The Book of the Earthworm

by Sally Coulthard

  Mucus is also the latest ingredient to be harnessed by the beauty industry. Earthworm slime – which is thought to contain useful peptides and enzymes – is washed from worm casts and extracted by submerging earthworms in water, and then added to anti-wrinkle and skin regeneration creams.

  CAN EARTHWORMS HEAR?

  Disdain thee! – not the worm beneath my feet!

  Percy Shelley, Hellas (1822)

  Earthworms don’t have ears but they are very sensitive to vibrations caused by sound. One of the loveliest images conjured up by Darwin during his observations involved him playing a number of musical instruments to his earthworm subjects.

  Despite his best efforts, ‘They took not the least notice of the shrill notes from a metal whistle, which was repeatedly sounded near them; nor did they of the deepest and loudest tones of a bassoon. They were indifferent to shouts, if care was taken that the breath did not strike them. When placed on a table close to the keys of a piano, which was played as loudly as possible, they remained perfectly quiet.’

  The earthworms did react, however, when they were placed on the piano in pots of soil. ‘When the note C in the bass clef was struck, both instantly retreated into their burrows. After a time they emerged, and when G above the line in the treble clef was struck, they again retreated.’ Darwin concluded that, although earthworms aren’t bothered by loud noises, they are very sensitive to sound vibrations (see Why do earthworms surface after rain? page 88).

  CAN AN EARTHWORM REGROW IF IT’S CUT IN HALF?

  One of the most common myths about earthworms is that if you cut one in half, both halves survive to become two new worms.

  Whether an earthworm will survive being chopped in two depends on where it’s cut. The Common earthworm (Lumbricus terrestris) will generally survive having a bit of its tail end cut off and can even regrow some of its segments, creating a new tail. The old, chopped tail end will die. Chop an earthworm anywhere at the front end between its saddle (clitellum) and its head, however, and you’ll kill it stone dead, as this is where its major organs are.

  Some species of earthworms are better at coping with being chopped in two than others, but all seem to have the capacity to regrow at least some segments of their tails. In fact, some species of earthworms can deliberately detach or self-amputate their tail if they have been caught by a predator, in a behaviour called autotomy (some species of reptiles do the same). Sometimes an earthworm will detach its own tail for different reasons: the compost earthworm Eisenia fetida, for example, uses the end of its own tail as a kind of refuse dump for insoluble waste, which gives it a characteristic yellow colour. When the worm can’t store any more waste in that section of the tail, like a spent rocket-ship it simply jettisons the tip.

  One worm can regenerate almost endlessly, however. Planarians are simple flatworms. They’re only loosely related to earthworms but have been shown to have spectacular regenerative powers. Cut a planarian up into pieces and each piece will grow into a completely new worm. In one experiment, a piece of planarian that was only 1/279th of the original worm regrew into an entirely new clone of itself. That’s like an entire person regrowing from a piece of human tissue the size of a hamster.*

  In 2015, thousands of earthworms rained from the skies over southern Norway. Freak weather events, such as water-spouts and tornados, are thought to pick up vegetation and small animals and carry them for miles before dropping them again. A similar event happened in 2011 at a Selkirkshire school in Scotland, when a teacher and his students had to take cover during a football game after earthworms began plummeting from the sky.

  * Calculation based on average human weight of 62 kg and an adult hamster weighing just over 200 g.

  Earthworm behaviour

  But it is surprising that an animal so low in the scale as a worm should have the capacity for acting in this manner

  Charles Darwin, The Formation of Vegetable Mould through the Action of Worms, with Observations on their Habits (1881)

  What does an earthworm do all day? Darwin was one of the first scientists to document, with any rigour, the idiosyncrasies of earthworms. Since then, subsequent studies and experiments have revealed some extraordinary behaviours. From ‘herding’ instincts to weather preferences, courtship rituals to defensive mechanisms, there’s much more to the average earthworm than meets the eye.

  Other questions, such as do earthworms sleep, can they feel pain, or how do they find a mate, are still being explored – it seems there’s still plenty we don’t fully understand about life underground.

  WHY DO EARTHWORMS SURFACE AFTER RAIN?

  The next time you go out on your morning or evening ramble, if you chance to see a worm in your path, do not kick it aside, not step over it; but take it from the ground, and lay it in the palm of your hand…

  James Samuelson, Humble Creatures (1858)

  Have you ever noticed that earthworms tend to head to the surface after a rainstorm? Scientists used to think that worms headed upwards through the soil to prevent them drowning in their water-filled burrows. We now suspect this isn’t true, as earthworms prefer moist soil and many species can actually survive submerged in oxygen-rich water for weeks (see Can earthworms survive underwater? page 74).

  We still don’t know for certain why earthworms head upwards during a rainstorm, but one theory is that it might be something to do with migration and that earthworms find it easier to move bigger distances across the wet soil surface than going through the soil, especially if it’s dry.

  It’s a risky strategy, however, as exposure to strong UV light can be lethal for an earthworm, so another more convincing theory is that the drumming of the rain mimics the vibrations of moles digging through the soil. In order to avoid being eaten, on hearing these vibrations earthworms move up through the soil to get out of the way. Certain species of birds, including seagulls, blackbirds and thrushes, actually mimic this vibration by tapping the ground with their feet to lure unsuspecting earthworms to the surface, where they are gobbled up. Worm charmers use the same trick.

  Scientists recently tested this hypothesis. At the Annual Sopchoppy Worm Gruntin’ Festival in Florida, worm charmers use a special trick to entice earthworms to the surface. They hammer wooden stakes about 30 centimetres into the soil and then scrape the stake with a long piece of steel called a ‘rooping iron’. The rhythmic, rasping noises that are created by steel scraping on the wood sound like low-pitched grunts (hence the name for the festival) and without fail seem to bring hundreds of earthworms to the surface.

  The researchers wanted to explain why the ‘rooping’ technique worked and set up seismic recording equipment to measure what was happening underground. It turned out that scraping with a rooping iron caused vibrations that registered about 100 hertz, an almost identical frequency to the sound of moles digging. The flight response of earthworms to the sound of moles is thought to be an evolutionary one, and deep-seated. In countries such as New Zealand, which don’t even have moles, earthworms seem to have a ‘memory’ of this predator and still behave in the same way if they hear vibrations.

  On 14 September 1972, Cleveland Airport was brought to a standstill when hundreds of thousands of earthworms appeared on the runway. Heavy rain had brought the worms to the surface of the ground and they’d inadvertently crawled on to the runway, causing aircraft to skid dangerously on landing.

  WHY IS IT DIFFICULT TO PULL AN EARTHWORM OUT OF A HOLE?

  It’s surprisingly difficult to pull an earthworm back out of the soil if it’s burrowing. Writing in the middle of the nineteenth century, the nature writer James Samuelson noted: ‘It has perhaps never occurred to you to enquire how it is, when you endeavour to draw a worm forth from the earth, that it can offer such resistance to your efforts, as almost to necessitate your tearing it in two before you can extract it.’23 Darwin also noticed that they ‘can seldom be dragged out of the ground without being torn to pieces’ and we can often observe garden birds struggling to winkle them out of the
soil.

  We already know that the earthworm’s body is actually covered in tiny stiff bristles called setae (see How do earthworms move? page 66). These setae face slightly backwards, helping the earthworm anchor itself into the ground as it pushes itself forwards through the soil. Without them, the earthworm would just slide around on the same spot. These little spikes also make it difficult to pull an earthworm out of its burrow, especially tail first, as the bristles dig in and create resistance. If a garden bird grabs a worm by its front end, it probably has a better chance of pulling it out of its burrow. In the struggle, the earthworm’s setae can fall off. If the worm manages to escape and live to tell the tale, its bristles will grow back, just like fingernails.

  How to help earthworms #5

  GROW GREEN 'MANURES'

  Research has shown that using cover crops is an easy way to introduce organic material to the soil and boost earthworm numbers by as much as 300 per cent.25

  ● Plant a cover crop in a vegetable garden at the end of summer. Also known as ‘green manures’, cover crops protect the otherwise bare soil over autumn and winter, reducing the amount of weeds and preventing the soil from becoming too cold. At the end of the winter season, cover crops are dug back into the soil, organically nourishing the earth and helping feed the plants that follow – for winter cover crops, look for plants such as grazing rye or winter vetch.

  ● You can also use cover crops during the growing season to ‘fill in’ spaces where you have lifted out your other vegetables and to attract pollinating insects – mustard, crimson clover and phacelia are lightning quick to germinate and will grow in just a few weeks.

  Experiments also suggest that garden birds, such as robins, carefully listen for cues that an earthworm is close by. Once the sound of an earthworm is located, some garden birds cock their heads to one side. We used to think this was so they could listen even more closely, but recent research suggests garden birds are head-cocking to locate the earthworm by sight, before they make their final, deadly strike.24

  DO EARTHWORMS LIKE WARM OR COLD WEATHER?

  We do not avoid evil by fleeing before it, but by rising above or diving below its plane; as the worm escapes drought and frost by boring a few inches deeper.

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

  Earthworms are at the whim of the weather. Unlike mammals, which can regulate their own body temperature, the earthworm is a cold-blooded creature and its temperature is directly affected by its surroundings.

  Different species of earthworms, however, prefer different temperatures. Surface dwellers, such as the composter’s favourite – the Tiger worm (Eisenia fetida) – can tolerate temperatures between 0°C and 35°C but seem happiest at a balmy 25°C. The Common earthworm (Lumbricus terrestris), on the other hand, prefers life deep in the soil, where it’s a cooler and more consistent temperature throughout the year.

  Deep soil temperature is remarkably stable. At the surface and just under, the soil temperature rises and falls in line with changes in the surrounding air temperature. The further down into the soil you go, however, the less the soil is affected by the temperature of the air. At 4 metres underground, the soil temperature is a fairly steady 10°C, at 1.5 metres underground it varies between about 5°C and 15°C, and at 50 centimetres underground, the temperature can fluctuate between just above freezing and just under 20°C.

  The full moon that appears during March is also known as the ‘Worm Moon’, named after the earthworms that re-emerge at the beginning of spring when the ground starts to thaw.

  Given that the Common earthworm’s optimum temperature is between 7°C and 12°C, it makes sense for the earthworm to move around in the soil, searching for the most comfortable temperatures. In winter, this means spending much of its time in the deeper layers of soil. The Common earthworm can’t tolerate temperatures below freezing but their egg cocoons can survive for weeks in soil as cold as –5°C. Each cocoon has a protective dehydration mechanism; in response to sub-zero temperatures, the cocoon begins to lose water, effectively freeze-drying itself until conditions improve and the weather warms up.

  Other species of earthworm, including Nordenskiold’s worm (Eisenia nordenskioldi), found across large parts of Russia, and the Octagonal-tailed worm (Dendrobaena octaedra), native to Eastern Europe and Western Siberia, have to cope with long, very harsh winters. They have evolved a neat trick whereby they rapidly increase the amount of glucose in their body fluids, which acts like a natural antifreeze and stops their bodies being destroyed by ice crystals.

  Back in temperate climates, if soil becomes too hot or too dry, some species – including the Common earthworm – will go into a period of dormancy called aestivation. The main aim of aestivation is to stay moist, so the earthworm coils itself into a tight knot to reduce its surface area, seals itself in a chamber lined with mucus to keep the humidity high, and lowers its metabolism so it doesn’t lose too much water. Then, in this ‘summer stasis’, the earthworm can sit out the scorching temperatures and dry soil for as long as three weeks, until things return to normal.

  WHAT DO EARTHWORMS EAT?

  It depends on the species of earthworm. Surface dwellers eat decaying organic matter – such as dead leaves, berries, rotting wood and flowers. Shallow-dwelling earthworms eat soil, which is high in organic matter. And deep-burrowing earthworms drag larger pieces of decaying plant material, such as leaves, into their burrows. Some species of earthworm also eat fungus and rotting animals.

  People sometimes call earthworms ‘omnivores’, but a more accurate term is a ‘detritivore’ or animal that feeds on dead organic material.

  One of the mysteries of earthworm eating habits was how they coped with toxic plants. Some plants are spectacularly poisonous and contain chemicals – called polyphenols – designed to stop them being stripped by herbivores. These toxins still carry on being potent even after the plant or its leaves are dead.

  Scientists at Imperial College, however, worked out how earthworms seem to be able to stomach almost anything, even if it’s poisonous. It turns out that earthworms’ guts contain molecules – called drilodefensins – that neutralise certain chemicals. No other animal on the planet has these toxin-busting molecules – drilodefensins are found only in the guts of earthworms.26

  What’s really interesting about earthworms, however, is what comes out of their back ends. Worm poo or ‘worm casts’ are wonderfully rich in nutrients and minerals, which are in a form readily available for plants to use. Compared to the surrounding soil, worm casts contain on average five times more nitrogen, seven times more phosphorus and eleven times more potassium.27

  The constant eating and pooing of earthworms moves an extraordinary amount of soil. Worm casts deposited on the soil surface slowly accumulate, burying whatever is lying on the surface – this is often how archaeological artefacts and ancient monuments get slowly submerged over centuries. It’s estimated that in just one single acre of land, earthworms add a fresh, 5-centimetre layer of topsoil every year. That’s about 8 tons of soil.

  DO EARTHWORMS BUILD HOMES UNDERGROUND?

  The very name Earthworm, so distinctive as it is of the habitat of these animals, seems to have been expressly invented in order to crystallise into one word the remarkable distributions of these creatures.

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

  Most earthworms don’t have homes. Both surface dwellers and shallow-dwelling species move through organic matter, eating and pooing it out behind them in the form of crumbly worm casts.

  Surface dwellers don’t create burrows at all – they don’t live in the soil – and prefer life under layers of moist, warm leaf litter. Shallow-dwelling species do create tunnels as they eat their way through the soil but quickly fill them up again with excreted worm casts as they move along.

  Deep-burrowing species, however, such as the Common earthworm (Lumbricus terrestris) create permanent burrows, which they keep clear of deb
ris. To do this, they leave their poo on the surface of the soil, at the entrance to their burrow, in piles known as ‘middens’. Unless it’s disturbed, the earthworm will stay in the same burrow its whole life and keep adding to the midden, creating an ever-increasing pile of poo. These permanent burrows are so well constructed they can remain stable for decades.

  Darwin noted that his Common earthworms sometimes even built little roofs over their burrows: ‘Where fallen leaves are abundant, many more are sometimes collected over the mouth of a burrow than can be used, so that a small pile of unused leaves is left like a roof over those which have been partly dragged in… They often or generally fill up the interstices between the drawn-in leaves with moist viscid earth ejected from their bodies, and thus the mouths of the burrows are securely plugged.’ Why they did this, Darwin wasn’t sure, but he suspected the leaves provided a means of regulating temperature and humidity within the burrows, acted as rain covers, and offered a level of concealment from hungry predators.

  ARE EARTHWORMS TERRITORIAL?

  While surface-dwellers and shallow-burrowing species of earthworm don’t have permanent burrows, there’s no place like home for the Common earthworm (Lumbricus terrestris).