worms in the rain

I spent much of my walk across town this morning dodging earthworms.

It has rained much of the past 24 hours,

and the worms were everywhere.

On the sidewalk, in puddles, on the street.

Many of us have seen so many earthworms emerging after rainstorms that we don’t pay much attention. Even the most curious urban naturalist probably doesn’t think about it much, because we already assume we know what’s going on. It is probably one of the first “nature lessons” we learned at school: earthworms have to come to the surface after rain because they’re drowning.

As is the case with so many nature stories, the real reason is likely more complicated than we imagine. Let’s dig deep, if you’ll pardon the pun, and look at what’s really happening when we see earthworms on a soggy sidewalk.

Conventional wisdom holds that earthworms head to the surface after rain because they can’t breathe. This is still taught to schoolkids, and you can find a lot of detailed explanation online. Most claim that worm trails and air pockets underground become submerged, and the earthworms can’t breathe. It makes sense.

Most researchers, though, dispute this explanation. earthworms breathe through their skin and require moisture to do so.

Humans drown when their lungs fill with water. This is not possible for earthworms as they lack lungs. Multiple studies have also shown that most earthworm species can survive being submerged in water for two weeks or more.

Lately, most popular scientific accounts write off the “drowning worm” idea as a myth. While it is certainly not likely the entire explanation, perhaps we shouldn’t entirely rule it out. for instance, found that worm behavior following rain depended on the species.

Two earthworm species with different life histories were investigated by the Taiwanese researchers.

One species consumed more oxygen at night than during the day, and the other consumed lower amounts of oxygen equally during a 24-hour period.

The species that consumed more oxygen at night tolerated water immersion poorly.

At night, when it needed oxygen the most, it was especially intolerant of rainfall and came to the surface.

The other species never came to the surface, even during the rain. Its consistent, lower intake of oxygen enabled it to better stand water immersion.

So it may well be that the earthworms you see on sidewalks and streets are those that need more oxygen. But many researchers doubt this is the entire explanation.

Another common explanation for worm emergence is that rain sounds like predators, so the worms come to the surface to escape. Moles, common earthworm predators, make vibrations in the soil as they hunt.

A tradition in the Appalachians and elsewhere, called worm grunting or fiddling, involves using a saw or stick to make vibrations on the soil’s surface. This brings worms to the top that are then harvested for bait.