Worms in the Rain

As temperatures rise, leaves start to peek out of buds, and the critters begin to reclaim the land. April brings precipitation to many spots in North America and warmer, rainy days solicit one creature, in particular: the earthworm.

Amble outside during a shower in the spring and you will likely encounter worms on the soil and on sidewalks.

Why do these animals emerge from the soil during and after rains?

The traditional view leans heavily on common sense. Not only do worms live in soil, they actually help create it. Eating detritus, worms transform natural waste into fertile dirt. They move through the soil, as well, digging burrows up to six and a half feet below the surface. When it rains, water seeps into the soil, flooding worm burrows. So, we believed worms needed to evacuate their homes when it rained to avoid drowning. Makes sense.

Scientists now know, however, that theory is wrong.

Worms have no lungs; they actually breathe through their skin via diffusion. They can even absorb oxygen from the water through their skin! Researchers have witnessed worms survive over multiple days in water.

An earthworm in its burrow - photo from USDA-NRCS

If it’s not drowning, why do the worms emerge in the rain?

At the moment, no definitive answer exists, though a couple of good ideas persist. The first involves movement. Though worms can and do move through the soil, they can move much more easily above ground. Worms produce a mucus layer around their bodies to make motion easier. This coating can disappear above ground in daylight. When it rains, they can move easily across the ground and won’t desiccate in the sun.

Why would they want to move more than usual? In addition to finding food sources, some scientists believe they use the freedom of movement to meet other worms. When it’s time to make new worms, it might be difficult to locate another in the underground dive bar; finding a mate in the rainy free-for-all might be more manageable!

NSFW - earthworms mating - photo by Sanctuary Soil

The second hypothesis centers around predators.

The main enemy of the earthworm is the mole. Moles terrorize yards and worms alike. Earthworms sense the presence of a mole through the vibrations of the larger organism moving. How does one escape becoming mole dinner? Worms move upward, out of the soil!

Some annelid scientists believe the tremors caused by precipitation (think rain falling on your roof) trigger worms into flexing their flight systems. This thesis does not convince everyone, however, as limited testing suggests the vibrations do not closely mimic those of moles. A one-to-one comparison is not necessarily disqualifying, though. Perhaps the getaway reflex does stem from vibrations, no matter the flavor.

Enterprising fishermen actually use this trick to lure worms to the surface:

The general locomotion theory with a side benefit of mating opportunities seems to be the most widely accepted.

Or maybe they just like the water!

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