Seasonal Observations: Earthworm Meet-Up

On Saturday, during the rain, I took a walk with my 2 year old. We didn’t cover much ground. But over the 500 metres we covered, we saw almost 30 earthworms. (I stopped counting at 20.) We were singing a made-up song about going on an earth worm hunt, “not to catch them, we just want to say hi,” so it is conceivable (in my rich fantasy life) they just thought my serenade was too good to miss. But I thought I’d Google it, anyway, and see if there was a more factual explanation for the abundance of lovely nematodes out for a wriggle in the rain.


This is what the entertaining WiseGeek had to say:

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.