All about earthworms

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“...It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures.” — Charles Darwin (1881)

The lowly earthworm, given such short shrift. With nary a thought from us, they quietly work magic day and night, right under our very feet. We should be celebrating earthworms, as the ancient Egyptians did, because they’re the unheralded champs of soil restoration. Aristotle called them “the intestines of the earth.”

Earthworms clean up debris and recycle it as fertilizer. Their tunnels aerate the soil, preventing erosion. Their labors benefit the food we eat, the flowers we love, the trees that shade us and the wildlife that live in our yards. The worms themselves are food for many animals. What’s more, around the world, agricultural areas are going “no-till,” where earthworms are valued as the primary force for churning crop residue into the soil and keeping it fertile. Worm poo is five times richer in nitrogen, seven times richer in phosphates, and 11 times richer in potassium than the surrounding soil, so it makes a great fertilizer.

They’re built perfectly for the work they do, and they’ve been doing it for a long time. In 2002, Australian researchers found a fossilized trail in sandstone they believe was made by a worm-like animal perhaps 1.2 billion years ago. The fossil is not only the oldest evidence of the earthworm’s existence, but it also marks it as earth’s oldest multi-celled animal.

Earthworms are classified in the phylum Annelida, along with leeches. The name comes from the Latin anellus for “little ring. Earthworms belong to the order Haplotaxida and the suborder Lumbricina. Today about 3,000 species of earthworms are known to exist, and they’re found on every continent, except Antarctica. In the United States, there are about 100 species of native earthworms and another 15 or so non-native species.

Physical description
Earthworms vary widely in size. Most are only a few inches long, with some less than an inch (25.4 mm). But, then there’s the Giant Gippsland Earthworm, an endangered (and harmless) Australian species that lives in the subsoil and grows to 6 feet (1.83 m) or more! The largest earthworm in the U.S. is the Nightcrawler, Lumbricus terrestris, which can measure up to 10 inches (25.4 cm) long.

Earthworms seem to look pretty much alike, don’t they? To experts, though, there are many differences. There’s body length, of course, but they vary also in such characteristics as coloration, number of body segments and type and location of body hairs (setae).

Image of an earthworm showing its setae, segments and clitellum

External body parts.(Iceclanl / Wiki; cc by-sa 3.0)



Earthworms range in color from reddish to gray to brown. Their skin (called a cuticle) is soft and very permeable. It’s also slimy. Slime is a mucus produced by a band of glandular tissue called the clitellum (kly-TELL-um), and it’s crucial: Earthworms breathe directly through tiny pores in their skin and this moisture allows oxygen to pass directly through and into their bloodstream. It also allows carbon dioxide to pass out. If the skin dries, the worm can’t breathe. The slimy skin also serves as a lubricant to help ease the worm through soil. Mucus has one more use: It forms the cocoons that hold worm embryos. 

Color illustration of the earthworm's body system

Internal anatomy of an earthworm. (KDS444 / Wiki; cc bysa 3.0)



Earthworms are simple animals with a body that’s basically a muscular cylinder of ring-like segments called annuli (ANN-u-lie), tapering off at both ends. Some species have as many as 150 segments. They have no skeleton; they’re soft through-and-through. Their body contains a simple circulatory system with two main blood vessels, five bands of tissue that pump blood, a digestive tract, reproductive organs and an anus. They have two heart-like organs and between them are glands that manage calcium in their diet.

Senses: Earthworms have a nervous system that reacts to heat, cold and touch. They can also feel vibrations and have chemoreceptors that detect odors. They don’t have eyes, ears or a nose, but they do have cells that detect light and changes in light intensity. They’ll move away from light (ultraviolet light rays can kill them).

Image of the head of a nightcrawler earthworm.

The head end of a Nightcrawler earthworm, Allolobophora chlorotica. The mouth is the rounded projection at the very tip. (Gilles San Martin / Flickr; cc by-sa 2.0)



Brain: You may wonder if earthworms have a head and brain. They do! The head, called the prostomium, is at the end that’s closest to the clitellum. They also have a mouth. A toothless one, but it has strong muscles for eating. And, eat they do — up to half or more of their body weight per day.

As for their brain, studies show that it holds a mere 302 neurons and 7,000 synapses. By comparison, the fruit fly has 250,000 neurons and 10 million synapses. (Neurons act as transmitters of sensory input to and from the skin and muscles; synapses are the various connections between nerve cells.) Despite its rudimentary size, another study¹ shows they can “think” and decide whether to investigate a particularly delicious odor or not. 

Mouth and digestion: Earthworms take in food with strong, muscular “lips” and a sucking action that pulls it into their mouth. The food is moistened (if it isn’t already wet) and swallowed, where it moves down the throat to a storage pouch called a crop. Then, on to their gizzard it goes, where powerful muscles grind and mix the food with a little help from fine sand particles that have also been consumed. Pulverized food, in the form of a thick paste, moves from there into the intestines. Most of the worm’s body is intestines. There, digestive juices break food down further and nutrients pass into the bloodstream. What’s left over, mostly revitalized soil, exits the body through the anus.

Image of earthworm castings.

Earthworm castings are rich in nutrients for plants. They make an excellent fertilizer. (Muhammad Mahdi Karim / Wiki; cc by-sa 2.0)



Worm poo is called castings. Castings are composed of dirt mixed with organic matter. Many people maintain a worm bin into which they throw leftover food. The resulting compost is used in potting soil, as a mulch, or made into a “tea” to sprinkle as a fertilizer. (Here’s one of the many sites that explain how to do vermicomposting.)

Movement
Earthworms move by expanding and relaxing muscles while gripping surfaces with tiny bristles, called setae (SEE-tee). Here’s how it works: A set of muscles around each segment contracts, which has the effect of squeezing the worm into a long, thin shape. It then anchors the front of its body using setae. Following that, other muscles that run the length of the body contract, which makes the worm short and fat and pulls the rear end forward. Setae at the rear then attach to the soil and the front releases and moves forward. The waves of alternately relaxing and contracting muscles give worms their characteristic wriggling motion. Earthworms typically move forward (which is one way to tell their head from their rear), but they’re capable of backward movement, too.

Excellent video showing earthworm movement   

You’ve likely heard that cutting an earthworm in two will produce two worms. That isn’t so. The front half might live and grow into a full-size body if it has the clitellum and 10 body sections behind it. If so, new growth will be a little smaller in diameter. The back half always dies.

Behavior and habitat

Color illustration of an earthworm burrow.

Earthworm burrow (USDA-NRCA)

In order to survive, earthworms need food, oxygen, moisture, a specific temperature range and to stay out of the sun. They’re cold-blooded (don’t generate their own body heat) and live closer to the surface in warm weather and deeper in winter. They’ll die if they freeze, so the colder it gets, the deeper they go to hibernate. Most earthworms prefer temperatures of between 50 and 70 degrees (10 and 21.1 C), but some species can tolerate a higher or lower range. If conditions become too dry or too hot for normal activity, they estivate, which is a stage that’s less deep than hibernation. Estivating earthworms can be easily identified, as they tie themselves up in a knot.

In the winter, some species move deeper into the soil, to below the frost line, and hibernate. Sometimes hibernating groups ball up together to reduce moisture loss. In spring, as the soil warms, they tunnel upward. Some species live exclusively near the surface and die when temperatures drop to freezing. 

Earthworms move through the earth in tunnels they create. Some of them burrow horizontally, some vertically. Either way, burrowing aerates the soil, allowing oxygen and water to penetrate to plant roots and soil microorganisms.

Earthworms eat soil and decomposing plant materials as they burrow. Good soil may hold up to a million earthworms per acre, eating as much as 10 tons of leaves and other plant matter per year. Their castings are rich in nitrogen, calcium, magnesium and phosphorus, all essential nutrients for healthy soil.

Earthworms don’t all perform the same kind of work or live at the same depth. There are three major groups:

  • Surface-soil species – These earthworms live just under the soil surface or within organic matter lying on top. Researchers have found that they’re most numerous in the crowns of grass clumps, the junction where stems and roots meet. They’re shaded there and hidden from birds while they feed on decaying leaves and roots. The grass also offers some protection from high temperatures. Typically small, these earthworms have adapted to more extremes of temperature and moisture than other species. Some surface-soil species live in the highly rich environment of compost piles and can’t survive in soil containing less organic matter.
  • Upper-soil species – These earthworms live close to the surface and feed on soil and whatever organic matter it contains. Their burrows are temporary and become filled with worm droppings, adding nutrients to subsoil.
  • Deep-burrowing species – Typically called Nightcrawlers, these earthworms form permanent, vertical burrows that may go down 8 feet (2.4 m) into the soil. They create “middens” at the entrance to the tunnels. 

Middens are piles of worm poo and plant residue. It isn’t exactly clear to experts just why worms build middens. But it’s probably to hide the burrow entrance while providing a handy pile of food for them to feed on from below, safe from sun and birds. Middens dissolve over time, and as they do, they fertilize the soil at the top and drain into the burrow, fertilizing that, as well. In soil that isn’t tilled, burrows are permanent and used for several years. The grass around a midden is sometimes greener and taller.

Image of estivating earthworm curled tightly up into a ball.

Estivating earthworm (Jtellerelsberg / Wiki; cc by-sa 3.0)



Earthworms can be found on sidewalks and driveways after a rain shower. Just why is another worm mystery. What is known for sure is that they don’t do it to avoid drowning. Edwin Berry, an entomologist who has studied earthworms for ten years, found they can live for weeks in water. Some experts believe rain or high humidity offers them a chance to disperse to new feeding areas, a way to move around “outdoors” without drying out. If you see one on the pavement, do it a favor by picking it up and placing it on the soil in a shady place — light paralyzes them after about an hour, leaving them unable to move to safety.

Reproduction
Image of two earthworms mating

Earthworms mating (Jack Hynes / Wiki)



Earthworms are hermaphrodites (her-MAFF-row-dites), which means they have both testes and ovaries. The male sex organs are in certain segments of the body and the female organs are in other sections. Despite this, it takes two worms to tango. 

You may be wondering how they find each other. It’s mostly by happenstance. Although they can’t see or hear, they can feel ground vibrations or the movement of leaves or grass when another earthworm is passing by. When that occurs, the worms start feeling around until they find each other. Precisely how fertilization takes place is complicated, but basically, they align their bodies so their heads are pointed in different directions, with the sex organs aligned for the transfer of sperm from one earthworm into the sperm receptacle of the other. Their setae hold their slippery bodies together. Following their pairing, they separate and both are ready to reproduce.

First, however, they must fertilize the eggs they’re carrying. In order to accomplish that, the clitellum secretes a ring of sticky mucus that slides forward, first over the segment containing eggs, which stick to it, and then to segments containing the sperm. There the sperm comes into contact with the eggs and fertilizes them. The clitellum continues its way forward, carrying the fertilized eggs until it slips right off the front end of the worm. It seals itself at both ends, forming a cocoon, and is left lying on or in the soil.

Image of many earthworm cocoons.

Earthworm cocoons. (© Soil and Water Conservation Society (SWCS). 2000. Soil Biology Primer. Rev. ed. Ankeny, IA: Soil and Water Conservation Society.)



Depending on the species: Earthworms produce between three and 80 cocoons a year. Those that live deeper in the soil produce fewer cocoons. A cocoon, about the size of a grain of rice and shaped like a lemon, may hold between one and 20 fertilized eggs. The embryonic stage lasts from three weeks to five months. Embryos can survive underground until temperature and moisture conditions are just right for hatching. In unsuitable situations, they may overwinter and hatch the following spring. Embryos develop in a way similar to that of a bird in an egg, by consuming nutritive matter within the cocoon. When it’s all used up, the babies are ready to hatch and come out one end of the cocoon. They become adults in 10 to 55 weeks. An earthworm without a noticeable clitellum is not yet a sexually mature adult. 

Lifespan
Earthworms live from four to eight years, depending on the species.

Food Sources
Earthworms living close to the surface eat plant matter — decaying roots, leaves, fruits, vegetables. Those living deeper mostly eat dirt, which contains fungi, bacteria, nematodes (a class of worms, usually microscopic in size), and other tiny organisms. So, either way, worm excrement contains nutrients beneficial to plants. Worms in compost bins feed on the vegetative garbage that gets tossed into the bin — bananas, tomatoes, corn, potatoes, etc.

Predators
Snakes, birds, toads, rodents, moles, foxes, certain beetles, slugs, humans and other animals prey on earthworms. Earthworms can be killed by a significant fluctuation in soil pH (they prefer neutral acidity), by oxygen-depleted soil (such as heavy clay soil), insecticides, vermicides and other chemicals, and by tilling

¹DailyMail.com

More reading:

Explore an insect-friendly yard   
How to improve clay soil   
Native plants for wildlife   

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