In Native American lore, the March full moon is called the Worm Moon, because the ground is softening as days get warmer, and earthworms reappear -- inviting the welcome return of American robins, a sure sign of spring.

In this winter's final days, several times I've stepped out of my car at Hogback Ridge Park in Sunbury to be greeted by the dusky aroma of earthworms. It's a common smell after a rain, even if the rain has not been heavy enough to drive all the worms out of their little tunnels and to the surface.

It's an aroma I also associate with damp woodlands, but I'm sure that worms are a primary cause. I think of the many times I've caught that whiff outside my former suburban home, with nary a woodland in sight.

Earthworms bring back many childhood memories: the wriggly, squiggly, squirmy feeling as they wove their way around my fingers; my initial hesitation in pinching off a section to bait my fishing line; a fascination in the fact that a worm cut in half will become two worms.

This latter "fact" is a myth, however. It's hard to see it, but a worm has a head and a tail.

If the worm is cut in half crosswise below the wide, smooth band called the clitellum, the half with the head can grow a new tail and continue its life.

But the tail end has just lost its head and its vital organs and will not live long. It will squiggle around for a while, giving the appearance of life, but that's about it. So much for that common myth.

Worms are fascinating for other reasons. About 6,000 species exist worldwide, and they range in length from less than a half-inch to nearly 10 feet. You'll have to go to the tropics to see any worm that long; your basic U.S. backyard earthworm might grow to 14 inches -- although even that sounds big to me.

Ever wonder how a robin can zero in on an underground worm so easily? Worms move around just under the surface of the soil, causing tiny vibrations and small disturbances in the look of the dirt. The robin can see and feel those changes, which tell it that dinner is at hand -- or beak, so to speak.

Dirt feels dry to my hands, but the worm slides through effortlessly, because a lubricating fluid makes moving through underground burrows easier and keeps the worm's skin moist.

Though I grew up thinking earthworms were soil-enriching powerhouses, they do have a down side. The glaciers that scoured much of Ohio -- including Delaware County -- during the most recent ice age wiped out earthworms in these areas. After the glaciers retreated, the northern forests evolved without earthworms and grew to require a deep layer of slowly decomposing leaves and other matter.

The worms we find today aren't native. They were brought in by European settlers who thought they would help the soil, or they hitched a ride in containers of plants or in dirt used as ballast. Invading earthworms eat the decomposing layer of the forest floor, depleting nutrients and compacting the soil.

This activity has damaged trees, such as the sugar maple, and many forest-floor plants such as trillium, trout lily and some ferns. That damage opens the forest to invasive plants that thrive in disturbed soil -- buckthorn and barberry, for example.

It's amazing to me that a seemingly benign little creature -- one I always thought of as food for robins and fish and entertainment for kids -- can have such wide-ranging effects. It's something I'll keep in mind when I transport new plants to my yard.

Yes, earthworms will continue to herald the start of spring and the return of the robins, but I'll never think of them in the same way, just as I'll never pinch one in half again and wait for both sides to thrive.

Sue Hagan is marketing and communications manager for Preservation Parks of Delaware County.