Q: I have a plastic half-barrel with water lilies growing in it. On one of the floating leaves I found a skinny black worm, thin as a hair. What is it?

A: It’s probably a harmless horsehair worm. Despite their diminutive size, these creatures are bad news for larger insects like crickets, cockroaches, and beetles. They eat an insect’s innards and emerge through their anus, well-fed and ready to court a horsehair worm of the opposite sex. They are harmless to humans.