Invasion of the Land

Sir David Attenborough © BBC

"The tiny creatures of the undergrowth were the first creatures of any kind to colonise the land. They established the foundations of the land's ecosystems and were able to transcend the limitations of their small size by banding together in huge communities of millions. If we and the rest of the back-boned animals were to disappear overnight, the rest of the world would get on pretty well. But if the invertebrates were to disappear, the land’s ecosystems would collapse. Wherever we go on land, these small creatures are within a few inches of our feet – often disregarded. We would do very well to remember them."
David Attenborough.

People often talk about the age of the dinosaurs, the age of the mammals or even the age of humans, where these groups of animals are thought of as ‘ruling the land’. In fact dinosaurs, birds, reptiles and mammals have only been able to survive and flourish thanks to the invertebrates. The invertebrates are the true ‘rulers of the land’ and have been since they first emerged from the sea about 450 million years ago.

Invertebrates are everywhere, and in vast numbers. There are an estimated one million known species on the Earth, plus another nine million that are yet to be discovered or described by science. Of all the animals on this planet 98% of them are invertebrates. They inhabit every environment from the bottom of the deep ocean to the peaks of the highest mountains, from freezing polar regions to hot, dry deserts.

Invertebrates are essential. They are the unsung toilers of the planet, they are the small things that run the world. Invertebrates perform most of the ecological tasks that we and all other animal species (and many plants) depend. They pollinate our crops, decompose our waste, maintain our soils, and are food for many mammals, birds, reptiles, amphibians and fish. Everything from the food we eat, the fish we catch, the birds we see, the flowers we smell and the hum of life we hear would all be lost without invertebrates.

Thea vigintiduopunctata © Dragisa Savic

In the series Life in the Undergrowth David Attenborough will concentrate on the terrestrial (land-living) invertebrates, and so will we in these accompanying web pages.

Programme 1 Invasion of the Land features slugs and snails, springtails, and centipedes and millipedes

Snails and slugs

Snails and slugs belong to a group of invertebrates known as Mollusca. This group also includes the bivalves or clams (cockles, mussels, oysters and scallops), and the cephalopods – octopus, nautilus, squid and cuttlefish.

Desmoulin`s whorl snail (Vertigo moulinsiana) © Roger Key

The bodies of molluscs are usually covered by a protective shell. Although this is absent in most land slugs, and sea slugs (nudibranchs). In the cephalopods the shell is inside the animal’s body rather than outside. The shell can offer protection from being squashed, protection from predators, and protection from desiccation (drying out).

Molluscs are found in the sea, in freshwater (including lakes, rivers and streams), and on the land. Within these environments they have exploited a wide range of habitats and evolved many different lifestyles. In the UK there are just over 200 species of land and freshwater molluscs, and many more in the sea.

Snails and slugs are members of the gastropod family – this name means stomach foot, they ‘walk’ on their bellies! They move by producing waves of muscular contractions with their foot, which is lubricated with mucus and leaves a characteristic slime trail.

Discus rotundatus © Roger Key

Image problem?

Snails and slugs have a bit of an image problem, especially with gardeners. However, only a small minority will eat healthy plants and have the potential to become garden pests. Most snails and slugs feed on dead and rotting vegetation, fungi, algae and lichens; a few are carnivorous (meat-eating).

Snails and slugs are incredibly important for a healthy environment. Through eating dead and rotting plant and animal material they play an important part in decomposing waste and in the recycling of nutrients. They are also food to many other animals. Many of the birds we like to watch feast upon slugs and snails. Song thrushes regularly eat Brown- and White-lipped snails (Cepea species). Fragments of brightly coloured yellow pink and banded shells are often found around ‘thrush anvils’ where the birds have bashed the snails open on stones. Water birds such as Moorhens and ducks eat water snails, as do several water beetles. Frogs, toads, slow worms and hedgehogs feed on slugs.

Invertebrates in Danger! Roman Snail

Roman Snail (Helix pomatia) © Dragiša Savic

The Roman snail Helix pomatia is our largest snail, it was introduced into the UK by the Romans. The reason the Romans brought it to this country was for food, in many parts of Europe (such as France and Italy) it is a gastronomic delicacy. Unfortunately this has lead to over collection and over exploitation of wild populations of this snail. The Roman snail is now endangered throughout its European range (it is listed in Appendix III of the Bern convention and on the IUCN Red List). In 2008, following a Buglife campaign, wild Roman snails were given legal protection from collection, killing and trade in the UK.

STAR FILE: Leopard slug

Leopard Slug (Limax maximus) © Roger Key

The Leopard slug Limax maximus is featured in programme 1 of Life in the Undergrowth. It is found in the UK and with its pinkish grey body and black spots it is one of our most attractive slugs. This is a fairly big slug for this country, they can grow up to 16cm long, and they can live for seven years or more. Leopard slugs are widespread and common throughout most of Britain and Ireland and can often be found in woodland and gardens.

The unusual mating behaviour of Leopard slugs is featured in Life in the Undergrowth. A pair of amorous slugs descend from a branch on a cord of mucus while twisting together in a mating dance.

Mating Leopard slugs © Dragiša Savic

Millipedes and centipedes

Millipedes and centipedes belong to a group called the myriopods. They inhabit the land and are secretive animals living in soil and leaf mould, and beneath stones, logs and bark.

White-legged snake millipede (Tachypodoiulus niger) © Roger Key

Millipedes have many legs, but not a thousand as their name suggests. Millipedes have long segmented bodies with two pairs of legs on each segment, hence the scientific name Diplopoda (diplo = two, poda = legs). Most millipedes feed on decaying vegetation. Compared to centipedes they are slow moving, their bodies are adapted for pushing through the soil or under stones. When threatened, most millipedes are able to release substances that taste or smell foul as a defence.

Centipedes have one pair of legs per body segment and are better adapted for running. They are predatory animals and have a large pair of poison claws for seizing and killing prey. Centipedes eat other invertebrates.

Centipede (Haplophilous subterraneus ) © Roger Key

FACT FILE: Bulldozers, tunnellers and wedgers

As mentioned above, millipedes have bodies that are adapted for pushing through soil or under stones. Their body forms are similar to bulldozers, with lots of short legs, ideal for forcing open crannies in soil and leaf-litter. There are several variations on this theme:

• Ram bulldozers. The snake millipedes tuck their head down and ram their way forwards on lots of closely spaced very short legs. The trunk of the cylindrical body is flexible, tough, yet sufficiently as not to be compressed (the double segments give strength and leg-power).
  
• Pill bulldozer. The pill millipede (Glomeris marginata) has short a hemi-spherical body and rather few legs. This is fine for rolling up into a defensive ball but serious burrowing is out of the question. These live in leaf-litter that is not over-compacted.
  
• Tunnellers. The pointed-head millipede (Polyzonium germanicum) forces its small head into crannies, then uses its muscles to pull in the progressively widening front body segments until the tunnel is forced wide enough for the rest of the body. The trunk 'segments' are rather more compressible for push-pull action.
  
• Wedgers. The flat-backed millipedes use their flat heads to lift open cracks in the soil or leaf litter and then work their flat body into crevices by opening up a wedge-shaped space. The legs are somewhat longer than other millipedes since less pushing power is needed in what is generally rather loose material.
  

Pill millipede (Glomeris marginata) © Roger Key

Springtails

Springtails are also known as Collembolans. They are primitive six-legged relations of the insects. Both collembolans and insects belong to a larger group called the Hexapoda (hex = six, pod = leg).

Collembolans Priosotoma minuta © Steve Hopkin

Springtails can be found all over the UK, however you may not have noticed them as they are incredibly small, as you will see from the programme they can be the size of a pin head! These beasts are so small they can utilise a wide range of tiny habitats. Some live on the underside of leaves, others live in the gutters of people’s houses, many live amongst leaf litter. Many springtails are beautifully coloured (white, yellow, orange, metallic green, lavender, red) but, because they are so small, this detail goes unnoticed by us humans.

Amazing athletes
Springtails are amazing athletes. They can jump incredibly high. If under threat from a predator they have an interesting escape mechanism, they use a special spring loaded organ to catapult themselves high into the air. This organ is a forked, tail-like structure (called a furcula) which is folded under the abdomen when the springtail is at rest. The furcula is held in place by a clasp like structure called a tenaculum. When the tenaculum is released the furcula thrusts downwards and backwards firing the springtail upward.

Springtails are masters of the high jump. A springtail 5-6mm in length can jump 75-100mm, that is the equivalent of a person jumping over the Eiffel Tower!! Some springtails that spend their lives floating on the surface of water can even jump on the water!

Dicyrtomina saundersi © Steve Hopkin