Charles Darwin spent 39 years studying the humble earthworm and concluded that they are very valuable to the Earth's processes.

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Introduction to How Earthworms Work

Back in 1881, the biologist and creator of the theory of natural selection, Charles Darwin, published a book on a subject very near and dear to his heart. "The Formation of Vegetable Mould, Through the Action of Worms, With Observations on their Habits" was the culmination of 39 years of research into what is often considered among the humblest of creatures, the earthworm.

"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," he concluded [source: Darwin].

Darwin was on the money; at least by sheer numbers alone, earthworms have a tremendous impact on local ecosystems (though not all of it is good, researchers have recently learned). There are more than 3,000 species of earthworm on the planet, and they range in average sizes from.4 inches (1 centimeter) to 9 feet (2.7 meters) in length, although outsized monsters are routinely reported around the world [source: Earthworm Society of Britain]. In a single hectare (2.47 acres) of land, you may find 500,000 to 2 million individual worms. You may further find that the total wet biomass of all these worms equals 10 times the total weight of all the animals living aboveground on that same plot of land [source: Green-Pik].

Yet as ubiquitous as earthworms are, there is much we only recently have come to learn about them. In some northern regions of North America, there are no earthworm species around just a few hundred years ago. That's because during the last ice age, most earthworm species native to North America died out due to the change in climate. The earthworms you'll find today in abundance in the temperate woodlands and forests of northern North America are relatively recent immigrants from Europe and Asia, who hitched rides in the soil attached to plants imported to North America in the 19th and 20th centuries. In this clime, they've flourished and spread even farther, due to humans who purchase them as fish bait and often release them into the wild or inadvertently give them long distance rides attached to the tread of car tires [source: Kinver].

If you thought the only cool thing about earthworms was their ability to regenerate, or perhaps that they play pinochle on your snout when you die, read on and have your socks knocked off.

Worms are generally classified by the position in the soil they inhabit.

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Classifying Earthworms

Earthworms typically fall into one of three categories based on where in the soil they live. If you took a spade and dug down a couple feet into soil that's covered with a top layer of dead leaves in just about any temperate woodland in North America or Europe, you'd likely encounter all three classifications of earthworms.

Around the point where the leaves sit just above ground and begin to mix with the topsoil, or litter horizon, you'll find the epigeic class of earthworms (epi means "above" and geic derives from "Gaia" or "Earth"). These worms play a big role in decomposing leaves and other organic matter that falls to the woodland floor simply by eating it. Redworms, also called manure worms, are among the comparatively small epigeic earthworms, and they're commonly sold for use in compost piles.

Beneath the soil surface, in the dark, rich topsoil layer, you'll find the endogeic class of earthworms. Endogeic earthworms generally move parallel to the surface, leaving horizontal burrows and eating organic material found only beneath the ground, like dead plant roots. As they spend their entire lives underground and out of the sun, they lack pigment and are usually pink, gray or white. Scientists know the least about the behavior and life cycle of endogeic worms.

The deep-dwelling anecic class is the most familiar types of earthworms. Their burrows are strong enough that they tend to be permanent and can reach several feet beneath the surface. They too feed on leaf litter, but anecic worms pull whole leaves down into their burrows where they feed at their leisure. In a single night, Lumbricus terrestris -- the common European earthworm, known in the U.S. as the night crawler -- can travel as far as 62 feet (19 meters) along the soil surface in search of food [source: Werner].

Earthworms are segmented, which means they belong to the phylum annelid, meaning "ringed worm" [source: Raskoff]. The 100 to 150 ringed segments that help to define earthworms are also its means of locomotion. Each ring can move as a muscle and independently from the rest of the segments, and the worms move through a process of expansion and contraction. At the front, or anterior, end of the worm are retractable bristles called setae. When extruded, the setae anchor the front end of the worm in place, which allows the worm to contract the segments in the rear, dragging them forward (they can also move backward). By extending its front, anchoring itself and then contracting the rear, the worm is able to move through the soil.

Danger, worm! There is plenty enough sunlight here to suffocate you.

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Earthworm Anatomy

As far as animals go, the earthworm is pretty no-frills. It's essentially an eating (and defecating) machine. A mouth at the front end of the earthworm leads to what amounts to a long tube where the organic matter and dirt from the worm's diet pass through until it exits the other end. Along the way, organic matter is pushed into the crop, where food is stored, and then into a gizzard, where tiny pebbles previously eaten by the worm are used to grind food for further digestion. The intestinal walls of the worm are lined with blood vessels that are effused with blood by one of the aortic arches, the earthworm's five hearts. The vessels absorb and distribute nutrients from the food. On its way out, microbes living in the worm may attach to the dirt and remaining organic material, and the entire package is deposited as worm feces, called castings. These castings may be deposited within the dirt or in tiny, cone-shaped piles with a hollow center on the surface of the ground.

That's pretty much the long and short of the earthworm's existence, but in addition to having five hearts, the earthworm has some other interesting anatomical features. Breathing through its skin is one. An earthworm lacks any kind of lungs, but like any other aerobic organism, it still needs oxygen to carry out essential processes and to rid itself of carbon dioxide that builds up as waste. Instead of inhaling and exhaling like us, the exchange of these gases in and out of the earthworm takes place passively through the skin. An earthworm can even survive submerged in water if it contains enough available oxygen.

For this breathing to occur, an earthworm's skin must always be moist. This need is generally assured by the mucus the worm excretes naturally through its skin. But earthworm slime is no match for hot, dry air. Without enough moisture at the skin, the gas exchange can't occur and the worm can't breathe. If you've ever seen a dead, desiccated earthworm curled up on a sidewalk on a warm day, you've likely met an earthworm that suffocated to death.

Because this need to avoid heat necessitates staying out of the sun, earthworms have evolved a means of determining if the sun's out. This leads us to yet another interesting earthworm fact: They don't have eyes but they can detect light. Specialized photosensitive cells on the earthworm's skin convert light into electrical impulses that the worm senses and reacts to, moving back below ground or under the cover of plant matter.

Earthworm reproduction or just unfortunately tangled up?

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Earthworm Reproduction

One of the most interesting aspects of earthworms is their sexuality. Earthworms are simultaneous hermaphrodites, meaning worms have both male and female reproductive organs. During sexual intercourse among earthworms, both sets of sex organs are used by both worms. If all goes well, the eggs of both of the mates become fertilized. You can imagine this is a highly efficient way of ensuring the survival of the species. Vermicomposters, people who raise worms and other organisms to compost, report that their earthworm populations typically double every 60 to 90 days [source: Werner].

To copulate, two worms line up against one another facing opposite directions. In this position, both worms excrete so much mucous, that what is called a slime tube forms around their bodies. Each worm ejaculates sperm from its sex organs into this slime tube and it is then deposited in the other worm's sperm receptacle. The act of mating is completed, but the process of reproduction still continues as each worm goes its separate way [source: Conrad].

You know the wide band near the front of any earthworm? That band is called the clitellum and it's responsible for producing another tube of mucus. This band is passed forward toward the mouth end of the worm. As it travels forward, the mucus passes over the sacs containing the worm's own eggs, which stick to the slime. Attached to the slime tube, the eggs then pass over the seminal receptacle, where the other worm's sperm is kept. The eggs and sperm come in contact in the slime tube and if all goes well, the eggs are then fertilized.

The band of slime is wriggled off the head of the worm and forms a cocoon in the shape of a lemon for the anywhere from four to 20 worm eggs that the common European earthworm typically lays. In about two to three weeks, the newborn worms will hatch and emerge from the cocoon into the soil. This cycle of reproduction can happen every week to 10 days, another reason earthworm populations can grow so quickly [source: Barrett].

Earthworms can also reproduce themselves if need be. They can regenerate new segments if they lose a few. Most earthworms are better at regenerating tails than heads, but some can. They don't reproduce asexually, however; only half (and likely the head half) of an earthworm split in two will regenerate into a full worm once again [source: Tomlin].

Moles are but one of the many predators that earthworms must learn to avoid. There are also plenty of parasites they have to endure as well.

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The Life and Death of the Typical Earthworm

From the time it emerges from its cocoon to the day it dies, an earthworm's life expectancy can vary widely, depending on the species. The night crawler has an average life span between six to nine years and has been reported to live up to 20 [source: Backman]. Red worms typically live between two and five years [source: Wormman.com]. Gray worms, which spend their entire lives beneath the soil surface, tend to live between 1.25 and 2.6 years on average [source: Muratake].

During these life spans, worms come to develop some preferences for food and habitats and they have a way of showing when they need perpetually moist environments, somewhere in the 50-percent to 90-percent humidity range. They are also sensitive to temperature changes aboveground, and are most active temperatures between 59 and 86 degrees Fahrenheit (15 to 30 degrees Celsius) [source: Edwards]. And while they'll eat just about any organic matter (from protozoa to leaves to animal carcasses) in just about any state of decay, earthworms also have preferences in their diets. For example, researchers found that earthworms prefer maple over oak leaves and clover over grass. In one study they left just .6 percent uneaten of the clover residue used in the experiment while 9 percent of the grass remained [sources: WSU, Bugg].

This means you can find an earthworm living anywhere it's relatively warm, moist and loaded with food -- from a forest in Wisconsin to a sewage treatment plant in Malaysia. When these conditions aren't right, earthworms can simply go dormant until things change more to their liking. In this dormant state -- called aestivation, which is similar to hibernation but is actually more efficient -- earthworms effectively stop living: They don't need food, move or reproduce. Instead they simply curl up in a ball to maintain moisture on their skin and go dormant until temperatures and other conditions improve [source: Southwest Wildlife].

Earthworms have to dodge plenty of natural predators during their lives. In addition to obvious ones, like birds and moles, they also have to look out for foxes, hedgehogs, turtles, slugs, beetles, snakes and leeches, all of which are happy to feed on them. What's more, they can serve as hosts to some of the same parasites that can infect humans, like nematodes, mites and flatworms, as well as cluster flies, which lay their eggs in earthworm burrows. When the eggs hatch, the maggots parasitically attach and feed on the earthworm [source: Tomlin].

All that black dirt -- that's red worm castings. Nitrogen-rich earthworm castings.

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The Helpful Earthworm

Just going about their own daily lives, earthworms provide vital benefits for local plants and animals. First and foremost, they carry out most of the decomposition of the leaves and litter that fall to the woodland floor.

Worms are eating machines. On the forest floor, redworms munch organic matter in any state of decomposition. Beneath the surface, earthworms like night crawlers eat leaves pulled into their burrows. As a worm consumes the organic matter, it breaks it down into smaller parts, releasing nutrients locked up in the leaf. The worm absorbs some, but not all, of these nutrients for itself. The castings excreted by earthworms are packed with nitrogen, a key element needed to sustain plant growth. Earthworms absorb only about 27 percent of the available nitrogen in their food, leaving the other 73 percent broken down and available as nutrients in the soil [source: Werner]. Charles Darwin calculated that 10 years' worth of worm castings from an acre of soil collected and spread evenly over that acre would form layer 2 inches (5.08 centimeters) thick [source: Conrad].

Earthworms also perform other services for its local ecosystem as well. You know how rainwater has a tendency to seep into the ground? You can thank deep-burrowing worms for that. As anecic worms like night crawlers move vertically to the ground, the mucus they produce not only helps them move more easily through dirt, it also acts as a stabilizer, kind of a slimy cement that helps maintain the structural integrity of the burrow. These burrows prevent flooding by also acting as channels for rainwater to percolate through the soil, which acts like a filter, cleaning out impurities as the water trickles down to aquifers and other reservoirs.

These same channels also aerate the soil and allow plants' roots to move into areas that would otherwise be too compacted had earthworms not already burrowed through it.

Tilling the soil is also a major service that earthworms perform. Deep burrowers move soil upward and downward, distributing nutrients more efficiently, breaking up compact soil and aerating it. Topsoil dwellers break up minerals in the soil and mix it together as well. You can kind of think of earthworms as natural earth movers or, as Aristotle put it perhaps best, "the intestines of the soil" [source: Tomlin].

Earthworms can reduce the understory in a woodland like this one. Without it, larger animals that feed on those plants can't thrive.

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The Harmful Earthworm?

Sure, they decompose organic matter, break it into usable nutrients for their local ecosystems and recycle compost. But as helpful as they are, researchers have recently come to see the earthworm in another way: as destructor.

The very same traits that make them beneficial can also make earthworms harmful. Study after study finds that earthworms are voracious eaters: One found they can break down about 90 percent of the surface leaf litter in an apple orchard in a single winter; another estimated that earthworms can consume about 9,240 pounds (4,200 kilograms) of organic litter for every 2.47 acres in 11 weeks [sources: Werner, Werner and Bugg]. But that same leaf litter that earthworms eat so efficiently is also the habitat for spiders, lizards, beetles and other arthropods, frogs, snails and innumerable other species. Put simply, earthworms eat these other animals and plants out of house and home.

The litter horizon also serves as protection for seeds that grow to form the understory plant community of a forest -- all the smaller plants and saplings that make up the lower canopy of vegetation near ground level. Studies have found earthworm activity can reduce both the total coverage and the diversity of plant species among the understory canopy by between 25 and 75 percent [source: University of Minnesota]. This effect ripples up the food chain to affect deer and other vertebrates that depend on that vegetation for food.

There is also evidence that earthworms also have a counterproductive effect on carbon sequestration. One of the major roles of soil is to act as a sink for storing carbon and prevent a disproportionate release into the atmosphere. That organic material earthworms eat have carbon locked up inside, in addition to nitrogen; worms unlock this carbon by breaking down the organic matter, and can contribute to as much as an additional 28 percent of carbon released from the soil by researchers from Colgate University's estimate [source: Kinver].

In North America, all of this is in line with the fact that, despite the many benefits of their presence, most earthworms are non-native, invasive species. The loss of earthworm species during the last ice age left northern woodlands to successfully adapt to conditions without them. With the reintroduction of earthworms to these ecosystems in the last few hundred years, the effects have been the same as with the introduction of any other invasive species: What was once the "natural" order is changing, as ecosystems evolve once again to adapt, this time to the presence, not the absence, of earthworms.

Lots More Information

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