BUTTERFLY LESSONS.(Polygonia c-album)

Polygonia c-album, generally known as the comma butterfly, spends most of its life pretending to be something else. In its larval, or caterpillar, stage, it has a chalky stripe down its back which makes it look uncannily like a bird dropping. As an adult, with wings folded, it is practically indistinguishable from a dead leaf. The comma gets its name from a tiny white mark shaped like the letter "c" on its underside. Even this is thought to be part of its camouflage--an ersatz rip of the sort that leaves get when they are particularly old and tatty.

The comma is a European butterfly--its American cousins are the hop merchant and the question mark--and it can be found in France, where it is known as le Robert-le-Diable; Germany, where it is called der C-Falter; and the Netherlands, where it is gehakkelde aurelia. The comma reaches the northern edge of its distribution in Britain. This is unremarkable--many European butterflies come to the end of their range in England--but from a scientific standpoint fortunate.

The English have been watching and collecting butterflies for centuries--some of the specimens in the British Natural History Museum date back to the seventeen-hundreds--and in the Victorian era passion for the hobby was such that even many small towns supported their own entomological societies. In the nineteen-sixties, Britain's Biological Records Centre decided to marshal this enthusiasm for a project called the Lepidoptera Distribution Maps Scheme, whose aim was to chart precisely where each of the country's fifty-nine native species could--and could not--be found. More than two thousand butterfly enthusiasts participated, and in 1984 the results were collated into a hundred-and-fifty-eight-page atlas. Every species got its own map, with black dots showing where it had been sighted. On the map for Polygonia c-album, the comma's range was shown to extend from the south coast of England up to Liverpool in the west and Norfolk in the east. Almost immediately, the map became out of date; in the years that followed, hobbyists kept finding the comma in new areas. By the late nineteen-nineties, the butterfly was frequently being sighted in the north of England, near Durham. By now, it is established in southern Scotland, and has been sighted as far up as the Highlands. The rate of the comma's expansion--some fifty miles per decade--was described by the authors of the most recent butterfly atlas as "remarkable."

Chris Thomas is a biologist at the University of York who studies lepidoptera. He is tall and rangy, with an Ethan Hawke-style goatee and an amiably harried manner. The day I met him, he had just returned from looking for butterflies in Wales, and the first thing he said to me when I got into his car was please not to mind the smell of wet socks. A few years ago, Thomas, his wife, their two sets of twins, an Irish wolfhound, a pony, some rabbits, two cats, and several chickens moved into an old farmhouse in the village of Wistow, in the Vale of York. The University of York has an array of thermostatic chambers where commas are raised under temperature-controlled conditions, fed carefully monitored diets, and measured on a near-constant basis, but, in the spirit of British amateurism, Thomas decided to turn his own back yard into a field lab. He scattered wildflower seeds he had collected from nearby meadows and ditches, planted nearly seven hundred trees, and waited for the butterflies to show up. When I visited the place in midsummer, the wildflowers were in bloom and the grass was so high that many of the tiny trees looked lost, like kids in search of their parents. The Vale of York is almost completely flat--during the last ice age, it formed the bottom of a giant lake--and from the yard Thomas pointed out the spires of Selby Abbey, built nearly a thousand years ago, and also the cooling towers of the Drax power plant, Britain's largest, some ten miles away. It was cloudy, and since butterflies don't generally fly when it's gray, we went inside.

Butterflies, Thomas explained after putting the kettle on for tea, can be divided into two groups. The so-called "specialists" require specific--in some cases unique--conditions. Specialists include the chalkhill blue (Polyommatus coridon), a large turquoise butterfly that feeds exclusively on horseshoe vetch, and the purple emperor (Apatura iris), which flies in the treetops of well-wooded areas in southern England. The "generalists" are less picky. Among Britain's generalists, there are, in addition to the comma, ten species that are widespread in the southern part of the country and reach the edge of their range somewhere in the nation's midsection. "Every single one has moved northward since 1982," Thomas told me. A few years ago, with lepidopterists from, among other places, the United States, Sweden, France, and Estonia, Thomas conducted a survey of all the studies that had been done on generalists that reach the northern limits of their ranges in Europe. The survey looked at thirty-five species in all. Of these, the scientists found, twenty-two had shifted their range northward in recent decades; only one had shifted south.

After a while, the sun emerged, and we went back outside. Thomas's wolfhound, Rex, a dog the size of a small horse, trailed behind us, panting heavily. Within about five minutes, Thomas had identified a meadow brown (Maniola jurtina), a small tortoiseshell (Aglais urticae), and a green-veined white (Pieris napi), all species that have been flitting around Yorkshire since butterfly recordkeeping began. Thomas also spotted a gatekeeper (Pyronia tithonus) and a small ...