This primeval rain forest, inhabited by monkeys, parrots and wild pigs with tusks, had hardly ever seen a human before. That is, until scores of scientists armed with butterfly nets began beating the bushes for bugs in what is being billed as "the greatest insect expedition ever mounted."

Entomologists from 18 countries, along with a detachment of British soldiers on "adventurous training," are hunting insects here on Celebes, one of Indonesia's main islands, in a year-long expedition known as "Project Wallace."

The expedition, six years in planning, is conducting research into conservation, medicine, agriculture and pure entomology. The scientists and the soldiers also are studying and surveying the ecosystem of the 625,000-acre Dumoga-Bone National Rain Forest Preserve. With chunks of rain forests the size of Wales being destroyed every day in the world's tropics, scientists want to uncover their secrets before they all disappear.

The Royal Entomology Society of Britain is sponsoring "Project Wallace" at a cost of about $500,000, in commemoration of its 100th anniversary, and says it has gone into debt because of it.

But it is the most ambitious project the society, or any like it, ever has launched, and its members say the project, which ends in January, is well worth the cost.

"Let it not be forgotten that entomologists made the near-eradication of malaria and the control of the locust plagues possible," said Royal Entomology Society President W.K. Knight in the project's manifesto.

The scientists on this project are quick to point out that three out of every four animals in the world are insects, those six-legged creatures that we often do not see until they bite us.

Rain forests around here are also home to exotic birds and several species of four-legged creatures found only on this orchid-shaped island, including the babirusa wild pig, the black-crested baboon, the pygmy buffalo, the giant palm civet, and the gremlin-like tarsier hornbills.

The scientists believe this rain forest contains many new species of insects, despite the fact that entomologists say there are about 30 million species that already can be classified. Some of them will also be testing the theories of the project's namesake, Sir Alfred Wallace, who traipsed through this part of northern Celebes more than 100 years ago.

Wallace is best known for publishing an essay on natural selection and the survival of the fittest, based on his findings in the Celebes, a year before Charles Darwin came out with his landmark "Origin of the Species" in 1859. Entomologists believe Wallace gave Darwin the courage to come out with his own version of the controversial theory.

Wallace is also the originator of the Wallace Line, an imaginary boundary that runs through the middle of Celebes and between the islands of Bali and Lombok. Flora and fauna west of this line belong to an Asian group of species; to the east are those with Australian links. On the line itself, in this region of Celebes, Wallace discovered that the indigenous animals and insects are as widely divergent as in any two widely separated parts of the world.

Many of the scientists here are trying to find out how the Wallace Line affects their particular insect specialty by capturing the ordinary, the weird and the unknown. Diane Calabrese, a biology professor at Dickinson College in Carlisle, Pa., is one of eight U.S. entomologists on Project Wallace. With a grant from the National Geographic Society, she is studying "water striders," a kind of dragon-fly. But Calabrese professes a larger interest in the "rain-forest biosphere."

"The value of this kind of expedition is understanding how diversity is established in this kind of habitat," Calabrese said. "So if we ever have to reestablish it, or just preserve it, we know what sort of mistakes not to make. Understanding the diversity helps humans to understand their place in the biosphere better, and helps to promote conservation."

John Feltwell, author of several books on botany, left the expedition in June carrying huge crates of seeds from the rain forest -- 20,000 seeds from each of 40 different plant species. He is giving the seed harvest to the Royal Botanical Gardens in London for their seed bank of plant species from around the world, which they hold so they could regenerate most habitats in the event of holocaust or pandemic disease.

"Mankind is dependent on 30 species of plants, most of them grasses. Should there be a world pandemic of grasses, or cereal, then we would need to find or create other species pretty fast. That's what you do with this kind of gene bank," Feltwell said.

Hans Huijbrechts, an entomologist with the Netherlands Museum of Natural History, is the butt of many good-natured jokes at the spartan base camp the scientists and soldiers have set up by the Bone River at the edge of the rain forest.

Huijbrechts is studying the dung beetle, which collects the fecal droppings of monkeys and other rain forest wildlife, lays eggs in it, and then buries the dung ball. This, Huijbrechts said, plays a big role in fertilizing and regenerating the rain forest.

Huijbrechts has had great success baiting his beetle traps with human waste from the base camp, with the beetles coming in droves to bury the prime waste. The Dutch scientist has, on occasion, received unsolicited donations of "bait" from the soldiers.

George Else, an entomologist with the British Museum of Natural History, goes into the woods with the feverish excitement of a man who enjoys his work. He has checked all his traps, which look like small pup tents and have been placed high up in the branches of the trees and in the underbrush below. Hiking back to base camp, he suddenly drops to all fours, his butterfly net swooping down on his prey.

"Yes, I've got it. Lovely," he says.

It was a very large, nasty-looking wasp, tangled in the confines of the net.

Mike Wilson, also with the British Museum of Natural History, and Australian biologist Alice Wells prefer a more romantic method of capturing bugs, called "night trapping." They go out into the gathering dusk to find a remote area of the forest, where they shine a powerful ultraviolet light on a hanging white sheet. The insects come in a frenzy to light on the sheet, where they are sucked into a tube by a suction gun.

Entomologists have their insect phobias as well as fanaticisms. Wells, for instance, says she keeps a live copperhead snake in the backyard of her home in Victoria, but that she stomps on cockroaches every chance she gets.

The other U.S. scientists on the expedition include:. Lance Durden, Vanderbilt University Medical School;. J. Hayes, Texas Tech University;. J.B. Heppner, Bureau of Entomology for the State of Florida; M.J.G. Hopkins, the New York Botanical Gardens; D.H. Kistner, California State University in Chico, Calif.; J.T. Polhemus, University of Colorado Museum; and C.W. Young of the Carnegie Museum of Natural History in Pittsburgh.

The soldiers, including a half dozen women, often accompany the scientists on their hikes and outings, driving the Land-Rovers, carrying the gear and picking up a little field science along the way.

"This is what the British Army calls 'adventurous training,' " said Maj. J.J. Brown, the field commander for this middle phase of the expedition.

It is voluntary training for the soldiers. All participants are selected according to their skills and are expected to pay for part of their expenses on the expedition. The idea is to take the soldiers out of the ordinary military environment and give them something challenging, both physically and mentally.

The data and specimens from the project will take several years to compile and evaluate, the scientists say. The Indonesian government is expected to benefit from data about the rain forest; the country has the second largest reserve of tropical forest in the world, after Brazil.

But the highest hope of most of the scientists here is that their specimens will turn up a new bug, beetle or fly that will help to explain more about the origin, and especially the diversity, of species.