hastens its early death. Contact with cold water apparently triggers the parasite into abandoning its host from the nearest orifice. If the worm s exodus is triggered by cold dew, like that condensing on a grasshopper, the evacuated worms will be fried and dried by sunlight and be dead by day's end.