There are more than 2,700 different species of terrestrial worms; they are among the most common and widespread organisms on the planet. Lowly but pervasive, slow moving but prolific, they inhabit every continent save Antarctica, squirming and undulating their way through all types of soil, whether sandy or silt.
A delight to birds, farmers and fishermen worldwide, earthworms are most active and noticeable in the spring, when the moisture content of the soil equalizes, creating an environment that is neither too dry nor too wet. Burrowing through the dirt, they aerate and mix the layers of soil, consuming their weight in food each day. What they leave behind as waste, called worm “castings,” are tiny water-soluble pellets containing more nutrients and beneficial bacteria than any other form of fertilizer, making them highly prized by gardeners. A single tablespoon of castings can nourish a large potted plant for several months.
Slender and cylindrical, with a taper at each end, earthworms vary in size and color, with the longest known species measuring almost 10 feet. One rare specimen, indigenous to the Palouse river region of Washington state, grows 12 inches long and is albino in complexion, earning it the nickname, Great White Worm.
Though there are more than 180 species in North America, at least 60 were introduced from the Old World, arriving to the continent during the days of exploration and conquest, first from the soil used as ballast in ships, and later as stowaways in the horticultural trade. One acre of soil can contain one million earthworms. The most recognizable species, and the one most likely to inhabit your own yard, is the European earthworm, known in the U.S. as the “nightcrawler.” Strong and quick to adapt, it can easily outcompete most native earthworms and in some regions is considered an invasive pest.
Though lacking true eyes, earthworms do have light-sensitive receptors, which alert these nocturnal eaters to retreat deep in the soil at daybreak. Only one hour in the sunlight can paralyze a worm, making it an easy catch for any proverbial early bird that may be standing by.
Seemingly simple, earthworms have long been considered vital to the planet at large. The biologist Charles Darwin thought them so important that he spent 44 years documenting their behavior, putting every last detail in a page-turning tome for the ages called “The Formation of Vegetable Mould Through the Action of Worms, with Observations on Their Habits.” Instantly popular, it sold thousands of copies within weeks of publication and inspired fan mail for years afterward.