Why Worms Appear When it Rains
What is the scientific reason worms come up onto the pavement or cement when it rains?
The misconception is that worms surface during rain storms because they would drown in their borrows if they did not. However, the truth is almost the exact opposite.
Worms don't have respiratory systems like humans. They take in oxygen through their skin which is covered in tiny mucus-secreting cells. For this reason, a worm's skin needs to stay moist for it to survive. Dry skin means death to a worm.
When it rains, worms leave the ground because the wet ground is much easier for them to navigate, plus they don't have to worry about drying out in the hot sun. Most often when they come to the surface they are doing so because it's easier to find mates on the flat open ground as compared to underground in their burrows.
Gardener John Mertus said it best in his article on earthworms: “To an earthworm, the wet ground is a wild singles bar.”
-The Fact Monster