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Curiosities: After rains, why do worms crawl out onto the pavement and “commit suicide”?

June 29, 2007

After a strong rain, the corpses of worms strewn across the pavement are a disgusting sight – or a pathetic one, depending on your sympathy for these slithery invertebrates. But what’s the advantage of suicide? Teri Balser, an associate professor of soil and ecosystem ecology at UW–Madison, says the answer starts with the fact that worms breathe through their skin.

“Oxygen from air or water passes directly from their outer cuticle into their blood vessels. Normally, soil has a mix of air and water — about 50 percent of the pore space in soil is air, the rest is water. Oxygen diffuses easily through air, and the soil stays aerobic because oxygen comes in from the surface.”

But after a rain, the soil pores and the worm burrows fill with water. Oxygen diffuses about a thousand times slower through water than through air, she says. “The worms can’t get enough oxygen when the soil is flooded, so they come to the surface to breathe.”

Beats drowning.

But why don’t the little darlings just slither back downstairs when the soil dries out? This is less clear. “Once at the surface, they seem to get confused about where their burrows are,” Balser says. “It may be that cars, lights and people disorient them. They move to seek safety, but sometimes they don’t make it back into the soil when the burrows drain, and it looks to us as if they are committing squirmy suicide.”

It’s not all gloom and doom, however: spring rains are open season for vermiphagia (eating worms).