Another Life: What can you do about the march of the millipedes?

A deep unease can be stirred in the Irish psyche by a novel and inordinate assembly of invertebrates. The swarming of bees is acceptable, because this is what they do – indeed, “a swarm of bees in June is worth a silver spoon” if you’re a beekeeper. And too many ants at once is usually one’s own fault for disturbing a nest.

But the march of the millipedes, gathering in phenomenal hundreds at dusk to climb the wall of one's house or even explore its interior, has a special potential for human aversion. Eye on Nature gets several pleas for advice annually, some from bungalow-dwellers in the west whose pebble-dashed gables are blackened at night by slowly undulating hordes of Tachypodoiulus niger. A caller to Derek Mooney's nature programme on RTÉ Radio 1 the other Friday found herself short of sympathy for her story of horror and disgust: why not just stay indoors after dark and watch telly?

Known simply as the black millipede (or, more scarily, as the white-legged snake millipede) T niger is among the largest and commonest of its family in Ireland, its narrow, segmented body rippling along on anything up to 400 closely spaced, very short legs to a maximum length of about four centimetres. Millipedes move slowly compared with their predatory centipede cousins, but T niger, in one fond description, "tucks its head down and rams its way forward" through the stones and litter of the soil surface.

Its role in life is as detritivore – a chewer of nature's dead leaves and algae, leaving the soil's microbes to finish recycling the nutrients. In tropical forests, where earthworms are few, millipedes are the main reducers of litter on the ground. Even when it's climbing trees, T niger is nibbling algae and lichens, and these flourish also on pebble-dashed bungalow walls in the moister west of Ireland.

This seems no explanation for the mass movement of millipedes, amounting to regular local plagues, that have been reported from all over Europe. In Scotland, in 2008, the conservationist John Muir Trust advised villagers at Sandwood in Sutherland, invaded annually by armies of T niger, to black out their windows after dark.

This “defence” was apparently inspired by experience in Australia, where an introduced black Portuguese millipede – a species we don’t have – has been a nuisance for more than half a century, even holding up trains by making the tracks too slippery. That must also smell horrible, as to squash a millipede is to release the repellent benzoquinones it uses as a defence.

In Poland it's the large, dark, red-striped millipede Ommatoiulus sabulosus (found in Ireland on dunes and moorland) that invades houses in spring and is classified as a pest. In 2002 it appeared in huge numbers in one big southern city, "causing panic among the inhabitants" as it wandered along streets and pavements and roamed into buildings. In Germany its mass assemblies, along with other millipede species, have often been concentrated at light-coloured house walls, often at dusk, in local swarmings repeated for several years.

So what is the common factor? Climatic conditions, mating behaviour and food shortage have all been considered. Population explosion may account for the numbers, but not for the lemming-like behaviour.

Damien Enright, my esteemed opposite number with the Irish Examiner, has been happy to accept the obvious, having watched T niger, over many springs, climbing up the side of his two-storey house in west Cork, crossing the roof, descending on the other side and trundling on down the garden. "If we left the front door and a back window open," he told his readers, "they'd march through."

The house, he conjectured, must have been built on an established millipede migration route – a thought suggesting some more eventful consequences of Ireland’s house-building boom. Migration following population explosions is one line of study, but it raises all kinds of questions about instincts, behaviour and compasses.

And the hedgehog? It’s the millipedes’ leading mass predator, quite ignoring the noxious chemicals, and thus to be encouraged and conserved. But hedgehogs, adept at climbing stone walls, or even stairs, cannot cope with sheer ascents of bungalow gables, even pebble-dashed for extra grip. If you absolutely must, take out the hoover.

Millipedes are among the most ancient of land animals, with species in thousands across the globe. They are not, however, insects, of which Ireland has well over 11,000. In a new book, Insects of Ireland (Collins Press, €14.99), the ecologists Stephen McCormack and Eugenie Regan have chosen more than 120 "most popular" insects, drawn from all the groups, with brilliant, often beautiful illustrations by Chris Shields. Backed by the knowledge in Ireland's biodiversity data banks and the authors' experience in the field, this is a timely, enjoyable and most welcome guide.