Earthworms live in narrow tunnels in the soil. Because they do not tolerate heat and light well, they only come to the surface of the soil at night. Another time they come out is when it rains. They are a few theories about why this happens.
One school of thought points to the fact that earthworms have no lungs and breathe through their skin. In their subsurface burrows, they obtain oxygen from the air between loose bits of soil. They do need a somewhat moist environment to allow the diffusion of gases across their skin membrane, but when the soil becomes saturated they begin to drown. Earthworms can still get a certain amount of oxygen out of the water, however, this soon runs out. That’s when the earthworms come up above ground to catch a breath of fresh air while they wait for their homes to dry out. Sounds plausible, but unfortunately, this theory is not applicable to certain earthworm species that can survive immersion for several days in oxygenated water.
Another theory is that the worms may be using the moist conditions on the surface to travel more quickly than they can underground, thus colonizing new areas more quickly and finding a greater selection of mates. Since the relative humidity is higher during and after rain, they do not become dehydrated. This is a dangerous activity in the daytime, since earthworms die quickly when exposed to direct sunlight with its strong UV content, and are more vulnerable to predators such as birds.
A further explanation is that, as there are many other organisms in the ground as well, and their respiration increases carbon dioxide, this gas may dissolve into the rainwater to form carbonic acid. As the soil becomes too acidic for the worms, they seek a more neutral environment on the surface.
Whatever the reason, they definitely do come out. Some scientists estimate there are as many as 50,000 earthworms per acre of moist soil. If you are going fishing and in need of bait, the morning after it rains is a good time to find them.
Facts about Earthworms
Earthworms are amazingly strong. They can easily shift stones 60 times their own weight. The largest earthworm ever was 22 feet long and was found in South Africa. Worms do not have eyes, but they can sense light. If they’re in light too long they’ll become paralyzed. Baby worms aren’t born; they hatch from a cocoon no bigger than a grain of rice.
Earthworms eat just about every other organism in the soil. They’re miniature topsoil factories—all the soil you have ever seen has passed through the stomachs of lots of earthworms. When they eat, they leave behind “castings,” which are high in organic matter and plant nutrients and are a valuable fertilizer.
Earthworms move through the soil creating tunnels—areas that can be filled by air and water. Fields that are “tilled” by earthworm tunneling can absorb water at a rate 4 to 10 times that of fields without worm tunnels. This reduces water runoff, restores groundwater, and helps store more water for dry spells.
This burrowing also helps nutrients enter the subsoil at a faster rate and opens up pathways for roots to grow into. During droughts the tunnels allow plant roots to penetrate deeper, to reach the water they need to thrive.
Earthworms help keep soil healthy by moving organic matter from the surface into the soil. Normally a tree leaf may take three to five years to decompose and be incorporated into the soil. In forests infested with night crawlers, this process can take as little as four weeks! By speeding up the breakdown of plant material, earthworms also speed up the rate at which nutrients are recycled back to the plants.
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