After it rains, aboveground conditions become humid, making it easier for earthworms to breathe and move through the soil.
Many people have noticed that after a rain, worms appear in gardens, on sidewalks, and spread through neighborhood yards, much to the delight of local birds. Modern humans are far from the first to make this observation: authors going back to the Renaissance noted the appearance of worms after a rain. There are several interrelated factors as to why they come out when it rains, but the short story is that the worms find the conditions above ground to be especially favourable.
Earthworms
A common myth about worms is that they are forced out of their burrows during rain because they would drown if they stayed below. In fact, this is not true. The worms breathe through gas exchange, absorbing oxygen directly through the skin. As long as the water has enough dissolved oxygen, the worms can live for several days completely submerged in the water, scientists have discovered.
Nightcrawlers can be used as bait to catch largemouth bass.
However, the way the worms breathe explains why they tend to accumulate on the ground after rain. The worms are covered in a mucus that facilitates gas exchange, and as a result they must remain moist. Most of the time, aboveground conditions are too dry and hot for the worms, and they dry out and die because they can’t breathe. After a rain, the environment is humid, which facilitates breathing and also crawling on the ground, so the worms are encouraged to emerge.
As for why the worms come out of the surface, they prefer to mate on the surface. They often go out after a rain in the hope of finding a mate, treating the surface world as a kind of nightclub, with a wide range of potential mates available. Learned observers may have noticed that the worms often gather in small groups above ground, illustrating the main reason they appear. The worms, by the way, are simultaneous hermaphrodites, so they both exchange sperm, which is used to fertilize the eggs.
In addition to coming back to the surface after a rain, the worms also come out at night. At night, the air is usually much cooler and the environment is often humid, making it hospitable to worms. People who use worms as bait for fishing usually choose the night hours to hunt “Nightcrawlers”, taking advantage of this characteristic to collect worms for fishing trips.