Backyard worms
Digging in the backyard garden or flowerbed, you can’t help but unearth a few worms in spring. A closer look reveals that their body is made of sections or segments. Earthworms are are in the phylum (group) Annelida, or segmented worms. Leeches are also in this group.
I grew up understanding that earthworms are great for soil, and they certainly are in gardens, lawns and flowerbeds. Their tunnels aerate the soil, and prevent compaction which allows water to penetrate to plant roots. Their castings (worm poop!) left on the surface are full of nutrients. Someone calculated that, in an acre (0.4 hectare), worms could eat and digest enough organic matter to bring 7200 kg of nutritious castings to the surface every year. So we can feel good about them in our yards and farm fields.
But here’s the twist: None of Ontario’s 19 species of earthworms are native! (two are from the United States and 17 are from Europe) After the last ice age wiped out any earthworms that were here, invertebrates like sow bugs and millipedes (and, of course, fungi) were the forest decomposers. Settlers brought most of our earthworms species over from Europe in ship ballast and among plant roots.
In a forest, these master diggers can do too good a job of decomposing fall’s soft and thick layer of tree leaves (called duff), which can leave large sections of soil bare and cause tree seedlings, wildflowers and ferns to decline.
An earthworm making its way deeper (in a plastic container). One of the two main blood vessels is visible. 
A large dew worm or night crawler, showing segments
But they are fascinating creatures. Just behind their prostonium (front segment), they have a mouth. Larger earthworms like Lumbricus terrestris (also called night crawlers or dew worms) come to the surface at night and drag piles of leaves back to their hole. It’s easy to see these tiny piles; carefully shift one and you will uncover the hole.
Then there’s the age-old question: Why do earthworms come to the surface when it rains? It’s not to avoid drowning, apparently – they can survive for long periods of time immersed in water since oxygen diffuses through their wet skin. The theory with the most evidence is that they do this to safely move more quickly to other areas, or find a mate or food.
And it’s pretty cool that gulls have been seen stamping the ground to imitate rain’s vibration, luring earthworms to the surface.
I’ve had fun observing my backyard worms, especially in the easy-to-make worm condo, which allows views underground (see Activities page for instructions). Watching them, it’s easy to agree that earthworms deserve their reputation as nature’s best soil engineers.
Dan the Nature Man 