The value of worms and a belted beauty

Athayde Tonhasca is a specialist adviser at Scottish Natural Heritage with a particular interest in invertebrates. Here he takes a look at some of the less glamorous species of the natural world – earthworms, and a rather curious moth.

In 1881, a few months before his death, Charles Darwin published his last book: The formation of vegetable mould, through the action of worms. The book was a huge success, selling 6000 copies in the first year; more than On the origin of species when it was first published. In the book’s final paragraph, Darwin said:

The plough is one of the most ancient and most valuable of mans (sic) inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earth-worms. It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organised creatures.

To most people, Darwin’s work on earthworms was a revelation about the importance of these discreet and humble animals. And his enthusiasm was well-justified; because of their burrowing, mixing and fertilizing, earthworms are vital to the process of soil formation, and consequently vital to plants and every organism that depends on them.

Earthworms also help maintain the structure of our soils; their burrowing creates tunnels that channel air, water and nutrients into deep layers of the soil. These burrows loosen up the soil to facilitate penetration by roots, and reduce runoff, thus decreasing the rates of erosion.

Different species (there are 26 in the UK) can be divided into three groups, based on their ecology and behaviour:

  • Epigeic earthworms (epigeic: refers to an organism that is active above the soil surface, from epi, meaning ‘above’ and geic meaning ‘earth’). These species live among leaf litter on the soil surface and usually do not to make burrows. They play an important role in decomposing leaves and plant matter on the ground.
  • Endogeic earthworms (endogenic: related to the interior of the earth). These worms live and feed underground, making burrows parallel to the surface. They eat only buried organic material, such as dead plant roots. As they spend their lives unexposed to light, they lack skin pigments and are usually pink, gray or white.
  • Anecic earthworms: These are the deep-dwelling species, and the most familiar types. They make build vertical burrows that can reach several metres down, into which they drag plant and other plant matter to feed. The common earthworm is their best known representative and the largest British earthworm.

By eating soil, dung, plant litter and other materials (depending on the species), earthworms break down organic matter into smaller fragments, helping bacteria and fungi decompose them and release their nutrients.

Earthworms are able to process 2–20 tonnes of organic matter per hectare each year, a digested volume that ends up as castings (worm excrement). Darwin calculated that in 10 years, worm castings from an acre of soil (0.4 ha) would form a 5 cm-thick layer of top soil (what he called ‘vegetable mould’). Castings are rich in nitrogen, phosphorus, potassium, magnesium and calcium, all minerals essential for plant growth, and also contribute to the physical properties of the soil.

Common earthworm (c) Laurie Campbell

Common earthworm (c) Laurie Campbell

With their relentless activity, earthworms gradually deepen the topsoil layer, mix soil and organic matter, increase soils capacity to absorb and hold water, and increase its fertility. Essentially, earthworms change the earth. It’s no wonder that Darwin called them ‘nature’s ploughs’.

Earthworms are something most of us are more likely to see than the belted beauty moth, which recently caused a deal of interest at our Rum National Nature Reserve. The female is particularly interesting for being wingless: and the belted beauty is one of several moth species characterised by reduced wings (brachyptery) or no wings at all (aptery).

Belted beauty

Belted beauty

Insects appeared in the Early Ordovician period (~479 million years ago), but the explosion of diversity that made them the dominant creatures on Earth happened when they acquired the ability to fly, thus becoming capable of exploring a three-dimensional world.

From then on, insects could occupy diversified niches, disperse and search for food more efficiently. In fact, it is believed that wings have contributed more to the success of insects than any other feature.

This raises the question of why the belted beauty, and some other moths, were selected to lose their wings or the ability to fly. This is a question that has puzzled biologists since Charles Darwin mused over the problem.

The answer may be related to the huge energetic cost of flying; flight muscles comprise 10-20% of the insect’s body weight. If flying does not give the species significant advantages, the energy required to sustain it could be diverted to some other function – such as producing more eggs, for example.

In fact, flightless insects are most often found in stable habitats (where dispersal is not necessary for the survival of populations), isolated areas (such as montane and coastal strands), and in areas where a great amount of energy is required for flight (for example, cold regions or areas subject to strong winds).

Conservation of energy in cold environments and adaptation to strong winds seem plausible explanations for the loss of wings of female belted beauty moths, but we can’t know for sure. What we do know is that the species is perfectly adapted to its environment. Wherever it occurs in Scotland (the west coast, Hebrides, Mull, Iona, Colonsay and Islay), females crawl by the dozens at the top of fence-posts, to disperse pheromones into the wind and so attract mates.


Further reading :

OPAL, a citizen science initiative, is open to all of those who would like to record their sightings.  Find out more @ .

By taking part in the above survey, you’ll help improve our knowledge of earthworms and the soils they live in – something we still know surprisingly little about. There is also a chance to learn some fascinating facts … for example it is often a surprise for people to learn that a hand full of soil has gone many time over through the gut of earthworns – they are the soil engineering, essential to healthy soil structure and areation.

Find out more about International Year of Soils 2015 at

And read all about the magnificent Rum National Nature Reserve at


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