Earthworms in the rain: What are they doing slithering along the sidewalks, driveways and through the grass?
The "conventional wisdom" always told us that all those earthworms came up out of the ground during heavy rains so that they would not drown. It turns out that the truth is different, and actually makes a lot of sense.
Earthworms are subterranean creatures, preferring to live their lives underground, digging earthworm tunnels and getting nutrition from what they find underground. Things like roots and leaves, decaying animals and also live organisms like bacteria, nematodes and fungi make up their diets.
Earthworms breath through their skin, which must remain moist so that they can breath. That is the reason they prefer the darkness of their subterranean world. On the surface, their skin would dry out, they would be unable to breath, and they would suffocate and die. Not something any creature wants.
Earthworms do come out at night, when the sun is down.
During times of rain conditions improve from the point of view of the earthworm. The sky is overcast, so there is no strong sunlight to threaten to dry out skin. So conditions are similar to night-lower light, cooler, moister. There is relatively little danger of becoming dehydrated.
Movement over the surface to a new area is faster than digging a new tunnel. Traveling can be further and faster on the surface as well. Moving along wet, damp ground is also easier for an earthworm than moving on dry ground. There is less friction for the skin of the worm. Also, moving over the surface during a time of rain is safer than doing so during strong sunshine. Getting to a new spot will bring the worm to new, richer sources of nutrients.
Another advantage to being on the surface is the fact that it is easier to meet and locate a mate on the surface. There is more "open space" to move around. Moving around in underground tunnels is more limiting, and it is harder for worms to meet other worms!
Another interesting fact about earthworms and water is the fact that, as long as the water has a high enough oxygen supply, worms can survive for several weeks under water.
Earthworms are a valuable resource for gardeners and farmers, because they help to aerate the soil, and devour and process old organic materials, breaking the materials down into nutrient rich "earthworm manure" that enriches the soil.