Imong stops many times along the trail to make observations. He points out a tree with bark that burns even when wet and another known as the joss-stick tree, whose incense was traded for porcelain with Chinese traders who visited the coastline three centuries ago. He plucks a particularly unspectacular leaf from another bush and announces that it is widely used to increase male potency.

Perhaps this leaf is favoured by the male proboscis monkey which boasts a permanent erection, even while leaping around the slender branches of the forest canopy, no doubt hoping to encounter a female from his sizeable harem. Then there's the python fat vine which drips a seemingly endless, potable flow of water when cut. Not that there's a shortage of water. Rainforest is aptly named; it rains an awful lot here, which means Sarawak is latticed with rivers curling their way to the sea. Until recently, all transport was waterborne.

AT Batang Ai, four hours' drive from Kuching, Sarawak's historic capital city, the Hilton group has built a hotel on the shores of a 90-square-kilometre lake formed by a massive hydro-electric project. It has been designed, somewhat incongruously, in the form of a longhouse - which is rather like a village under one roof, and the traditional communal dwelling of river tribes in this region. From the luxury of the hotel, a two-hour ride by longboat takes tourists through spectacular scenery to a genuine Iban longhouse at Sumpa, where they are greeted by an aged, tattooed headman and several naked children leaping into the muddy river screaming with delight.

The longhouse is 100 metres in length - but pacing it from end to end is believed to bring bad luck to the community. Visitors will be offered copious amounts of the local alcoholic jungle juice,tuak, but take care not to drink too much, as it will give you a bad headache. Then again, you can't refuse it altogether.

Only the T-shirt and the outboard motor have changed the lives of these people who, 100 years ago, were still hunting human heads. It was, hard though it is to believe, a great honour to have your head lopped off and carried home by a tribal warrior. The forest gods would be appeased, the young man would have a fine trophy to present to his bride-to-be and the victim's skull spirit would be revered as it hung, slowly growing black in the hot air of the perpetual fire of the longhouse.

Nowadays, the Iban no longer hunt heads, having been converted to Christianity by the 19th-century missionaries who followed James Brooke, the young English naval adventurer, the protagonist of one of Asia's most romantic colonial stories. He arrived in Kuching, and on behalf of the ruling Sultan of Brunei, managed to quell the endemic piracy and the revolt along the Sarawak river. The Sultan rewarded Brooke by ceding Sarawak to him in 1841; the state was governed by his family for the next 100 years.

Next to the Sumpa longhouse, a travel lodge has been built by Borneo Adventure, one of Sarawak's leading eco-tour operators, where visitors can stay the night before returning to the luxury of the Hilton longhouse. The round trip can be done in a day, including a picnic of barbecued silver catfish, rice cooked in a young bamboo stem and steamed bamboo shoots hacked from the forest floor by the longhouse shaman, who is also the cook and tuak barman.

PEBBLES were speeding past in the clear shallow water and on the river bank roots clung for survival, looking like the nerve endings of the earth. Imong picked up the smell first, long before our city-dulled senses were aware of anything different in the air. Five minutes later, we rounded a bend in the narrow stream and the rest of us caught a whiff of what he was getting so excited about: a hunter was singeing the carcasses of a wild boar and a barking deer on a fire before taking them back to the longhouse.

In former times deer used to gather in the caves at Mulu, one of the largest known underground cavern systems. Local tribes had known of the caves' existence for centuries but it was not until the 1960s that the area began to be seriously explored by scientists. What they found was the largest natural cavern in the world, known as Sarawak Chamber. Its limestone roof spans a staggering 16 hectares, which could comfortably house 40 jumbo jets with room to spare. Unfortunately, it is too difficult to reach for most tourists, and is far too large to be lit (it had to be measured by sonic apparatus). Deer Cave, however, can be visited.

One of four spectacular caverns in the Gunung Mulu National Park in north-east Sarawak, bordering Brunei, Deer Cave can be reached from the oil and timber town of Miri by a 20-minute flight or an eight-hour drive. Its entrance is not particularly impressive but once inside, the limestone roof draws back into an enormous vault, 120 metres high and 150 metres wide. This one could, apparently, accommodate London's St Paul's Cathedral five times over.

Millions of wrinkle-lipped bats live inside and the acrid smell of their droppings permeates the air. At dusk, the bats leave home to feed outside in the dark, forming a swirling ribbon which continues for half an hour. Each bat will fly 100 kilometres and consume precisely 10 grammes of insects before dawn: any less and it would be too weak to fly home; any more and it would be too heavy. That's three million bats and 30 tonnes of insects. And each day a few more tonnes of droppings descend from the roof to add to the piles on the floor, which teems with millipedes, crickets, spiders and other cave creepies.

The revelations which unfurl from a trip to Sarawak are boundless and varied. Some are deadly serious: could the smooth bark and leaves of the bitangor tree really provide the cure for AIDS, as is hoped by scientists currently conducting laboratory tests at the US National Cancer Institute in Maryland? Others are trivial: the word Kuching, the capital, means cat in Malay. No one is sure why but it is often supposed that it was named after mata kuching (cat's eyes), a fruit closely related to the lychee, which once grew profusely in the area.

Today, Kuching is a colourful, easy-going river city with, among other things, a museum devoted to cats. Here you can learn about the world's great cat lovers: Florence Nightingale owned 60; Churchill's ginger attended wartime cabinet meetings and Ernest Hemingway wrote that he liked cats 'for their absolute emotional honesty'. Learn the difference between a Japanese bobtail, shorthair and a Maine coon. Read about Towser, who had dispatched 25,726 mice by his 23rd birthday, and the four-month-old kitten who, in 1950, followed a team of climbers 4,500 metres above sea-level to the summit of the Matterhorn.

You could dine out on Sarawak for months.

HOW TO GET THERE MAS and Dragonair fly a joint service non-stop to Kuching twice a week. Round-trip economy packages, including three nights' accommodation (extendable) start at $4,470.