By PATRICIA CLEVELAND-PECK

AS we look up into the clear blue sky, the air is filled with literally myriads of delicate, orange-and-black creatures. On the ground they are so thickly gathered to drink at every stream and puddle that it is hard not to tread on them. At first I think the great protrusions from the fir trees, which our guide insists on calling "nests", are dead leaves. But gradually I am aware that, on the contrary, not a leaf is visible. The cluster is solid butterfly. Individuals detach themselves as the sun warms them and flutter down, as Malcolm Lowry put it, "like torn up love letters".

Michoacan, in central Mexico has a considerable number of natural wonders, perhaps the most unusual being the sight of the migrating monarch butterflies. That this is really something special has been endorsed this year by the placing of the Monarch Butterfly Biosphere on the Unesco World Heritage List.

The butterflies which congregate high in the forests here each winter have probably been making their way here since the Pleistocene era, at least 10,000 years ago. But it was not until they were tracked by the zoologist Fred Urquart in the 1970s that it became apparent that these delicate insects make a 3000-mile trip from eastern Canada and North America (those from the west of the Rockies migrate to California) in what must be one of the most arduous endeavours in the insect kingdom. Equally amazing is the fact that the butterflies which return northwards are, in the main, the offspring of those which made the original journey. In turn, the ones which return to Mexico the following autumn, are the third-generation offspring - guided only by instinct, Earth's magnetism and wind currents.

So it was with great excitement that I found myself setting off to witness this phenomenon. First we drove to the old mining town of Angangueo from where we hired a local driver and high-suspension vehicle (of indeterminate vintage and utterly devoid of anything resembling a safety belt) to take us up 12 kilometres of precipitous, bumpy mountain track to the El Rosario Butterfly Sanctuary. As we ascended through Alpine-type scenery we were afforded not only spectacular views but also glimpses of the tiny farming communities which eke out a living in these highlands. Arriving at the sanctuary, a collection of newly refurbished buildings, we learned that there was a farther steep climb of two klometres on foot to where the butterflies congregate. As we were already at 3000 metres, the air was thin, so it was quite a struggle, even for the younger members of the party.

But it was worth it. Here we are, at the top, looking up at these amazing creatures as they fill the air above us, cling to the oyamel fir trees and carpet the forest floor. Their story is moving. Some 300 million make this journey. They fly by day at an average speed of 20-km per hour over deserts, mountains, cities and motorways. Many starve or are buffeted to death on the way - those that survive conserve their energy by clustering together, eating and drinking little. Even so, many are dying here before our eyes. Mating uses up the last of the male's energy. They all die - as do many of the females when they've laid their eggs under the leaves of the milkweed plant. After 10 days, the caterpillars hatch and, once developed, spin their silk cocoons and begin the metamorphosis which will take them back north as the next generation of butterflies.

To see them is an unforgettable experience. But if you want to do so, come quickly. Attempts are made by the Monarch Butterfly Biosphere Reserve, via notices and word of mouth, to emphasise the fragility of this environment and its need for protection. But the authorities have a difficult act to balance here if tourism is not to kill the thing it loves. To some extent, the difficulty of the terrain may preclude mass tourism, but that visitors are sought is indicated not only by the fact that Michoacán has adopted the monarch butterfly as its symbol but also by the investment in big, new interpretative building and retail outlets at El Rosario.

Another threat to the butterflies' habitat is illegal logging of the forestry in which the butterflies shelter. But many local people are unhappy that they are now forbidden to cut down trees. They, therefore, feel they are entitled to a piece of the tourist cake, too: as we ascend in our rattletrap vehicle we come across two women who have thrown a rope across the track and are demanding a few pesos "toll" to pass over their land. Our driver pays but comments that the cost of this journey has almost doubled since last year.

There is, of course, much more to Michoacán than butterflies. Morelia the state capital, declared a "national treasure" and also on the Unesco World Heritage List, is a pretty colonial town with convents, churches and palaces built of a rose-coloured cantera stone. The lakeside town of Patzcuaro is equally attractive but with a quite different atmosphere; here are whitewashed adobe buildings with red paintwork, a pretty square with portales and a lively market. On the lake is the island of Janitzio, famous now for its Day of The Dead ceremonies.

NEED TO KNOW

Patricia Cleveland-Peck travelled with Journey Latin America, 020 8747 8315 or 0161 832 1441, www.journeylatinamerica.co.uk.

A 10-night tailor-made holiday to Mexico's colonial heartland starts from £1215 per person (based on double occupancy), including all transfers. This itinerary includes first-class hotels and a visit to Mexico City, Cuernavaca, Morelia, Patzcuaro, Guanajuato, San Miguel Allende and Queretaro. A side trip to the El Rosario butterfly sanctuary can be added to the itinerary in season (November to March).

Travelling independently, return flights from Edinburgh or Glasgow via Amsterdam are from £666 per person, including taxes. El Rosario Sanctuary can be reached from Anganguero, Ocampo or Zitácuaro. The Serra Chincua sanctuary can be reached from Anganguero. Here the hike is three kilometres. It is longer but gentler than El Rosario. Horses are sometimes available. Hotels and local tourist offices can arrange local guides and vehicles.

The four-star Hotel Rancho San Caytano at Zitácuaro is situated in 13 acres of woodland garden, and has a swimming pool, nine double rooms and three small cottages. Horseback expeditions to the butterflies can be arranged to Cerro Pélon sanctuary. Check www.ranchosancayetano.com for more information.

February is the best time to see the butterflies, but the sanctuaries are open from mid-November to mid-March from 9am to 5pm. When visiting the butterflies it is advised to set off early in the morning, so aim at staying near the sites. The butterflies do not fly in cold or wet weather.

CARBON FOOTPRINT: Approximately 1.8 tonnes of C02 on a return flight from Glasgow to Mexico City via Amsterdam.