Invertebrates
Earthworm
Lumbricus terrestris · also called common earthworm, nightcrawler, lob worm, dew worm
The earthworm is easy to overlook — a soft, blind, boneless tube that lives in the dark and eats dirt. But Charles Darwin spent the last years of his life studying earthworms and concluded that few animals have done more to shape the land we live on. By endlessly feeding and burrowing, worms build the soil that grows almost everything we eat.
Overview
Earthworms are segmented worms, and the familiar large one across Europe and much of North America is the common earthworm or nightcrawler, up to about 25 to 30 cm long. It has no lungs, breathing through its moist skin, and no eyes, sensing light through its skin instead. It lives in a vertical burrow, coming to the surface at night to pull dead leaves down to eat.
How it lives
Every earthworm is a hermaphrodite. Two worms line up head to tail, exchange sperm inside a slime tube secreted by the pale saddle, and both later produce egg cocoons. A cut worm does not become two — the head end may regrow a short tail, but the tail end dies.
Darwin’s engine of the soil
In an 1881 book, Darwin argued that earthworms, simply by dragging leaves underground and casting soil on the surface, slowly bury stones and ruins and create fertile topsoil across whole landscapes. It is now a founding idea of soil science.
Helper and invader
In gardens and farmland, worms improve the soil. But northern North America lost its native earthworms to the ice ages, and introduced European and Asian worms are now stripping the leaf litter from hardwood forests, harming native plants, salamanders, and birds.
Status
Earthworms as a group are Not Evaluated. Common species are abundant; some large or narrow-range natives, like Australia’s giant Gippsland earthworm, are threatened.
Earthworm: common questions
How does an earthworm move through soil?
It uses two sets of muscles working against a fluid-filled body. Circular muscles around each segment squeeze to make the worm long and thin, pushing the front end forward into a crack in the soil; longitudinal muscles running along the body then contract to make it short and fat, anchoring the front while the rear is pulled up. Tiny stiff bristles called setae, four pairs on most segments, grip the burrow wall so the worm does not simply slide back. Some worms, including the common earthworm, also eat their way through firmer ground, passing soil right through the gut. Waves of this stretching and gripping run down the body and drive the worm along.
Why do earthworms come to the surface when it rains?
The old explanation, that worms surface to avoid drowning in flooded burrows, is probably wrong, because earthworms breathe through their skin and can survive submerged in oxygenated water for a long time. Better-supported ideas are that rain lets worms move over the surface without drying out, so they use wet nights to travel and disperse, and that the vibration of rain, or of a person walking, resembles the vibration made by a digging mole, sending worms up to escape. The trick of "worm charming", drumming or vibrating the ground to bring worms up, works for the same reason.
What did Darwin discover about earthworms?
Over more than forty years, Charles Darwin ran patient experiments in his garden and fields, measuring how fast worms buried objects left on the surface, weighing the casts they threw up, and testing what they would drag into their burrows. He concluded that earthworms, simply by feeding and tunnelling, gradually turn over the entire upper layer of soil in a region, bury stones and archaeological remains under a slowly rising blanket of worm-worked earth, and are the main biological engine that creates and maintains fertile topsoil. His 1881 book on the subject was a bestseller and is regarded as a founding work of soil science.
Are earthworms always good for the soil?
In gardens, farmland, and grassland they generally are, improving drainage, mixing organic matter into the soil, and making nutrients available to plants, which is why gardeners welcome them. But "good" depends on context. Large parts of northern North America had no earthworms after the last ice age, and their forests developed a thick layer of slowly rotting leaves that many native plants and animals depend on. European and Asian earthworms introduced by settlers and anglers are now spreading through these woods and eating that layer away within a few years, drying the forest floor, releasing carbon, and reducing the diversity of wildflowers, amphibians, and birds. There, earthworms are an invasive problem.