When does a worm turn?

If the earthworm detects an unfamiliar object next to its skin, the muscles will contract immediately and the body will turn to avoid the object. The worm will also turn in response to heat and, in a lesser degree, to light and sound.

Even the heat given out by an ordinary match some inches away, will cause a worm to retreat. Sound waves will also lead the earthworm to change direction rather than approach what, to human ears, many seem only a slight noise. Light, on the other hand, usually attracts the worm. It turns inquisitively to inspect the brighter patch, even although, of course, earthworms can live underground for long periods.

The phrase “even a worm will turn” is used in the sense that even the humblest of creatures will eventually rebel if goaded or pushed too hard. But the phrase has no real relationship to the activities of worms as such.