The Book of the Earthworm

by Sally Coulthard

  GREEN

  • Grass clippings

  • Raw vegetable and fruit waste/peelings

  • Nettles

  • Dead flowers

  • Tea leaves

  • Herbivore droppings e.g. horse/sheep manure (but never cat or dog faeces)

  • Bird droppings

  • Coffee grounds

  • Pet/human hair

  BROWN

  • Shredded woody stems

  • Prunings and hedge trimmings

  • Wood/bark chippings

  • Dead leaves

  • Scrunched-up cardboard, brown paper, toilet roll tubes and egg boxes (crumpled up, these provide useful air pockets that help the decomposition process)

  • Straw or hay

  • Sawdust

  * Rozier met his own, rather sticky end, when he was blown up by a bomb during the Siege of Lyons during the French Revolution.

  The earthworm’s body

  I don’t want to be a fly!

  I want to be a worm!

  Charlotte Perkins Gilman, ‘A Conservative’ (1915)

  Earthworms are essentially one long digestive system with a mouth at one end and a bum at the other. They are invertebrates, which means they don’t have a skeleton, so their bodies are made up of disc-like segments filled with muscles and fluid. Different species of earthworm have different numbers of segments – an adult Common earthworm (Lumbricus terrestris), for example, has between one hundred and one hundred and fifty segments.

  DIGESTIVE SYSTEM – Earthworms are part of a group of segmented worms called Annelids (which means ‘little rings’) and are closely related to leeches and ragworms. Down the centre of each earthworm runs a digestive system – a mouth, followed by an oesophagus, crop (which stores food), gizzard (a kind of stomach that grinds food), intestine and finally the anus.

  LUNGS AND HEART – Instead of having lungs, the earthworm breathes through its skin. It does this by having five pairs of simple ‘hearts’ called aortic arches, which sit near the head of the earthworm and pump blood around its body. When the blood flows close to its outer surface, it absorbs oxygen and releases carbon dioxide through a thin layer of skin. For this to happen, the earthworm’s skin needs to be moist so, as well as living in damp conditions, the earthworm also coats itself in slime from its own mucus-secreting cells. If an earthworm dries out, it dies.

  NERVOUS SYSTEM – Earthworms also have a brain and nervous system, which controls their movements and helps them detect environmental stimuli such as heat, chemical changes, vibrations, temperature and light. Their central nervous system consists of a simple brain and a long nerve cord, which runs almost the entire length of the earthworm’s body. Along this nerve cord are little swellings, or gangla, in each individual segment. These small swellings act like mini-computers, each controlling its own segment. The nerve cord also has smaller nerves coming off it, which connect to the muscles and sensors of the worm, and together this system constantly ‘reads’ the earthworm’s environment, helping it to move through the soil.

  CAN EARTHWORMS SEE?

  If you live underground, eyes aren’t much use. That doesn’t mean that earthworms can’t ‘see’, or at least detect, light. Rather than a pair of eyes, like mammals, earthworms have cells – called photoreceptors – on their skin. These light-sensing cells can tell whether it’s light or dark. This helps the earthworm know whether it’s above or below ground and, if the earthworm needs to visit the surface, how bright the sunlight is.

  Earthworms soon dry out and die if they’re exposed to warm, sunny weather. Much of the activity of the Common earthworm (Lumbricus terrestris), for example, happens at night-time, away from the heat and light of the day. Under cover of darkness, they come to the surface to feed and mate, and can hopefully avoid predators (although many worm-devouring creatures, including hedgehogs, have also evolved to use this dusk-to-dawn window).

  The earthworm’s light receptors can also tell the difference between different coloured lights. They move away from white or blue light – the colours of daylight – but don’t seem to react to red or orange lights, the colours of late-evening or early-morning light.

  WHY DO EARTHWORMS SQUIRM IN DAYLIGHT?

  …the earth without worms would soon become cold, hardbound, and void of fermentation; and consequently sterile…

  Gilbert White, The Natural History of Selborne (1777)

  Earthworms are so sensitive to light that even just an hour in daylight leaves them paralysed. In experiments that have looked into the effects of UV radiation on earthworms, scientists have found that different species have different tolerances. Unsurprisingly, surface dwellers typically can cope with more UV than deep or shallow burrowers. When exposed to light, however, no earthworm can cope indefinitely. Almost immediately, earthworms will start to have abnormally strong muscle contractions, often making those characteristic ‘S’-shaped movements or seeming to jump and flick about – we don’t know why they do this ‘dance of death’ but it may be something to do with UV light causing earthworms to become confused and poorly coordinated.18 After only a short period, however, UV light starts to damage the earthworm’s skin cells – the earthworm needs its skin to breathe and so, in effect, it suffocates to death. That’s ultimately why, come the morning’s bright sunshine, an earthworm, as Darwin noted, ‘dashes like a rabbit into its burrow’.

  WHICH END OF AN EARTHWORM IS WHICH?

  The easiest way to tell the head from the tail is to look for something called the clitellum. This is a fleshy ring around the worm, which is also called the ‘saddle’. The clitellum is always nearer the head end.

  Young earthworms don’t have a clitellum. These only appear when the earthworm is sexually mature. So, if an earthworm has no saddle (and about 50 per cent of earthworms are juveniles), look which way the worm is travelling, as they tend to go head first. Earthworms can move backwards but usually only do it if their head touches something noxious.19 If you look really closely at an earthworm, you might even be able to make out the prostomium on its head – a kind of shelf-like lip that covers its mouth.

  HOW DO EARTHWORMS MOVE?

  Earthworms move along their burrows by contracting and expanding muscles. Each earthworm has two sets of muscles: the first set encircles each of their segments, like a body-length corset; the second set runs from head to tip, like a long rubber band. Contracting these sets of muscles, at different times, helps the earthworm move forwards.

  When an earthworm wants to move in a forwards direction, it squeezes the circular muscles around each of its segments; contracting these corset-like muscles allows the earthworm to stretch out forwards and become longer and thinner.

  Now the earthworm is stretched out to its fullest, it needs to pull the back end of its body forwards. To do this, it contracts the rubber-band-like muscles – which run lengthwise down its body (i.e. parallel to the earthworm’s body) – making its body fatter and shorter again.

  This constant stretching and pulling its body forwards, however, only works if the earthworm can somehow anchor itself in the soil. If it couldn’t, the worm would constantly slip on the same spot. So, to anchor itself into the soil, the earthworm has a special trick – it has tiny retractable bristle-like structures called setae along its body, which it can push and pull – like climbers’ crampons – out of the soil. Each segment of the worm has eight of these bristles.

  So, to move forwards, one complete cycle of an earthworm stretch would go like this:

  1. Contract the corset-like muscles to extend the body forwards.

  2. Grip the soil with the setae at the front end of the body.

  3. Contract the rubber-band-like muscles to ‘pull’ the back end of the worm forwards.

  4. Grip the soil with the setae on the back end of the body and extend the body forwards again.

  HOW STRONG IS AN EARTHWORM?

  To move through the earth, earthworms burrow by forcefully enlarging tiny crevices and cracks in the soi
l. This can take an enormous amount of pressure and strength – in experiments that have measured the force needed for an earthworm to tunnel its way through the ground, it was found that large adults could push ten times their own body weight. That’s the equivalent of a human pushing a large polar bear or bison out of his or her way.

  What’s even more extraordinary is that tiny hatchlings – baby earthworms – can push five hundred times their own body weight – that’s the same as a person casually shoving a humpback whale to one side.

  HOW FAST CAN AN EARTHWORM TRAVEL?

  A worm is as good a traveler as a grasshopper or a cricket, and a much wiser settler.

  Henry David Thoreau, A Week on the Concord and Merrimack Rivers (1849)

  The Common earthworm (Lumbricus terrestris) is generally a slow mover, meandering gently through the soil, but the speed at which it travels depends on a number of factors, including the size of the worm, the structure of the soil and why the earthworm is moving.

  Few people have studied how fast earthworms travel. However, one study, published in the Journal of Experimental Biology, provides some interesting insights.20 The first is that, perhaps not surprisingly, larger earthworms can go faster than smaller earthworms. To pick up pace, earthworms of all sizes seem to increase their stride length, i.e. they stretch out further with each movement. Smaller worms, however, also have a tendency to increase the frequency of their strides to go faster, while large worms take fewer but comparatively longer strides.

  How to help earthworms #4

  LOVE LEAVES

  ● Fallen leaves have long been regarded by gardeners as something to be tidied away, to be scraped off the lawn, bagged up and thrown in the bin. Not only are we missing out on a free source of nutrients for our soil, we are removing a key habitat for a wide range of creatures who need leaf litter to survive.

  ● Dead and decaying leaves are a vital source of food and shelter for many invertebrates, especially over winter. The insulating effect of a carpet of leaves allows spiders, beetles, flies, snails and many other creatures to endure cold temperatures; some overwinter as eggs, others ride out the weather as larvae or cocoons, while many species survive in adult form, waiting for the return of spring by wrapping themselves up in leaves or snuggling below the blanket of litter. Larger animals also rely on piles of leaves – hedgehogs, mice and other mammals use the leaves as bedding or nesting material – while garden birds love nothing more than searching for bugs among the debris.

  ● Any area of the garden that has a high density of leaf litter is also an absolute haven for surface-dwelling earthworms (epigeic earthworms). Plants that produce an abundance of falling leaves – hedges, deciduous trees, shrubs – will attract surface-dwelling earthworms in large numbers. If you can’t leave the leaves where they fall, try and pile them somewhere else in the garden – you’ll not only be creating a habitat for wildlife but, over time, you’ll also end up with a rich source of leaf mould for your garden soil.

  Using the data from the study – which measured earthworms ranging in size from a minuscule 0.2 grams to a healthy 8 grams – it has been calculated that the Common earthworm travels at the following speeds, depending on its relative size:

  • Tiny earthworms can crawl about 12 centimetres per minute or 7.2 metres per hour.

  • Medium-sized earthworms can crawl about 90 centimetres per minute or 54 metres per hour.

  • Large earthworms can crawl about 1.2 metres per minute or 72 metres per hour.

  In effect, larger Common earthworms can travel ten times faster than the smaller ones. Whether earthworms do actually travel at these speeds consistently when they’re underground, we don’t know. The texture and composition of the soil, for instance, radically affects how fast earthworms can burrow – it can take deep-burrowing worms four or five times longer to move through clay than light loam.

  What we do know, however, is that earthworms can move really fast if they want to. Earthworms have superfast nerve impulses, which can race down the entire length of their body at speeds as high as 600 metres per second. Touch an earthworm when it’s poking its head out of the soil and it’ll retreat, quick as a flash.

  DO WORMS HAVE TASTE BUDS?

  The short answer is no. Earthworms don’t have a tongue, so they don’t have taste buds. However, they do have a sense of taste and smell – it’s just different from ours.

  In earthworms, the same receptors serve for the senses of taste and smell. They’re called chemoreceptors and can detect different chemical stimuli. Experiments have shown that earthworms have these special receptors inside their mouths and on their prostomium – a firm lip-like projection over their mouths that they use to push their way through the soil. They use these receptors to seek out and choose food, to sense how much water there is in the soil, and to find other worms to mate with.

  Darwin spent a great deal of time trying to establish whether the Common earthworm (Lumbricus terrestris) had any food preferences. Of the foods he offered to his subterranean ‘dinner guests’, he concluded that ‘Cabbage-leaves are much liked by worms; and it appears that they can distinguish between different varieties’ and that carrots, onion leaves, wild cherry leaves and celery were particular favourites. His conclusions were startling, not because Darwin established the menu of choice of earthworms (he only offered a limited range of food options) but because he proved earthworms were making definite selections of one food type over another (see What Do Earthworms Eat? page 98).

  CAN EARTHWORMS SURVIVE UNDERWATER?

  ‘…there are many arguments which tend to show that these purely land-dwellers have grown out of exclusively water-dwellers… For there are here and there vestiges of structures which seem only fitted for an aquatic life…’

  Frank E. Beddard, Earthworms and their Allies (1912)

  In 1874, a scientist called Edmond Perrier tested how long Common earthworms (Lumbricus terrestris) could survive submerged in tap water. To his surprise, if the water was frequently changed, the worms lasted for more than four months. Similar results have been reported for a number of other earthworm species.

  What’s interesting is that, although earthworms have become an important land animal, they originally evolved in the ocean. Most annelids, the family to which earthworms belong, still live in wet environments, whether it’s oceans, fresh water or damp soil. In fact, many species of earthworm are so adept at surviving in waterlogged conditions or aerated water that earthworms used in water submersion experiments usually died of starvation rather than ‘drowning’.

  When you see dead or dying earthworms on the ground surface after a heavy rainstorm, this is not because they have drowned, but more likely because of exposure to daylight (see Why do earthworms surface after rain? page 88).

  WHY ARE EARTHWORMS SLIMY?

  Earthworms secrete ‘slime’ or mucus for a number of reasons: the first is so they can breathe through their skin. Rather than use lungs to breathe, earthworms diffuse oxygen and carbon dioxide through their skin and for this to happen the surface has to be moist.

  They also coat themselves in mucus to help them move more easily through soil. A recent study showed that earthworm mucus is incredibly effective at stopping soil particles sticking to their bodies. This sliminess reduces drag as the earthworm travels through the soil, making its movements quicker and more agile. In fact, earthworm mucus is so efficient at this process that researchers are looking into how they can apply this lubricating quality to farmers’ soil-tillage implements, which often struggle when soil sticks to their surface.21

  Earthworms also produce lots of mucus during sex. When two earthworms are ready to reproduce, they cover themselves in a layer of slime so they can stick together and swap sperm (see How do earthworms have sex? page 113). Deep-burrowing species, such as the Common earthworm (Lumbricus terrestris), also use mucus to cement the walls of their burrows to stop the tunnels from collapsing.

  In-depth studies of one exotic species – the large Ne
w Zealand earthworm (Octochaetus multiporus) – suggest that we’re only just beginning to understand what a powerful substance earthworm slime is. Scientist Anna Palmer, who has been studying earthworms since she was a child, investigated the mucus of the Octochaetus multiporus and found a number of remarkable qualities: the first was that the mucus was extremely toxic to soil bacteria, keeping the earthworm’s body relatively free from disease. The second was that the mucus contained trace elements of thirty-three metals and minerals, including magnesium, potassium and calcium. These last three elements are vital for healthy plant growth and may explain, in part, why plants grown in earthworm-rich soil do so much better than in worm-free ground.

  The Octochaetus multiporus earthworm also has another trick up its sleeve – bioluminescent slime. Its glow-in-the-dark mucus indicates the age of the earthworm as it matures, changing from blue to yellowy-orange over the course of its life. Octochaetus multiporus also squirts out bioluminescent fluid when it is accidentally disturbed. It’s not clear why these earthworms, and a small number of other species of worm, have this glow-in-the-dark function. Depending on how it is deployed, it may be a lure or a deterrent. Gentle, glowing bioluminescence can be a way of attracting a potential mate – the changing colour of the Octochaetus multiporus slime may signal sexual maturity to another eager worm. But fast-response, sudden squirts of their glow-in-the-dark slime might be used, instead, as an effective deterrent against predators. Whether its purpose is to startle and confuse a potential attacker, or warn a predator of its toxicity, we still don’t know.22