After rain storms, here is why worms are scatted on sidewalks.

Watch where you step! April showers bring May flowers, but spring rains typically resurface a more immediate creature that will help your plants bloom.

That's right, we're talking about the earthworm.

These little toothless, earless, five-hearted critters serve as "ecosystem engineers" and make for a good asset to compost piles. There are at least 20,000 different species, but the main three types include flatworms, roundworms and segmented worms, according to Britannica. The earthworm, the worms you commonly see after it rains, belongs to the segmented worm family.

Des Moines has seen a little more than 2 inches of rain in April, according to the National Weather Service. Whenever it rains, earthworms will often appear on sidewalks. Whether they creep you out or help your garden, worms seem all too common after a warm rain.

Why do worms come out when it rains?

Earthworms breathe through their skin, according to the Iowa Department of Natural Resources. So when the ground is wet, the moist soil can inhibit their breathing. This brings the invertebrate out from under the dirt and onto sidewalks for a breath of fresh air.

Other theories suggest that earthworms use the rain for migration. It is easier and faster for worms to travel above ground until it dries up again, according to the Columbia Daily Tribune.

More: NWS: There were at least 17 tornados in Iowa last week. Here's what we know so far.

How do worms breathe?

With no lungs, one might wonder why worms need to migrate from underground to breathe. Earthworms use their skin to breathe oxygen. However, earthworms can still breathe underwater and can do so for up to two weeks.

Are worms native to Iowa?

No, the worms you see on the ground came from Asia and Europe. They were brought here through horticultural trade, according to the DNR. The Pleistocene glacier wiped out any native worms from the Midwest more than 10,000 years ago.

What other insects come out in the rain?

Ants are another insect that commonly appears after a storm. Only they aren't just on the sidewalk, they will often march into your house because they nest in the ground, according to pest control company Flick. Other pests that you may encounter after it rains include mosquitos and mice.

More: Spring is coming and so are critters. Here are 5 insects and animals to watch for in Iowa.

Kate Kealey is a general assignment reporter for the Register. Reach her at kkealey@registermedia.com or follow her on Twitter at @Kkealey17.

This article originally appeared on Des Moines Register: Why do worms come out when it rains? There's a simple answer.