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A familiar sight in gardens across the UK, earthworms wriggle around in the soil and dig their way deeper into the earth if we accidentally disturb them. Far more than tiny invertebrates, they play a vital role in improving soil conditions, especially for the organic gardener. Earthworms help to create healthy and fertile soil naturally by eating decaying plant material, while producing good soil structure through their movement and supporting soil micro life, such as bacteria and fungi. An important part of the food chain, they are also a main source of food for birds.   What is an earthworm? Ranging from around 10mm to 25mm long on average in the UK; Britain has around 27 species of earthworms and they can live in most types of soil. In mainland Europe and other continents, earthworms can grow much bigger. Native to western Europe; the average Lumbricus Terrestris earthworm grows to around 360mm long. The biggest earthworm recorded is the Amynthas Mekongianus, found on the banks of the Mekong River in south-east Asia. It can grow to a massive three metres long in the mud and isn't the kind of worm you'd wish to find in your garden! The basic shape of an earthworm is a long, slender, cylindrical tube, divided into many segments known as metamerisms. With visible furrows on their body, marking out the segments, they exude a fluid to moisten and protect their surface, which allows it to breathe. Found within the worm's first body segment is its mouth and a lobe called the prostomium, which protects the entrance to the mouth when the earthworm is at rest. When the worm is moving around, it is used to feel and sense its surroundings. Some species use their prostomium to grab food items, such as grass and leaves, and drag them into the burrow. From the mouth to the anus: the worm's digestive tract runs straight through the centre of its body, without coiling. Worms reproduce through egg capsules. Tiny baby worms emerge from the egg capsule fully formed, developing into an adult in around 60 to 90 days, but continuing to grow for up to one year. The lifespan of an earthworm varies greatly, depending on its species. Scientists say the average for most worms found in UK gardens is up to two years, but many won't live that long, as they are eaten by birds, moles and other predators. Larger worms, known as Night Crawlers, which like moist soil and can even survive in water, can live for up to nine years in ideal conditions.   Where can you find earthworms? You can find earthworms living in soil just about anywhere. They live in most soils, but may not be as prevalent in acidic types, or those prone to getting waterlogged. Active all year round, they may not be seen as much during very cold weather, or in hot and dry weather, such as heatwaves or droughts. If you have a new house and garden, you don't need to introduce earthworms, as they will find their way into the soil naturally and will soon colonise. Some species, including the Allolobophora Nocturnal, live in permanent burrows. You may find worm casts produced by these species on the surface of the soil. Others emerge mainly at night, when there is less chance of being eaten by birds, to feast on dead plant materials on the surface. Not all birds eat worms as their main food source, although they need protein and worms are readily available in nature and easy to catch. Robins, blackbirds and plovers are particularly fond of worms and other species will eat them occasionally.   How important are earthworms? Earthworms are vital to the wellbeing of our gardens, soil and other wildlife. If you have plenty of worms in your garden, you know the soil is healthy. Worms create good soil structure by moving through the earth as they create their burrows. These tunnels create pores, through which water and oxygen can travel. Earthworms make vertical and horizontal burrows, some of which are quite deep. If your soil has plenty of worm burrows, it will be less likely to flash flood. Waste matter, such as worm casts, also help to create a fine structure in the soil. The casts can contain seven times more phosphorus, five times more nitrogen and 1,000 times more beneficial bacteria than soil alone! Earthworms also help to decompose organic matter, as their tunnels in the earth are lined with bacteria and digested leaf litter, bonded by secretions from the worms. Decaying leaves and roots, materials from the compost bin and animal manure will decompose naturally in the soil. This releases nutrients, which are used by the plants. Worms help this process by eating the organic matter, breaking it down further into smaller pieces and enabling fungi and bacteria to feed on it. They mix the soil layers through their movement and combine the organic matter back into it, which supports the important soil micro life, making it more active. These minute life forms are a vital food source for many other creatures, including insects and animals that live in the soil and garden. In winter, some species of earthworm struggle more than others to survive freezing temperatures. However, they lay eggs that are encased and protected in tiny cocoons. Other species, such as the Night Crawler, survive winter conditions by burrowing deep into the soil, below the frost line - the level at which groundwater freezes.   How to bring more earthworms to the garden Wrap up well in warm clothing and boots and make your garden into a worms' winter haven by building a compost heap. You'll be glad you did next spring! Keep a lookout for earthworms in winter and keep your compost heap topped up to give them the best chance of survival. If you're out doing some winter gardening, take care not to push your spade or hoe too hard into the earth, as you may inadvertently kill worms sheltering from the elements.