If you’ve seen one then you’ll know what I’m talking about. On a warm day in March, maybe your first afternoon without a coat, a flash of brilliant yellow crosses your path. It takes a split second to realise that this is a butterfly, and by then, flying above head height, the brimstone has streaked away faster than you can run. Aptly named after the sulphurous flare in an alchemist’s laboratory, the brimstone butterfly is one of the most dramatic signs of spring. A blaze of colour on a still drab world, it promises that the base metal of winter is over, the gold of spring here. With such distinctive tincture, many people consider brimstones to be the main candidate for clearing up the mystery of that most beloved word – butterfly. In a certain light, especia