Earthworms (Lumbricidae) are a family of invertebrates belonging to the ringworms (Annelida)and are common worldwide, except in arid climates and in Antarctica.
They are usually found in large numbers, some species have even been released to improve soil structure. Earthworms have a great impact on soil structure by digging tunnels and breaking down plant material. About 670 species of earthworms are known worldwide, ranging in length from a few centimeters to decimeters.
When soil vibrates, earthworms crawl out of the soil, this can be caused by the walking movements of animals or by human activity such as using a stick or handle. Under natural conditions, earthworms surface by digging movements of moles. Earthworms also surface during rainy weather to mate, they are otherwise active mainly at night. In winter, they crawl deeper into the ground and go into a kind of hibernation or diapause. Some vertebrates rely heavily on earthworms because they are an important food source, examples are the mole and the blackbird.
Distribution and habitat
Earthworms have holarctic origins: from North America to eastern Europe, Asia and the Middle East. Some species have a worldwide distribution, such as the common earthworm, and others are found only in a certain area. The species Eisenia japonica, for example, is found only in Japan.
Earthworms do not all live underground, many species are deep burrowers that make long vertical corridors such as the common dew pier or common earthworm (Lumbricus terrestris), which grows 9 to 30 cm long and the red worm Lumbricus rubellus which grows up to 15 cm long. The common earthworm lives just below the soil surface and leaves piles of excrement on the soil surface. There are also those that live in the litter layer such as the dung worm Eisenia fetida, which grows 6 to 13 cm long and is called tiger worm in America because of its red color and sometimes orange transverse bands. Furthermore, the gray-blue Allolobophora caliginosa is common.
Although the species have different preferences, one of the most important factors is the acidity of the soil. Most earthworms prefer pH-neutral to slightly acidic (pH 5.4) soil. However, the species Dendrobaena octaedra is still present at pH 4.3 and some representatives of the family Megascolecidae have even specialized in extremely acidic soil. The more acidic the soil the sooner earthworms go into diapause. At a pH of 6.4, earthworms remain in diapause the longest. Earthworms cannot survive in overly acidic environments, such as peat soils. As a result, plant residues are not converted to fertilizer on a large scale, allowing peat to form.
Earthworms have an elongated, typically worm-like body without legs and also lack other appendages. The common earthworm reaches a total body length of thirty centimeters, this is considerably longer than similar earthworms such as Lumbricus rubellus (up to 15 cm) and Lumbricus castaneus (up to 5 cm). Some representatives of other soil-dwelling worms grow even longer than earthworms, such as Megascolides australis from Australia that can reach a length of three meters. This species is called “giant earthworm” but, on the contrary, does not belong to the earthworms (family Lumbricidae), but to the family Megascolidae.
The body of the earthworm can consist of more than a hundred segments, the common earthworm has about 150. The number of segments increases with age because new segments develop in a special growth zone near the tip. In front, after about the thirtieth segment, adult earthworms have a thickening called the clitellum or saddle.
The skin layer is called the epidermis; on the outside is a single-celled layer of epidermal cells, which contains some glandular and sensory cells. The epidermis is covered with a hardened layer called the cuticle, this wax-like cuticle protects the worm from drying out. Below the epidermis is a layer of transverse muscle tissue, with a thick layer of smooth muscle tissue underneath. Close under the skin of the earthworm’s entire body is a layer of dermal muscles.
Like all ringworms, the body of earthworms is made up of segments or rings. The common earthworm has 110 to 180 of them; the number varies slightly among species. The segments are separated inside the body by a segment wall, or septum. These segment walls are penetrated by the digestive column, the veins, the nerve cord and also by the nephridia. The segment in which a nephridium is located contains the vesicle and duct of the excretory organ, but there is always a spur present that drains into the segment before it.
The anterior segment contains the mouth opening. Before the anterior segment, the mouth flap is present, this is called the prostomium which literally means before (pro) the mouth opening (stoma). An obsolete name for the prostomium is “acronym. The prostomium is not a real segment but a so-called pseudosegment, it is an outgrowth of the mouthparts. Within the earthworm family there are species in which the prostomium is a lobe of the first segment (epiloob) and there are species in which the prostomium has a connection to the second segment (tanyloob). This is shown on the right in the diagram. A third group is zygoloob, in which the first segments have no connection with each other. Behind the prostomium is present the first true segment, the peristomium, in which the mouth opening is located.
The segments of all earthworms have paired openings behind the front, starting from the fourth or fifth segment, on the ventral side called the nephridiopores. These pores are the exits of the nephridia, which are excretory glands that play a role in water balance and primary metabolism.
The body segments of earthworms always feature small, hardened and brush-like protrusions called the setae. In worms, the name chaetae is also used for these thick, hair-like structures. They consist of chitin and proteins. The earthworm always has four pairs of brushes per body segment; only the first and last body segments lack such structures. In the common earthworm, the bristles are grouped in rows; a double row on the ventral side, the ventral bristles, and a row on each body side, the lateral bristles. Other species of earthworms have a different configuration of bristles, such as all rows on the underside or the bristles are more dispersed throughout the body.
The bristles of some segments are distinctly longer than those of most others; these chaetae are used to anchor the body to a mate during mating. These chaetae are therefore called the sex chaetae, the number and position on the body segments of these brushes are different for each species.
The movements of the earthworm are caused by muscle contractions that move the body. The position of the brushes plays a role regarding the direction of body movement. The brushes give the earthworms grip when digging, they are usually pointed backwards. If one slides an earthworm between the fingers from front to back the brushes give little resistance but if one tries this from back to front the presence of the brushes becomes noticeable. The brushes can be protruded a bit to increase grip. Because the brushes prevent the body from moving backward, the worm naturally crawls forward. If the worm wants to move backward, such as to retreat into the burrow, the position of the brushes is changed where they point forward and prevent the body from moving forward. The way earthworms move forward is called peristalsis.
The nervous system consists of a nerve cord that runs throughout the body and has branches in each segment. A large branch is present at the border of two segments and a second branch sprouts in the middle of each segment.
The earthworm has two types of muscles under its skin; those that surround the segments are called the ring muscles and those that are longitudinal are the longitudinal muscles. When digging or crawling, the bristles are directed backward so that the body is anchored. The longitudinal muscles relax while the ring muscles contract, making the body both longer and thinner.
The food of the earthworm is absorbed through the mouth, after which it is passed through various organs and the waste is excreted at the rear. The head of the earthworm is always on the side of the clitellum, the head is otherwise hardly distinguishable from the rear. Under a microscope the structure of the head becomes more visible, the worm has a kind of patch in front of the mouth with which to grasp food. Hardened structures such as teeth or jaws, however, are missing. An earthworm does not have eyes or ears but can probably detect certain chemical compounds. This is how the worm knows if the food being eaten is suitable; there is a preference for leaves over twigs.
The worm’s “lip” is called the prostomium and hangs over the mouth at the head end. The mouth opening begins just after the first segment and continues into the gut, which runs throughout the earthworm’s body. The beginning of the gut consists of a muscleless throat (pharynx), which turns into an esophagus (esophagus). This esophagus consists successively of calcareous sacs, a muscleless “crop” and muscular stomachs. In this, the vegetable food is evenly ground with the help of grains of sand (in the same way as in chickens but they do it using pebbles). Next is the long midgut, which has a bulge along its entire length on the dorsal side called the tyflosole. This structure gives the midgut an internally larger surface area. Present on the outside of the tyflosole are the chloragogen cells, which are yellow-brown in color and have a function as an excretory organ.
In each segment except the first three to five and the last segment, the excretory organs are located on either side of the intestine. These are called the nephridia and they consist of a long canal folded into loops with a vesicle at the end. A funnel with eyelashes located in the anterior segment catches the excreta after which they enter the duct. Each excretory organ is surrounded by blood vessels for oxygenation. In the nephridia, many conversion processes take place that require energy, which explains the blood vessels. In the nephridia, substances such as uric acid, salts and ammonia are disposed of, and in the organs, water is recovered from the waste products. The nephridia are similar to the kidneys of higher vertebrates; they do not directly interact with the digestion of food but process the compounds obtained from food to make them suitable for the worm’s bodily processes. The excretory organ of the food is called the anus and is located at the rear on the last segment, this is called the pygidium. Often found in the feces are nematodes, small worms, such as species from the genus Rhabditis.
Between the gut and the dermal muscle layer in each segment is the secondary body cavity, the coelom. The secondary body cavities of the adjacent segments are separated by thin transverse walls (dissepimental). The secondary body cavities are filled with a fluid and are under pressure, forming a hydrostatic skeleton that gives the earthworm the necessary strength.
Earthworms have a highly developed nervous system, like other invertebrates they do not have a brain but a number of nodes where nerves converge. Such a node is called a ganglion, plural ganglia. The earthworm has such nodes at the front of the head, these are called the brain ganglia. The ganglia lie in the third segment just before the beginning of the throat (pharynx) on the dorsal (dorsal) side of the intestine. From there, numerous nerves travel to the upper lip or prostomium. The prostomium is an important body part because it serves as a sense of touch when searching for food. The upper lip also serves as a grasping organ when picking up food. The cerebral ganglia merge into a main nerve cord that extends on the ventral side to the back of the body. The central nerve cord takes the form of a pairwise rope ladder nerve system. In each segment are branches to the dermal muscle layer to control the locomotor and brush muscles. These partial ganglia are also called the segmental ganglia. They are very small and when dissected with the naked eye can hardly be seen.
Earthworms never have eyes, but they are sensitive to light. They are negatively phototactic; that is, they try to avoid light. Light-sensitive sensory cells are located on both the front and the back. These cells are also called the photoreceptors. The earthworm is also sensitive to vibration; this serves to detect burrowing animals eating worms so that the earthworm can quickly flee upward.
Respiration and blood circulation
The earthworm has no special respiratory organs, but it does have a highly branched closed blood vessel system, through which oxygen is absorbed by the skin and nutrients absorbed from the gut are transported. Earthworms have two blood vessels that run throughout the body, on the upper side is located the dorsal vessel that propels blood from the back to the front. On the ventral side is located the abdominal vessel through which blood flows from the front to the back. These blood vessels are connected in each segment by intermediate channels, the annular vessels. The dorsal vessel, by contraction of certain ring vessels, pumps the blood forward into the hearts. The annular vessels are greatly enlarged at the site of the digestive organ and have a pumping function. They are called lateral hearts or pseudoharts. The number of hearts varies among earthworms; the common earthworm (Lumbricus terrestris) has five pairs and thus ten hearts in total.
To provide oxygen to the body, earthworms, like vertebrates, have hemoglobin as an oxygen carrier. The hemoglobin is a pigment that easily binds and can release oxygen; in vertebrates, it is stored in special cells, the red blood cells. In earthworms there are no red blood cells, the oxygen is dissolved in the blood with the help of proteins. The hemoglobin is bound to a protein complex called erythrocruorin. These proteins are very complex and can be very large, the set of proteins can consist of 36 different parts and serve as a kind of framework to bind the hemoglobin molecules. Because the molecules are always divided into 12 rows of 12, the protein complex always contains 144 hemoglobin molecules.
Because earthworms sometimes end up in low-oxygen conditions, such as during prolonged flooding of the land, the blood has a relatively high oxygen-binding capacity.
The blood fluid also contains cells that belong to the immune system, called phagocytes. These break down foreign and unwanted particles, such as waste products from cells and microorganisms.
Earthworms are hermaphrodites, meaning they possess both male and female sex organs. The earthworm has a distinct thickening at the front of the body, which is often lighter in color compared to the rest of the body. This band is called the saddle or, more scientifically, the clitellum. In the common earthworm, the clitellum is always located between the 32nd and 37th body segments, as seen from the head. In other species, the position of the clitellum is different, so they can be distinguished by this.
It is often thought that the prominent, thickened saddle or clitellum contains the sexual organs but this is incorrect. The clitellum is a clump of mucus-producing cells and while it plays a role in the formation of a cocoon and the deposition of eggs, it is not itself part of the sexual apparatus. The mucus secreted by the clitellum serves as a “test tube” in which the eggs and sperm congregate and later dries into a cocoon that protects the eggs from drying out.
The internal testis and an external orifice form the male organ. Ovaries, fallopian tubes and an external opening form the female organ. In addition, they have sperm sacs, in which the partner’s sperm is collected. Dewlap copulates above the ground, other species in the ground.
Way of life and ecology
Earthworms are classified into three ecological groups based on their burrowing behavior and way of life. The epigean species live on the soil in the litter layer. They remain small and do not dig burrows; the body is usually pigmented. The epigean species reduce leaf litter on the soil. An example is the species Lumbricus rubellus. The endogeic species live in the top layer of the soil. They dig tunnels that are often horizontal and just below the surface. Earthworms belonging to this group grow medium-sized and lack pigmentation, are often pale in color. Endogeic worms break down leaf litter and provide aeration of the topsoil. An example is the species Allolobophora chlorotica. The third group are the anecic species, which are worms that dig deep, vertical tunnels. The anecic species have pigmentation and become the largest. The deep tunnels improve soil aeration and water drainage. An example is the common earthworm (Lumbricus terrestris).
Earthworm populations depend on both physical and chemical properties of the soil, such as soil temperature, moisture, acidity (pH), salts, aeration and texture. In addition, sufficient food must be present and the species must be able to reproduce and disperse. To achieve this, organic material must be added to the soil regularly by introducing it into the soil or sprinkling it on top.
The earthworm plays a very important role in improving soil structure. The earthworm digs long tunnels into which atmospheric air can penetrate, thus aerating the soil. As a result, aerobic bacteria can live deeper in the soil; such bacteria further accelerate the decomposition of organic matter. As earthworms move in their corridors, air is displaced in the soil, which aids aeration. The use of heavy agricultural machinery results in air being forced out of the soil, but the work of earthworms makes the soil more suitable for plants to grow. Earthworm tunnels are appreciated by plants: they can easily penetrate their roots in them.
In addition, the water balance of the soil also improves when earthworms are present in it. The long tunnels allow the soil to absorb, drain and retain more water, depending on the conditions. Another characteristic of the earthworm that benefits soil conditions is eating dead, fallen plant parts. The earthworm converts these into minerals that are very important for plants. In the process, it partially digests the organic material and releases the remaining soil well-mixed. Earthworms were described by Charles Darwin as a “natural plow,” he estimated that every few years the entire surface layer of the world passes the stomach of an earthworm.
In particular, the earthworm plays such a major role in soil ecology because the species occurs in enormous numbers. In humus-poor soil, 62 worms per square meter can occur and in humus-rich soil even more than 432 worms per m². According to one estimate, earthworms constitute about half of the biomass in soil. In an area of 1,000 square meters, an average of about 125 kilograms of earthworms occur; in fertile soil, this can be as much as 3 tons of earthworms, although this concerns not only the common earthworm but all types of earthworms. In fields where many worms are present, the yield is considerably higher than fields in which there are no worms. Weighing the worms’ excrement on an area of one square meter during one year showed that the animals produce 4.4 to 8 kilograms, representing an annual processing of 44 to 80 tons of earth per hectare.
Reproduction and development
Earthworms reproduce sexually producing eggs that are deposited in a slime-like mass that eventually dries into an egg cocoon. Earthworms are hermaphroditic (hermaphrodite), but an earthworm cannot fertilize its own eggs. Each earthworm has both a male and a female sex organ. Another peculiarity is the fact that although the worms have true mating, the eggs are still fertilized outside the body during cocoon formation.
Earthworms can basically reproduce throughout the year. In many regions the winters are too harsh or the summers too dry to be active and here earthworms reproduce mainly in spring and autumn, the rest of the year they have to hide.
Most other species of earthworms are known to mate underground but the common earthworm is one of the few species that mates above ground. Under humid conditions, the animals crawl upwards and emerge from their passage in search of a mate. Because they are sensitive to dehydration, this usually occurs in the morning or evening or during the day after a rainstorm. Other earthworms live permanently on the surface and mate under leaf litter or underground in a corridor.
During mating, the worms position themselves so that the animals are with their bodies against each other with their fronts facing opposite sides. During mating, sperm are secreted to the partner. Skin brushes near the clitellum (the saddle) hook into the partner’s skin at the front of the clitellum. Mucus is then formed by glands near the saddles; the mucus layer eventually forms a kind of “wrap” around the worms’ male sex organs. The purpose of this slime wrap is to prevent the seed from mixing and the worm from ingesting only the partner’s seed. Mating of the earthworm takes about three to four hours.
The male sex organs of the earthworm are located internally and have a connection to an opening on the abdominal side, called the genital pore. The sperm cells that are secreted end up in a groove that runs backward. Because this groove is surrounded by mucus during mating, the groove is closed and forms a channel through which the semen flows to the partner’s seminal vesicles. These seminal vesicles are called spermatotheca, in these bottle-shaped body cavities the sperm cells are kept alive. So earthworms do not fertilize each other during mating but only exchange sperm, the actual fertilization takes place later, outside the body.
The sperm remains in the spermatotheca until the eggs mature. This sperm supply allows an earthworm to produce eggs for several months after a mating without the need for another mating. Often, during a second mating, sperm cells from a previous partner are still present in the spermatotheca, creating a kind of competition between sperm cells.
When the eggs are sufficiently developed, fertilization finally takes place. Here again a slime layer is formed around the clitellum, which is like a belt around the worm. The slime belt is formed on segments 8 through 38, but is thickest around the clitellum. The mucus contains proteins that serve to provide food for the developing embryos. The mucus also contains a protein that serves as a signaling substance. Once the mucous band is formed, it is exited backward by the worm, thus moving the mucous band to the front of the body. Once the slime band is over the female sex opening, eggs are deposited in the slime. The female sex opening is called the gonopore and is located on the fourteenth body segment in the common earthworm.
After the slime tube is provided with (as yet unfertilized) eggs, it is further stripped off and a little later emerges above the spermatothecae. In the common earthworm, the posterior is located between segments 10 and 11 and the anterior is located between segments 9 and 10. As soon as the mucus tube is above the openings, the vesicles contract, releasing the sperm and thus fertilizing the eggs.
Once the worm has completely stripped the mucus tube from its body, it dries out and forms a hardened cocoon. The cocoon is about the size of a pea and often has a characteristic lemon shape. The cocoon has a yellow to reddish-brown color. Thus, although the cocoon contains several viable eggs, only one young worm crawls out of most cocoons. In other earthworms such as Eisenia fetida, there are two to five eggs in a cocoon.
The common earthworm crawls out of the cocoon after one to five months, depending on conditions. Six months to a year and a half later, the worm is sexually mature and can reproduce. The lifespan of the common earthworm in the wild is several years, but few specimens live long enough to reach the maximum length of 30 centimeters. Observations of the worm in captivity show that an age of six years can be reached. In other species of earthworms, development time differs somewhat; smaller species of earthworms often mature earlier but have a lower life expectancy. Very large earthworms such as the species Allolobophora longa have been known to reach an age of more than ten years.
Earthworms have a fairly high capacity for recovery or regeneration, which is the phenomenon that an animal can replace lost body parts. The earthworm can lose a part of its body due to predators biting off a piece, but many specimens are also cut in half by, for example, mechanical farming techniques used by humans. It has been claimed of the earthworm that a worm cut in two parts develops into two “new” worms, but this is incorrect. In practice, the earthworm usually dies anyway when the body is broken, because the broken part is too large for either part to be viable. Moreover, often the internal damage is also great in such an incident, for example, if a worm is pulled apart by a bird. Only if a rupture is caused by a non-traumatic separation, such as a cut, can the earthworm survive provided the part taken is not too large.
Species with red pigmented skin do not possess this ability. Regeneration occurs almost exclusively at injuries to the abdomen. Because blood vessels at the site of the injury contract, red blood cells clog the broken vessels, after which recovery can occur. It takes a long time after regeneration (2-3 months) before the regrown part regains its original color. At the front, up to four segments can be removed and then simply re-formed. If more than 15 segments are removed, they cannot all grow back. If more segments are removed, the earthworm will die. At the rear end, many more segments can be removed. The internal organs are known to be able to regenerate the nervous system, the primary sexual organs are not. Regeneration occurs only under favorable conditions, especially during the warm summer months, but by no means always.
Earthworms are waste eaters that live on dead organic plant material. Plant debris and leaves lying on the ground are pulled into a burrow and eaten. Earthworms are part of the soil fauna, contributing to a more aerated soil structure. The airier soil structure provides more oxygen in the soil, which benefits accelerated biodegradation of organic matter, which consists of the metabolism (cellular respiration) of all oxygen-loving soil organisms, both invertebrates and heterotrophic microorganisms or reducers. This decomposition leads to humus formation in the soil.
During burrowing, most of the material is taken up but the soil contains only part of the food required. Plant parts drawn into the burrow are pre-digested by secretions from their own mouths and from soil bacteria before entering the digestive system. An experiment done by Charles Darwin showed that when triangular food parts are offered, they are usually pulled down a corridor with the tip down. Earthworms could therefore be attributed some form of intelligence, according to Darwin. It later became clear that earthworms simply pulled on a food particle until the least resistance was reached. Thus, the earthworm finds a point to a food particle by much trying and not by intelligence.
Earthworms avoid eating the feces of their peers. They do sometimes eat droppings of other animals because they contain many digestible substances. Soft parts of dead animals are also eaten, including those of other earthworms.
Earthworms are at the base of many food chains and serve as food for many bird species, such as the blackbird, crow, robin. Earthworms are also on the menu of hedgehogs, badgers and moles. Various insects (including beetles), slugs and flatworms have earthworms on their menu.
Earthworms have many enemies; many animals that eat earthworms have specialized in tracking and trapping them. The earthworm particularly has to fear small mammals such as mice and moles. Examples are the shrew and the European mole, which eat large quantities of earthworms. Several birds also intensively hunt the earthworm, an example being the blackbird. Even large birds of prey such as the buzzard are known to eat large amounts of earthworms during food-deficient periods. If an earthworm is chopped into two parts by an enemy, the lost part can be replaced, see also under regeneration.
A new threat is the importation of exotics such as the New Zealand flatworm Arthurdendyus triangulatus, which lives on earthworms and has no enemies outside its natural range. The flatworm was accidentally imported into England with contaminated soil in 1960 and is now a pest. Earthworms are parasitized by algae (protozoa), Platyhelminthes and nematodes. They are found in the blood, muscle, stomach and intestine of the adult earthworm and in the cocoons.
In addition to vertebrates, many invertebrates prey on earthworms. A notorious enemy are ground beetles such as leafhoppers and especially their predatory larvae. Centipedes such as the garden centipede (Geophilus longicornis) also eat earthworms. Also some leeches – which, like the earthworm, belong to the ringworms – prey on earthworms, swallowing the worm in one go. Some species of houseflies reproduce by laying eggs on earthworms, such as the cluster fly (Pollenia rudis).
In addition to diseases and predation, earthworms are threatened by the use of fertilizers and pesticides. Earthworms can store toxins in their bodies, which can poison animals that use worms as food.
Earthworms get their name from the fact that they can be seen mainly when it rains and only then crawl over the soil surface. Earthworms can perceive a rainstorm by the vibrations in the soil caused by falling raindrops. A common misconception about earthworms is that they try to escape the rain because they may drown if their burrow fills up. The earthworm often lives in watery conditions such as below the water table. They absorb oxygen through the thin skin, however, the respiratory apparatus also works under water. Since rainwater is rich in oxygen, earthworms do not have much to fear from a shower. However, if oxygen-poor groundwater comes up, an earthworm can drown and will crawl to the surface.
Earthworms surface during a shower because they mistake rain for the vibrations of an enemy, such as a burrowing mole. These vibrations can be mimicked by sticking a stick in the ground and vibrating it. The earthworms will then crawl upward en masse, regardless of weather conditions. The earthworm can move over land without danger, and mating also takes place above ground under wet weather conditions. Earthworms are much more sensitive to dehydration than to drowning. A major enemy of the earthworm is the weather. If it is too dry, the worms must take shelter deeper in the ground to avoid drying out. The worm maneuvers its body into a ball shape and forms a layer of excrement around the body so that the worm is in a kind of cocoon. Here the earthworm hides until better times arrive. In winter, the worms crawl deeper into the soil to escape freezing.
The earthworm and humans
Earthworms play a major role in soil ecology and are known as one of the most important natural soil improvers. In particular, they play an important biological role as decomposers of dead remains. The earthworm is very important in breaking down organic matter (composting). In this process, dead material is converted into humus, and humus-rich soil is important for good plant growth. Excrement of worms sometimes contains 40% more humus than the top layer of soil, in which the earthworm lives. Earthworms pull leaves, manure and other dead organic material into the soil at night to later serve as food and for lining the nest. The earthworm can swell the anterior end bud-shaped, surrounding the mouth with a suction cup. With the help of the throat, the earthworm sucks in and crawls backward, pulling the leaf into the ground. Excretions from the throat glands promote digestion.
Its usefulness as a tidier was somewhat qualified by German research on the common earthworm; worms would also eat live seeds and seedlings. This poses a potential danger to plants from areas where the species occurs as an exotic; however, for most areas they are useful. Chemically, earthworms also alter the soil; along with organic material, the earthworm also eats soil particles. In the crop or pharynx, the grains of sand present there grind the soil particles into a fine paste, making the minerals more available to the plant. Research in the U.S. has shown that earthworm feces contain up to five times more nitrogen, seven times more phosphate and 11 times more potassium than the surrounding soil. It is estimated that earthworms convert 85 to 170 kg of organic nitrogen per hectare per year into mineral nitrogen (absorbable by the plant) in this way in Dutch pastures.
If earthworms live in large numbers in clay soil, they can have a detrimental effect on the soil of arable areas. The slime trails of the earthworms harden (seal), creating large clumps of clay that are difficult to separate from the produce to be harvested. Also, plowing in such soil becomes more difficult. One possible solution is to spread fire lime on the land, which is a mixture of calcium oxide and magnesium oxide, which loosens the clay. On the earthworms themselves, fire lime has no effect.
Several species are used to break down organic waste in a worm bin, among other things; this is also called a vermiculture. The most suitable for a worm bin is Eisenia fetida, which is distinctly different from soil-dwelling earthworms.
Earthworms are sold around the world as natural soil conditioners. In 1980, for example, 370 million worms were purchased by the United States from Canada. Earthworms are also used as bait in sport fishing. Especially the species Eisenia hortensis and Eisenia fetida are suitable for this purpose. Finally, earthworms are also an ideal food for some birds and some captive amphibians, such as salamanders.
Earthworms, as well as all other deep-dwelling earthworms, play a larger role in culture than one might think at first glance. Not only are they important soil improvers that can increase yields, but they also undermine objects lying on the soil so that they become part of the soil. These may be stones, which are undercut and slowly but surely sink into the soil, all kinds of human objects such as jewelry and coins have also ended up in the soil thanks to the activity of earthworms and are thus preserved.
Naming and taxonomy
The scientific name of the group to which earthworms belong, Oligochaeta, refers to the rows of small bristles on the body. Compared to other worms, earthworms have relatively fewer bristles, oligo meaning “few” and chaeta meaning “brushes. Another group of ringworms are the bristle worms (Polychaeta) and these worms actually have many bristles, poly meaning many. The common earthworm’s scientific name is Lumbricus terrestris and this literally means worm (Lumbricus) of the earth (terrestris).
Earthworms, like all other animals, are classified into different groups, such as tribe, class and family. The highest animal group to which earthworms are included is the tribe of ringworms (Annelida). Most ringworms live in the sea, but certain groups can live in freshwater or even on land. Earthworms are further classified with the class Clitellata or saddle-bearers and the subclass Oligochaeta or brushless ringworms, respectively. The representatives of the Oligochaeta that have segmented bodies, such as earthworms, are further classified in the order Haplotaxida.
The haplotaxids are a diverse group of worms that live both in water and on land. The terrestrial species always live in moist environments, such as mud, manure or compost. The earthworms (family Lumbricidae) are only one of more than ten families of the Haplotaxida. Other known families are the Naididae, which includes the streamling worms of the genus Tubifex and the family Enchytraeidae which includes all enchytreans (potworms or white worms). Species in the family Haplotaxidae are inhabitants of groundwater and deep lakes, these species are long, up to thirty centimeters, but are only a few millimeters in diameter.
Anatomically, the differences between families of the Haplotaxida are sometimes far-reaching. Certain body parts are absent or, on the contrary, others are extremely developed. An example is the representatives of the family Criodrilidae, which have no typhosole and also lack a gizzard, they do, however, possess a horn-like spermatophore which is unusual in ringworms.
Other families not directly related to earthworms actually look very similar in appearance to earthworms, such as the brittle siltworm (Lumbricilus variegatus), which belongs to the family of Lumbriculidae. Representatives of this group live in very humid conditions and can be classified as water-dwelling rather than soil-dwelling. However, species in this family do not have a distinct saddle or clitellum.