Here’s a modern scientific interpretation, courtesy of science journalist Nicholas DeMarino: “Flies rub their limbs together to clean them. This may seem counterintuitive given these insects’ seemingly insatiable lust for filth and grime, but grooming is actually one of their primary activities. It gets rid of physical and chemical detritus and clears up their smell receptors—all of which is important for flying, finding food, courting mates and just about everything that a fly does.”