<Li> A species of newt, frog or eel (there is disagreement on this point) found in the Lincolnshire Fens had yellow undersides . </Li> <Li> Bacon hung up and stored for a long time turned yellow (reasty). </Li> <Li> The backs of farm workers who stripped to the waist in hot weather turned dark brown but their bellies turned yellow . </Li> <Li> Opium extracted from poppy heads, and taken to relieve malaria that was prevalent in the fens in earlier centuries, turned the skin a shade of yellow . </Li>

Where does the phrase yellow bellied come from