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Why Do Worms Appear on Sidewalks When It Rains?

By DNAinfo Staff on June 12, 2015 5:40am  | Updated on March 27, 2017 3:44pm

 A worm slithers on the sidewalk after rain in Chicago.
A worm slithers on the sidewalk after rain in Chicago.
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DNAinfo/KylaGardner

CHICAGO — Why did the worm cross the sidewalk?

It's complicated.

When it rains heavily in Chicago, worms make their way onto sidewalks, and some never make their way off.

"When you see a lot of worms on the sidewalk, they get squished, or dried out, but there are a lot that make that transition pretty successfully," said Steve Sullivan, senior curator of urban ecology at the Peggy Notebaert Nature Museum.

Kyla Gardner debunks some of the worm myths out there:

With all the risks of shoes and sun involved, what are they doing up there?

Sullivan said answers vary between species, and there isn't enough research to definitively say.

It was once thought that worms might drown in soil overflowing with water, but that myth has been busted, according to Accuweather.

A worm's "skin" — its outer membrane — is much like the inside of your lungs, Sullivan said.

For worms to breathe, which they do by passing oxygen and carbon dioxide through their skin, they need that moisture, according to magazine Mental Floss.

But requiring moisture might be one reason they take advantage of slick surfaces after a rainfall, hightailing it over normally dry and unfriendly concrete.

"If you’re close to the top of the soil to eat and renew your vigor, and all of the sudden it's raining, and you want to get the next part of the buffet, you can squirt across the ground because of the moisture," he said. "They may simply be trying to move from one side of lawn to the other, and this is the easiest way to do it."

Sullivan noted that "earthworm" is a catch-all term that could refer to a lot of different species of worm found in Illinois.

There's Lumbricus terrestris, a fat, grayish worm, and the redworm (Eisenia fetida), which is more tapered and, of course, red.

Sullivan hypothesized that some worms could be irritated by lawn fertilizers, herbicides or road salts dissolving into the soil when it rains heavily, and be trying to get away from those chemicals.


In Illinois, you may see Lumbricus terrestris (left), a fat, grayish worm, and the redworm (right) (Eisenia fetida), which is more tapered and, of course, red. [Wikimedia Commons]

Spring rains may also coincide with the season of "mating events."

"The risk of predators is there, that’s why these adult worms are coming out in mass, so they can breed," Sullivan said. "If you’re breeding in a large mass, the risk to a given individual is low, so you’re going to have successful breeding."

So, why did the worm cross the road? Future scientists of America, you tell us.

"It's an area that’s very much open to study," Sullivan said. For biology-minded students in "high school or just starting college, I would encourage them to jump on the worm bandwagon and start studying worms."

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