Photo of a dead worm in an S shape

After it rains, people often step outside and think, “It smells like worms.”

(The dead worm above looks like an S. Take the kids for a shape walk to see what else you might discover.)

The smell is more likely caused by soil bacteria released after heavy downpours, but perhaps we think it smells like worms because we see worms more often after it rains.

Why is it that we see worms after it rains?

First, let’s talk about different types of earthworms. There are three types of earthworms in Iowa — anecic, endogeic and epigeic.

Graphic about earthworm types

Anecic earthworms are capable of burrowing up to 6 feet below the surface, building permanent burrows in mineral layers of the soil. They drag organic material from the surface into the burrow for food. Examples of anecic earthworms are nightcrawlers, bait worms and dew worms.

Epigeic worms live on the soil surface and don’t form permanent burrows. These worms, such as red worms, manure worms, red wigglers and compost worms, feed on decaying organic matter.

Endogeic earthworms build non-permanent burrows in the upper mineral layer of the the soil and feed on organic material. They live in the soil but are usually not noticed until after a heavy rain, when they come to the surface.

There is speculation as to why worms come to the surface when it rains.

The reasoning for a while was that worm burrows would fill with water, and because oxygen is not diffused as well through water as through air that the worms would come up to the surface to prevent drowning. However, worms can survive for several days fully submerged in water, so that theory was debunked.

Scientists have thought perhaps it is to help with travel, because it is easier to move great distances on the surface than through the earth. They normally wouldn’t have enough moisture on the surface, because they have to be damp in order to absorb oxygen through their skin. But after the rain, they have enough surface moisture to migrate.

Another theory is that the vibration of the rain on the surface sounds like a predator, such as a mole, so the earthworms are trying to get out of the path of their predator. Humans create vibrations when searching for bait earthworms, and even birds will drum their feed to create vibrations and bring worms to the surface.

Even if only a small percentage of earthworms are tricked by rain’s vibrations, there is as many as one million worms in an acre of soil, so even a small percentage can seem like a lot!

(You also see plenty of leaves on the ground. Learn how to identify what trees they came from here.)

Up-and-coming invasive species

Environmentalists have been watching and waiting as new invasive species begin to take hold and move across the country and across Iowa. Jumping worms, the brown marmorated stink bug, the gypsy moth and the spotted lanternfly. Let’s take a look at a few of these up-and-coming invasives. Jumping worms Most earthworms are not actually native…

Read More

Reasons why insects are beneficial

Insects are beneficial. For some reason, most people grow up thinking “Ew” whenever they think “Insect.” However, the vast majority of insects are actually beneficial to humans and the environment. Even those that we think of as annoying, such as mosquitoes, can actually be beneficial in some way. Mosquitoes are actually pollinators. A type of…

Read More

Four types of wetlands

Bog, marsh, swamp, fen. Often these words are used interchangeably, but in reality, each is its own type of wetland, which is a word used to refer to water-saturated landscapes. (Watch: What is a wetland?) A marsh is a wetland that is continually full of water. If you have been to the Florida Everglades, then…

Read More

7 ugly mammals in Iowa

Iowa is full of adorable mammals, so many that’s its hard to choose which are the cutest, but the state also has so uggos. Our environmental education coordinator likes to call these mammals non-charismatic, because ugly tends to turn people off. Although perhaps not the most attractive, the following mammals do still have a important…

Read More

What is permeable skin? A video experiment

Amphibians have permeable skin, meaning they can absorb oxygen through their skin. However, it also means that chemicals and other harmful things from the environment can also be absorbed. Learn more about permeable and impermeable skin with this science experiment led by environmental education coordinator Bryanna Kuhlman. See more lessons and animal videos here.

Read More

Dig a little deeper, paint a little browner

I (Kiley) know I made this exhibit at the Dickinson County Nature Center, but this might be the most boring thing I’ve ever written. Thirty-six inches down in our soil, you will find loam which is characterized by common, fine, distinct brownish yellow mottles, a weak fine prismatic structure…yikes. It continues but I won’t bore…

Read More

2 Comments

  1. David on August 14, 2019 at 8:28 pm

    Very helpful identifier with illustrations

  2. Jan Grant on September 21, 2018 at 12:56 pm

    Very interesting!

Leave a Comment