Friday, May 23, 2008
So it's quite late and I'm not sleeping, which means I'm looking at random articles in WikiPedia. One piece of very interesting information that I came across is that we actually have no idea why worms come up from the ground after it rains.
BehaviorAll of these hypotheses seem to contradict each other or just are not convincing on their face. It's always very interesting to me to realize what we still don't know about the world.
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Earthworms are seen out of the dirt after large rain storms because the soil becomes too moist for them to survive. They need a moist environment to allow the diffusion of gases across their skin membrane, however if the soil becomes too moist the earthworms begin to drown in the water. To protect themselves from drowning they find higher, dry ground. This is why they are seen in places like driveways after a storm. However, after the storm they are sometimes unable to return to the moist soil and they dry up, and because their body is no longer moist enough to allow the diffusion of gases, the earthworms suffocate. However, this theory is rejected by some because earthworms can survive underwater for several hours if there is oxygen in the water.
An alternative theory concerning this behaviour is that some species (notably Lumbricus terrestris) come to the surface to mate. Since this behaviour is limited to a few species, as well as the fact that mating is not connected to rain, this theory does not seem very likely.
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. 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.
Another theory is that as there are many other organisms in the ground as well and they respire as any animal does; the carbon dioxide produced dissolves into the rainwater; it forms carbonic acid and the soil becomes too acidic for the worms and they come seek neutral nourishment on the surface.