Originally published June 2012
Hello everyone! As hard as it is to believe, we are halfway through 2012 and, more to the point, we have not had a TQW ‘show of hands’ moment so far this year! So, in order to correct this situation, let us see who can link together, the founding father of evolution theory, an unlikely omelette and a creature with an admirably high number of body organs?
The answer is, arguably, the most important animal in terestrial ecology; our commonal garden Earthworm. I know that all you keen taxonomists will be crying out for some Latin at this point, but classification of these humble animals is not quite so simple. The Earthworm Society of Britain (yes, there really is one!) differentiates between four groups or ecotypes of Earthworm. Each one contains a number of Genera and several Species; they do not even belong to the same Family. However these wriggling wonders are all members of the venerated Sub-Order Lumbricina, of the Grand Order of the Haplotaxida.
The first group is very much associated with human activity as it is comprised of the Compost Earthworms; those worms that specialise in reducing our organic waste. These red, stripy worms reproduce very quickly and, given the right conditions, will turn a bin full of vegetable matter and old cardboard into a writhing, eco-factory of decomposition within a few days. The second group, the Epigeic Worms (from Greek epi, upon + gaia, earth) contains the non-burrowing Earthworms. The leaf litter is their domain; breaking it down whilst another group, the Anecic Worms (from Greek anekas, upward), transports it beneath the surface via their vertical chambers. The Endogeic ecogroup (from Greek endon, within + gaia) consists of the pale-coloured, deep-burrowing Earthworms who may never get to see the light of day.
Between them, the various Earthworm species carry out a whole range of vital tasks from the decomposition and movement of organic matter to the aeration and irrigation of the soil column. Organic particles and soil are siphoned in through the mouth and excreted at the other end as the familiar worm castes found on the surface of lawns in the morning. It was this mixing effect that led Charles Darwin to refer to Earthworms as ‘nature’s ploughs’. The great man both started and finished his illustrious career with in-depth studies of the creatures. These resulted in the publication of that well thumbed volume The Formation of Vegetable Mould through the Actions of Worms a year before he died in 1882. He estimated that in one year alone around fifteen tons of worm castings are brought to the surface on an acre of land; a ceaseless workforce breaking down and enriching our soil.
Physiologically an Earthworm can be described as a tube-like digestive tract encased in muscle. However they do have five pairs of aortic arches (hearts) pumping blood throughout the segmented body and several pairs of nephridia (a basic kidney) to filter out waste products. Worms have no eyes as such but can tell the difference between day and night due to a number of light receptors located at either end of the body. These sensors are not responsive to light in the red part of the spectrum; a fact exploited by anglers as they search for bait using red-lensed torches. Earthworms also lack ears, though their bodies are highly sensitive to vibration. The familiar ‘Seagull Dance’ is an example of a predator participating in a bit of ‘worm charming’ for their meal. This involves sending vibrations down through the ground and fooling the worm into thinking that a Mole is on the prowl. The fact that the worm will surface to evade the myopic digger says something about the Mole’s consummate hunting technique; it also suggests that Earthworms may be all heart but very little brain!
Earthworms are the food of choice for a great many animals, rich as they are in protein. Our local Brocks can cope with 200 or more in one sitting, turning a night ramble into a veritable feast. The worms have the disadvantage of having to surface to mate and are often snapped up mid hermaphroditic coupling. Those who survive the nocturnal rampages of the predators will go their separate ways where they discard a fertilised mucous tube. This dries out in the soil to become an egg capsule from which the mini worms are hatched. If an Earthworm manages to avoid being eaten by bird or beast, or impaled on the end of a hook, it can live up to eight years depending on species. However there is one predator we have not mentioned so far, chiefly out of consideration for those of a weaker disposition. Ask Ray Mears, Andy Mcnab or maybe even the Hairy Bikers how to make an Earthworm omelette and they will provide some survivalists’ cuisine that may appeal to Gollum but very few others!
So there is a very brief look at the role of the humble Earthworm in the scheme of things. As you stroll around our little reserve just consider how indebted we are to these night-crawling wrigglers as they continue to make the top few inches of the planet’s surface fit for us to survive upon.
I doubt whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures.
Charles Darwin (1809-1882)
Image 1: Earthworm Allolobophora chlorotica by Gilles San Martin, used under Creative Commons License 2.0 / Image 2: Compost with earthworms by SusanA Secretariat, used under CC2.0 / Image 4: Mole by Mick Talbot, used under CC2.0