By Environmental Management System, Health & Safety Branch
After a spring or summer North Carolina thunderstorm, it’s not uncommon to see earthworms crawling on sidewalks and other hard surfaces. Sometimes you’ll find enough worms that it looks abundant enough to look like a swarm, although the technical terms for a large group of earthworms are a bed, clew, bunch, or clat of worms. But why do worms actually come to the surface and crawl around?
You may have heard that the worms come to the surface to breathe after the ground becomes saturated with rainwater, but that is an old wives’ tale. Worms don’t have oxygen breathing lungs as humans do; in fact, as worms take in oxygen through their body walls, they need to continuously stay wet to breathe and survive dehydration. Worms can even remain viable for several days when submerged in oxygenated water! Worms produce mucus to move up and down in soil to stay moist. The UV rays produced by the sun, however, will dehydrate worms.
While scientists think that mating may have something to do with earthworms surfacing, soil experts are confident that earthworms come out of the saturated soil for migration purposes. While earthworms can move through soil underground, they are able to move longer distances on top of the soil. As they need to stay consistently wet, earthworms wait until the top layer of soil will be wet – from a rainstorm – to migrate. Worms on the sidewalk are likely attempting to move from one patch of soil to another one close by.
If you’re curious about the migratory or mating rituals of earthworms, please read “ Why Do Earthworms Surface After a Rainstorm” in Scientific American for further information.