The chances of living to be 100 have never been better in Britain, according to the Office for National Statistics' latest data. There are 9,300 centenarians in the country now, compared with 100 a century ago.

But at the same time, through one of science's stranger connections, the chances of spotting a famous British insect have sunk by a similar ratio. We trade our longer lives for a decline, which looks terminal, in the melanistic - or dark-coloured variety - of the Peppered Moth.

The relation of older people to creepie-crawlies may sound curious, but it is based on pollution. Smog allowed Biston betularia carbonaria to escape bird predators through camouflage at the same time as it suffocated the elderly. Meanwhile, the standard Peppered Moth, a bright, light creature with a dusting of black spots, which account for its name, was hopelessly vulnerable when it perched on trees or walls in sooty old Britain. The link was established in the 1960s by the geneticist Sir Cyril Clarke, as clean air laws relieved humanity but made life deadlier for the dark version of the moths.

Through apparently trivial experiments breeding swallowtails, Clarke had already helped solve the rhesus negative blood condition that killed or injured thousands of babies. With the centenarians, he enterprisingly correlated telegrams from Buckingham Palace with the contents of moth traps.

Has the trend continued? "Absolutely," says Dr Mike Majerus of Cambridge University's department of genetics, who updated Clarke's work in 1993 and has since kept tabs on the moths. "We predicted that the melanistic version would reach less than 1% of Britain's Peppered Moth population by 2019. Here in Cambridge, it already has."

That counts effectively as extinction, within 80 years of carbonaria's glory days, when it formed between 95% and 98% of all British Peppereds. The form now clings on resolutely in smokier parts of the north-east, where it has only just fallen below 20%, and similarly, pensioners' numbers have seen a more modest rise.