The City Naturalist

Earthworm

Article by Leslie Day, Drawing by Jonah Nishiura

EARTHWORMS: Lumbricus terrestris

"It may be doubted whether there are many other creatures which have played so important a part in the history of the world." Charles Darwin, 1881

Some scientists estimate that there are approximately 50,000 earthworms per acre of moist soil. Earthworms live in deep, dark, long, and narrow tunnels or burrows under the ground, They cannot tolerate heat and sun and so during the summer they come up to the surface only at night. They also leave their burrows when it rains because it is easier for them to move on the wet surface. After a rain you will notice multitudes of earthworms on the surface. The wet ground allows them to move without drying out.

BENEFITS OF EARTHWORMS: Gardeners, farmers, foresters and soil scientists all love the earthworm because of the good they do for flowers, crops, and plants and animals of the forest. Earthworms are active animals and feed by bringing organic debris into their burrows from the surface and by eating their way through the soil. The leaf litter (dead leaves and animals) they digest contains nutrients made by plants during photosynthesis and includes calcium, nitrogen, potassium and phosphorus, and organic minerals and nutrients from dead animals. Their excrement, called castings, is deposited on the surface and is rich in nutrients, providing food for other animals and microorganisms. This organic material is then further broken down by microorganisms of the soil, releasing nutrients in a form available for absorption by plants.

In this way, earthworms have helped produce the fertile humus that covers the land. As a result the layers of soil are thoroughly mixed, seeds are covered and enabled to germinate, and over long periods of time stones and other objects on the surface are buried. This process has even buried and preserved ancient buildings. Each year earthworm castings cover each acre with as as much as 18 tons of rich soil.When earthworms die, usually in the dry summer, the organic material making up their bodies is gradually released providing additional nutrients for plants. These minerals are essential to healthy plant growth.

EARTHWORM BURROWS: the tunnels earthworms make beneath the topsoil do a tremendous service to the trees and plants above. Their burrowing aerates the soil, which is why earthworms are called "nature's plough". They not only help bring oxygen down into the soil, but their tunnels allow rainwater carrying organic and inorganic nutrients down deep into the soil where the roots lie. The roots then take up the water and the minerals and recycle them back to the herbaceous plants and woody trees.

DESCRIPTION: If you watch an earthworm move, you will most likely see it move forward, with its pointy end in the front. This is its mouth and prostomium (area in front of the mouth). There is a concentration of sensory cells at this anterior end around the prostomium. And though it has no eyes, it possesses light sensitive cells and can "sense" light. As mentioned above, it cannot hear, but feels vibrations of animals moving nearby.

The worm's body is divided into 100 or more body segments. As the worm works its way forward, successive peristaltic or contracting waves of thickening and thinning (7-10 per minute) pass down the body. At each place where the body bulges out at a given moment, the bristles, or setae, are extended and grip the burrow walls. Setae, which are not true legs but pairs of bristles attached to each segment, push against the ground with each contraction and help the animal move.

When a Robin tries to pull an earthworm out of the ground, the worm uses these bristles to hold on tight to the wall of its home. Sometimes the worm holds on so tight and the Robin pulls so hard that the worm comes apart. The Robin keeps the front end and the hind end wriggles back into its burrow. If a bird pulls off the first 7 or 8 rings of the worm's body, new segments will grow. If a worm is pulled in half, the head end will grow back.

The earthworm has no lungs and takes in oxygen through its moist skin - it is a skin breather. If it dries out it will suffocate. Its skin is covered by mucus-secreting cells. The mucus serves not only in respiratory exchange, but it also lubricates the worm's body and eases passage through the burrow. The mucus covered skin helps bind soil particles together and prevents the walls of the burrow from collapsing.

LIFE CYCLE: Earthworms are hermaphrodites with both male and female reproductive organs. On warm, moist spring and summer nights, you can often seen hundreds of mating worms coming up out of their burrows. Once they have mated, the girdle like ring around the front of an earthworm, called the clitellum slides along the worm's body, picking up fertilized eggs. When it finally falls off the worm into the soil, it forms a well protected nest or egg case within which the embryo worms develop.

PREDATORS: Because the body of the earthworm is 70% protein, they are a sought after prey by birds, especially Robins, and by burrowing animals like moles. If you watch a Robin hunting, it pauses, cocks its head, then strikes with its bill, pulling a worm from the ground. The Robin, with its keen eyesight, detects the earthworm's movement in the grass. The earthworm, both sightless and ear-less, can feel the vibrations of the bird on the surface of the ground.

HISTORY: Earthworms were brought to North America by the early European settlers in the 17th and 18th centuries. If earthworms existed in North America prior to this, they were probably wiped out during the last ice age, 10,000 to 50,000 years ago

BIBLIOGRAPHY:

Earthworms, Dorothy Childs Hogner, 1953, Thomas Y. Crowell, New York

Earthworms, John Mertus, 1993

Living Invertebrates, Editors Pearse and Buchsbaum, 1986, Boxwood Press, Pacific Grove, Ca.

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