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Now that Canada Day has passed, a dedicated band of volunteers are heading for field and fen to begin the enumeration of those Canadians who are members of the order Lepidoptera: The great butterfly census of July has begun.

In Prince Edward County, an island on the northeastern shore of Lake Ontario, someone will be sucking in his breath at a giant swallowtail, a bruiser with a 15-centimetre wingspan. They are the largest butterflies in Canada and the United States - ravishing black-and-yellow animals that graze among the goldenrod and milkweed. Until a few years ago, their Canadian range was a pocket of exotic forest in the extreme south of Ontario, but for a variety of possible reasons - warming climate; the spread of their larval food plants, the prickly ash and the hop tree - their range has drifted east and north. The census-takers will find out if they are ranging farther.

Some of those who study butterflies see a darkening future for the insects and, by extension, for all life. In Britain, where butterflies and moths have been studied more intensively than anywhere, an accelerating decline in abundance from habitat loss has led experts to extrapolate a dreadful cull of species generally.

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"If insects elsewhere in the world are similarly sensitive," nine scientists wrote in the journal Science, "the known global extinction rates of vertebrate and plant species have an unrecorded parallel among the invertebrates, strengthening the hypothesis that the natural world is experiencing the sixth major extinction event in its history."

The British study found that over 20 years, 71 per cent of all butterfly species had declined. Birds and plants declined as well, but butterflies fared worst of all. In a trope coined by Sandy Knapp, a scholar at London's Natural History Museum, Britain is the canary in the coal mine for the rest of the world, with effects detected in that country likely to turn up elsewhere. The best way to watch for this is to go out and count.

Known as the Canada Day count, the tallies actually take place on, before or after July 1. They provide a snapshot of numbers of species and butterfly abundance across a span of time when the insects are likely to be flourishing.

Each census covers the area of a 24-kilometre-wide circle. The Toronto West count, set for next Saturday, includes urban streets, the peninsula of the Leslie Street Spit, High Park and the scruffy, butterfly-rich strips of unused railway lines. Among newcomers from the south are the little yellow and the fiery skipper. The silvery blue, now an established colonist, arrived from the other direction. "They pushed down from the north," says John Carley, the leader of the Toronto West count, "following the spread of their food supply, crown vetch."

At the other end of town, the Toronto East count, led by Tom Mason, plunged into the Rouge River Valley and assorted creeks on July 1 and came up with 43 species, including 36 tiger swallowtails, 176 little wood satyrs and a painted lady. There were no silvery checkerspots, but none was expected: They vanished from Taylor Creek eight years ago. Nor was there a meadow fritillary, which Mr. Mason used to find in the Rouge.

Peter Hall, a research associate at the Canadian National Collection of Insects and one of the authors of Butterflies of Canada, leads a count west of Ottawa and has found 70 species over the years. These days, the haul could include a Delaware skipper, not seen in Eastern Ontario until 1998, when Mr. Hall found the first.

"I could take you out right now," he says, "and find it at 15 locations."

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The census area also contains an unusual, limestone habitat called an alvar that attracts the mottled duskywing, a kind of butterfly known as a skipper for its rapid, darting flight. The mottled duskywing has a striking purplish gloss and light yellow-brown highlights on the wings. It is rare in Canada and may soon be listed as endangered.

Although Mr. Hall was often alone when he first began to take the census, he expects about 40 counters to join him today or, if it's raining, tomorrow. Even though interest in butterflies is growing in Canada, some country's populations are much better known.

"I lived in England," Mr. Hall says, "and if you go out to a small woodlot when there's something like a purple emperor to be seen, you'll run into 100 people, all looking for the same butterfly."

Canada has 297 species of butterfly and Britain about 60, so it is perhaps natural that the latter's passion for Lepidoptera should spill over into moths - as when, last month, National Moth Night in Britain drew a group of the smitten to London's Holland Park.

At 9:15 on the appointed night, we trooped up a grassy hill to the edge of a wood. A peacock screeched in the twilight. In the background, an orchestra thundered its way through Il Trovatore in the summer-opera tent, but the moth group might have been a hundred miles away - nine people chatting in low voices as Tim Freed unpacked his apparatus.

Dr. Freed is a cheerful, methodical man who has been tracking the fortunes of British Lepidoptera since his parents dropped The Observer's Book of British Butterflies into his lap when he was 5. "They couldn't get me to read," he says, "so they tried that."

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He finished setting up his Robinson trap - a plastic tub with a Plexiglas cover - and switched on the 125-watt mercury vapour light. Such bulbs emit a light especially attractive to moths, which fly through a funnel into the trap.

One of the species we hoped to find was Monopis fenestratella, a tiny moth with little translucent panes in its front wings. Unlike its enormous distant cousin, the death's-head hawkmoth, or the brilliant daylight-flying cinnabar moth, fenestratella has no common name, and as things are going it may vanish before it gets one: The Lilliputian creature is on Britain's Red Data List of endangered species.

Many handsome little animals flew into the trap that night. In the morning light, we looked at them through Dr. Freed's field glass: the sliver of blazing silver that is the apple leaf miner; a light emerald, trembling in a glass tube, its wings a pale, dusty pistachio. We found a blotched emerald, a marbled orchard tortrix, a small magpie and a treble-bar. We did not find fenestratella.

"I'm disappointed," Dr. Freed confessed. "I've had it here before."

He began the laborious business of packing up his traps to drive to his next location, Buckingham Palace Garden. Many wonders are to be found inside the 39-acre private park behind the palace - the flamingo lake, an ancient mulberry tree, a 20-ton marble urn known as the Waterloo Vase, and the plumed fan-foot.

"It's colonized the garden," Dr. Freed said. "I've been running a trap at the palace for two years, and I've been getting the plumed fan-foot and nobody else has. It's on the Kentish coast and one or two other places and that's it."

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The tabulations of this year's moth count are still months away. Last year, 870 traps at 761 locations caught more than 75,000 moths representing 887 of Britain's roughly 2,500 species.

From such snapshots, British experts conclude that the island's moth numbers and diversity are in decline.

"Between 1968 and 2002," says Richard Fox of Butterfly Conservation, which advises the British government on biodiversity, "the total abundance of moths in Britain decreased by 31 per cent. We examined population trends for each of a group of 337 common moths, and found that two-thirds of the species had declined, many of them severely. Seventy-five species had declined by more than 70 per cent and another 57 had decreased by more than half."

The morning after he had trapped in the palace garden, I called Tim Freed on his cell. I asked if he had found the plumed fan-foot. He had not.

SPECIES ON THE MOVE

Moths and butterflies are exceptionally fragile: A hiccup in the weather can exterminate a colony. It is not always certain why they pop up here or there, but a warming climate is increasingly suspected as a shifter of species. A review of 20 years of data shows that two-thirds of European butterfly species have moved their ranges north, and only a relatively small number made a similar range extension to the south.

Peter Hall, who is writing a report on Canada's butterfly populations, has identified the same trend in Ontario. In the past 15 years, the pearl crescent and the eastern tailed blue have joined the Delaware skipper in a range shift to the north. In addition, the tawny-edged skipper, which used to cycle through two generations in a single summer in Southwestern Ontario, but only a single generation at the more northerly latitude of Ottawa, now goes through two generations in both locations. Yet while some butterflies find climate opportunities, others vanish altogether in the general attrition of animal species.

"The tawny crescent may be completely gone from the Eastern United States, where it used to be widespread," Mr. Hall says. "It's in the Ottawa Valley, because I actually went out the other day looking for it. I knew where there was a colony and it's still there. But who's to say how long?"

Ross Layberry, a veteran lepidopterist and one of Mr. Hall's Butterflies of Canada co-authors, says large parts of North America lack the observational data to support climate-change conclusions about butterflies.

"Until 2004, there were no resident collectors/observers in the Northwest Territories or Nunavut, an area bigger than Europe. There are now three in the Territories, and surprise, surprise, they are finding all kinds of new records. But only three are possibly the result of northward movement. The cabbage white has been found in Yellowknife, Fort Simpson and Nahanni Butte, right in the towns, where you would expect them if the larvae came in on cabbages. But also in the middle of Nahanni National Park, where no collecting had ever been done until 2005. Who knows how long it took to spread that far?

"A local in Yellowknife has in his collection a Peck's skipper, and I caught a long dash skipper near Hay River. Both of these were first records for the Northwest Territories, but they have been long known in extreme northern Alberta, just 100 miles or so further south."

Peter Hall knows of no butterfly species that has disappeared from Canada, although there are 17 listed for protection under Canada's Species at Risk Act. Most of the negative pressure on butterfly populations, says Mr. Hall, appears to come from human development and consequent loss of butterfly habitat. In fact, a recent report from the Commission for Environmental Cooperation in Montreal warned that migrating monarchs are increasingly threatened by pesticides and urban sprawl.

The problem of how to weigh an apparent butterfly bounty from climate warming against such negative factors as habitat loss has been studied in Britain, where researchers found an ultimately negative effect. Some butterflies thrive; most suffer.

"The same situation holds today," says Jeremy Thomas, professor of ecology at Oxford and a fellow at the government-funded Centre for Ecology and Hydrology, "and there's no evidence of a change in the downward long-term trend in most U.K. butterfly species' populations.

"Moreover, there is growing evidence that similar, if not worse, declines are occurring in other monitored insects, such as moths and bumblebees. It used to be thought that butterflies were not the same as other insects, but we have found that, in fact, they are pretty representative. What's happening to them is happening to other insects too, and insects are half of all known species of animal in the world."

"Over all," says Mr. Fox of Butterfly Conservation, "things are bad for butterflies. It's true that Britain is a pretty messed-around environment, but if you look around the world, that's what you mostly see, and often worse."

Matthew Hart is a Canadian writer based in London.

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