Desire takes to the air - by the million

If, in the excited days coming up to the eclipse, Dubliners found themselves consulting the sky more keenly than usual, they may also have found themselves spying on another little wonder of nature - one that can be relied upon to happen around the start of every August, rather than once in a lifetime.

It is signalled by flocks of black-headed gulls, circling high over the city, by the hawking of swifts in among them, and also, perhaps, by squadrons of swooping starlings. All are feasting on columns of winged ants, soaring up for a sexual orgy more daintily known as the "nuptial flight". Glance around at the ground at this time and you may catch the glitter of tiny wings as black garden ants, Lasius niger, pour out of their underground nests to take off.

The wonder is not so much in the flight as in its extraordinary synchrony: the way in which millions of ants from thousands of colonies of a particular species make their winged emergence during the same two or three afternoons, usually late in July or early August. They are triggered by an internal body clock, along with climatic conditions: low windspeed, high humidity, and a balance of warmth between the nest and the air just above the ground.

In city parks and suburbs the black ant is generally the common one, while in the countryside it is mostly the yellow meadow ant, Lasius flavus, that spirals up above the fields. Much earlier this century, the naturalist Robert Lloyd Praeger took notes of an experience with winged yellow ants one August day, as he walked into Blessington, Co Wicklow:

"They became more abundant on nearing the village, and I roughly estimated that on the last half-mile of road over a million-and-a-half of ants were crawling, while in the air above the road they were so abundant that my clothes were quite brown with them.

"The fields on each side were thick with them, the windows of the hotel were alive with them, and my tea was richly flavoured with them. Unless they were confined to a narrow line down which I had the fortune (?) to walk, there must have been hundreds of millions of them within a mile of Blessington."

Such an awesome swarming is less probable today, given the changes in our grassland. Yellow ant colonies, left undisturbed, carry up particles of soil in their mandibles to make free-draining mounds that absorb the sun's heat and help the incubation of the brood. A study on the Gower Peninsula in Wales found almost 500 ant-hills in 1,000 square metres.

But this sort of density is achieved only on old, undisturbed and permanent grassland with a good diversity of plants and insects. Today's intensive silage fields, ploughed and reseeded with a rye-grass monoculture and compacted by farm machinery, are no landscape for ant-hills. Only in places such as the Burren is there the ecological stability that encourages long-lived ant-colonies, their dry mounds of fine soil covered in summer with purple-flowering thyme.

A colony of ants grows from the eggs laid by a mated queen. These hatch initially into workers, a fraction of her size. Like their fellow Hymenoptera, the worker bees, their job is to forage outside, and to feed and look after the larvae. As the colony matures in size, it stops producing more workers and prepares to reproduce itself sexually by producing winged ants - both males and virgin queens. As the big day approaches, they are not allowed out of the nest until the workers have tested the earth and air and found conditions just right.

The males have no other purpose than to copulate high in the sky - a once-in-a-lifetime fling. The queens, too, have only the one maiden flight, and may mate more than once and with different partners. When they spin back to earth and cut off their wings, they have stored enough sperm to keep laying thousands of eggs underground for the next 10 to 20 years.

The synchronised flights within a species may help to prevent in-breeding. And millions of ants on the wing give the queens the best chance of escaping the birds. In some species, mated queens help each other for a time in setting up a new home. The males, predictably, are dead within hours.

City suburbs, with their concrete paths and walls, are rich in sun-warmed nest sites, and this can bring the black ant, in particular, wandering into human homes. It likes sweet things, and in nature licks the anuses of aphids to obtain "honeydew", the excess plant sugar these sap-sucking insects excrete - indeed, ants herd them like livestock and even thatch them little shelters at the foot of trees and plants.

As most children seem to know, the black ant doesn't hurt you, but its red-brown, narrow-waisted neighbours in the garden, the Myrmica species, do have an irritating sting. Our biggest species, the wood ant, Formica rufa, can squirt formic acid from its rear end to immobilise its prey, but it is still an insect we should hope will increase with the progress of global warming. It is presently scarce throughout Ireland and little is known of its distribution: only in the ancient woodlands of the Killarney valley have its prominent nests come under notice.

These are big mounds of leaves (or, in conifer forests, pine needles), sometimes looking like the thatched roofs of early round-houses and built at woodland edges and in clearings. As Terry Carruthers reports in his Natural History of Kerry, the Killarney ants nip off the ferns which might shade the mound and so lower the temperature in the nest.

A big mound can hold 100,000 individuals, and while the wood-ant may use trees as aphid-farms, it clears woods and forests of immense numbers of other pests. Some European countries actually protect the species by law. Coillte, too, may come to welcome the ant as a biological control that costs nothing.