Why worms come out on the road when it rains

Wee see a lot of earthworms on sidewalks and roads on rainy days. But what exactly makes them crawl out of their safe soil?

There is a version that the worms swim to the surface to avoid drowning in their burrows, but this is not true. A recent study has shown that the most likely explanation is that earthworms surface on rainy days to move faster on wet ground instead of burrowing slowly in the soil as usual, Live Science writes.

The fact is that worms do not have lungs like humans. Therefore, they are able to live in soils that have been completely saturated with water for 31 to 50 weeks. Thus, earthworms absorb oxygen through their skin, and they can do so from water as well as from air. Researchers realized this when they conducted a study in 1956. "I have kept earthworms in water for days, and they do not die," explained Kevin Butt, an earthworm ecologist at the University of Central Lancashire in Preston, England.

However, during a study in 2008, scientists found that there are two species of earthworms for which oxygen consumption may play a role in why some, but not all, earthworms tend to surface on rainy days.

"The species that didn't require such high oxygen levels was less likely to come to the surface. Thus, some species of worms are more susceptible to low oxygen levels, and may be more likely to leave their burrows due to a heavy rain, while others may remain happily belowground," commented Thea Whitman, a soil scientist at the University of Wisconsin-Madison.

According to Kevin Butt, the most likely explanation is that earthworms surface on rainy days to move more quickly on the wet ground, instead of burrowing slowly in the soil as they normally would. "They require moist environments to survive, and so wet days could allow for them to travel above the soil," Thea added.

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