IF YOU VISIT A TRADITIONAL lambic brewery in Belgium, you’ll see spiders spinning webs among the casks. They are not a nuisance, and brewers don’t swat them away. The spiders are there by design to protect the fruity beer from fruit flies. The webs do, however, create a fitting environment for lambic, which can seem a little like magic. Using the oldest of all modern brewing styles, brewers summon wild yeast, resulting in funky, sour beers.