Mystery science: Why do earthworms emerge from the ground during rainfall?

One of the great mysteries of science is why during rainstorms, earthworms crawl out of their underground abodes and expose themselves to danger from birds, pedestrians and bicyclists. To be certain, earthworms are not known for their brain capacity; in fact when describing worm anatomy, it’s brain is usually put in quotes, because it nothing more than a barely elevated appendage of its nervous system. 

Most people don’t really think of it all; it is just a peculiarity of nature. It might be assumed that it has something to do with the ground being so waterlogged that the worms are forced to evacuate so that the ground doesn’t become a watery grave by drowning. Still, it must be observed that moles do not seem to have this problem. Mole hills often appear after a wet evening because moist soil allows them to burrow closer to the surface; during dry weather, they seek moister soil deeper underground.  

In the case of earthworms, the notion of drowning in wet ground is just a folk tale. They do not possess lungs, and must breathe through pores in their skin. If left in a dry environment, earthworms will die because the mucous membranes in their skin are not moist enough to keep these pores open. One may observe while on a fishing expedition that a worm will continue to writhe for several hours even if left on fishing hook in water, and earthworms can in fact survive for days submerged in water. Still, a few scientists conjecture that since some varieties of earthworms, like the “Alabama Jumper,” require more oxygen at night, during times when their diurnal rhythm (i.e. “biological clock”) is disturbed, this may explain why earthworms tend to emerge at night when it rains rather than during the day.

Thus the threat of “drowning” is likely not the reason why earthworms emerge during and following a rainstorm. The problem is that scientists don’t really know; the can only posit guesses and assumptions. For example, it is believed that worms can somehow “sense” the rain, and that they “know” they can travel greater distances on the surface when it is wet. Yet the question is then why they need to do this; certainly during rainy weather they can travel below ground faster than when it is dry. Yet why do they need to do this? They “eat” dirt, and there was plenty in the place they just left. When they are crawling over sidewalks and onto roads, they certainly are not going toward more “fertile” feeding areas—particularly if they end-up in the place that was already occupied by worms that are themselves on the move.

It is also posited that earthworms mistake “vibrations” made by rainfall for the approach of moles and other animals that feed on them. I suppose that millions of years of evolution has made this an “instinctive” reaction—or maybe not. After all, earthworms potentially expose themselves to even greater danger when they surface, which is kind of “dumb.” If you are collecting bait for fishing, it is much more efficient to wait for rain rather than wasting time with a shovel. Other scientists claim that during rainstorms, earthworms take the opportunity to mate, although this is only true of a few species and not so the common variety around here.

The other night I actually took the time to observe an earthworm crawling along the pavement at a fairly “rapid” clip, and decided to see what it would do once it reached soil. This proved to be a more interminable “study” than I liked. When I observed it poking its head beneath a clutch of grass, I assumed it was burrowing in the root system, since presumably the soil would be more brittle. I waited, and waited, and waited some more to see if the rest of it would begin to disappear into the earth (or at least start to, since this was starting to become a little boring). But then its snout emerged out of the grass and “inched” forward to nowhere in particular again. I'll just leave it as another "mystery" of science.