Nick Baker’s Bug Book

Slugs and snails belong to a huge group of animals known as molluscs, which includes clams, mussels, chitons, octopuses and squid. They belong to a class known as the gastropods, which is a cool name that means ‘belly foot’.

Slugs and snails are not a very popular group of creepy-crawlies. This is partly because many people think they all want our lettuce and strawberries and have a penchant for our pansies. They are also slimy, which doesn’t do much for their appeal.

But get to know them and they are a fascinating bunch. Of the 80 types of snail and 20 slugs in the UK, very few species actually cause damage to our gardens and some are even quite pretty.

The only real difference between slugs and snails is that snails have a hard shell that they can withdraw into while slugs do not. However, some slugs do have tiny shells on their backs and most species have a little one inside their bodies. And a few snails do not fit inside their own shells.

All slugs and snails are quite slimy. The slime slows down the rate at which water is lost from the surface of their skin. Because slugs do not have a shell they have much thicker slime to stop them drying out. Even so, some tough slugs and snails can lose around 50% of the water in their bodies and still survive.

The mantle is the part of the body that secretes the shell in snails and forms a curtain that the snail hides behind when it retreats into its own shell. It can also produce a screen of bubbles that are blown at any intruder or curious conchologist (someone who studies slugs and snails).

Snails’ shells protect them from predators and make them a difficult mouthful to swallow. They also act as a kind of survival capsule.

Slugs and snails lose a lot of water every day through their skin and the production of slime uses up a fair bit of water too, so they need to conserve it. Snails use their shell to help them do this. Slugs do not have this advantage and that is the main reason why you can find snails in drier habitats than slugs.

When conditions get very cold in winter or extremely dry in summer snails can retreat into their shells and seal off the entrance with a thick mucous, which dries to form a waterproof seal called an epiphragm.

Shell The shell of a snail is instantly recognisable, even when the animal is long dead and gone.

Glands on the skin The bit of the slug or snail that is exposed to the outside world is covered in tiny glands that secrete a thick mucous. This slime runs and oozes along a network of channels (which you can see if you use a microscope).

Tactile, tasting, tentacles All land slugs and snails have four tentacles that stick out of the head region.

Mantle This is the hump that contains all the gubbins of a slug, and the thicker, tough, rubbery flesh that can be seen around the mouth of the shell in a snail.

Breathing pore This is always found on the right of the mantle. It can be opened and closed by the snail, which is why sometimes you can look for it and not find anything!

If you look at a snail’s shell closely enough you will notice little ridges and sometimes a slightly different colour to sections of the shell.

Snails grow by adding new material to the mouth of the shell, which is formed with two layers of building material. The outer layer appears papery and thin at the lip of the shell as it starts growing in early spring. It acts like a waterproof varnish for the tough chalky layer that grows underneath. In a similar way to trees, snails have good times, where food is plentiful and they grow well, followed by hard times like winter or a really dry spell when the snail hardly grows at all. The ridges and wrinkles in the shell are formed when conditions change.

The breathing pore is a hole leading to a chamber called the mantle cavity, which is used a little bit like a lung. Air wafts in and oxygen is taken up by a network of blood vessels in the skin. Some pond snails use this technique too. If you keep a few Great Pond Snails in a jam jar next to your bed at night you can actually hear them breathing, making a kind of popping noise as they come to the surface and open their breathing pore.

Some other water snails have gills and fill their mantle cavity with water, from which they extract oxygen. Others have a combination of both. All slugs and snails can also breathe through their skin.

Look closely and you will see a dark spot right at the very tip of the tentacles. This is the eye, but it can’t see very much. (About as much as you could see if you stuck greaseproof paper over your own eyes. You could see the difference between light and dark but that’s about all!)

Snails are very sensitive to light and they follow the low light of the night out into the garden to feed. Tentacles are also used to feel around, just like you might use your hands if you were blindfolded. You can steer a slug or snail by gently touching the tentacles one side at a time.

Each of the four tentacles is covered in taste buds. The lower ones are sensitive to food up to 20 cm away, while the two larger top tentacles work over distances of 50 cm or more.

Because they are so important, the tentacles can be sucked back into the body, in the same way a glove finger can be turned inside out. Gently tap a tentacle with your finger and watch as it rolls in on itself. When the coast is clear the mollusc squeezes blood back into its tentacles and they roll out again. If they do get damaged it still is not the end of the world as new ones grow to replace them.

Like the worms in the previous chapter, there is no such thing as a he or she snail or slug. Each animal is both, which is helpful for such famously slow-moving animals. If they do bump into each other they make the most of it! Both partners will be fertilised, so both can lay eggs.

Take a torch out on a damp and warm night–you never know what you might see. Snail and slug love can be as simple as a kind of mollusc kiss chase, with one animal picking up the trail of the one it fancies and following it until the potential mate is found. After a quick embrace both animals go on their way having exchanged sperm via a pore on the side of their bodies.

Certain snails have a surprisingly passionate courtship. When a Garden Snail encounters a mate they meet each other head on, rear up and with their mouths pressed together they ‘kiss’. Then they fire love darts at each other. (I’m not kidding – such passion!) After stabbing each other with these sharp shards of shell-like material, they mate. Nobody is really sure what these darts do; they could stimulate the other snail into producing sperm or they could inhibit the other snail from mating with another.

If you poke around in the soil and explore beneath bark and stones, sooner or later you will stumble upon what looks like a cluster of mini ping pong balls. These are the eggs of slugs and snails, which are laid in damp crevices so they won’t dry out. The size of the clutch varies a lot, but anywhere between 10 and 100 eggs is common. It is fun to collect these eggs and watch them hatch, which will take a few weeks, depending on the temperature. You will see the little molluscs inside the egg just before they break free.

The eggs of pond snails are very different and have a jelly-like coating for protection. They are usually found on pond weed or on the undersides of lily pads and stone surfaces.

The Great Grey Slug has some of the most spectacular sex in the animal world and if you are ever fortunate enough to witness it you will feel like giving them a round of applause when they have finished. It goes a bit like this.

First the slugs meet, they run each other around and work up a bit of a lather, producing loads of mucous, then one slug initiates the act of ‘going upstairs’. They climb up a vertical surface, still chasing each other. When they find a mutually acceptable spot for their affair they start tickling each other with their tentacles. As they get more and more involved with each other they continue to ooze slime and further entwine until they make the ultimate lovers’ leap!

A lot of people will throw down slug pellets and slowly turn the subject of this chapter into a slimy lifeless blob. This may get rid of the nightly nibblers of prized seedlings, but for every slug maliciously murdered in this way, I bet there are several other animals going hungry that we would rather like to see in our gardens.

Not every animal hates slugs: some positively love them. And remember that any slugs and snails that are slowly dying can be consumed by other animals. By using chemicals and poisons on any pest, you could also be poisoning animals you would love to have in your garden.

One of the top mollusc mashers leaves vital clues at the scene of the crime. Shards and splinters of snail shells scattered around prominent rocks, stones, paths and patios are the work of the Song Thrush. This bird uses ‘anvils’ to dash the snail’s shell to pieces and get to the succulent morsel inside. Both Mistle Thrush and Blackbirds also put away large numbers of slugs and snails. They just haven’t invented such an effective way to get the wrappers off.

Look under corrugated tin sheets, often the home of Field and Bank Voles. Here you will find neatly piled clusters of snail shells, stacked like broken crockery. Shrews and Hedgehogs will also snack on snails. Toads also provide a great slug removal service, as does the Slow Worm.

All of the above slug guzzlers will eat the animals whole, but the last thing a slug wants to see is a Ground Beetle. These have vicious mandibles that can slice open a slug or crunch through a snail shell like a pair of bolt cutters.

Even worse is a Glow Worm larva. These unusual beetles prey almost entirely on slugs and snails. They repeatedly stab their mollusc victim with venom-laden mouthparts, and when it is not able to run away or even twitch, the larvae eat the animal alive.

Slugs and snails can defend themselves pretty well from all these attackers. The obvious defence for a snail is to withdraw its soft parts into its protective shell. If you pick up a snail and poke it, eventually it will belch forth an impressive quantity of bubbles. This sticky green froth gets everywhere and will certainly put off many predators that might otherwise crunch the shell.

Slugs, despite appearing quite vulnerable without a shell, are not as defenceless as you might at first think. They can hunch up into a ball shape in seconds, withdrawing their tentacles and exposing large hump of its mantle, which is covered in a thicker leathery skin.

This posture also has the effect of making the slug harder to pick up or bite into, especially when it starts to produce a thicker slime in its defence. Try picking one up and you will find the goo is like glue.

I once saw a Blackbird pick up a Great Black Slug but it soon lost interest and spent the next five minutes trying to wipe the mucous off its beak.

This same species of slug also starts rocking when it is threatened. Quite how or why this is scary I’m not sure but it certainly looks fairly funny if you get a chance to see it happening.

YOU WILL NEED:

small piece of clear glass or perspex • a slug or snail • misting spray of water • magnifying hand lens

Take a snail or a slug – it doesn’t matter which. If you are not too keen to touch them you can use a couple of spoons to pick up most species and avoid getting sticky fingers. Place your chosen animal onto the piece of horizontal perspex or glass and wait for it to start moving. This sounds easier than it actually is. If after a few minutes your gastropod refuses to emerge from its shell, or even shake a tentacle, it is time for you to get persuasive!

Most slugs and snails behave like this when the air is too dry for them. You can reassure them that conditions are good by lightly spraying them with water. (Run your fingers through a wet nail brush to produce a fine mist of water if you don’t have a spray-mister.)

When your mollusc is moving gently lift up the clear sheet and watch from below. You should see dark and light bands moving along the underside of the foot. These are bands of muscles, the dark ones are raised and ‘stepping’ forward, the pale bands are in contact with the surface. But the mollusc wouldn’t get anywhere without the essential ingredient – slime. Glands on the underside of the snail produce loads of this stuff, which lubricates the ground and allows the mollusc almost to surf along through the garden.

The first bit of this next experiment needs you to be a top tracker. You have to find a ‘roost’ of snails. Garden Snails are the best, because they are big, darkish in colour and common. Think about what a snail needs: shelter from predators and the sun. Look under piles of rocks and logs, walls, flower pots, overhanging plants, even ivy.

Once you have found your snails, make a note of where they are and then using enamel paint, mark each individual snail’s shell with a number, making sure you do not get any paint on their soft parts.

Return the next day and see how many are still there. Mark any new ones. You can even look for other day-time hide-outs in the same garden or park and mark these with another colour. Do the snails mix up or do they return to the same place? If you do this well, soon you will know all the snails in your own patch personally, where they live and how many there are. If you are really keen, you can go out at night with a torch and look for them, plotting their positions on a map so you can work out how far they travel during the night.

		Garden Snail A very common snail, second only to the rare Roman Snail in size. Its large appetite for garden plants makes it unpopular but it is beautiful to anyone prepared to look at it the right way.
		Strawberry Snail A very common small snail found all around gardens and in greenhouses.
		Door Snail Many snails have a corkscrew swirl to their shells. The Door Snail is so called because of a ‘door like’ mechanism that shuts the shell.
		Great Pond Snail The commonest water snail in areas of hard water, found in ponds, ditches and lakes.
		Great Black Slug This handsome beast is usually glossy black, but it also comes in a bright orange form. A fully grown one will reach 15 cm in length.
		Hairy Snail Yes, snails can be fluffy!
		Ramshorn Snail One of a large group of molluscs with flattened shells, which look a bit like the coiled horns of male sheep.
		Shelled Slug Quite common, but rarely seen, this slug spends most of its time underground chasing worms and other slugs. It has a tiny shell.

YOU WILL NEED:

blender • lettuce or grass or cornstarch • cuttlefish bone (from a pet shop) • small piece of perspex or glass • paint brush • snail

There are lots of different ways of seeing a slug or snail’s radula, but you will need a good magnifying lens and a cooperative snail to get a really good look. The best chance is with our old friend the Garden Snail. Not only has it got a big mouth, it will eat almost anything.

First of all you need to tempt your snail to eat by making a tasty soup. I’ve tried lettuce, which works well, but you could experiment with other greens. Place the lettuce in a blender and turn it into a runny liquid, add a bit of water and a bit of chalk in the form of cuttle bone (snails need the calcium for their shells).

Using the paint brush, paint this liquid onto one side of the clear perspex or glass and leave it to dry. Repeat this a couple of times to build up a thicker layer.

If you grow pond snails in a small aquarium or tub that is kept under bright light and make sure they are fed well, they tend to grow quickly and their shells tend to be thin. This is handy for the snail enthusiast. Use a magnifying lens to see into the shell of a living snail and you will notice the mantle cavity and beating away next to it the two chambers of the heart.