Why do Worms Come out When It Rains?

Many people have noticed that after a rain, worms appear in gardens, on sidewalks, and scattered across neighborhood lawns, much to the delight of local birds. Modern humans are far from the first to make this observation: authors dating back to the Renaissance have noted the appearance of worms after a rain. There are several interconnecting factors to explain why they come out when it rains, but the short story is that worms find conditions above ground to be especially favorable then.

A common myth about worms is that they are forced to come out of their holes during the rain because they would drown if they stayed below. In fact, this is not true. Worms breathe through gas exchange, absorbing oxygen directly through their skins. As long as water has sufficient dissolved oxygen, worms can actually live for several days fully immersed in water, as scientists have discovered.

The way in which worms breathe does explain why they tend to congregate above ground after the rain, however. Worms are covered in a mucus that facilitates gas exchange, and as a result, they must stay moist. Most of the time, conditions above ground are too dry and hot for worms, and they will dry out and die because they cannot breathe. After a rain, the environment is moist, facilitating breathing and also making it easier to crawl along the ground, so worms are encouraged to surface.

As for why worms come out above ground at all, worms prefer to mate above ground. They often come out after a rain in the hopes of finding mates, treating the above-ground world like a sort of worm discotheque, with a wide sampling of potential mates available. Studious observers may have noted that worms often congregate in small groups above ground, illustrating their primary reason for surfacing. Worms, incidentally, are simultaneous hermaphrodites, so both partners exchange sperm, which is used to fertilize eggs.

In addition to surfacing after a rain, worms come out at night as well. During the evening hours, the air is typically much cooler, and the environment is often moist, making it hospitable to worms. People who use worms for fishing bait often choose the evening hours to go hunting for “nightcrawlers,” taking advantage of this trait to collect worms for fishing trips.

Worms come out during rain primarily to avoid drowning. Their burrows fill with water, and since they breathe through their skin, the worms must stay moist to absorb oxygen. Heavy rains make the soil too waterlogged for them to breathe effectively, so they surface to find oxygen while also taking advantage of the moist conditions above ground.

Yes, worms can drown if they don't come out of the soil during heavy rain. Although they need a moist environment to breathe, too much water can fill their burrows and cut off their oxygen supply. By surfacing, they avoid drowning while still maintaining the moisture needed for respiration.

While worms do need a moist environment to mate, coming to the surface during rain is not primarily for reproduction. However, the wet conditions post-rain can be opportune for worms to mate since they require moisture to exchange sperm. The rain provides ideal conditions for them to move around and find a mate.

Worms have developed behaviors to prevent being washed away in heavy rains. They often stay close to their burrow entrances or move to areas with less water flow, such as under leaves or debris. Their bodies also produce a slime that helps them adhere to surfaces, reducing the risk of being swept away.

Worms can survive being submerged in water for a short period, as they can absorb oxygen through their skin. However, extended submersion without access to air can be fatal. The duration worms can survive underwater varies by species, but it is generally a matter of a few hours.

After the rain stops, worms will often return to their burrows to continue their life underground. The moisture on the surface allows them to move easily back into the soil. If they remain on the surface too long after the rain and the environment dries out, they risk desiccation and predation.